Half-metallic Electrodes Research Articles

Designing High-Performance Nanoscale Spin-MOSFET Devices by Using Two-Dimensional Half-Metallic Cr2Se3.

Constructing nanoscale spin devices has been a crucial pursuit in the field of nano spintronics. Here, by using the density functional theory (DFT) and nonequilibrium Green's function (NEGF) method, high-performance nanoscale spin-MOSFET devices using half-metallic 2D Cr2Se3 as electrodes are theoretically designed. Specifically, seven typical two-dimensional (2D) semiconductors, Sb, Bi, BP, BAs, MoTe2, WTe2, and WSeTe (with two different contacting surfaces), are considered here as the channel materials. The properties of contact interfaces between these 2D semiconductors and half-metallic 2D Cr2Se3 are first investigated. It is found that except BP and BAs (having Schottky contacts with Cr2Se3), the other 2D semiconductors have vertical Ohmic contacts with Cr2Se3 among which Cr2Se3/Sb, Cr2Se3/MoTe2, Cr2Se3/WTe2, Cr2Se3/WSeTe-Se, and Cr2Se3/WSeTe-Te retain the half-metallic characteristic. Then, these 2D semiconductors with Ohmic vertical contacts are further used to construct spin-MOSFET devices. The results show that devices constructed by half-metallic vertical contacting systems have nearly 100% SIE and therefore giant MR (>107%) when the gate voltage varies. Furthermore, four designed spin-MOSFET devices, namely, Cr2Se3/MoTe2, Cr2Se3/WTe2, Cr2Se3/WSeTe-Se, and Cr2Se3/WSeTe-Te spin-MOSFET have high efficient gate modulations on the magnitude of completely spin-polarized source-drain current with Cr2Se3/WTe2 having the smallest SS value of 134.1 mV/dec. The calculations suggest that Cr2Se3 is a good candidate for constructing spin-MOSFET devices. Our study sheds light on the design of high-performance nanoscale spin-MOSFET devices by using two-dimensional half-metallic electrodes.

Improvement in CPP-GMR read head sensor performance using [001]-oriented polycrystalline half-metallic Heusler alloy Co2FeGa0.5Ge0.5 and CoFe bilayer electrode

ABSTRACT A current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) device with a half-metallic electrode is one of the most promising candidates of next-generation read head for hard disk drive. In this study, we fabricate [001]-oriented polycrystalline CPP-GMR devices with the normal ferromagnet (NFM) CoFe/half-metallic ferromagnet (HMFM) Co2FeGa0.5Ge0.5 (CFGG) bilayer electrodes to enhance the magnetoresistance (MR) ratio by large interfacial spin-dependent scattering at the NFM/HMFM interface. The CoFe/CFGG bilayer electrode provides the additional large interfacial spin-dependent scattering and achieves high MR ratio of 22.7% with the CoFe(4.5 nm)/CFGG(2.5 nm) bilayer electrodes, which is almost three(two) times larger than the MR ratio with the single CoFe(CFGG) (7 nm) electrodes. The bias voltage dependent study revealed an additional advantage of increasing the output voltage |ΔV| by using the CoFe/CFGG bilayer due to the improvement of the endurance against spin-transfer torque under high bias current. A maximum output voltage Δ V max of 6.5 mV was obtained with the CoFe(5.5 nm)/CFGG(1.5 nm) electrodes, which is the highest ever reported in the CPP-GMR devices with a uniform metallic spacer including high-quality epitaxial devices.

Implementation of excellent spin-filtering effect in half-metallic electrode-based single-molecule optoelectronic devices

The application of half-metallic materials in single-molecule optoelectronic devices opens a promising way in advancing device performance and functionality, thus addressing a research question of significance. Here we propose a series of single-molecule devices with half-metallic FeN4-doped armchair graphene nanoribbon as electrodes and metalloporphyrin (MPr) molecules as photoresponsive materials for photon harvesting, which are driven by photogalvanic effects (PGEs). Through the quantum transport simulations, we systematically investigated the spin-polarized photocurrents under the linearly polarized light illumination in these devices. Since the exclusive opening only exists in the spin-up channel of the half-metallic nanoribbons, these devices can generate a large photocurrent in the spin-up direction whereas suppressing the spin-down photocurrent. Consequently, they exhibit an effective spin-filtering effect at numerous photon energies. Our study unveils the excellent spin-filtering effect achieved in single-molecule optoelectronic devices with half-metallic electrodes, showing instructive significance for the future design of new optoelectronic devices.

