MTJ的温度效应

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We demonstrate a significant effect of atomic-scale MgO insertion layers on the tunnel magnetoresistance (TMR) in epitaxial magnetic tunnel junctions (MTJs) using a small bandgap oxide MgGa2O4. An enhanced TMR ratio of 151% at room temperature (resistance area product, RA: 23 kΩ ⋅ μm2) and 291% at 5 K (RA: 26 kΩ ⋅ μm2) were observed using 0.3 nm MgO insertion layers at the bottom and top barrier interfaces in Fe/MgGa2O4/Fe(001) MTJs with a total barrier thickness of 2.3 nm. The TMR showed a strong MgO thickness dependence. Microstructure analyses revealed that after MgO insertion, a homogeneous rock-salt structured Mg0.55Ga0.45O(001) barrier is formed, which differs from the nominal spinel crystal MgGa2O4. Elemental mapping of the MTJ showed that Ga diffusion into the adjacent Fe can be effectively suppressed while maintaining perfect lattice-matching at the Fe/barrier interfaces, thereby improving effective tunneling spin polarization through the barrier. The RA of the Mg0.55Ga0.45O (2.3 nm) MTJ is smaller than that of a comparable MgAl2O4 barrier (2.3 nm), thanks to the lower barrier height of the Mg0.55Ga0.45O as confirmed by the current–voltage characteristics.

We demonstrated a magnetic tunnel junction (MTJ) consisting of Fe/MgO-Si-MgO/Fe. Si layer was deposited at room temperature and at 700 °C; when deposited at 700 °C, Si diffused into the MgO layer. The MTJ with silicon deposited at 700 °C attained high MR ratios of up to 38.7 and 2.9% at t Si = 0.19 and 1.3 nm, respectively. Low-temperature measurements established that the temperature dependence of the MR ratio and resistance between MTJs with and without diffused silicon are significantly different. This behavior confirms that the Si-MgO channel acts as an impurity semiconductor in the MTJ.

In STT-MRAM, breakdown of the nanometer thin MgO barrier layer in Magnetic Tunnel Junctions (MTJ) is impacted by self-heating due to high operating currents. The simulated temperature increase at breakdown conditions can reach 200◦C, which dominates breakdown acceleration. The self-heating depends on area, which results in the absence of VBD-area scaling. In addition, process variability introduces variation in Resistance-Area product (RA) and critical dimension (CD), both impacting reliability. We propose an empirical model to fit both RA and CD dependence. This model offers a fair benchmark for stack and process optimization. Moreover, we demonstrate improved reliability for MTJs with W-based spacers and MTJs with end-of-line anneal at 400°C, 180 min.

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Flexible spintronic devices hold significant potential for applications in wearable electronics and other emerging fields due to their excellent mechanical flexibility. However, most studies on the temperature dependence of magnetic tunnel junctions (MTJs) have primarily focused on rigid silicon-based substrates, while systematic investigations of flexible MTJs remain limited. This study systematically examines the temperature dependence of the tunnel magnetoresistance (TMR) ratio, the parallel (P) and the antiparallel (AP) resistance, and dynamic conductance of MTJs fabricated on polyimide (PI) substrates. Our experimental results demonstrate that the temperature-dependent behavior of TMR in flexible MTJs closely resembles that of conventional silicon-substrate MTJs. This finding suggests that optimization strategies for rigid MTJs structures can be applied to flexible MTJs, accelerating the development of flexible spintronics. We also observe a significant reduction in the coercivity of flexible MTJs at low temperatures, a trend that contrasts with the coercivity variation in silicon-based MTJs. This distinct feature provides an alternative approach for tuning flexible MTJs and provides new design insights for future stress-controlled spintronic devices.

Temperature dependence of retention energy of MTJs with strain-induced magnetic anisotropy (SIMA-MTJs) was estimated by performing repeated measurements on the coercivity of the storage layer and fitting the switching probability data obtained from them with Sharrock’s formula. The retention energy of SIMA-MTJs at <inline-formula> <tex-math notation="LaTeX">$85~^{\circ }$ </tex-math></inline-formula> C increased by 14% compared with that at room temperature on average. Also, over 300% of tunnel magnetoresistance (TMR) was demonstrated with 1.9 nm-thick storage layer. Furthermore, it was estimated that the switching current for 1.9 nm-thick storage layer can be lowered to the level of <inline-formula> <tex-math notation="LaTeX">$20~\mu $ </tex-math></inline-formula> A with a retention energy of 80 kBT by practically available values of interface magnetic anisotropy, <inline-formula> <tex-math notation="LaTeX">$K_{s} =1.6$ </tex-math></inline-formula> erg/cm2, and spin-Hall angle of 0.3.

In recent years, two-dimensional (2D) multiferroic tunnel junctions (MFTJs) have attracted much attention due to their characteristics and potential application value. However, many current 2D MFTJs encounter the challenge that high tunneling electroresistance (TER) and high tunneling magnetoresistance (TMR) cannot coexist with a low resistance-area (RA) product simultaneously. Particularly, a high RA product tends to result in a low absolute variation in conductance, which limits the device's applicability. In addition, the phase transition temperature of the magnetic layer of some 2D MFTJs is not high enough. These have limited their practical applications to a certain extent. In this study, we theoretically constructed MFTJs using the Janus ferroelectric (FE) materials α-In2S2Se (ISS-1) and monolayer α-In2SSe2 (ISS-2) as the intermediate insulating layer, with the high Curie temperature ferromagnetic (FM) material Fe3GaTe2 (FGT) on both sides. This strategy concurrently achieved a maximum TER of 136%, a maximum TMR of 523%, and a minimum RA product of 0.06 Ω·μm2 under zero bias. Meanwhile, the maximum conductance variation caused by polarization reversal ΔGP reached 1.02 μS and caused by magnetization configuration change ΔGM reached 1.70 μS. Furthermore, the variations in the corresponding properties under nonzero bias have been calculated. Our strategy not only provides a new means to realize the TER effect but also provides some theoretical guidance for realizing 2D MFTJs with better performance.

A promising area for developing devices beyond conventional metal-oxide-semiconductor (CMOS) technology is spintronics, which uses the spin degree of electrons as the information vector. The advent of 2-dimensional materials has given spintronic devices unique platforms, and numerous innovative and novel application concepts have been presented. This article evaluates the effect of adding composite dielectrics (ZrO2 or TiO2 or Ta2O5) to the MgO dielectric layer to create a MgO-ZrO2/TiO2/Ta2Os-MgO composite dielectric layer. Non-equilibrium Green's function (NEGF) driven simulator is used to run the simulations. The important transport properties of the MTJ, such as the anti-parallel and parallel resistances, anti-parallel and parallel differential resistances, TMR, differential TMR, and both the out-of-plane and in-plane spin transfer torques (STT) have been calculated using simulator based on NEGF quantum transport. To examine the reliability and performance of the MTJ model under the temperature variation, the temperature-related analysis of TMR has been analyzed for both the conventional and proposed MTJ models. Interestingly, the maximum TMR at 13 K and at room temperature is obtained for the Tu2O5-based composite dielectric MTJ model, with 549% and 282% respectively. Hence, the design of the MTJ memory with a Taz2O5-based composite dielectric composition is found to be superior.

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Due to their compact size and exceptional sensitivity at room temperature, magnetoresistance (MR) sensors have garnered considerable interest in numerous fields, particularly in the detection of weak magnetic signals in biological systems. The "magnetrodes", integrating MR sensors with needle-shaped Si-based substrates, are designed to be inserted into the brain for local magnetic field detection. Although recent research has predominantly focused on giant magnetoresistance (GMR) sensors, tunnel magnetoresistance (TMR) sensors exhibit a significantly higher sensitivity. In this study, we introduce TMR-based magnetrodes featuring TMR sensors at both the tip and midsection of the probe, enabling detection of local magnetic fields at varied spatial positions. To enhance detectivity, we designed and fabricated magnetrodes with varied aspect ratios of the free layer, incorporating diverse junction shapes, quantities, and serial arrangements. Utilizing a custom-built magnetotransport and noise measurement system for characterization, our TMR-based magnetrode demonstrates a limit of detection (LOD) of 300pT/Hz at 1 kHz. This implies that neuronal spikes can be distinguished with minimal averaging, thereby facilitating the elucidation of their magnetic properties.

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In the field of spin caloritronics, spin-dependent transport phenomena are observed in a number of current experiments where a temperature gradient across a nanostructured interface is applied. The interpretation of these experiments is not clear as both phonons and electrons may contribute to thermal transport. Therefore, it still remains an open question how the temperature drop across a magnetic nanostructured interface arises microscopically. We answer this question for the case of a magnetic tunnel junction (MTJ) where the tunneling magneto-Seebeck effect occurs. Our explanation may be extended to other types of nanostructured interfaces. We explicitly calculate phonon and electron thermal conductance across Fe/MgO/Fe MTJs in an ab initio approach using a Green function method. Furthermore, we are able to calculate the electron and phonon temperature profile across the Fe/MgO/Fe MTJ by estimating the electron-phonon interaction in the Fe leads. Our results show that there is an electron-phonon temperature imbalance at the Fe-MgO interfaces. As a consequence, a revision of the interpretation of current experimental measurements may be necessary.

