Do Bang

Mirror-symmetric magneto-optical Kerr rotation using visible light in [(GeTe)2(Sb2Te3)1]n topological superlattices.

Interfacial phase change memory (iPCM), that has a structure of a superlattice made of alternating atomically thin GeTe and Sb2Te3 layers, has recently attracted attention not only due to its superior performance compared to the alloy of the same average composition in terms of energy consumption but also due to its strong response to an external magnetic field (giant magnetoresistance) that has been speculated to arise from switching between topological insulator (RESET) and normal insulator (SET) phases. Here we report magneto-optical Kerr rotation loops in the visible range, that have mirror symmetric resonances with respect to the magnetic field polarity at temperatures above 380 K when the material is in the SET phase that has Kramers-pairs in spin-split bands. We further found that this threshold temperature may be controlled if the sample was cooled in a magnetic field. The observed results open new possibilities for use of iPCM beyond phase-change memory applications.

Current-Induced Domain Wall Motion in Perpendicular Magnetized Tb?Fe?Co Wire with Different Interface Structures

Current-induced domain wall motion has been investigated in perpendicular magnetic anisotropy SiO2/TbFeCo/Pt (asymmetric interface) and Pt/TbFeCo/Pt (symmetric interface) wires. The domain wall is found to be moved in opposite to the electron-flow direction in the asymmetric interface wire whereas it is kept same direction in the symmetric one at room temperature. As an alternative to the conventional spin-transfer torque, the Rashba field at the TbFeCo/Pt interface and the spin Hall current injected from the Pt layer are possibilities of the reversed domain wall motion in an ultrathin magnetic film sandwiched by dissimilar materials.

High efficiency of the spin-orbit torques induced domain wall motion in asymmetric interfacial multilayered Tb/Co wires

We investigated current-induced DW motion in asymmetric interfacial multilayered Tb/Co wires for various thicknesses of magnetic and Pt-capping layers. It is found that the driving mechanism for the DW motion changes from interfacial to bulk effects at much thick magnetic layer (up to 19.8 nm). In thin wires, linearly depinning field dependence of critical current density and in-plane field dependence of DW velocity suggest that the extrinsic pinning governs field-induced DW motion and injecting current can be regarded as an effective field. It is expected that the high efficiency of spin-orbit torques in thick magnetic multilayers would have important implication for future spintronic devices based on in-plane current induced-DW motion or switching.

Reversal of Domain Wall Motion in Perpendicularly Magnetized TbFeCo-Based Wires: Size Dependence

Current-induced domain wall (DW) motion has been investigated in interfacial asymmetric SiO2/TbFeCo/Pt wires with different wire widths and thicknesses. The reversed DW motion was observed in these interfacial asymmetric wires at zero field and room temperature. This reversal of DW motion was attributed to the structural inversion asymmetry-induced spin–orbit torques (SOTs) from the Pt/TbFeCo interface and Pt layer of the wire. We found that the reversed DW velocity strongly depends on the size of wires, typically large for thin and narrow wires, suggesting that the SOTs effectively enhance the reversed DW velocity in these asymmetric TbFeCo wires.

Strain-induced reversible modulation of the magnetic anisotropy in perpendicularly magnetized metals deposited on a flexible substrate

In this study, the strain-induced change in the magnetic anisotropy of perpendicularly magnetized thin metals (TbFeCo and Pt/Co/Pt) deposited on a polyethylene naphthalate flexible substrate was investigated. The in-plane uniaxial tensile strain was reversibly applied up to 2%. The magnetic anisotropy was reversibly changed in both samples with applied stress. In the TbFeCo film, a marked change in magnetic anisotropy energy of 1.2 × 105 J/m3 was observed. In the Pt/Co/Pt film, where the thickness of Co was 2–4 monolayers, the stress-induced changes in interface and volume contributions to magnetic anisotropy were individually determined.