MXene-Based Sc2CHO/Sc2NHO/Sc2CHO Magnetic Tunnel Junctions for Multi-Value Logic Computing Devices: A First Principles Study

The rapid development of technologies such as machine learning and artificial intelligence has further accelerated the growth of multi-value logic (MVL) computing nanodevices. However, current MVL nanodevices based on conventional materials face the challenges of low peak–valley ratio (PVR) and unstable spin filtering effects. Therefore, the intrinsic half-metal nanoscale MXene has become the answer to the development bottleneck of MVL nanodevices for its outstanding properties such as high Curie temperature and an appropriate band structure. The present work attempts to design spintronic magnetic tunnel junctions (MTJs) based on new predicted Sc2CHO as half-metallic electrodes and Sc2NHO as semiconductor-based scattering regions. Furthermore, we simulate its transport properties from density functional theory combined with nonequilibrium Green’s function. And we have considered the effect of the length of the scattering region on MTJ performance. According to our calculations, the spintronic transport of the MTJs demonstrates up to perfect 100% polarizability spin currents stably due to the single-channel conduction capability of half-metal Sc2CHO. The MTJs exhibit a significant negative differential resistance (NDR) phenomenon, with the PVR of the type 2N reaching 1.28 × 106, the highest value among the two-dimensional spintronic nanodevices currently being theoretically explored. MTJs with a high PVR will be beneficial for the application and development of MVL nanodevices. The spin filtering effect and NDR phenomenon can be well-maintained as the scattering region length increases, and the PVR usually almost always remains 104. This is well reflected in the transmission spectra and molecular projection self-consistent Hamiltonian. The tunneling magnetoresistance phenomenon is also observed.

Lateral magnetic tunnel junctions with a heterointerface‐induced half‐metallic electrode

The electronic properties of monolayer MnGeSe3 and MnGeSe3/CrI3 heterostructures were investigated by density functional theory (DFT) calculations. In the MnGeSe3/CrI3 heterostructure, the half-metal MnGeSe3 can induce the half-metallic properties of CrI3 by charge transfer. Based on the charge transfer effect, we designed lateral magnetic tunnel junctions (MTJs) in which MnGeSe3 attaches to the bottom of the detached CrI3 crystals. This model can form half-metallic-CrI3/insulating-CrI3/half-metallic-CrI3 MTJs, thereby avoiding the loss of half-metallic properties caused by defects and lattice distortion at the interface of traditional half-metallic MTJs. According to the extension length of the CrI3 layer, they can be divided into heteroelectrode MTJs and MnGeSe3-electrode MTJs. Half-metallic electrode MTJs are famous for their ultrahigh tunnel magnetoresistance (TMR), and the ability to maintain a stable TMR under bias has become the other significant target of half-metal MTJs. We find that the half-metallic gap of the electrode material can affect the threshold bias voltage in antiparallel configuration (APC) when opening and closing. The larger the half-metallic gap is, the larger the bias voltage required to generate the tunneling current, and correspondingly, the better its ability to maintain a high TMR. Therefore, it is imperative to find two-dimensional half-metallic materials with large half-metallic gaps. Our results provide a new idea for designing half-metallic electrode MTJs and provide a new understanding of the mechanism of half-metallic electrode MTJs under bias.

Spin transport properties of highly lattice-matched all-Heusler-alloy magnetic tunnel junction

A highly lattice-matched all-Heusler-alloy magnetic tunnel junction (MTJ) is proposed by associating half metallic CoFeTiSi electrode and non-magnetic semiconductor Fe2TiSi barrier. Based on the non-equilibrium Green's function combined with first-principles calculations, spin transport mechanism is studied by analyzing the transmission coefficient, local density of states, and scattering states. The calculated equilibrium tunnel magnetoresistance (TMR) ratio in CoFeTiSi/Fe2TiSi/CoFeTiSi reaches up to about 3.30 × 108%, which is much higher than in CoFeTiSi/MgO/CoFeTiSi. The calculated scattering states reveal that CoFeTiSi/Fe2TiSi/CoFeTiSi MTJ possesses two transport channels, and the contribution of channel I to the total transport is about 80%, which plays a dominate role. The analyses on non-equilibrium spin transport properties reveal that CoFeTiSi/MgO/CoFeTiSi MTJ can output a high intensity transport current with complete spin polarization, and it can output a stable and highly efficient signal. The TMR ratio possesses an extremely large value of about 2.4 × 108% and even the bias voltage increases to 0.2 V, exhibiting a robust current-driven stability.