Improving the thermal resilience of magnetic tunnel junctions (MTJs) broadens their applicability as sensing devices and is necessary to ensure their operation under harsh environments. In this work, we are address the impact of temperature on the degradation of the magnetic reference in field sensor stacks based on MgO-MTJs. Our study starts by simple MnIr/CoFe bilayers to gather enough insights into the role of critical morphological and magnetic parameters and their impact in the temperature dependent behavior. The exchange bias coupling field (H ex), coercive field (H c), and blocking temperature (T b) distribution are tuned, combining tailored growth conditions of the antiferromagnet and different buffer layer materials and stackings. This is achieved by a unique combination of ion beam deposition and magnetron sputtering, without vaccum break. Then, the work then extends beyond bilayers into more complex state-of-the-art MgO MTJ stacks as those employed in commercial sensing applications. We systematically address their characteristic fields, such as the width of the antiferromagnetic coupling plateau ΔH, and study their dependence on temperature. Although, [Ta/CuN] buffers showed higher key performance indications (e.g. H ex) at room temperature in both bilayers and MTJs, [Ta/Ru] buffers showed an overall wider ΔH up to 200 °C, more suitable to push high temperature operations. This result highlights the importance of properly design a suitable buffer layer system and addressing the complete MTJ behavior as function of temperature, to deliver the best stacking design with highest resilience to high temperature environments.

Significance One of the most promising emerging memory technologies is magnetic random access memory (MRAM), which promises to be a high-performance, nonvolatile memory. The essential feature of MRAM is the efficient switching of the magnetic tunnel junction (MTJ) memory cell between two distinct resistance states associated with the relative magnetic orientation of the ferromagnetic electrodes which sandwich the tunnel barrier by using magnetic fields, charge currents, or pure spin currents. In this paper, a temperature difference of just a few kelvin across an ultrathin (∼1 nm) MgO tunnel barrier is found to generate giant spin currents sufficient to significantly influence the switching of the MTJ. These spin currents get created only in those devices that show a large asymmetry in their tunneling conductance across zero bias voltage. Spin-polarized charge currents induce magnetic tunnel junction (MTJ) switching by virtue of spin-transfer torque (STT). Recently, by taking advantage of the spin-dependent thermoelectric properties of magnetic materials, novel means of generating spin currents from temperature gradients, and their associated thermal-spin torques (TSTs), have been proposed, but so far these TSTs have not been large enough to influence MTJ switching. Here we demonstrate significant TSTs in MTJs by generating large temperature gradients across ultrathin MgO tunnel barriers that considerably affect the switching fields of the MTJ. We attribute the origin of the TST to an asymmetry of the tunneling conductance across the zero-bias voltage of the MTJ. Remarkably, we estimate through magneto-Seebeck voltage measurements that the charge currents that would be generated due to the temperature gradient would give rise to STT that is a thousand times too small to account for the changes in switching fields that we observe.

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Striking advancement of science over the last few decades has doubled the need of having faster and more efficient electronic devices. Magnetic tunnel junction-based molecular spintronic devices (MTJMSDs) are potential platforms for futuristic computers and may significantly reduce power consumption and enhance processing speed [1], [2]. Using transport properties of electrons, MTJMSD creates conductive molecular channels between two FM electrodes (FMEs). In our previous work, we investigated the effect of several factors on MTJMSDs' magnetic properties through Monte Carlo Simulation (MCS). Our results showed that i) Molecule-FMEs' coupling strength and nature ii) FMEs' length and thickness and iii) thermal energy have determinative effect on MTJMSD magnetic behavior [3]. For our initial comprehension, we constrained our earlier studies to just electrons' transport properties via molecular channels. In this research we took one step further towards realization of MTJMSD magnetic properties and investigated the effect of spin fluctuation (SF) as well. Here, we report the result of an extreme case where molecules made a strong antiFM coupling with one electrode and a strong FM coupling with another one at room temperature (KT=0.1 of the Curie temperature) for a fixed device size. Our preliminary results show that MTJMSD's need more iteration counts to attain equilibrium state in the presence of SFs. According to our MCS results, 16 molecules can induce antiFM coupling between FMEs in both with and without SF cases. However, the spatial orientation of M is noisier in the presence of SF despite doing 500 million simulation counts. The correlation results agree with spatial orientation of electrodes and molecules' magnetic moment. Based on our observation, there is a strong negative/antiferromagnetic correlation between FMEs when there is no SF. However, there are multiple pockets of average to high negative correlation between FMEs and molecules while applying SF effect. To complement our study and gain a better understanding of the role of SF on MTJMSD's magnetic properties, we will also investigate time evolution of energy, magnetic susceptibility and coupling energy required for transition from low to high magnetization.

Temperature sensors are becoming an increasingly important component in System-on-Chip (SoC) designs with increasing transistor scaling, power density and associated heating effects. This work explores a compact nanoelectronic temperature sensor based on a Magnetic Tunnel Junction (MTJ) structure. The MTJ switches probabilistically depending on the operating temperature in the presence of thermal noise. Performance evaluation of the proposed MTJ temperature sensor, based on experimentally measured device parameters, reveals that the sensor is able to achieve a conversion rate of 2.5K samples/s with energy consumption of 8.8 nJ per conversion (1–2 orders of magnitude lower than state-of-the-art CMOS sensors) for a linear sensing regime of 200–400 K.

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We developed a tunnel magnetoresistance (TMR) sensor consisting of a CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) and a CoFeSiB amorphous soft magnetic layer. This multilayer structure is promising for a high-sensitivity sensor because a giant TMR ratio of the MTJ and a small anisotropy field Hk of the free layer can be obtained simultaneously. However, the soft magnetic properties of the CoFeSiB layer disappear when it is annealed at above the crystallization temperature (around 300 °C), which determines the thermal tolerance of the TMR sensor and limits improvements to the sensor's sensitivity and applications. In this study, we doped the CoFeSiB layer with various amounts of Ta to raise its crystallization temperature. TMR sensors using the Ta-doped CoFeSiB layers showed thermal tolerance to annealing temperatures above 425 °C, whereas the sensor with the undoped CoFeSiB layer was tolerant to annealing temperatures up to 325 °C. As well, the Ta doping effectively reduced Hk of the CoFeSiB layer, which resulted in a sensitivity of 50%/Oe, over three times higher than the sensor with the undoped CoFeSiB layer. These results pave the way toward next-generation TMR sensors having higher sensitivity and wider applicability.

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Nanoscale molecular spintronics devices (MSDs) are poised to transform logic and memory technologies. Paramagnetic nanostructures, such as molecules exchange-coupled to ferromagnetic electrodes, can yield novel magnetic material properties. Break-junction nanogap-based MSDs enable single-molecule devices, while magnetic tunnel junctions (MTJs) with sub-2 nm gaps allow for thousands of spin channels, potentially transforming MTJs. However, the role of varying numbers of paramagnetic nanostructures within the device and its stability over time and temperature remains unexplored. This study uses an MTJ-based MSD (MTJMSD) architecture with a Heisenberg model to simulate the magnetic effects of molecular nanostructures along the MTJ perimeter. By varying molecular populations, we analyze the MTJMSD’s magnetic moment and spatial correlations under different thermal energies. The findings reveal atomistic spatio temporal insights into the impact of molecular population and thermal factors on MTJMSD and its components, two ferromagnetic electrodes, and molecular analog.

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We investigate the response of magnetic tunnel junction (MTJ) devices based on GlobalFoundries 22FDX <xref ref-type="fn" rid="fn1">1</xref> embedded-magnetic random access memory (MRAM) technology to external thermal and magnetic stress. An anomalous reversal of the reference system was observed in some devices when subjected to a constant static external magnetic field at temperatures as high as <inline-formula> <tex-math notation="LaTeX">$150~^{\circ }$ </tex-math></inline-formula>C. The strength of the external magnetic field, ambient temperature, MTJ diameter, and composition of the synthetic antiferromagnet (SAF) reference system all affect the severity of the reference system’s instability. In this study, we show that while a SAF reversal in single-bit MTJ devices reverses the direction of their <italic>R</italic>–<italic>H</italic> hysteresis loop and so their switching field and offset field polarity, it does not significantly impact their electrical switching behavior. Furthermore, we experimentally show that the functionality of 40-Mbit MRAM arrays with a pitch of approximately 200 nm remains unaffected by the SAF configuration and consequent offset field polarity of the individual devices.

