スピントロニクスとは、電子の持つ「電荷」と「スピン」の両方の性質を利用する電子工学・技術分野です。本グループでは、強磁性体スピンの不揮発性を利用した高集積固体メモリ素子や不揮発ロジック素子等に関する材料開発や新機能開拓の研究を行っています。加えて、スピンを利用した新しい原理で動作するデバイスの基盤的研究や関連する評価技術の研究開発も行っています。現在、電子情報技術における省エネと低環境負荷が世界的な喫緊の課題となっており、電子デバイスの低消費電力化に寄与するこれらの新規素子に大きな期待が寄せられています。

Large perpendicular magnetic anisotropy at Fe/MgO interfaces

Enhanced interface perpendicular magnetic anisotropy

Vector measurments of current induced torques in magnetic heterostructures

Enhanced tunnel magnetoresistance in a spinel oxide tunnel barrier with cation site disorder

Spin Hall angle of Pt and Pd

Magnetic Tunnel Junctions with Perpendicular Co₂FeAl Heusler Alloy

Resonant domain wall nucleation

Spin-transfer switching in full-Heusler Co₂FeAl-based magnetic tunnel junctions

Realtime detection of the Spinmotive force

Spatial control of magnetic anisotropy

Perpendicular magnetic anisotropy in Co₂FeAl Heusler alloy

Narrow linewdith spin torque oscillators using Co₂Fe(Ge_0.5Ga_0.5) Heusler alloy

Spin dependent resonant tunneling

Magnetic tunnel junctions with Spinel MgAl₂O₄ barrier

Spin transfer magnetization switching in full-Heusler Co₂FeAl_0.5Si_0.5 spin valves

- Fe(5ML)/MgO[100] bilayer structures were prepared.
- Small amount of absorbate on Fe(100) enhances interface perpendicular magnetic anisotropy.
- Maximum PMA energy density of Keff = 14 Merg/cm³ was obtained.

Reference
J.W. Koo, S. Mitani, T. T. Sasaki, H. Sukegawa, Z. C. Wen, T. Ohkubo, T. Niizeki, K. Inomata and K. Hono, Large perpendicular magnetic anisotropy at Fe/MgO interfaces, Appl. Phys. Lett. 103, 192401 (2013).


- Interface perpendicular magnetic anisotropy increases upon doping the Ta underlayer with nitrogen.
- Nitrogen doped Ta shows smaller magnetic dead layer thickness than that of pure Ta.
- Out of plane bulk magnetic anisotropy found in many films.

Reference
J. Sinha, M. Hayashi, A. Kent, S. Fukami, M. Yamanouchi, H. Sato, S. Ikeda, S. Mitani, S. Yang, S. S. P. Parkin and H. Ohno, Enhanced interface perpendicular magnetic anisotropy in Ta-CoFeB-MgO using nitrogen doped Ta underlayers, Appl. Phys. Lett. 102, 242405 (2013).


- Current induced effective field in Ta|CoFeB|MgO magnetic heterostructure.
- Vector measurements of the effective field (fields parallel and transerse to the current direction).
- Effective field increases with Ta thickness.
- Sign changes at low Ta thickness.
- Transverse field is larger than the longitudinal field for thick Ta regime, i.e. a large field-like term.
- Field like term is opposite to the conventional field like term for spin Hall spin torque.

Reference
J. Kim, J. Sinha, M. Hayashi, M. Yamanouchi, S. Fukami, T. Suzuki, S. Mitani, and H. Ohno, Layer thickness dependence of the current induced effective field vector in Ta|CoFeB|MgO, Nature Mater. 12, 240 (2013).


- Large tunnel magnetoresistance of 308% at RT (479% at 15 K) was obtained by introducing cation site disorder into spinel oxide (MgAl₂O₄) tunnel barrier.
- The enhancement can be interpreted as a band-folding effect in Fe.

Reference
H. Sukegawa, Y. Miura, S. Muramoto, S. Mitani, T. Niizeki, T. Ohkubo, K. Abe, M. Shirai, K. Inomata and K. Hono, Enhanced tunnel magnetoresistance in a spinel oxide barrier with cation-site disorder, Phys. Rev. B 86, 184401 (2012).


