Large Tunneling Magnetoresistance in Nonvolatile 2D Hybrid Spin Filters.

Excellent spin-filtering and giant tunneling magnetoresistance in a dual-electrode van der Waals magnetic tunnel junction based on ferromagnetic CrSe2

Van der Waals (vdW) magnetic tunnel junctions (MTJs), with a two-dimensional (2D) material barrier between two vdW ferromagnetic electrodes, present unprecedented opportunities to design innovative spintronic devices. In this study, we employ density functional theory and non-equilibrium Green's function methods to investigate the spin-dependent electronic transport properties of a vdW MTJ, Fe3GaTe2/InSe/Fe3GaTe2. The MTJ with a monolayer InSe barrier demonstrates nearly 100% spin filtering and a large tunneling magnetoresistance (TMR) of 7.48 × 105%, where the resistance changes nearly 10 000% as the magnetization alignment of the electrodes transitions from parallel (P) to antiparallel. When the barrier layer increases from monolayer InSe to bilayer InSe, the TMR ratio (3.64 × 107%) is significantly enhanced. The large TMR originates from the high spin polarization of the magnetic electrodes, Fe3GaTe2. Our results highlight that room-temperature vdW MTJs pave the way for potential applications of nonvolatile spintronic devices.

Calculation of the tunneling magnetoresistance (TMR) of an Fe/Ag/MgO/Fe(001) magnetic junction is reported. The magnetoresistance is determined without any approximations from the real-space Kubo formula using tight-binding bands fitted to an ab initio band structure. It is shown that the calculated TMR oscillates as a function of Ag interlayer thickness between positive values in excess of 2000% and negative values of the order of ?100%. The oscillation period is determined by the spanning vector of the Ag Fermi surface. The large positive TMR and the changes in its sign are due to resonant enhancement of the tunneling conductance of majority-spin carriers in the ferromagnetic configuration and of the conductance of carriers tunneling in the antiferromagnetic configuration from the minority-spin channel in the Fe electrode adjacent to the Ag layer to the majority-spin channel in the other Fe electrode. The resonant enhancement occurs because the Ag interlayer creates potential steps for electrons in both the ferromagnetic and antiferromagnetic configurations of the junction. This mechanism, which results in a very large TMR, is quite different from the mechanism that causes large TMR in the standard Fe/MgO/Fe(001). It offers the possibility of tuning the magnitude and sign of the TMR by the choice of the interlayer thickness. A Lateral supercell method was also used to investigate the effect of interfacial roughness on the resonant tunneling in an Fe/Ag/MgO/Fe(001) junction. It is found that, in contrast to the Fe/MgO/Fe(001) junction whose TMR is reduced drastically by disorder, the junction with a silver interlayer is much less affected by interfacial roughness.

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.

In recent years, writing data in magnetic random access memory (MRAM) utilizing voltage controlled magnetic anisotropy (VCMA) has attracted much attention for its potential low power consumption [1]. We proposed voltage-control spintronics memory (VoCSM) which had high-efficient and deterministic writing properties [2]. In order to realize those memories, three features of a large VCMA, a large surface anisotropy Ks, and a large tunneling magnetoresistance (TMR) should coexist. In addition, a large spin-Hall angle is a must for VoCSM. Many challenges based on MgO tunneling barrier/ferromagnetic layer (FL) such as CoFeB thin films combined with various materials as an insertion layer at the MgO/FL interface or as an underlayer of FL showed improved VCMA but were concerned to fail in the coexistence of the feature because of very thin storage-layer or degraded lattice growth between MgO and CoFeB [3]–[5]. As a result, none of them have had a practical meaning as a memory cell so far. In this study, the experiments were conducted in which the insertion position of Ir was changed in MgO/CoFeB/Ta thin films. Each of the interface layer, the interlayer and the underlayer of Ir showed an increase in VCMA, and the largest VCMA was obtained in the case of inserting the Ir interlayer into the CoFeB layer. In addition, both the resistance-area product (RA) and TMR ratio decreased greatly when using the Ir interface layer, but clearly improved by employing the Ir interlayer. The base multilayer structure for VCMA measurement was Ta (5 nm)/MgO $(\sim 3$ nm)/CoFeB (1–2 nm)/Ta (5–8 nm), which was deposited on a thermally oxidized Si substrate. The CoFeB layer was set to in-plane magnetization, and the base stack of IrMn/ CoFe/Ru/CoFeB /MgO/CoFeB/Ta with a reference layer was prepared for RA and TMR measurement by using current in-plane tunneling (CIPT). The multilayers for VCMA were patterned and etched into the device size with one side of 3 to $50 \mu \mathrm {m}$ and their hysteresis curves were measured using the magneto-optical polar Kerr effect. The effective perpendicular magnetic anisotropy field Hk $_{eff}$ of the CoFeB layer was measured while bias voltage was applied to the device, and the variation of Ks depending on the electric field E was evaluated as the VCMA coefficient. Figure 1 shows the VCMA coefficients (–dKs/dE) of the MgO/CoFeB/Ta thin films as the “Base” sample, “Interface” sample in which Ir (0.2 or 0.3 nm) is layered at the MgO/ CoFeB interface, “Interlayer” sample in which Ir (0.3 nm) is inserted in the middle of the CoFeB layer, and “Underlayer” sample in which Ir (0.5 nm) is formed between the CoFeB and the Ta layer. All coefficients of the “Interface”, “Interlayer”, and “Underlayer” samples increased more than that of the “Base” sample in terms of each average value, although each coefficient had a certain degree of dispersion. The Ks in the “Interface” sample also increased more than in the “Base” sample at each average value, however, the largest Ks (maximum of 2.2 erg/cm $^{2})$ and VCMA (maximum of 190 fJ/ Vm) were obtained in the “Interlayer” sample. The relationship between RA and TMR ratio in the MTJ samples similar to Fig. 1 with the reference layer is plotted in Fig. 2. Both RA and the TMR ratio in the “Underlayer” sample were almost the same as those in the “Base” sample, but both decreased in the “Interface” sample and further decreased by increasing the Ir layer thickness from 0.2 to 0.3 nm. In the “Interlayer” sample, the deterioration of RA was not observed, and although the TMR ratio decreased, it still showed a high value of more than 120%. By comparison at the Ir thickness of 0.3 nm, it can be seen that both RA and TMR are clearly improved by changing from the Ir interface layer to the Ir interlayer. In summary, we successfully found the practical MTJ structure as a memory cell which realized coexistence of a large VCMA, a large Ks, and a large TMR for the first time. The structure is expected to have a large spin-Hall effect as well. This work was partly supported by the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