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We study tunneling magnetothermopower (TMTP) in CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by an electric heater line. Thermopower voltages up to a few tens of μV between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence the thermal gradient. The thermopower signal varies by up to 10  μV upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresponds to a large spin-dependent Seebeck coefficient of the order of 100  μV/K and a large TMTP change of the tunnel junction of up to 90%.

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Recently, double barrier magnetic tunnel junction (DBMTJ) can be applied for spin transfer torque magnetic random access memory (STT-MARM) technology. DBMTJ provides a higher thermal stability factor and a lower critical current when comparing with the single barrier magnetic tunnel junction (SBMTJ). However, the double MgO layers in structure can affect the high temperature during switch process. Therefore, this work explored the different effect of temperature increment on both SBMTJ and DBMTJ during the switching current flow. Structures for study consist of the SBMTJ with MgO thickness of 0.9 nm, the DBMTJ with MgO thickness of 0.9 nm (DBMTJ(A)) and the DBMTJ with MgO thickness of 1.3 nm (DBMTJ(B)). Simulation can be achieved by 3D finite element method. The results show that although the DBMTJ(B) has the minimum critical current when comparing with DBMTJ(A) and SBMTJ, the maximum temperature increment is found with DBMTJ(B) at the same switching current in the switching process. Hence, the thermal reliability of DBMTJ device applied for in-plane MTJ in STT-MRAM is interesting for improvement of the memory technology.

While magnetoresistive random-access memory (MRAM) stands out as a leading candidate for embedded nonvolatile memory and last-level cache applications, its endurance is compromised by substantial self-heating due to the high programming current density. The effect of self-heating on the endurance of the magnetic tunnel junction (MTJ) has primarily been studied in spin-transfer torque (STT)-MRAM. Here, we analyze the transient temperature response of two-terminal spin–orbit torque (SOT)-MRAM with a 1 ns switching current pulse using electro-thermal simulations. We estimate a peak temperature range of 350–450 °C in 40 nm diameter MTJs, underscoring the critical need for thermal management to improve endurance. We suggest several thermal engineering strategies to reduce the peak temperature by up to 120 °C in such devices, which could improve their endurance by at least a factor of 1000× at 0.75 V operating voltage. These results suggest that two-terminal SOT-MRAM could significantly outperform conventional STT-MRAM in terms of endurance, substantially benefiting from thermal engineering. These insights are pivotal for thermal optimization strategies in the development of MRAM technologies.

The exceptional properties of two‐dimensional (2D) magnet materials present a novel approach to fabricate functional magnetic tunnel junctions (MTJ) by constructing full van der Waals (vdW) heterostructures with atomically sharp and clean interfaces. The exploration of vdW MTJ devices with high working temperature and adjustable functionalities holds great potential for advancing the application of 2D materials in magnetic sensing and data storage. Here, we report the observation of highly tunable room‐temperature tunneling magnetoresistance through electronic means in a full vdW Fe3GaTe2/WSe2/Fe3GaTe2 MTJ. The spin valve effect of the MTJ can be detected even with the current below 1 nA, both at low and room temperatures, yielding a tunneling magnetoresistance (TMR) of 340% at 2 K and 50% at 300 K, respectively. Importantly, the magnitude and sign of TMR can be modulated by a DC bias current, even at room temperature, a capability that was previously unrealized in full vdW MTJs. This tunable TMR arises from the contribution of energy‐dependent localized spin states in the metallic ferromagnet Fe3GaTe2 during tunnel transport when a finite electrical bias is applied. Our work offers a new perspective for designing and exploring room‐temperature tunable spintronic devices based on vdW magnet heterostructures.

Magnetic tunnel junctions (MTJs) are the elemental devices for advanced spintronic technologies, where tunneling magnetoresistance (TMR) serves as one of the key performance metrics. Here, we used the topologically nontrivial magnetic insulator CrVI6 and magnetic metal Fe3GeTe2 to fabricate CrVI6/Fe3GeTe2 heterojunctions and Fe3GeTe2/CrVI6/Fe3GeTe2 MTJs. In the heterojunctions, the addition of CrVI6 led to a 180% coercive field enhancement of Fe3GeTe2 near the Curie temperature (TC) of CrVI6, which originated from the antiferromagnetic coupling between them. More importantly, a temperature-dependent TMR sign reversal in the Fe3GeTe2/CrVI6/Fe3GeTe2 MTJ was observed, gradually transforming from a negative value at low temperature to a positive value above 60 K, close to the TC of CrVI6. The results demonstrate that the spin-filtering effect, with -opposite polarity of CrVI6 to Fe3GeTe2, constitutes the primary mechanism for temperature-dependent TMR sign reversal. Furthermore, under combined temperature and bias voltage modulation, we observed the coexistence of both positive and negative TMR. This finding suggests the potential extension of conventional bistate MTJ operation to multi-state functionality. This research broadens the controllable degrees of freedom in MTJs and advances van der Waals magnet-based spintronic devices.

Magnetic tunnel junctions (MTJs) based on van der Waals (vdW) heterostructures with sharp and clean interfaces on the atomic scale are essential for the application of next-generation spintronics. However, the lack of room-temperature intrinsic ferromagnetic crystals with perpendicular magnetic anisotropy has greatly hindered the development of vertical MTJs. The discovery of room-temperature intrinsic ferromagnetic two-dimensional (2D) crystal Fe3GaTe2 has solved the problem and greatly facilitated the realization of practical spintronic devices. Here, we demonstrate a room-temperature MTJ based on a Fe3GaTe2/WS2/Fe3GaTe2 heterostructure for the first time. The tunneling magnetoresistance (TMR) ratio is up to 213% with a high spin polarization of 72% at 10 K, the highest ever reported in Fe3GaTe2-based MTJs up to now. A tunneling spin-valve signal robustly persists at room temperature (300 K) with a bias current down to 10 nA. Moreover, the spin polarization can be modulated by bias current and the TMR shows a sign reversal at a large bias current. Our work sheds light on the potential application of low-energy consumption in all-2D vdW spintronics and offers alternative routes for the electronic control of spintronic devices.

Spintronic circuits exploit electronic spin other than charge as an information carrier, consuming several orders of magnitude less energy than conventional electronic circuits. 2D ferromagnets and topological material harbor spin polarization and spin dependent transport properties, bearing great application potential in spintronic devices. Among them, Fe3GeTe2 is a well‐known van der Waals ferromagnetic material. Nb3Cl8 is an emerging topological semiconducting material with a van der Waals layered structure, which enables 2D heterostructure composition and CMOS fabrication compatible. Here, the observation of antisymmetric tunneling magnetoresistance is reported in van der Waals Fe3GeTe2/Nb3Cl8/Fe3GeTe2 heterostructures with a semiconductor as the intermediate tunnelling layer. Unlike previous studies, the antisymmetric magnetoresistance can be tuned to be symmetric through both temperature or bias current. Such an antisymmetric magnetoresistance phenomenon can be explained by the dominant spin splitting in the electronic band structure of the Nb3Cl8 barrier layer. Meanwhile, the bias current across the heterojunction affects the coercive field of Fe3GeTe2 with varying thicknesses and leads to the emergence of exchange bias, resulting in the magnetoresistance transitioning from antisymmetric to symmetric. The work thus not only provides a new material platform but also shines light on the physical mechanism of exotic magnetic tunnel junction devices for spintronic applications.

Objectives . This work aims to theoretically and experimentally investigate the specific features of magnetoresistance temperature dependence in nanostructured films of doped manganites. The temperature dependence of electrical resistance for La 0.67 Ba 0.33 MnO 3 manganite films, grown by laser ablation on various dielectric substrates, is investigated over a wide temperature range. Methods . Epitaxial La 0.67 Ba 0.33 MnO 3 films with a thickness of 80 nm were grown by pulsed laser ablation using an АrF excimer laser (a laser wavelength of 247 nm) on single-crystalline SrTiO₃ and ZrO 2 (Y 2 O 3 ) substrates. The magnetoresistance properties were measured using a two-probe DC method. The measurements were conducted in magnetic fields up to 8 kOe applied in the film plane, across a temperature range of 80–350 K. To accomplish the research objectives, an empirical magnetoresistance model was applied in two distinct temperature regions: near the magnetic phase transition temperature and in the ground-state region. Results . Empirical relations for temperature dependence of magnetoresistance for nanostructured La 0.67 Ba 0.33 MnO 3 films were established, encompassing both the Curie temperature region and the ground-state regime. Our studies revealed that the magnetoresistance of epitaxial single-crystalline La 0.67 Ba 0.33 MnO 3 films exhibits a sharp peak exclusively near the Curie temperature while remaining negligible in other temperature ranges. Conversely, La 0.67 Ba 0.33 MnO 3 films with a variant structure demonstrate significant low-temperature magnetoresistance. This effect arises from magnetic-field-induced modifications of the high-frequency conductivity, which results from spin-polarized electron tunneling across structural domain boundaries. A unified empirical model to describe various mechanisms of magnetoresistance in doped manganites is proposed. Conclusions . For the first time, an empirical model to describe both the colossal and tunneling magnetoresistance in thin films of doped manganites has been developed. This model demonstrates excellent agreement between experimental and calculated data for La 0.67 Ba 0.33 MnO 3 films with and without a variant structure. The simulation results agree well with experimental data. The findings elucidate the understanding of magnetoresistance mechanisms, contribute to the development of the magnetorefractive effect theory for thin-film manganites, and inform new approaches for controlling charge carrier dynamics in strongly correlated magnetic oxides.