Giant magnetoresistance in spin valves realized by substituting Y-site atoms in Heusler lattice

‘All-Heusler’ spin-valve constructed by two half-metallic Heusler electrodes and a non-magnetic Heusler spacer contains two interfaces that have a crucial influence on the magnetoresistance. In order to reduce the disorder at the interface and protect the half metallicity of the electrode at the same region, we propose a scheme to construct a spin valve by replacing the Y-site atoms in the half-metallic Heusler electrode to obtain the corresponding non-magnetic spacer based on the Slater–Pauling rule. In this way, the lattice and band match of the two materials can be ensured naturally. By using Co2FeAl as electrode and Co2ScAl as the spacer materials, we construct the Co2FeAl/Co2ScAl/Co2FeAl(001)-spin valve. Based on the first-principles calculation, the most stable FeAl/CoCo-interface is determined both from the phonon spectra and the formation energy when the spacer Co2ScAl grows on the FeAl-terminated (001) surface of electrode material Co2FeAl. By comparing the projected density of states of the interfacial atoms with the corresponding density of states of the bulk electrode material, only the value of spin-up state of Al changes from 0.17 states/atom/eV to 0.06 states/atom/eV before and after substitution, the half metallicity at the interface is maintained. As a result, the spin-dependent transport properties show significant theoretical magnetoresistance MRop which can reach up to 1010% and much larger than 106% reported before.

Elucidation of the strong effect of an interfacial monolayer on magnetoresistance in giant magnetoresistive devices with current perpendicular to the plane

Electronic band matching at the heterointerface between magnetic and nonmagnetic metals influences spin-dependent transport through that interface. However, the impact of atomic-level interfacial structure on the electronic band matching and the resultant transport properties have not been clarified. Here, the authors have fabricated high-quality epitaxial giant magnetoresistance devices with half-metallic Heusler electrodes with interfacial ultrathin Ni insertion layers. Through state-of-the-art microstructure analysis, they demonstrate that even a monolayer of Ni strongly affects interfacial band matching and giant magnetoresistance.

Spin field effect transistors and their applications: A survey

Among the various devices being researched as possible alternative for conventional scaled down transistor, the spin-FET (Datta-Das transistor) held some early promise. Various research groups have proposed the complementary spin-FETs viz., parallel and anti-parallel spin-FETs, later designed some basic and higher order logic circuits using them, and compared the performance with the conventional CMOS design. Various bulk materials like InAs, InAlAs etc. and 2D materials like graphene, silicene etc. have been used as channel materials in spin-FETs by many researchers. Different ferromagnetic and half-metallic electrodes like cobalt, nickel, CrO2 have also been employed in the spin-FETs. Several research groups have worked on the usage of multi-gate and multifunctional logic using spin-FET devices. In this paper, a review of the development of spin-FETs and spin-FET based design has been performed. In addition, the various applications of spin-FETs and the challenges faced for the implementation of spin-FETs are highlighted in the paper.

Experimental observation of large tunneling anisotropic magnetoresistance in a magnetic tunnel junction without heavy metals

Tunneling anisotropic magnetoresistance (TAMR) in the magnetic tunnel junctions (MTJs) driven by spin–orbit coupling (SOC) has attracted much attention for potential spintronic applications based on magnetic manipulation of electric transport. However, it has been believed that heavy metals are indispensable for the large TAMR. Here we experimentally show a TAMR of up to 46% in a La0.7Sr0.3MnO3 (LSMO) based MTJ without heavy metals. This value represents the largest TAMR for MTJs with half-metallic electrodes. We demonstrate that the TAMR ratio is enhanced by over one order of magnitude by tuning interface termination layer, achieving quasilocalized states at the interface Fermi level on the basis of magnetization orientation. These results are also supported by our first-principles density functional calculations. This finding illustrates a crucial role of interface tuning in the TAMR effect, opening a route for engineering the large anisotropic magnetoresistance by interface termination control and manipulating high spin polarization without using heavy metals.

