Large Magnetoresistance Ratio Research Articles

Spin transport through a junction entirely consisting of molecules from first principles

Using first-principles calculations based on density functional theory combined with the nonequilibrium Green's function formalism, we studied the spin transport through a single molecular junction which consists of a single 1,4-benzenedithiolate (BDT) molecule and two ferromagnetic electrodes [(Ge5)Fe]∞. A large magnetoresistance ratio (MR) of 21100% was found in the [(Ge5)Fe]∞-BDT-[(Ge5)Fe]∞ molecular junction at small bias voltage, and the MR value decreased with the increase in the applied bias voltage. For the parallel magnetization configuration, the molecular junction showed outstanding spin injection effects. Negative differential resistance was observed for the antiparallel magnetization configuration. Spin dependent transmission spectra at different bias voltages were used to explain the calculated results.

Spin transport properties based on spin gapless semiconductor CoFeMnSi

Spin gapless semiconductors have been regarded as the most promising candidates for spin injection materials due to the complete (100%) spin polarization and the conductivity between half-metals and semiconductors. To explore the potential spintronic applications of the quaternary Heusler alloy CoFeMnSi (CFMS), a recently fabricated spin gapless semiconductor with a high Curie temperature of 620 K, we design the GaAs/CFMS heterostructure and the CFMS/GaAs/CFMS magnetic tunnel junction (MTJ). It is found from the first-principles calculations combined with nonequilibrium Green's function that the heterostructure exhibits an excellent spin filtering effect and spin diode effect and the MTJ has a large tunnel magnetoresistance ratio (up to 2 × 103), which are explained from the calculated spin-dependent band structure and transmission spectrum. These perfect spin transport characteristics make CFMS a promising candidate for spintronic applications.

Enhancement of current-perpendicular-to-plane giant magnetoresistive outputs by improving B2-order in polycrystalline Co2(Mn0.6Fe0.4)Ge Heusler alloy films with the insertion of amorphous CoFeBTa underlayer

We studied the origin of the enhancement of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) effect by inserting a thin amorphous CoFeBTa (CFBT) underlayer below a Co2(Mn0.6Fe0.4)Ge (CMFG) Heusler alloy ferromagnetic (FM) layer. Large magnetoresistance ratio of ∼25% and resistance change-area product of ∼7.5 mΩ μm2 were obtained at room temperature by inserting a CFBT (1.2 nm) underlayer. X-ray diffraction (XRD) and transmission electron microscope analyses confirmed that the CMFG FM layer deposited on the CFBT underlayer was amorphous in the as-deposited state and crystallized to a B2-ordered polycrystalline film by annealing at 300 °C. The degree of B2 order (SB2) of the CMFG films was estimated by anomalous XRD using x-ray energies around the Co K-absorption edge. SB2 of the CMFG film deposited on the amorphous CFBT (1.2 nm) underlayer was ∼0.76, much larger than that of the CMFG film deposited on a crystalline CoFe underlayer (SB2 ∼0.47). First-principles calculations indicated that the spin polarization of the sp-conduction electrons in CMFG increases with increasing SB2, which accounts for the enhanced CPP-GMR effect in the pseudo spin-valve by inserting an amorphous CFBT underlayer.

L10-MnGa based magnetic tunnel junction for high magnetic field sensor

We report on the investigation of the magnetic tunnel junction structure designed for high magnetic field sensors with a perpendicularly magnetized L10-MnGa reference layer and an in-plane magnetized Fe sensing layer. A large linear tunneling magnetoresistance ratio up to 27.4% and huge dynamic range up to 5600 Oe have been observed at 300 K, with a low nonlinearity of 0.23% in the optimized magnetic tunnel junction (MTJ). The field response of tunneling magnetoresistance is discussed to explain the field sensing properties in the dynamic range. These results indicate that L10-MnGa based orthogonal MTJ is a promising candidate for a high performance magnetic field sensor with a large dynamic range, high endurance and low power consumption.