The problem of the ballistic electron tunneling is considered in magnetic tunnel junction with embedded non-magnetic nanoparticles (NP-MTJ), which creates additional conducting middle layer. The strong temperature impact was found in the system with averaged NP diameter dav < 1.8 nm. Temperature simulation is consistent with experimental observations showing the transition between dip and classical dome-like tunneling magnetoresistance (TMR) voltage behaviors. The low temperature approach also predicts step-like TMR and quantized in-plane spin transfer torque (STT) effects. The robust asymmetric STT respond is found due to voltage sign inversion in NP-MTJs with barrier asymmetry. Furthermore, it is shown how size distribution of NPs as well as quantization rules modify the spin-current filtering properties of the nanoparticles in ballistic regime. Different quantization rules for the transverse component of the wave vector are considered to overpass the dimensional threshold (dav ≈ 1.8 nm) between quantum well and bulk-assisted states of the middle layer.

The magnetic tunnel junction (MTJ), one of the most prominent spintronic devices, has been widely utilized for memory and computation systems. Electrical writing is considered as a practical method to enhance the performance of MTJs with high circuit integration density and ultralow-power consumption. Meanwhile, a large tunneling magnetoresistance (TMR), especially at the non-equilibrium state, is desirable for the improvement of the sensitivity and stability of MTJ devices. However, achieving both aspects efficiently is still challenging. Here, we propose a two-dimensional (2D) MTJ of 1T-MnSe2/h-BN/1T-MnSe2/h-BN/1T-MnSe2 with efficient electrical writing, reliable reading operations and high potential to work at room temperature. First, for this proposed MTJ with a symmetrical structure and an antiparallel magnetic state, the degeneracy of the energy could be broken by an electric field, resulting in a 180° magnetization reversal. A first principles study confirms that the magnetization of the center 1T-MnSe2 layer could be reversed by changing the direction of the electric field, when the magnetic configurations of the two outer 1T-MnSe2 layers are fixed in the antiparallel state. Furthermore, we report a theoretical spin-related transport investigation of the MTJ at the non-equilibrium state. Thanks to the half-metallicity of 1T-MnSe2, TMR ratios reach very satisfactory values of 2.56 × 103% with the magnetization information written by an electric field at room temperature. In addition, the performance of the TMR effect exhibits good stability even when the bias voltage increases gradually. Our theoretical findings show that this proposed MTJ is a promising high performance spintronic device and could promote the design of ultralow-power spintronic devices.

This study investigates the effects of annealing on the tunnel magnetoresistance (TMR) ratio in CoFeB/MgO/CoFeB-based magnetic tunnel junctions (MTJs) with different capping layers and correlates them with microstructural changes. It is found that the capping layer plays an important role in determining the maximum TMR ratio and the corresponding annealing temperature (Tann). For a Pt capping layer, the TMR reaches ~95% at a Tann of 350 °C, then decreases upon a further increase in Tann. A microstructural analysis reveals that the low TMR is due to severe intermixing in the Pt/CoFeB layers. On the other hand, when introducing a Ta capping layer with suppressed diffusion into the CoFeB layer, the TMR continues to increase with Tann up to 400 °C, reaching ~250%. Our findings indicate that the proper selection of a capping layer can increase the annealing temperature of MTJs so that it becomes compatible with the complementary metal-oxide-semiconductor backend process.

Summary Two-dimensional (2D) magnetic materials and their van der Waals (vdW) heterostructures offer a unique platform for investigating low-dimensional magnetic interactions and developing next-generation spintronic devices. This study employs electronic tunneling spectroscopy to systematically explore the interfacial magnetic coupling mechanisms in CrI3/CrBr3 vdW heterostructures. Experimentally, we observe complex tunneling magnetoresistance (TMR) behaviors, including multi-step jumps and asymmetric hysteresis, with their magnetic origins confirmed through temperature-dependent measurements. Theoretically, we develop a one-dimensional (1D) spin-chain model that successfully reproduces the TMR characteristics, unveiling the synergistic role of interfacial Dzyaloshinskii-Moriya interaction (DMI, ∼10.8 μeV) and ferromagnetic exchange (∼13.5 μeV) in governing magnetic configurations. We find that interfacial DMI, driven by broken inversion symmetry, induces spin canting in the ferromagnetic layers, profoundly influencing TMR behavior. This work not only provides a novel physical framework for understanding interfacial coupling in vdW magnetic heterostructures but also charts a pathway for designing multi-state memory devices through interfacial engineering.

Nb and its compounds are widely used in quantum computing due to their high superconducting transition temperatures and high critical fields. Devices that combine superconducting performance and spintronic non-volatility could deliver unique functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m^2 was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was evaluated in junctions with different thickness combinations and different annealing conditions. An optimized magnetoresistance of 120% was obtained at room temperature, with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition, spin-transfer torque switching has also been successfully observed in these junctions with a quasistatic switching current density of 7.3 $$\times \;10^{5}$$ × 10 5 A/cm^2.

In this study we have designed a spin caloritronic device based on boron doped armchair graphene nanoribbons (B2-7AGNR). In presence of ferromagnetic (FM) graphitic-carbon nitride (g-C4N3) electrodes the spin-thermoelectric features of the device, both for FM and antiferromagnetic (AFM) states, are studied using first principle calculations. The spin polarized transmission peaks and the presence of density of states near the Fermi level indicate that the system have large spin-thermoelectric figure of merit. In addition, it is observed that the system has a large tunneling magnetoresistance due to the difference in total current between FM and AFM configurations. Further studies reveal that the spin component of the Seebeck coefficient of the device is much higher than the other zigzag and armchair nanoribbons. When the spin magnetic moments of the electrodes are aligned in parallel manner, spin-thermoelectric figure of merit of the system becomes significantly high. It has also been found that on decreasing temperature the efficiency of the device increases. As a whole, the numerical results show that g-C4N3-B2-7AGNR-g-C4N3 system in FM configuration is an efficient low temperature thermoelectric device.

Helical magnets are emerging as a novel class of materials for spintronics and sensor applications; however, research on their charge- and spin-transport properties in a thin film form is less explored. Herein, we report the temperature and magnetic field-dependent charge transport properties of a highly crystalline MnP nanorod thin film over a wide temperature range (2 K < T < 350 K). The MnP nanorod films of ~100 nm thickness were grown on Si substrates at 500 °C using molecular beam epitaxy. The temperature-dependent resistivity ρ(T) data exhibit a metallic behavior (dρ/dT > 0) over the entire measured temperature range. However, large negative magnetoresistance (Δρ/ρ) of up to 12% is observed below ~50 K at which the system enters a stable helical (screw) magnetic state. In this temperature regime, the Δρ(H)/ρ(0) dependence also shows a magnetic field-manipulated CONE + FAN phase coexistence. The observed magnetoresistance is dominantly governed by the intergranular spin dependent tunneling mechanism. These findings pinpoint a correlation between the transport and magnetism in this helimagnetic system.

Heusler alloy-based magnetic tunnel junctions have the potential to provide high spin polarization, small damping, and fast switching. In this study, junctions with a ferromagnetic electrode of Co2FeAl were fabricated via room-temperature sputtering on Si/SiO2 substrates. The effect of boron doping on Co2FeAl magnetic tunnel junctions was investigated for different boron concentrations. The surface roughness determined by atomic force microscope, and the analysis of x-ray diffraction measurement on the Co2FeAl thin film reveals critical information about the interface. The Co2FeAl layer was deposited on the bottom and on the top of the insulating MgO layer as two different sample structures to compare the impact of the boron doping on different layers through tunneling magnetoresistance measurements. The doping of boron in Co2FeAl had a large positive impact on the structural and magneto-transport properties of the junctions, with reduced interfacial roughness and substantial improvement in tunneling magnetoresistance. In samples annealed at low temperature, a two-level magnetoresistance was also observed. This is believed to be related to the memristive effect of the tunnel barrier. The findings of this study have practical uses for the design and fabrication of magnetic tunnel junctions with improved magneto-transport properties.