Equiatomic quaternary Heusler compounds (EQHCs) generally have the advantages of high Curie temperature, large spin polarization and long spin diffusion length, and they are regarded as one of the most promising candidates for spintronics devices. Herein, we report a theoretical investigation on an EQHC CoRhMnGe based magnetic tunnel junction (MTJ) with (i) MnGe-terminated interface and (ii) modified pure Mn terminated interface, i.e., MnMn-terminated interface. By employing first principle calculations combined with non-equilibrium Green's function, the local density of states (LDOS), transmission coefficient, spin-polarized current, tunnel magnetoresistance (TMR) ratio and spin injection efficiency (SIE) as a function of bias voltage are studied. It reveals that when the MTJ under equilibrium state, TMR ratio of MnGe-terminated structure is as high as 3,438%. When the MTJ is modified to MnMn-terminated interface, TMR ratio at equilibrium is enhanced to 2 × 105%, and spin filtering effects are also strengthened. When bias voltage is applied to the MTJ, the TMR ratio of the MnGe-terminated structure suffers a dramatic loss. While the modified MnMn-terminated structure could preserve a large TMR value of 1 × 105%, even bias voltage rises up to 0.1 V, showing a robust bias endurance. These excellent spin transport properties make the CoRhMnGe a promising candidate material for spintronics devices.

We propose, by performing advanced ab initio electron transport calculations, an all-oxide composite magnetic tunnel junction, within which both large tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects can coexist. The TMR originates from the symmetry-driven spin filtering provided by an insulating BaTiO(3) barrier to the electrons injected from the SrRuO(3) electrodes. Following recent theoretical suggestions, the TER effect is achieved by intercalating a thin insulating layer, here SrTiO(3), at one of the SrRuO(3)/BaTiO(3) interfaces. As the complex band structure of SrTiO(3) has the same symmetry as that of BaTiO(3), the inclusion of such an intercalated layer does not negatively alter the TMR and in fact increases it. Crucially, the magnitude of the TER also scales with the thickness of the SrTiO(3) layer. The SrTiO(3) thickness becomes then a single control parameter for both the TMR and the TER effect. This protocol offers a practical way to the fabrication of four-state memory cells.

Both theoretical and experimental studies of the epitaxial Fe/MgO/Fe(001) trilayers have shown new spin dependent tunnelling processes due to the conservation of the (001) crystallo-graphic orientation in the entire stack. In addition the MgO barrier acts as a spin filter : the tunnelling electrons are more or less attenuated in the barrier as a function of their symmetry. The very high tunneling magnetoresistance (TMR) value calculated and obtained at room temperature is a consequence of both this spin filtering effect and the fact that Fe(001) is half metallic as regard s the Delta1 symmetry. The theoretical predictions were performed at the equilibrium (symmetrical barrier). Very recent works have shown that the study of the conductance in Fe/MgO/Fe(001) tunnel junctions with large barrier thickness and out of equilibrium (for biased junctions) reflects the Fe(001) band structure in the Delta direction. At low bias voltage (in the range of plusmn1V) the parallel conductance is governed by the Delta1 Bloch states whereas the antiparallel conductance is governed by the Delta5 symmetry. Here, we are interested in the conductance very far from the equilibrium i.e. at high bias voltage (more than 1.5 V). We observed an inversion of the sign of the TMR for -1.8 V The specific voltage of this crossing point can be understood from the analysis of the Fe band structure well above the Fermi level and can be related to the opening of a new conduction channel.