Magnetic-proximity-induced magnetoresistance on topological insulators

We theoretically study the magnetoresistance (MR) of two-dimensional massless Dirac electrons as found on the surface of three-dimensional topological insulators (3D TIs) that is capped by a ferromagnetic insulator (FI). We calculate charge and spin transport by Kubo and Boltzmann theories, taking into account the ladder-vertex correction and the in-scattering due to normal and magnetic disorder. The induced exchange splitting is found to generate an electric conductivity that depends on the magnetization orientation, but its form is very different from both the anisotropic and spin Hall MR. The in-plane MR vanishes identically for non-magnetic disorder, while out-of-plane magnetizations cause a large MR ratio. On the other hand, we do find an in-plane MR and planar Hall effect in the presence of magnetic disorder aligned with the FI magnetization. Our results may help understand recent transport measurements on TI|FI systems.

Large magnetocaloric and magnetoresistance effects in metamagnetic Sm0.55(Sr0.5Ca0.5)0.45MnO3 manganite

The metamagnetic transition, magnetocaloric and magnetoresistance effects are investigated in polycrystalline Sm0.55(Sr0.5Ca0.5)0.45MnO3 (SSCMO) manganite. A sharp magnetization jump at Curie temperature (TC) 73.5K with large thermal hysteresis is observed. Magnetic measurements and Arrott plots analysis indicate that the transition is first order in nature. Under a low magnetic field change of 1T, the magnetic entropy change exhibits a peak value of −ΔSMmax(T)=4.01J/kg K with relative cooling power (RCP) value of 44.1J/kg in the vicinity of TC. The mechanism of charge conduction in insulator phase is polaron transport. Large negative magnetoresistance ratio with value of ~99% is obtained within a broad temperature range below metal insulator transition temperature under 1T magnetic field. These results indicate the potential applications of SSCMO in magnetic refrigeration and spintronics devices.

Low noise all-oxide magnetic tunnel junctions based on a La0.7Sr0.3MnO3/Nb:SrTiO3 interface

All-oxide magnetic tunnel junctions with a semiconducting barrier, formed by the half-metallic ferromagnet La0.7Sr0.3MnO3 and n-type semiconductor SrTi0.8Nb0.2O3, were designed, fabricated, and investigated in terms of their magneto-transport properties as a function of applied bias and temperature. We found that the use of the heavily Nb-doped SrTiO3 as a barrier results in significant improvement in the reproducibility of results, i.e., of large tunnel magnetoresistance (TMR) ratios, and a spectral noise density reduced by three orders of magnitude at low temperature. We attribute this finding to a considerably decreased amount of point defects in SrTi0.8Nb0.2O3, especially oxygen vacancies, compared with the conventional insulating SrTiO3 barrier.

Enhanced tunneling electroresistance in multiferroic tunnel junctions due to the reversible modulation of orbitals overlap

We report a first-principles study of the ferroelectricity and spin-dependent transport through Co/BaTiO3/CoO/Co multiferroic tunnel junctions (MFTJs). We find the coexistence of large tunneling magnetoresistance (TMR) ratio and large tunneling electroresistance (TER) ratio in the MFTJs. The large TMR effect originates from the spin-filter tunneling through the BaTiO3 barrier, while the TER effect is due to the modulation of orbitals overlap by polarization reversal. The microscopic physics of TER are identified and understood through the analysis of metal-oxygen relative displacements, local polarization magnitude, transmission in momentum space and real space scattering states. Our results provide a practical way to achieve the coexistence of large TER and TMR effects in MFTJs.