2D van der Waals magnets have been widely studied in spintronics because of their unique electronic properties, no dangling bonds, and ultra-clean interfaces. However, most of them possess low Curie temperatures. Motivated by the recent discovery of a near-room-temperature ferromagnetic semiconductor in monolayer GdI2, we proposed the Au/GdI2/Au vertical van der Waals junction and investigated the bias-voltage- and temperature-gradient-dependent spin transport characteristics using density functional theory and the non-equilibrium Green's function method. It is found that, like bulk GdI2, the four-layer GdI2 in the central scattering region of the junction exhibits intralayer ferromagnetism with weak interlayer antiferromagnetic coupling. An almost 100% spin polarization can be obtained whether at a bias voltage or at a temperature gradient for the junction, while high tunneling magnetoresistances are observed in a large bias voltage range or in a large temperature gradient range, which can reach 29000% and 3600%, respectively. The junction also exhibits a thermal spin diode effect. These versatile bias voltage- and temperature gradient-driven spin transport properties are understood from the calculated spin-dependent band structure of layered GdI2 and the spin-dependent transmission spectrum and density of states of the junction. The present work highlights layered GdI2 as a promising magnetic tunnel barrier for van der Waals spintronic devices and spin caloritronic devices.

Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr2Ge2Te6 and CrI3. Of particular interest in semiconductors is the interplay between magnetism and transport, which has remained unexplored. Here we report magneto-transport measurements on exfoliated CrI3 crystals. We find that tunneling conduction in the direction perpendicular to the crystalline planes exhibits a magnetoresistance as large as 10,000%. The evolution of the magnetoresistance with magnetic field and temperature reveals that the phenomenon originates from multiple transitions to different magnetic states, whose possible microscopic nature is discussed on the basis of all existing experimental observations. This observed dependence of the conductance of a tunnel barrier on its magnetic state is a phenomenon that demonstrates the presence of a strong coupling between transport and magnetism in magnetic van der Waals semiconductors. Layered van der Waals compounds offer opportunities to visit new physical phenomena in two dimensional materials. Here the authors report large tunneling magnetoresistance through exfoliated CrI3 crystals and attribute its evolution to the multiple transitions to different magnetic states.

No abstract available

After the demonstration of the giant tunnel magnetoresistance (TMR) ratios at room temperature (RT) using a crystalline MgO barrier based magnetic tunnel junctions (MTJs) in 2004 [1] , [2] , an application range of spintronic devices, including read-heads of hard disk drives (HDDs) and cells of magnetoresistive random access memories (MRAMs), has been significantly boosted. Use of MgO barriers enabled to obtain large electric output from MTJ structures since a large TMR ratio over 100% can be easily achieved due to the ∆ 1 band preferential coherent tunneling mechanism [3] . Fe/MgO/Fe(001) is known as one of the most basic structures exhibiting the giant TMR effect [2] . The structure is very simple, and thus many theoretical calculations have been demonstrated so far [3] , [4] . However, experimental TMR ratios (180~220% at RT and 290-370% at low temperature) are much smaller than those of the predictions like >1,000%. In order to create novel MTJ-based applications, such as high-capacity MRAMs, magnetic logics, and brain-morphic devices, further improvement in an experimental TMR ratio is needed. Especially, it should be essential to understand the origin of the TMR gap between experiments and theories. In this study, we revisited the simplest "Fe/MgO/Fe(001)" MTJ to obtain much larger TMR ratios by careful tuning of growth conditions for all the layers, combining sputtering and electron-beam evaporation, and MgO barrier interface modifications [5] .

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here, we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in α′-FeRh-based junctions driven by the magnetic phase transition of α′-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one α′-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the α′-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics. Tunneling anisotropic magnetoresistance is promising for next generation memory devices but limited by the low efficiency and functioning temperature. Here the authors achieved 20% tunneling anisotropic magnetoresistance at room temperature in magnetic tunnel junctions with one α′-FeRh magnetic electrode.

No abstract available

Large low‐field magnetoresistance (LFMR, < 1 T), related to the spin‐disorder scattering or spin‐polarized tunneling at boundaries of polycrystalline manganates, holds considerable promise for the development of low‐power and ultrafast magnetic devices. However, achieving significant LFMR typically necessitates extremely low temperatures due to diminishing spin polarization as temperature rises. To address this challenge, one strategy involves incorporating Ruddlesden–Popper structures (ABO3)n:AO, which are layered derivatives of perovskite structure capable of potentially inducing heightened magnetic fluctuations at higher temperatures. Here, a remarkable LFMR of up to 1.0×103% is obtained in the layered (NdNiO3)n:NdO films with a high and wide temperature range (190–240 K). This finding underlines that the layered (NdNiO3)n:NdO (n = 1) structure show a complex magnetic structure above TMI of perovskite NdNiO3, where small ferromagnetic domains are embedded in the antiferromagnetic domains, raising the tunneling barriers and magnetic fluctuations at high temperatures. Furthermore, applying a low magnetic field (<0.1 T) near TMI effectively mitigates the disruption of antiferromagnetic order structures at boundaries, then a higher temperature is required to break the inhibition of ferromagnetic to antiferromagnetic phase transition. The results contribute significantly to the advancement of magnetic devices capable of achieving substantial LFMR at room temperature.

No abstract available

Magnetic tunnel junctions (MTJs) serve as essential platforms for investigating spin transport properties, magnetic phase transitions, and anisotropy in magnetic materials. Recent advancements have employed two-dimensional van der Waals antiferromagnetic insulators like chromium chloride (CrCl3) or chromium iodide (CrI3), to develop spin-filtering magnetic tunnel junctions (sf-MTJs), enhancing device performance for material property exploration and spintronic applications. However, it is crucial to recognize that the spin-filtering effect is not the sole determinant of tunneling magnetoresistance (TMR) in these junctions; interface magnetic exchange interactions and tunable electrode density of states (DOS) fluctuations, responsive to applied electric or magnetic fields, can also influence the tunneling current. In this study, we fabricate MTJ devices using mechanically-exfoliated few-layer CrCl3 as the tunnel barrier and few-layer graphene (FLG) as electrodes through dry transfer technique. Conducting low-temperature quantum transport measurements, we observe unconventional TMR behaviors, including bias voltage-dependent TMR, oscillatory tunneling current under high magnetic fields, and tunable tunneling current via gate voltage. A qualitative model of elastic tunneling current is employed to analyze the spin and band characteristics of our MTJ device. The observed bias-voltage-dependent TMR is attributed to changes in the tunneling mechanism due to magnetic proximity effect, which induces magnetization in the FLG electrodes near the FLG/CrCl3 interface. The antiparallel alignment of polarized spins with CrCl3’s magnetization results in a higher tunnel barrier for injected charge carriers, leading to negative TMR at lower bias. As the bias voltage increases, the proximity effect lessens, and the device reverts to its conventional spin-filtering functionality. The oscillatory tunneling current is explained by the graphene electrode’s quantum oscillatory density of states behavior under vertical magnetic fields, which can be controlled by the applied gate voltage. This study contributes to the understanding of previously unexplored TMR phenomena in two-dimensional MTJs, deepening our insights into carrier transport properties in these heterostructures and broadening avenues for investigating the physical properties of two-dimensional magnetic materials and their spintronic applications.

The physical systems with ferromagnetism and ‘bad’ metallicity hosting unusual transport properties are playgrounds of novel quantum phenomena. Recently EuTi1−xNbxO3 emerged as a ferromagnetic system where non-trivial temperature dependent transport properties are observed due to coexistence and competition of various magnetic and non-magnetic scattering processes. In the ferromagnetic state, the resistivity shows a T2 temperature dependence possibly due to electron–magnon scattering and above the Curie temperature , the dependence changes to T3/2 behaviour indicating a correlation between transport and magnetic properties. In this paper, we show that the transport spin-polarization () in EuTi1−xNbxO3, a low Curie temperature ferromagnet, is as high (∼40%) as that in some of the metallic ferromagnets with high Curie temperatures. In addition, owing to the low Curie temperature of EuTi1−xNbxO3, the temperature (T) dependence of could be measured systematically up to which revealed a proportionate relationship with magnetization versus T. This indicates that such proportionality is far more universally valid than the ferromagnets with ideal parabolic bands. Furthermore, our band structure calculations not only helped to understand the origin of such high spin polarization in EuTi1−xNbxO3 but also provided a route to estimate the Hubbard U parameter in complex metallic ferromagnets in general using experimental inputs.

Half-metallic ferromagnets exhibit a perfect spin-polarization at the Fermi energy. Among many candidates, Co2MnSi Heusler alloy is the most investigated material due to its half-metallic nature and high Curie temperature (TC). Magnetic junction devices using Co2MnSi show remarkable performance at low temperatures. However, the performance is significantly degraded at room temperature, which requires a detailed understanding of the temperature-dependent electronic structure of Co2MnSi films. Here, using surface-sensitive spin- and angle-resolved photoelectron spectroscopy combined with first-principles calculations, we verify the temperature- and momentum-dependent spin-polarization of Co2MnSi thin-film. The recorded spin-polarization reaches ~ 60-75% at 50 K, while it reduces ~ 30-50% at 300 K. The observed surface-specific spin-depolarization behavior can be described by the thermally excited magnon model even well below TC, and we conclude that the spin-fluctuation is markedly enhanced on its surface. Our findings provide insights into the temperature-dependent electronic structure of half-metallic Heusler films, which could have significant implications for future spintronic applications. Co2MnSi Heusler alloy is a well-known half-metallic ferromagnet exhibiting a perfect spin-polarization (100%) and high Curie temperature ( ~ 985 K). Here, using surface sensitive spin- and angle-resolved photoelectron spectroscopy, the authors investigate the temperature evolution of the spin-polarized electronic structure of Co2MnSi films and reveal the surface-specific spin depolarization mechanism around room temperature.