Oscillations of tunneling magnetoresistance on bias voltage in magnetic tunnel junctions with periodic grating barrier

A spintronic theory is developed to describe the effect of bias voltages on the magnetic tunnel junctions (MTJs) with a single-crystal barrier. The theory is founded on a conventional optical diffraction method and has already explained the barrier thickness effect, the temperature effect, and the half-metallic electrode effect in the MTJs with a periodic grating barrier. We find that the tunneling magnetoresistance (TMR) will oscillate with the bias voltage. This theoretical result can interpret the bias dependence observed in experiments. The range of bias voltage where the oscillations arise can be regulated by the barrier thickness and the spin polarization of the electrodes. In particular, it demonstrates that the bias voltage smaller than 100mV can hardly change the properties of TMR oscillations on the barrier thickness, which is in agreement with the experiments. Finally, a practical method is proposed to enhance and optimize the output voltage.

Effect of Lattice Distortion on the Magnetic Tunnel Junctions Consisting of Periodic Grating Barrier and Half-Metallic Electrodes**Supported by the National Natural Science Foundation of China under Grant Nos. 11704197 and 61574079, and the NUPTSF under Grant No. NY217046.

A spintronic theory is developed to study the effect of lattice distortion on the magnetic tunnel junctions (MTJs) consisting of single-crystal barrier and half-metallic electrodes. In the theory, the lattice distortion is described by strain, defect concentration and recovery temperature. All three parameters will modify the periodic scattering potential, and further alter the tunneling magnetoresistance (TMR). The theoretical results show that: (1) the TMR oscillates with all the three parameters; (2) the strain can change the TMR about 30%; (3) the defect concentration will strongly modify the periodic scattering potential, and further change the TMR about 50%; and (4) the recovery temperature has little effect on the periodic scattering potential, and only can change the TMR about 10%. The present work may provide a theoretical foundation to the application of lattice distortion for MTJs consisting of single-crystal barrier and half-metallic electrodes.

Magnetic tunnel junctions consisting of a periodic grating barrier and two half-metallic electrodes

We have developed a spintronic theory for magnetic tunnel junctions consisting of a single-crystal barrier and two half-metallic ferromagnetic electrodes. Radically different from the conventional theories, the barrier is now regarded as an optical diffraction grating, and treated by the traditional optical scattering method, i.e. Bethe theory and two-beam approximation. After tunneling, the electrons can thus possess high coherence. In the case that the electrodes are both half-metallic, the conventional theories give an infinite tunneling magnetoresistance (TMR). By contrast, in the Bethe theory and two-beam approximation, there can exist the scattering channels of nonconservation of energy. Therefore, the TMR can still be far away from infinity, which is in accordance with experiments. Also, we find that, due to the half-metallicity of the electrodes, the parallel conductance oscillates with temperature whereas the antiparallel conductance will increase other than oscillate with temperature. That is in agreement with experiments, too. Finally, two applications of the present theory are discussed with regard to the material design and engineering: one is how to choose appropriate materials for the barrier to realize infinite TMR; the other is a criterion for judging whether a material is half-metallic or not.

Large magneto-Seebeck effect in magnetic tunnel junctions with half-metallic Heusler electrodes

Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make wasted heat in electronic devices useable or to provide new read-out mechanisms for information. However, only few materials have been studied so far with Seebeck voltages of only some microvolt, which hampers applications. Here, we demonstrate that half-metallic Heusler compounds are hot candidates for enhancing spin-dependent thermoelectric effects. This becomes evident when considering the asymmetry of the spin-split density of electronic states around the Fermi level that determines the spin-dependent thermoelectric transport in magnetic tunnel junctions. We identify Co2FeAl and Co2FeSi Heusler compounds as ideal due to their energy gaps in the minority density of states, and demonstrate devices with substantially larger Seebeck voltages and tunnel magneto-Seebeck effect ratios than the commonly used Co-Fe-B-based junctions.