Li-substituted MgAl2O4 barriers for spin-dependent coherent tunneling

Epitaxial magnetic tunnel junctions (MTJs) with a Li-substituted spinel MgAl2O4 barrier were prepared by sputtering and plasma oxidation of an Mg/LiAl bilayer. The formed MTJ with Fe(001) electrodes showed a relatively large tunnel magnetoresistance (TMR) ratio of 120% (174%) at room temperature (3 K) and two local minima in the tunneling conductance spectra (parallel magnetization), revealing the occurrence of the coherent tunneling effect through the Li-substituted spinel barrier. The results are likely to pave the way for a variety of quaternary spinel oxide barriers in which structural and transport properties of MTJs can be engineered in the coherent tunneling regime.

Large magnetoresistance in Heusler-alloy-based epitaxial magnetic junctions with semiconducting Cu(In0.8Ga0.2)Se2 spacer

We investigated the structure and magneto-transport properties of magnetic junctions using a Co2Fe(Ga0.5Ge0.5) Heusler alloy as ferromagnetic electrodes and a Cu(In0.8Ga0.2)Se2 (CIGS) semiconductor as spacers. Owing to the semiconducting nature of the CIGS spacer, large magnetoresistance (MR) ratios of 40% at room temperature and 100% at 8 K were obtained for low resistance-area product (RA) values between 0.3 and 3 Ω μm2. Transmission electron microscopy observations confirmed the fully epitaxial growth of the chalcopyrite CIGS layer, and the temperature dependence of RA indicated that the large MR was due to spin dependent tunneling.

MgAl2O4(001) based magnetic tunnel junctions made by direct sputtering of a sintered spinel target

We developed a fabrication process of an epitaxial MgAl2O4 barrier for magnetic tunnel junctions (MTJs) using a direct sputtering method from an MgAl2O4 spinel sintered target. Annealing the sputter-deposited MgAl2O4 layer sandwiched between Fe electrodes led to the formation of a (001)-oriented cation-disorder spinel with atomically sharp interfaces and lattice-matching with the Fe electrodes. A large tunnel magnetoresistance ratio up to 245% at 297 K (436% at 3 K) was achieved in the Fe/MgAl2O4/Fe(001) MTJ as well as an excellent bias voltage dependence. These results indicate that the direct sputtering is an alternative method for the realization of high performance MTJs with a spinel-based tunnel barrier.

Enhanced High-Frequency Magnetoresistance Responses of Melt-Extracted Co-Rich Soft Ferromagnetic Microwires

We present the relationships between the structure, magnetic properties and high-frequency magnetoresistance (MR) effect in melt-extracted Co68.2Fe4.3B15Si12.5 microwires subject to thermal annealing. In order to release residual stresses to improve the magnetic softness while retaining the good mechanical property of an amorphous material, microwire samples were annealed at different temperatures of 100°C, 200°C, 350°C, 400°C, and 450°C for 15 min. We have shown that relative to an as-cast amorphous microwire, annealing microwires at T a = 100°C, 200°C, and 350°C improved both the magnetic softness and the MR effect, while an opposite trend was observed for the microwires annealed at T a = 400°C and 450°C. We have observed a distinct difference in the frequency dependence of MR response (ξ) for dc applied magnetic fields below and above the effective anisotropy field of the microwires. While the microwire annealed at 200°C shows the largest MR ratio (~580%) at 100 MHz, the highest value of ξ (~34%/Oe) has been achieved at 400 MHz for the microwire annealed at 350°C. These results indicate that the optimally annealed Co68.2Fe4.3B15Si12.5 microwires are attractive candidates for high-frequency sensor applications.