Altermagnetism, an emerging magnetic order state that combines the spin polarization of ferromagnets with the antiferromagnetic resistance to magnetic perturbations, provides a new route to overcome the performance bottleneck of traditional spintronic devices. This Letter focuses on the altermagnetic material CrSb, revealing the spin-polarized band structure protected by real space rotation and mirror operations in the three-dimensional Brillouin zone of CrSb through symmetry analysis and density functional theory calculations, as well as the role of spin–orbit coupling in regulating the degeneracy of the bands. Further, by combining the non-equilibrium Green's function method, we examine the quantum transport properties of altermagnetic CrSb-based magnetic tunnel junction (MTJ). Our results show that the tunneling magnetoresistance (TMR) near the Fermi level reaches 60% for a vacuum barrier of 2.196 Å. When the vacuum is replaced by an h-BN monolayer whose effective thickness is 4.4 Å, the TMR decreases to 20%, breaking the limitation of spin-polarized current dependence in traditional ferromagnetic MTJs. The research combined “Néel spin current” and clarified the cooperative transport mechanism of the spin channels of the altermagnetic sublattices. It not only broadens the theoretical framework of altermagnetic materials but also lays an important foundation for the design of spintronic devices based on g-wave symmetry altermagnetism.

No abstract available

This paper investigates the impact of electrode materials on the Tunnel Magneto-Resistance (TMR) ratio of Magnetic Tunnel Junction (MTJ) device. Four different structures of MTJ have been simulated by using cobalt (Co), Nickel (Ni), Iron (Fe) and two alloy materials of nickel-iron (NiFe) and cobalt-iron (CoFe). These materials have been used as ferromagnetic electrodes. Mulliken population and transmission spectrum observed in both parallel and antiparallel configurations of these devices to understand the spin transport properties and Tunnel Magneto-Resistance (TMR) ratio has been estimated. The first principal study was performed based on density function theory (DFT) and Non-equilibrium Green’s Function (NEGF) computational methods using the QuantumATK simulation tool to study properties such as band structure, the density of states (DOS), Spin Transfer Torque (STT), I-V characteristics and TMR. This study explores how different electrode materials affect the Tunnel Magneto-Resistance (TMR) ratio in Magnetic Tunnel Junction (MTJ) devices. With these results, it is observed that cobalt-based MTJ devices (that is Co-MgO-Fe and CoFe-MgO-CoFe) exhibit higher TMR ratio as compared to Nickel- and Iron-based MTJ devices (that is NiFe-MgO-NiFe and Ni-MgO-Fe). As Cobalt has a high spin polarization this property makes it suitable for use in spintronics devices like MTJs, where the manipulation of electron spins is essential for data storage and information processing. These findings can be employed to improve the performance characteristics of the MTJ-based Random Access Memory (MRAM) in the field of spintronics.

No abstract available

Altermagnetic (AM) materials have recently attracted significant interest due to their nonrelativistic momentum-dependent spin splitting of their electronic band structure which may be useful for antiferromagnetic (AFM) spintronics. So far, however, most research studies have been focused on conducting properties of AM metals and semiconductors, while functional properties of AM insulators have remained largely unexplored. Here, we propose employing AM insulators (AMIs) as efficient spin-filter materials. By analyzing the complex band structure of rutile-type altermagnets MF2 (M = Fe, Co, Ni), we demonstrate that the evanescent states in these AMIs exhibit spin- and momentum-dependent decay rates resulting in momentum-dependent spin polarization of the tunneling current. Using a model of spin-filter tunneling across a spin-dependent potential barrier, we estimate the tunneling magnetoresistance (TMR) effect in spin-filter magnetic tunnel junctions (SF-MTJs) that include two magnetically decoupled MF2 (001) barrier layers. We predict a sizable spin-filter TMR ratio of about 150-170% in SF-MTJs based on the AMIs CoF2 and NiF2 if the Fermi energy is tuned to be close to the valence band maximum. Our results demonstrate that AMIs provide a viable alternative to conventional spin-filter materials, potentially advancing the development of next-generation AFM spintronic devices.

Half-metallic Co-based full Heusler alloys have captured considerable attention of researchers in the realm of spintronic applications, owing to their remarkable characteristics such as exceptionally high spin polarization at the Fermi level, ultra-low Gilbert damping, and a high Curie temperature. In this comprehensive study, employing the density functional theory, we delve into the electronic stability and ballistic spin transport properties of a magnetic tunneling junction (MTJ) comprising a Co2MnSb/HfIrSb interface. An in-depth investigation of k-dependent spin transmissions uncovers the occurrence of coherent tunneling for the Mn-Mn/Ir interface, particularly when a spacer layer beyond a certain thickness is employed. It has been found that the Co-terminated Co2MnSb/HfIrSb interface shows perpendicular magnetic anisotropy, while those with Mn-Sb and Mn-Mn termination exhibit in-plane magnetic anisotropy. Furthermore, our spin-dependent transmission calculations demonstrate that the Mn-Mn/Ir interface manifests strain-sensitive transmission properties under both compressive and tensile strain and yields a remarkable three-fold increase in majority spin transmission under tensile strain conditions. We find a tunnel magnetoresistance of ∼500% under a bi-axial strain of -3%, beyond which the tunnel resistance is found to be theoretically infinite. These compelling outcomes place the Co2MnSb/HfIrSb junction among the highly promising candidates for nanoscale spintronic devices, emphasizing the potential significance of the system in the advancement of the field.

We investigate the suitability of nearly half-metallic ferrimagnetic quaternary Heusler alloys, CoCrMnZ (Z=Al, Ga, Si, Ge) to assess the feasibility as electrode materials of MgO-based magnetic tunnel junctions (MTJ). Low magnetic moments of these alloys originated from the anti-ferromagnetic coupling between Mn and Cr spins ensure a negligible stray field in spintronics devices as well as a lower switching current required to flip their spin direction. We confirmed mechanical stability of these materials from the evaluated values of elastic constants, and the absence of any imaginary frequency in their phonon dispersion curves. The influence of swapping disorders on the electronic structures and their relative stability are also discussed. A high spin polarization of the conduction electrons are observed in case of CoCrMnZ/MgO hetrojunctions, independent of terminations at the interface. Based on our ballistic transport calculations, a large coherent tunnelling of the majority-spin $s$-like $\Delta_1$ states can be expected through MgO-barrier. The calculated tunnelling magnetoresistance (TMR) ratios are in the order of 1000\%. A very high Curie temperatures specifically for CoCrMnAl and CoCrMnGa, which are comparable to $bcc$ Co, could also yield a weaker temperature dependece of TMR ratios for CoCrMnAl/MgO/CoCrMnAl (001) and CoCrMnGa/MgO/CoCrMnGa (001) MTJ.

We present a first-principles investigation of the spin-dependent electronic structure and quantum transport properties of the van der Waals (vdW) ferromagnets Fe3GeTe2, Fe4GeTe2, Fe5GeTe2, and Fe3GaTe2, motivated by their growing use as electrodes in vdW magnetic tunnel junctions (MTJs). Using density functional theory combined with the non-equilibrium Green’s function formalism within the linear-response regime, we analyze their Fermi surfaces, transmission coefficients, and orbital-resolved densities of states. Our results show that Fe3GeTe2, Fe4GeTe2, and Fe3GaTe2 exhibit a Fermi surface dominated by spin-up states, leading to nearly half-metallic out-of-plane conductance with spin polarizations exceeding 90% in the bulk. Among these compounds, Fe3GaTe2 stands out as the most robust case, with the Fermi energy lying deep within the spin-down transmission gap. For Fe5GeTe2, we compare the crystal structure adopted in previous theoretical studies with the recently reported experimental structure and show that the latter is expected to support a high spin polarization. We further investigate bilayer heterostructures as minimal MTJs, where the vdW gap acts as the tunneling barrier. The high spin polarization of the bulk materials is preserved in these bilayers, resulting in large tunneling magnetoresistance ratios on the order of several hundred percent. These findings underscore the promise of these materials, and in particular of Fe3GaTe2, for spintronics applications.