Magnetic tunneling junctions (MTJs) continue to receive considerable attention because of their large tunneling magnetoresistance (TMR) effect not only for the depth of fundamental phenomena available, but also for the potential and proven applications [1]–[4]. An MTJ consists of an $FM_{1}-I_{3}-FM_{4}$ structure, where $FM_{1}$ and $FM_{4}$ are ferromagnetic electrodes and $I_{3}$ is a thin insulator. The TMR effect originates from spin-dependent tunneling (SDT) in a MTJ. The change of SDT in the MTJ is measured by the TMR ratio. In the $FM_{1}-I_{3}-FM_{4}$ MTJ, SDT is extremely sensitive to the interface structures between the insulator and each electrode. Thus, modulating one of the interfaces can change the TMR. One way to achieve this is to insert a thin nonmagnetic metallic layer $(M_{2})$ between one of the ferromagnetic electrodes, $FM_{1}$ or $FM_{4}$ , and the insulator $I_{3}$ . The $M_{2}$ -inserted MTJ is the $FM_{1}-M_{2}-I_{3}-FM_{4}$ structure. The inserted $M_{2}$ layer leads to a severe alteration of the $M_{2}-I_{3}$ interface, thus resulting in a dramatic effect on SDT and the TMR ratio. The excellent experimental work of Yuasa et al. shows how the TMR is affected by the inserted Cu metal in a high-quality Co-Cu-A1 2 O 3 -NiFe MTJ [1]. A distinct attenuated oscillation of the TMR ratios with increasing the Cu thickness is observed at room temperature when the inserted Cu thickness is less than to 29A. Here, we propose a method of changing the $M_{2}-I_{3}$ interface. The proposed method uses a gate voltage on the $M_{2}$ layer to alter the potential profile at the $\text{M}_{2}-\text{I}_{3}$ interface, and to modulate the TMR ratio in the $FM_{1}-M_{2}-I_{3}-FM_{4}$ MTJ.

A body-centered cubic (bcc) FeCo(B) is a current standard magnetic material for perpendicular magnetic tunnel junctions (p-MTJs) showing both large tunnel magnetoresistance (TMR) and high interfacial perpendicular magnetic anisotropy (PMA) when MgO is utilized as a barrier material of p-MTJs. Since the p-MTJ is a key device of current spintronics memory, i.e. spin-transfer-torque magnetoresistive random access memory (STT-MRAM), it attracts attention for further advance to explore new magnetic materials showing both large PMA and TMR. However, there have been no such materials other than FeCo(B)/MgO. Here, we report, for the first time, PMA in metastable bcc Co-based alloy, i.e. bcc CoMnFe thin films which are known to exhibit large TMR effect when used for electrodes of MTJs with the MgO barrier. The largest intrinsic PMAs were about 0.6 and 0.8 MJ/m3 in a few nanometer-thick CoMnFe alloy film and multilayer film, respectively. Our ab-initio calculation suggested that PMA originates from tetragonal strain and the value exceeds 1 MJ/m3 with optimizing strain and alloys composition. The simulation of the thermal stability factor indicates that the magnetic properties obtained satisfy the requirement of the data retention performance of X-1X nm STT-MRAM. The large PMA and high TMR effect in bcc CoMnFe/MgO, which were rarely observed in materials other than FeCo(B)/MgO, indicate that bcc CoMnFe/MgO is one of the potential candidates of the materials for X-1X nm STT-MRAM.

Magnetic tunnel junctions (MTJs) based on novel states of two-dimensional (2D) magnetic materials will significantly improve the value of the tunneling magnetoresistance (TMR) ratio. However, most 2D magnetic materials exhibit low critical temperatures, limiting their functionality to lower temperatures rather than room temperature. Moreover, most MTJs experience the decay of TMR ratio at large bias voltages within a low spin injection efficiency (SIE). Here, we construct a series of MTJs with Weyl half-semimetal (WHSM, e.g. MnSiS3, MnSiSe3, and MnGeSe3 monolayers) as the electrodes and investigate the spin-dependent transport properties in these kind of lateral heterojunctions by employing density functional theory combined with non-equilibrium Green’s function method. We find that an ultrahigh TMR (∼109%) can be obtained firmly at a small bias voltage and maintains a high SIE even at a large bias voltage, and MnSiSe3 monolayer is predicted to exhibit a high critical temperature. Additionally, we reveal that the same structure allows for the generation of fully spin-polarized photocurrent, irrespective of the polarization angle. These findings underscore the potential of WHSMs as candidate materials for high-performance spintronic devices.