Inelastic tunneling spectra of MgO barrier magnetic tunneling junctions showing large magnon contribution

We investigated bias-voltage and temperature dependence of conductivity arising from the magnon contribution in MgO-based magnetic tunneling junctions (MTJs) with different ferromagnetic electrodes. Second derivative conductance curves showed broad peak structure, which extends from 5 to 200 mV, accompanied by additional peaks at around 23, 54, and 85 mV. The peak intensities were larger for antiparallel configuration than for parallel configuration except that at 85 mV. This difference in the peak intensity was observed to be larger for the MTJs having higher tunneling magnetoresistance ratio, indicating a magnetic origin of these peaks. Abrupt increase in the second derivative conductance at very low biasing voltage in the antiparallel configuration suggests the important role of the surface magnon excitation.

Domain wall motion in Tb/Co multilayer wires with a large domain wall depinning field

We have investigated domain wall (DW) motion in Tb/Co multilayer wires with a large DW depinning field ∼3.0 kOe and different wire widths. In this system, the DW depinning field was decreased as a linear dependence of injecting current. A high effective efficiency e ∼ 5 × 10−13 T m2 A−1 was obtained. Threshold current density for the DW motion was found to be approximately 3 × 1011 A/m2 for the wires. High DW velocity up to 90 m/s was observed. In addition, the DW velocity showed a strong dependence on perpendicular (easy-axis) magnetic field. These results suggest that the Tb/Co wire is an excellent candidate for the high-speed and stability of stored information in DW-controlled devices.

Spin-torque diode spectrum of ferromagnetically coupled (FeB/CoFe)/Ru/(CoFe/FeB) synthetic free layer

We investigated systematically the spin torque diode spectrum of a ferromagnetically coupled (FeB/CoFe)/Ru/(CoFe/FeB) synthetic free layer in an MgO-based magnetic tunnel junction. In the spectra, we observed single peaks shifted to higher frequency with increasing the in-plane magnetic fields, as expected from the ferromagnetic resonance of the FeB/CoFe adjacent to the MgO tunnel barrier. On the other hand, under the perpendicular fields, we observed several peaks below 6 GHz and around 10 GHz, which were rather insensitive to the field. These behaviors are different from our simple calculation taking account the interlayer coupling, suggesting that the excitation of magnetizations occurs in a complicated manner for the synthetic free layers.

Effect of microstructure on magnetism and transport properties of Cu–Mn–Al granular ribbons

The microstructure and the magnetic and transport properties of as-quenched and annealed Cu2MnAl granular ribbons have been investigated. The appearance of nonmagnetic β-Mn and γ-Cu9Al4 phases is attributed to the decomposition reaction of the Cu2MnAl phase after the annealing process. Negative magnetoresistances (MRs) above 5% have been observed for the annealed ribbons. This high value of MR is attributed to spin-dependent impurity scattering within the granules and interface scattering at the nonmagnetic boundaries of the granules.

Strong quantum interference effect in fully epitaxial Cr/Fe/MgO/Fe magnetic tunnel junctions with ultrathin-Fe electrodes at room temperature

Spin-dependent transport in high quality fully epitaxial magnetic tunneling junctions with structure of ultrathin-Fe(001)/MgO(001)/Fe(001) on a crystalline Cr(001) buffer which is employed as a spin-reflection layer has been investigated. We successfully observed an oscillation of conductance with respect to the Fe-wedge thickness with about a 2 monatomic-layer period. Also, we observed clear quantum interference effect both in tunneling spectra and differential tunneling magnetoresistance curves of the samples even at room temperature. The tunneling magnetoresistance is clearly modulated at the bias voltages corresponding to the resonant peaks. Spin-torque diode spectra of the junctions with ultrathin-Fe electrodes were also measured under various bias voltages. From these results, we proved that the spin-dependent transport in the magnetic tunnel junctions can be modulated by introducing quantum well states in the structure.