The seamless integration of two-dimensional (2D) ferromagnetic materials with similar or dissimilar materials can widen the scope of low-power spintronics. In this regard, a vertical van der Waals (vdW) heterostructure of 2D ferromagnets with semiconducting transition metal dichalcogenides (TMDCs) forms magnetic junctions with exceptional stability and electrical control. Interestingly, 2D metallic Fe3GeTe2 (FGT) reveals above room temperature Curie temperatures and has large magneto anisotropy due to spin-orbit coupling. In addition, it also possesses topological states and a large Berry curvature. Herein, we designed the FGT/WSe2/FGT vdW heterostructure with a uniform and sharp interface so that FGT could maintain its inherent electronic properties. Also, the uniform thickness of the barrier provides a smooth flow of spins through the junctions as tunneling exponentially decays with an increasing barrier thickness. However, strong energy-dependent spin polarization is crucial for achieving optimum spin valve properties, such as large tunneling magnetoresistance (TMR) along with the manipulation of the magnitude and sign reversal. We have observed a shifting of high-energy localized minority spin states toward low-energy regions, which causes spin polarization fluctuation between -42.5% and 41% over a wide range of bias voltage. This leads to a negative TMR% of ∼-100% at 0.1 V Å-1 and also a large positive TMR% at 0.2 V Å-1 and -0.4 V Å-1. Besides, the system exhibits a highly tunable large anomalous Hall conductivity (AHC) of 626 S cm-1. Interestingly, such unprecedented electronic behaviour with large and switchable spin polarization, anomalous Hall conductivity and TMR can be incorporated into MTJ devices, which provide electrical control and long-range spin transport. Additionally, the system emerges as a standout candidate in low-power spintronic devices (e.g., MRAM and magnetic sensors) owing to its distinctive energy-dependent electronic structure with a wide range of external bias.

2D materials offer the ability to expose their electronic structure to manipulations by a proximity effect. This could be harnessed to craft properties of 2D interfaces and van der Waals heterostructures in devices and quantum materials. We explore the possibility to create an artificial spin polarized electrode from graphene through proximity interaction with a ferromagnetic insulator to be used in a magnetic tunnel junction (MTJ). Ferromagnetic insulator/graphene artificial electrodes were fabricated and integrated in MTJs based on spin analyzers. Evidence of the emergence of spin polarization in proximitized graphene layers was observed through the occurrence of tunnel magnetoresistance. We deduced a spin dependent splitting of graphene’s Dirac band structure (∼15 meV) induced by the proximity effect, potentially leading to full spin polarization and opening the way to gating. The extracted spin signals illustrate the potential of 2D quantum materials based on proximity effects to craft spintronics functionalities, from vertical MTJs memory cells to logic circuits.

We report on spin transport in WS2-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS2 mono-, bi-, and trilayers are derived by a chemical vapor deposition process and further characterized by Raman spectroscopy, atomic force microscopy (AFM), and photoluminescence spectroscopy. The WS2 layers are then integrated in complete Co/Al2O3/WS2/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface and its evolution as a function of WS2 layer thicknesses. For monolayer WS2, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of PCo/WS2 = 12%. Interestingly, for bi- and trilayer WS2, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of ab initio calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS2 band structure while going from monolayer (direct bandgap) to bilayer (indirect bandgap). These experiments illustrate the rich potential of the families of semiconducting 2D materials for the control of spin currents in 2D-MTJs.

Spin-charge conversion in NiMnSb films was clarified via the interplay between interface and bulk (magnon) contributions. Half-metallic Heusler alloys are attracting considerable attention because of their unique half-metallic band structures, which exhibit high spin polarization and yield huge magnetoresistance ratios. Besides serving as ferromagnetic electrodes, Heusler alloys also have the potential to host spin-charge conversion. Here, we report on the spin-charge conversion effect in the prototypical Heusler alloy NiMnSb. An unusual charge signal was observed with a sign change at low temperature, which can be manipulated by film thickness and ordering structure. It is found that the spin-charge conversion has two contributions. First, the interfacial contribution causes a negative voltage signal, which is almost constant versus temperature. The second contribution is temperature dependent because it is dominated by minority states due to thermally excited magnons in the bulk part of the film. This work provides a pathway for the manipulation of spin-charge conversion in ferromagnetic metals by interface-bulk engineering for spintronic devices.

Half-metallic ferromagnetic CrO2 has attracted much interest due to its 100% spin polarization and high Curie temperature. CrO2 films have been fabricated on a TiO2 (100) substrate. However, there have been no reports on the spin transport properties of devices based on a CrO2 electrode and TiO2 barrier. In this work, we use first-principles calculations combined with a nonequilibrium Green's function method to investigate the bias-voltage-dependent spin transport properties for the CrO2/TiO2 (100) heterostructure and the CrO2/TiO2/CrO2 (100) magnetic tunnel junction (MTJ). Our results reveal the excellent spin filtering effect and spin diode effect in the heterostructure as well as the high tunnel magnetoresistance ratio (up to 4.48 × 1014%) in the MTJ, which indicate potential spintronic applications. The origins of these perfect spin transport characteristics are discussed in terms of the calculated spin-dependent electrode band structures, the spin-dependent transmission spectra and semiconductor theory.

Monolayer transition metal dichalcogenides have attracted great attention for potential applications in valleytronics. However, the valley polarization degree is usually not high because of the intervalley scattering. Here, a largely enhanced valley polarization up to 80% in monolayer WS2 under nonresonant excitation at 4.2 K is demonstrated using WS2 /LaMnO3 thin film heterostructure, which is much higher than that for monolayer WS2 on SiO2 /Si substrate with a valley polarization of 15%. Furthermore, the greatly enhanced valley polarization can be maintained to a high temperature of about 160 K with a valley polarization of 53%. The temperature dependence of valley polarization is strongly correlated with the thermomagnetic curve of LaMnO3 , indicating an exciton-magnon coupling between WS2 and LaMnO3 . A simple model is introduced to illustrate the underlying mechanisms. The coupling of WS2 and LaMnO3 is further confirmed with an observation of two interlayer excitons with opposite valley polarizations in the heterostructure, resulting from the spin-orbit coupling induced splitting of the conduction bands in monolayer transition metal dichalcogenides. The results provide a pathway to control the valleytronic properties of transition metal dichalcogenides by means of ferromagnetic van der Waals engineering, paving a way to practical valleytronic applications.

Two-dimensional (2D) van der Waals (vdW) heterostructures, known as layer-by-layer stacked 2D materials in a precisely chosen sequence, have received more and more attention in spintronics for their ultra-clean interface, unique electronic properties and 2D ferromagnetism. Motivated by the recent synthesis of monolayer 1T-VSe2 with ferromagnetic ordering and a high Curie temperature above room temperature, we investigate the bias-voltage driven spin transport properties of 2D magnetic tunnel junctions (MTJs) based on VSe2 utilizing density functional theory combined with the nonequilibrium Green's function method. In the device 1T-MoSe2/1T-VSe2/2H-WSe2/1T-VSe2/1T-MoSe2, the tunneling magneto-resistance (TMR) is incredibly satisfactory up to 5600%. Based on the analysis of evanescent states, this large TMR is attributed to the spin filter effect at the interface between 1T-VSe2 and 2H-WSe2, which overcomes the low spin polarization of 1T-VSe2. Furthermore, by inserting 2H-MoSe2, the spin filter effect is enhanced with decreasing current and the TMR is drastically improved to 1.7 × 105%. This work highlights the feasibility of 2D vdW heterostructures for ultra-low power spintronic applications by electronic structural engineering.

CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) have revolutionized modern spintronics, driving extensive efforts to optimize their properties such as spin polarization, perpendicular magnetic anisotropy, and switching speed. Here, a novel MTJ architecture is demonstrated, integrating ferrimagnetic CoTb to the conventional CoFeB/MgO/CoFeB MTJ structure, achieving both superior device performance and unique functionalities. The key innovation lies in the realization of sign‐tunable tunneling magnetoresistance (TMR), where the TMR ratio undergoes a dramatic transition from +33% (300 K) to ‐58% (30 K). This sign reversal, occurring at the ferrimagnetic compensation temperature (TM = 212 K), stems from strong ferromagnetic (CoFeB‐Co sublattice) and antiferromagnetic (CoFeB‐Tb sublattice) couplings in the hybrid CoFeB/CoTb layers. Around TM, a distinctive spin‐flop mediated TMR sub‐loop is further observed at high field which provides additional resistance states. These resistance states can not only be switched by external magnetic field but also by thermal operations. Furthermore, energy‐efficient field‐free switching is demonstrated through synergistic spin‐orbit torque (SOT) and spin‐transfer torque (STT) effects, achieving all‐electrical switching of MTJ at JSOT = 5 MA cm−2 with a minimal 4.1% STT current incorporation. This innovative ferrimagnetic MTJ architecture establishes a new platform for developing next‐generation spintronic devices with superior functionality, operational versatility, and performance metrics.