Spin-polarized transport utilizing d0 ferromagnetism: an ab initio study of CaC/MgS/CaC (0 0 1) heterojunctions

The electronic structure and magnetic properties of rocksalt-type CaC compound are studied by means of first-principles calculations. The B1-type calcium monocarbide is a metastable, nearly half-metallic ferromagnetic compound, with a magnetic moment of 1.81 μB/f.u. and a negative Fermi level spin-polarization of 50%. The estimated Curie temperature exceeds the room temperature. The electronic, magnetic and spin-polarized transport properties of CaC/MgS/CaC (0 0 1) heterostructures are investigated by using a first-principles Green’s function technique for layered structures in conjunction with the Kubo–Landauer formalism. The magnetism of CaC (0 0 1) electrodes is robust while the Fermi level spin-polarization of interfacial CaC layers at CaC/MgS (0 0 1) interfaces increases over the bulk value. The localized states formed at CaC/MgS (0 0 1) interfaces strongly enhance the minority-spin ferromagnetic state conductance. The current spin-polarization reaches values as high as 99%. Very large magnetoresistance ratios, above 104%, are in evidence. The highly magnetoresistive effect observed for CaC/MgS/CaC (0 0 1) heterojunctions together with the robust ferromagnetism of CaC (0 0 1) electrodes make this system attractive in the context of spin-electronics.

Large tunnel magnetoresistance ratio in Fe/O/NaCl/O/Fe

Magnetic tunnel junction (MTJ) is an important device element for many practical spintronic systems. In this paper, we propose and theoretically investigate a very attractive MTJ Fe(001)/O/NaCl(001)/O/Fe(001) as a two-terminal transport junction. By density functional theory total energy methods, we establish two viable device models: one with and the other without mirror symmetry across the center plane of the structure. Large tunnel magnetoresistance ratio (TMR) is predicted from first principles, at over 1800% and 3600% depending on the symmetry. Microscopically, a spin filtering effect is responsible for the large TMR. This effect essentially filters out all the minority spin channels (spin-down) from contributing to the tunnelling current. On the other hand, transport of the majority spin channel (spin-up) having Δ1 and Δ5 symmetry is enhanced by the FeO buffer layer in the MTJ.

Investigation on Spin Dependent Transport Properties of Core-Shell Structural Fe₃O₄/ZnS Nanocomposites for Spintronic Application.

The core-shell structural Fe3O4/ZnS nanocomposites with controllable shell thickness were well-fabricated via seed-mediate growth method. Structural and morphological characterizations reveal the direct deposition of crystalline II-VI compound semiconductor ZnS shell layer on Fe3O4 particles. Spin dependent electrical transport is studied on Fe3O4/ZnS nanocomposites with different shell thickness, and a large magnetoresistance (MR) ratio is observed under the magnetic field of 1.0 T at room temperature and 100 K for the compacted sample by Fe3O4/ZnS nanocomposites, which is 50% larger than that of sample with pure Fe3O4 particles, indicating that the enhanced MR is contributed from the spin injection between Fe3O4 and ZnS layer.

Spin-Dependent Conductance in Co/C60/Co/Ni Single-Molecule Junctions in the Contact Regime

The spin-dependent conductance in Co/C60/Co/Ni single-molecule magnetic tunnel junctions has been measured by combining spin-polarized scanning tunneling spectroscopy and current-displacement measurements using an ultrahigh vacuum low temperature scanning tunneling microscope (STM) at 5 K. With an electrical contact between molecule and electrodes, the measured junction conductance is 0.02–0.13 G0 in the low-conductance state and 0.12–0.76 G0 in the high-conductance state, respectively. The investigated single-molecule junctions exhibit large tunnel magnetoresistance (TMR) ratios higher than −60%. A variation of TMR from −63% to −94% has been observed due to different Co/Ni electrodes.

The vacancy defect in graphene nano-ribbon field-effect transistor in the presence of an external perpendicular magnetic field

In this paper, we investigate effect of vacancy defect on graphene nano-ribbon field effect transistor (GNR-FET) in the presence of an external perpendicular magnetic field. Our simulation is based on Non-Equilibrium Green's Function (NEGF). The nearest neighbor tight-binding model based on pz orbital construct the device Hamiltonian. Also we calculate magnetoresistance (MR) for different position vacancies in presence B-field. We find an external B-field causes the conductance of the GNR increases, giving rise to the MR effect. The simulation results indicate in the presence of a small B-field the current is degraded because the vacancy defect act as scattering and the conductance decreases due to defects compared with the perfect nanoribbons and the MR is reduced but in the hall quantum regime the probability of being scattered by the vacancy reduced due to the electrons are more localized at edge states and this phenomena to enhance the MR ratio of the device. The results indicate that the defect near drain and near edge has the largest MR ratio in the presence of a small B-Field and in the hall quantum the defect near source has the largest MR ratio.

