Seiji Mitani

A 4‐Fold‐Symmetry Hexagonal Ruthenium for Magnetic Heterostructures Exhibiting Enhanced Perpendicular Magnetic Anisotropy and Tunnel Magnetoresistance

A 4-fold-symmetry hexagonal Ru emerging in epitaxial MgO/Ru/Co2 FeAl/MgO heterostructures is reported, in which an approximately Ru(022¯3) growth attributes to the lattice matching between MgO, Ru, and Co2 FeAl. Perpendicular magnetic anisotropy of the Co2 FeAl/MgO interface is substantially enhanced. The magnetic tunnel junctions (MTJs) incorporating this structure give rise to the largest tunnel magnetoresistance for perpendicular MTJs using low damping Heusler alloys.

Low-resistive monocrystalline Mg-Al-O barrier magnetic tunnel junctions for spin-transfer magnetization switching

Monocrystalline Mg-Al-O barriers for magnetic tunnel junctions (MTJs) were fabricated by natural oxidation of Mg-Al thin films. The naturally oxidized Mg-Al-O barrier had a cation-disorder MgAl2O4 structure, and Fe/Mg-Al-O/Fe(001) MTJs showed a tunnel magnetoresistance (TMR) ratio of over 180% at room temperature. In addition, the natural oxidation process resulted in tunnel barriers with a resistance area product (RA) of less than 5 Ω·μm2. We observed clear magnetization switching in a CoFe/Mg-Al-O/CoFe MTJ by spin-transfer torque. These results indicate that the natural oxidation of Mg-Al alloy is an effective method for processing cation-disorder MgAl2O4 barrier for coherent TMR with low RA.

Chemical ordering and large tunnel magnetoresistance in Co2FeAl/MgAl2O4/Co2FeAl(001) junctions

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.

Post-oxidized Mg-Al-O(001) coherent tunneling barrier in a wide range of resistance-area products

We fabricated epitaxial Mg-Al-O(001) tunnel barriers using direct/indirect plasma oxidation and natural oxidation of an MgAl layer for use in Fe/Mg-Al-O/Fe magnetic tunnel junctions. All the oxidation processes formed epitaxial Mg-Al-O barriers, and a wide resistance area (RA) product range (101–106 Ω·μm2) and large tunnel magnetoresistance (TMR) ratios (185%–212%) at room temperature were achieved by optimizing the MgAl thickness (tMgAl). Near optimum oxidation conditions and tMgAl, small bias voltage dependence of the TMR ratio, and distinct local minima in the dI/dV spectra for the parallel magnetic configuration were observed, indicating that coherent tunneling transport was significant. This study showed that Mg-Al-O coherent tunnel barriers have excellent formability in a wide RA product range.

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 5u2009K) 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 (≤1.5u2009nm) CoFe layer between the CFA and Mg-Al-O, the TMR ratio further increased up to 280% at RT (453% at 5u2009K), which reflected the importance of controlling barrier-electrode interface states other than the lattice matching.

Spin-transfer magnetization switching in ordered alloy-based nanopillar devices

This paper reviews spin-transfer magnetization switching in ordered alloy-based nanopillar devices. L10-ordered FePt was used for one of the earliest demonstrations of spin-transfer switching in perpendicularly magnetized systems. The behaviour of magnetization switching deviates from the predictions based on a macro-spin model, suggesting incoherent magnetization switching in the system with a large perpendicular magnetic anisotropy. The effect of a 90° spin injector on spin-transfer switching was also examined using L10-ordered FePt. Full-Heusler alloys are in another fascinating material class for spin-transfer switching because of their high-spin polarization of conduction electrons and possible small magnetization damping. A B2-ordered Co2FeAl0.5Si0.5-based device showed a low intrinsic critical current density of 9.3 × 106u2009Au2009cm−2 for spin-transfer switching as well as a relatively large current-perpendicular-to-plane giant-magnetoresistance (CPP-GMR) up to ~9%. The specific physical properties of ordered alloys may be useful for fundamental studies and applications in spin-transfer switching.

The spin Nernst effect in tungsten

The spin Nernst effect, direct conversion of heat current to spin current, is observed in W/CoFeB/MgO heterostructures. The spin Hall effect allows the generation of spin current when charge current is passed along materials with large spin-orbit coupling. It has been recently predicted that heat current in a nonmagnetic metal can be converted into spin current via a process referred to as the spin Nernst effect. We report the observation of the spin Nernst effect in W. In W/CoFeB/MgO heterostructures, we find changes in the longitudinal and transverse voltages with magnetic field when temperature gradient is applied across the film. The field dependence of the voltage resembles that of the spin Hall magnetoresistance. A comparison of the temperature gradient–induced voltage and the spin Hall magnetoresistance allows direct estimation of the spin Nernst angle. We find the spin Nernst angle of W to be similar in magnitude but opposite in sign to its spin Hall angle. Under an open-circuit condition, this sign difference results in the spin current generation larger than otherwise. These results highlight the distinct characteristics of the spin Nernst and spin Hall effects, providing pathways to explore materials with unique band structures that may generate large spin current with high efficiency.

Interdiffusion in epitaxial ultrathin Co2FeAl/MgO heterostructures with interface-induced perpendicular magnetic anisotropy

The structures of epitaxial ultrathin Co2FeAl/MgO(001) heterostructures relating to the interface-induced perpendicular magnetic anisotropy (PMA) were investigated using scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray magnetic circular dichroism. We found that Al atoms from the Co2FeAl layer significantly interdiffuse into MgO, forming an Al-deficient Co-Fe-Al/Mg-Al-O structure near the Co2FeAl/MgO interface. This atomic replacement may play an additional role for enhancing PMA, which is consistent with the observed large perpendicular orbital magnetic moments of Fe atoms at the interface. This work suggests that control of interdiffusion at ferromanget/barrier interfaces is critical for designing an interface-induced PMA system.

Perpendicular magnetic anisotropy at lattice-matched Co2FeAl/MgAl2O4(001) epitaxial interfaces

We report perpendicular magnetic anisotropy (PMA) induced at Co2FeAl/MgAl2O4(001) epitaxial interfaces prepared by magnetron sputtering and post-oxidation of MgAl layers. A PMA energy density of more than 4 Merg/cm3 for 1-nm-thick Co2FeAl layers and an effective interface PMA energy density of 1.6u2009erg/cm2 were achieved by controlling the interfacial oxidation states through fine-tuning of oxidation processes and annealing temperature. Cross-sectional scanning transmission electron microscopy imaging revealed a lattice-matched Co2FeAl/MgAl2O4 interface, which may be responsible for the large PMA energy due to a reduction of the bulk anisotropy contribution.

Ferromagnetic MnGaN thin films with perpendicular magnetic anisotropy for spintronics applications

Perpendicularly magnetized flat thin films of antiperovskite Mn67Ga24N9 were grown on an MgO(001) substrate by reactive sputtering using an argon/1% nitrogen gas mixture and a Mn70Ga30 target. The films showed a saturation magnetization of 80u2009–100u2009kA/m, an effective perpendicular magnetic anisotropy (PMA) energy of 0.1–0.2u2009MJ/m3, and a Curie temperature of 660–740u2009K. Upon increasing the N composition, the films transformed from ferromagnetic to antiferromagnetic as expected in the stoichiometric Mn3GaN phase. Point contact Andreev reflection spectroscopy revealed that the ferromagnetic MnGaN has a current spin polarization of 57%, which is comparable to D022-MnGa. These findings suggest that MnGaN is a promising PMA layer for future spintronics devices.