No abstract available

Thermally assisted spin transfer torque [TAS + STT] is a new switching approach for magnetic tunnel junction [MTJ] nanopillars that represents the best trade-off between data reliability, power efficiency and density. In this paper, we present a compact model for MTJ switched by this approach, which integrates a number of physical models such as temperature evaluation and STT dynamic switching models. Many experimental parameters are included directly to improve the simulation accuracy. It is programmed in the Verilog-A language and compatible with the standard IC CAD tools, providing an easy parameter configuration interface and allowing high-speed co-simulation of hybrid MTJ/CMOS circuits.

Magnetite is a half-metal with a high Curie temperature of 858 K, making it a promising candidate for magnetic tunnel junctions (MTJs). Yet, initial efforts to exploit its half metallic nature in Fe3O4/MgO/Fe3O4 MTJ structures have been far from promising. Finding suitable barrier layer materials, which keep the half metallic nature of Fe3O4 at the interface between Fe3O4 layers and barrier layer, is one of main challenges in this field. Two-dimensional (2D) materials may be good candidates for this purpose. Molybdenum disulfide (MoS2) is a transition metal dichalcogenide (TMD) semiconductor with distinctive electronic, optical and catalytic properties. Here, we show based on the first principle calculations that Fe3O4 keeps a nearly fully spin polarized electron band at the interface between MoS2 and Fe3O4. We also present the first attempt to fabricate the Fe3O4/MoS2/Fe3O4 MTJs. A clear tunneling magnetoresistance (TMR) signal was observed below 200 K. Thus, our experimental and theoretical studies indicate that MoS2 can be a good barrier material for Fe3O4 based MTJs. Our calculations also indicate that junctions incorporating monolayer or bilayer MoS2 are metallic.

Half-metallic chromium dioxide (CrO2) is an ideal spintronic material due to its near-full spin polarization and ultralow Gilbert damping at room temperature. Based on theoretical calculations, we found that the tunneling magnetoresistance (TMR) ratios of the CrO2/XO2/CrO2 (X= Ti and Sn) magnetic tunnel junctions (MTJs) can reach up to the order of magnitude of 105%, and the magnetoresistance (MR) ratio of CrO2/RuO2/CrO2 magnetic junctions (MJs) can reach the order of magnitude of 104%. In addition, we succeeded in fabricating epitaxial CrO2-based MTJs (CrO2/TiO2/CrO2 and CrO2/TiO2/Co2FeAl) with TiO2 tunnel barriers of varying thickness. Evident TMR effects were observed for all CrO2-based MTJs with the highest MR ratio of 8.55% for the CrO2/TiO2/Co2FeAl MTJ at 10 K. The MR ratios of CrO2-based MTJs in our studies were lower than theoretical expectations, which could be due to the possible mixture of interface atoms and Cr magnetization reversal. Moreover, the existence of oxygen vacancies in the TiO2 tunnel barrier also weakened the TMR effect significantly due to increased spin scattering, and the annealing treatment in an oxygen atmosphere led to an increase in the MR ratio of the CrO2/TiO2/Co2FeAl MTJ by about 33% in comparison with the unannealed MTJ, which is consistent with theoretical calculations.

Spin-transfer torque magnetic random-access memory (STT-MRAM) is a promising candidate for high-density storage-class memories by using magnetic tunnel junctions (MTJs) with small diameters. However, it is reported that reducing the MTJ diameter can result in resistance drift and degradation of tunnel magnetoresistance (TMR) ratios, which can lead to readout errors. In this study, we investigated the mechanism of the time-dependent degradation of MgO barrier and proposed a method to suppress it. Resistance drift and degradation of TMR ratio were experimentally observed in scaled MTJs under a voltage stress. The degradation can be explained by the current-induced generation of oxygen Frenkel defects at the Fe–MgO interface using microscopic calculations. The reduction of the initial oxygen vacancies in MgO can suppress degradation. Our findings elucidated on the improved reliability in high-density STT-MRAM for storage class memory applications.

The discontinuity of a spin-current through an interface caused by spin-orbit coupling is characterized by the spin memory loss (SML) parameter δ. We use first-principles scattering theory and a recently developed local current scheme to study the SML for Au|Pt, Au|Pd, Py|Pt, and Co|Pt interfaces. We find a minimal temperature dependence for nonmagnetic interfaces and a strong dependence for interfaces involving ferromagnets that we attribute to the spin disorder. The SML is larger for Co|Pt than for Py|Pt because the interface is more abrupt. Lattice mismatch and interface alloying strongly enhance the SML that is larger for a Au|Pt than for a Au|Pd interface. The effect of the proximity-induced magnetization of Pt is negligible.

The two-dimensional electron gas (2DEG) formed at the interface between SrTiO3 (STO) and LaAlO3 (LAO) insulating layer is supposed to possess strong Rashba spin-orbit coupling. To date, the inverse Edelstein effect (i.e., spin-to-charge conversion) in the 2DEG layer is reported. However, the direct effect of charge-to-spin conversion, an essential ingredient for spintronic devices in a current-induced spin-orbit torque scheme, has not been demonstrated yet. Here we show, for the first time, a highly efficient spin generation with the efficiency of ∼6.3 in the STO/LAO/CoFeB structure at room temperature by using spin torque ferromagnetic resonance. In addition, we suggest that the spin transmission through the LAO layer at a high temperature range is attributed to the inelastic tunneling via localized states in the LAO band gap. Our findings may lead to potential applications in the oxide insulator based spintronic devices.

In this article, we have theoretically investigated spacer-thickness-dependent spin transport across organic magnetic tunnel junctions (MTJs) [x/Rubrene/Co, ${x} =$ La2O3, LaMnO3, La0.7Ca0.3MnO3 (LCMO), La0.7Sr0.3MnO3 (LSMO)] using non equilibrium Green’s function (NEGF). Parallel and antiparallel resistances have been found to be thickness-independent due to high magnetic coupling and minimum interfacial defects at low thickness region. Sharp rise of differential antiparallel resistance in La2O3 device has been attributed to large change in defect state depth with spacer thickness. Saturation of spin current at large spacer thickness has been attributed to trapping of spins at large defect depths with strong pinning strength. Spacer thickness modifies magnetic coupling between electrodes thereby modulating tunnel magnetoresistance (TMR) response across the devices. Reduction of spin transfer torque (STT) with thickness resulted in spin damping thereby quenching TMR response across the devices.

I show that recent experiments of inelastic scanning tunneling spectroscopy of single and a few magnetic atoms are modeled with a phenomenological spin-assisted tunneling Hamiltonian so that the inelastic dI/dV line shape is related to the spin spectral weight of the magnetic atom. This accounts for the spin selection rules and dI/dV spectra observed experimentally for single Fe and Mn atoms deposited on Cu2N. In the case of chains of Mn atoms it is found necessary to include both first and second-neighbor exchange interactions as well as single-ion anisotropy.

The vibrational excitation related transport properties of a manganese phthalocyanine molecule suspended between the tip of a scanning tunneling microsope (STM) and a surface are investigated by combining the local manipulation capabilities of the STM with inelastic electron tunneling spectroscopy. By attachment of the molecule to the probe tip, the intrinsic physical properties similar to those exhibited by a free standing molecule become accessible. This technique allows one to study locally the magnetic properties, as well as other elementary excitations and their mutual interaction. In particular a clear correlation is observed between the Kondo resonance and the vibrations with a strong incidence of the Kondo correlation on the thermopower measured across the single-molecule junction.

Detection of a single nuclear spin constitutes an outstanding problem in different fields of physics such as quantum computing or magnetic imaging. Here we show that the energy levels of a single nuclear spin can be measured by means of inelastic electron tunneling spectroscopy (IETS). We consider two different systems, a magnetic adatom probed with scanning tunneling microscopy and a single Bi dopant in a silicon nanotransistor. We find that the hyperfine coupling opens new transport channels which can be resolved at experimentally accessible temperatures. Our simulations evince that IETS yields information about the occupations of the nuclear spin states, paving the way towards transport-detected single nuclear spin resonance.

The recent experimental conductance measurements taken on magnetic impurities on metallic surfaces, using scanning tunneling microscopy technique and suggesting occurrence of inelastic scattering processes, are theoretically addressed. We argue that the observed conductance signatures are caused by transitions between the spin states that have opened due to, for example, exchange coupling between the local spins and the tunneling electrons, and are directly interpretable in terms of inelastic transitions energies. Feasible measurements using spin-polarized scanning tunneling microscopy that would enable new information about the excitation spectrum of the local spins are discussed.

The excitation of the spin degrees of freedom of an adsorbed atom by tunneling electrons is computed using strong coupling theory. Recent measurements [Heinrich, Science 306, 466 (2004)] reveal that electron currents in a magnetic system efficiently excite its magnetic moments. Our theory shows that the incoming electron spin strongly couples with that of the adsorbate so that memory of the initial spin state is lost, leading to large excitation efficiencies. First-principles transmissions are evaluated in quantitative agreement with the experiment.