Spin Transport in H2O Adsorbed SiCNT Based Magnetic Tunnel Junction Using Half Metallic Ferromagnetic Electrodes

Spin Transport in H₂O Adsorbed SiCNT Based Magnetic Tunnel Junction Using Half Metallic Ferromagnetic Electrodes

Influence of image forces on the interlayer exchange interaction in magnetic tunnel junctions with a ferroelectric barrier

We study interlayer exchange interaction in magnetic tunnel junctions with ferroelectric barrier. We focus on the influence of image forces on the voltage dependence of the interlayer magnetic interaction (magneto-electric effect). The influence of the image forces is twofold: 1) they significantly enforce magneto-electric effect occurring due to the surface charges at the interface between ferroelectric and ferromagnets; 2) in combination with voltage dependent dielectric constant of the ferroelectric barrier image forces cause an additional contribution to the magneto-electric effect in magnetic tunnel junctions. This contribution can exceed the one coming from surface charges. We compare the interlayer exchange coupling voltage variation with spin transfer torque effect and show that for half-metallic electrodes the interlayer exchange coupling variation is dominant and defines the magnetic state and dynamics of magnetization in the tunnel junction.

Magnetization and Spin Polarization of Heusler Alloys Co $_{2}$TiSn and Co$_{2}$ TiGa$_{0.5}$Sn $_{0.5}$

The substitution effect on half-metallicity for Co-based Heusler alloys Co 2 TiGa 1-x Sn x was investigated by experimental measurements and first-principles band calculations. The synthesized alloys crystalized in the ordered L2 1 crystal structure. The spontaneous magnetization obeyed the Slater-Pauling rule, indicating the possibility that Co 2 TiSn and Co 2 TiGa 0.5 Sn 0.5 are half-metals. The first-principles band calculations revealed that Co 2 TiSn is half-metal and Co 2 TiGa 0.5 Sn 0.5 is nearly half-metal. The spin polarization P was determined by the Andreev reflection technique. The intrinsic P values were 58.9% for Co 2 TiSn and 58.2% for Co 2 TiGa 0.5 Sn 0.5 , respectively. The P values of Co 2 TiGa 1-x Sn x were relatively higher than other Co-based Heusler alloys. Thus, Co-based Heusler alloys Co 2 TiGa 1-x Sn x are possible candidates for half-metallic electrode materials in spintronic devices.

Fully magnetic manganite spin filter tunnel junctions

In this paper we demonstrate spintronic devices which combine magnetic tunnel junctions with a spin-filtering tunnel barrier. These consist of an ultrathin ferromagnetic insulating barrier, Sm0.75Sr0.25MnO3, sandwiched between two ferromagnetic half-metallic manganite electrodes, La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3, in a nanopillar structure. Depending on the relative magnetic configurations of barrier and electrode layers, three resistance states are well defined, which therefore represent a potential three-state memory concept. These results open the way for the development of spintronic devices by exploiting the many degrees of freedom of perovskite manganite heterostructure systems.

Optimization of spin injection and spin detection in lateral nanostructures by geometrical means

Lateral spin devices are an important concept in nowadays all-metallic spintronic devices. One of the key problems is to obtain large spin injection and detection efficiency. Several concepts has been envisaged, such as to use half-metallic ferromagnetic electrodes or spin-polarized interface barriers. Within this work, we address the optimization of spin devices (namely optimization of spin current density, spin current and spin accumulation) based on adjustment of the geometry (dimensions) of the lateral device, material selection of spin conductors, jointly with optimization of the interface resistance.

Tunneling anisotropic magnetoresistance in a magnetic tunnel junction with half-metallic electrodes

Tunneling anisotropic magnetoresistance (TAMR) is the difference in resistance of a magnetic tunnel junction due to a change in magnetization direction of one or both magnetic electrodes with respect to the flow of current. We present the results of first-principles density functional calculations of the TAMR effect in magnetic tunnel junctions with $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) electrodes and a $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO) tunneling barrier. We find an $\ensuremath{\sim}500%$ difference in resistance between magnetization in the plane and out of the plane. This large TAMR effect originates from the half-metallic nature of LSMO: When magnetization is out of plane spin-orbit coupling (SOC) contributions to the transmission come only from spin-flip scattering, which is intrinsically small due to the half-metallicity. For in-plane magnetization, however, there is a large non-spin-flip SOC contribution to the conductance. The large magnitude of the effect stems from the additional fact that there is an inherent polar discontinuity between LSMO and STO which leads to quasilocalized states at the interface whose influence on tunneling is strongly dependent on the magnetization orientation.