Magnetoresistance effect in rubrene-based spin valves at room temperature.

We fabricate spin-valve devices with an Fe3O4/AlO/rubrene/Co stacking structure. Their magnetoresistance (MR) effects at room temperature and low temperatures are systemically investigated based on the measurement of MR curves, current-voltage response, etc. A large MR ratio of approximately 6% is achieved at room temperature, which is one of the highest MR ratios reported to date in organic spin valves. With decreasing measurement temperatures, we observe that the MR ratios increase because of decrease in spin scattering, and the width of the MR curves becomes larger owing to increase in the coercivity of the electrodes at low temperature. A nonlinear current-voltage dependence is clearly observed in these organic spin valves. From the measurement of MR curve for the spin valves with different rubrene layer thickness, we observe that the MR ratios monotonously decrease with increasing rubrene-layer thickness. We discuss the spin-dependent transport mechanisms in these devices based on our experimental results and the present theoretical analysis. Moreover, we note that the devices exhibit smaller MR ratios after annealing compared to their counterparts without annealing. On the basis of atomic force microscopy analysis of the organic films and device resistances, we deduce that the increase of interface spin scattering induced by large surface roughness after annealing most probably leads to reduction in the MR ratios.

Oscillatory exchange coupling and strong direct tunnelling with AgCl based heterojunctions

The structural stability and ground state electronic properties of Fe/AgCl/Fe (001) junctions with c(2×2) Fe/AgCl (001) heterointerfaces are studied by means of ab␣initio surface Green’s function technique for surface and interfaces. The spin-resolved ballistic conductances across Fe/AgCl/Fe (001) heterojunctions are determined by using Kubo–Landauer approach. Total energy calculations show that sharp Fe/AgCl (001) interfaces are possible. The interfacial charge transfer is interface sensitive and generates electrostatic interactions that contribute to the stabilization of Fe/AgCl (001) interfaces. The magnetism of interfacial Fe layers is robust but is responsive to the interfacial structure. The exchange coupling between Fe electrodes across AgCl barriers shows unexpected oscillations with respect to the spacer thickness that may be related in part with the amount of metal induced gap states in AgCl spacers. A powerful Δ1 spin filter effect is evidenced across AgCl barriers. Depending on the interface structure, large magnetoresistance ratios, above 3100%, are predicted. Interfacial interdiffusion may affect the magnetoresistive properties. The lower lattice mismatch at c(2×2) Fe/AgCl (001) interfaces combined with the strong Δ1 direct tunnelling across AgCl barriers make Fe/AgCl (001) interfaces attractive in the context of magnetoelectronics.

Lattice-matched magnetic tunnel junctions using a Heusler alloy Co2FeAl and a cation-disorder spinel Mg-Al-O barrier

Perfectly lattice-matched magnetic tunnel junctions (MTJs) consisting of a Heusler alloy B2-Co2FeAl (CFA) electrode and a cation-disorder spinel (Mg-Al-O) barrier were fabricated by sputtering and plasma oxidation. We achieved a large tunnel magnetoresistance (TMR) ratio of 228% at room temperature (RT) (398% at 5 K) for the epitaxial CFA/MgAl-O/CoFe(001) MTJ, in which the effect of lattice defects on TMR ratios is excluded. With inserting a ultrathin (≤1.5 nm) CoFe layer between the CFA and Mg-Al-O, the TMR ratio further increased up to 280% at RT (453% at 5 K), which reflected the importance of controlling barrier-electrode interface states other than the lattice matching.