Experiments on magnetic tunnel junctions (MTJs) show that the use of a (001)-oriented spinel ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ barrier improves the robustness of the tunneling magnetoresistance (TMR) ratio against bias voltage [Sukegawa et al., Appl. Phys. Lett. 96, 212505 (2010)]; however, the maximum TMR ratio is very small compared with that of the $\mathrm{Mg}\mathrm{O}$-based MTJ. To overcome this problem, we propose a MTJ with a trilayered tunnel-barrier junction, $\mathrm{Fe}$/$\mathrm{Mg}\mathrm{O}$/${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$/$\mathrm{Mg}\mathrm{O}$/$\mathrm{Fe}$, from first-principles calculations. The presence of the $\mathrm{Mg}\mathrm{O}$ interlayer between $\mathrm{Fe}$ and ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ has the effect of enhancing the TMR ratio to more than $1000\mathrm{%}$ at zero bias. The large TMR is maintained under a bias voltage. The results indicate the potential of a hybrid-type tunnel barrier that combines the advantages of MTJs containing a single $\mathrm{Mg}\mathrm{O}$ barrier (high TMR) and a single ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ barrier (robustness to bias voltages). The $\mathrm{Mg}\mathrm{O}$ interlayer is found to play a key role in suppressing the transmittance of the minority-spin channel, and thus, the tunneling conductance of antiparallel magnetization is significantly reduced.

A systematic study is carried out on the spin-filter (SF) tunneling magnetoresistance (TMR) occurring in a ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal (FM/FI/FM) tunnel junction. The theoretical investigation gives a unified and compact description on the SF and TMR effects in this structure, and qualitatively explains the relevant experiments in this area. Specifically, due to the strong SF effect, the TMR can be separately controlled by the extended Slonczewski's polarization factors, leading to both the barrier-height and bias-voltage induced sign-change behavior. It is also proved that this structure can provide a positively or negatively large and stable TMR, which does not vary appreciably with increasing the bias. These features are very prominent compared with an FM/I/FM conventional magnetic tunnel junction and are believed to be of practical use in designing spintronic devices.

The two-dimensional magnetic van der Waals heterojunctions have opened unprecedented opportunities to explore new physics due to their potential for spintronic applications. Here, combing density functional theory with non-equilibrium Green’s function technique, we systematically investigate the spin-polarized transport properties of Cu/FeX2/h-BN/FeX2/Cu (X = Cl, Br, I) magnetic tunnel junctions (MTJs). It is found that the maximum tunneling magnetoresistance of Cu/FeCl2/h-BN/FeCl2/Cu, Cu/FeBr2/h-BN/FeBr2/Cu, and Cu/FeI2/h-BN/FeI2/Cu MTJs can reach 3443%, 3069%, and 1676%, respectively. In the parallel state, the resistance area products at zero bias for Cu/FeCl2/h-BN/FeCl2/Cu, Cu/FeBr2/h-BN/FeBr2/Cu, and Cu/FeI2/h-BN/FeI2/Cu MTJs are 0.92, 0.47, and 0.32 Ω⋅μm2, respectively. More interestingly, our results indicate that Cu/FeX2/h-BN/FeX2/Cu (X = Cl, Br, I) MTJs can realize spin filtering effect, while Cu/FeCl2/h-BN/FeCl2/Cu and Cu/FeI2/h-BN/FeI2/Cu MTJs exhibit negative differential resistance. Our results demonstrate that large tunneling magnetoresistance, negative differential resistance effect, low resistance area product as well as excellent spin filtering effect coexist in Cu/FeCl2/h-BN/FeCl2/Cu and Cu/FeI2/h-BN/FeI2/Cu MTJs, and that the feasible tunability of such a kind of van der Waals magnetic tunnel junctions is beneficial to designing next-generation logic devices.

Epitaxial magnetic tunnel junctions (MTJs) with a Co2FeAl/CoFe (0.5 nm)/MgAl2O4/Co2FeAl(001) structure were fabricated by magnetron sputtering. High-temperature in situ annealing led to a high degree of B2-order in the Co2FeAl layers and cation order of the MgAl2O4 barrier. Large tunnel magnetoresistance (TMR) of up to 342% was obtained at room temperature (616% at 4 K), in contrast to the TMR ratio (%) suppressed by the band-folding effect in Fe/cation-ordered MgAl2O4/Fe MTJs. The present study reveals that the high degree of B2-order and the resulting high spin polarization in the Co2FeAl electrodes enable us to bypass the band-folding problem in spinel barriers.