- Thickness dependence of the spin current-induced ferromagnetic resonance in Ni₈₀Fe₂₀/nonmagnetic (Pt,Pd) bilayer thin films.
- Spin Hall angle of Pt~0:022 and Pd~0:008.
- Spin diffusion length of Pt~1:2nm and Pd~2:0nm.

Reference
K. Kondou, H. Sukegawa, S. Mitani, K. Tsukagoshi, S. Kasai, Evaluation of spin Hall angle and spin diffusion length by using spin current-induced ferromagnetic resonance, Appl. Phys. Express 5, 073002 (2012).


- Perpendicularly magnetized magnetic tunnel junctions (p-MTJs) with an ultrathin Co₂FeAl (CFA) full-Heusler alloy electrode.
- Out-of-plane tunnel magnetoresistance (TMR) ratio of 53% at room temperature in CFA/MgO/Co₂₀Fe₆₀B₂₀ p-MTJs.
- TMR ratio enhanced to 91% by inserting a 0.1-nm-thick Fe layer between the MgO and /Co₂₀Fe₆₀B₂₀ layers.
- Bias voltage dependence of differential conductance did not clearly show coherent tunneling characteristics.

Reference
Z.C. Wen, H. Sukegawa, S. Kasai, M. Hayashi, S. Mitani, and K. Inomata, Magnetic Tunnel Junctions with Perpendicular Anisotropy Using a Co₂FeAl Full-Heusler Alloy, Appl. Phys. Exp. 5, 063003 (2012).


- Domain wall nucleation with microwave field in permalloy nanowires.
- Resonance frequency coincides with the excitation frequency for the FMR uniform mode.
- Nucleation field drops by a factor of two compared to off resonant nucleation.

Reference
M. Hayashi, Y. K. Takahashi, and S. Mitani, Microwave assisted resonant domain wall nucleation in permalloy nanowires, Appl. Phys. Lett. 101, 172406 (2012).


- Magnetic tunnel junctions (MTJs) with a full-Heusler alloy Co₂FeAl (CFA).
- "CFA-free": CFA(1.5 nm)/MgO/CoFe(4 nm) and "CFA-reference": CFA(30 nm)/MgO/CoFeB(2 nm) MTJs on Cr(001) layer.
- "CFA-free": JC0=2.9x10⁷ A/cm².
- "CFA-reference": JC0=7.1x10⁶ A/cm².
- Large Gilbert damping of ~0.4 in "CFA-free" due to the Cr layer.

Reference
H. Sukegawa, Z. C. Wen, K. Kondou, S. Kasai, S. Mitani, and K. Inomata, Spin-transfer switching in full-Heusler Co₂FeAl-based magnetic tunnel junctions, Appl. Phys. Lett. 100, 182403 (2012).


- Time resolved measurements of the voltage (spinmotive force) generated when a domain wall propagates along a magnetic nanowire.
- Spinmotive force in permalloy nanowires.
- Generated voltage scales with the driving magnetic field.
- Voltage sign changes when the domain wall propagation direction is reversed.
- Scaling of the voltage with magnetic field indicates current spin polarization of 0.55-0.69.

Reference
M. Hayashi, J. Ieda, Y. Yamane, J.Ohe, Y. K. Takahashi, S. Mitani, and S. Maekawa, Time-Domain Observation of the Spinmotive Force in Permalloy Nanowires, Phys. Rev. Lett. 108, 147202 (2012).


- Spatial patterning of magnetic anisotropy in CoFeB|MgO.
- Anisotropy of the CoFeB layer determined by the amount of etching of the MgO layer.
- Regions where MgO is half-etched show in-plane easy axis, otherwise it is out of plane.
- In-plane|out of plane domains form 90 deg domain walls.
- Presence of the 90 deg domain walls allows current induced injection of domain walls.

Reference
M. Hayashi, M. Yamanouchi, S. Fukami, J. Sinha, S. Mitani, and H. Ohno, Spatial control of magnetic anisotropy for current induced domain wall injection in perpendicularly magnetized CoFeB|MgO nanostructures, Appl. Phys. Lett. 100, 192411 (2012).


- Perpendicular magnetization in full-Heusler alloy Co₂FeAl (CFA) films using CFA|MgO and MgO|CFA structures.
- Interfacial magnetic anisotropy at the CFA|MgO interface.
- Perpendicular magnetic anisotropy energy density (KU) of 2-3x10⁶ erg/cm³.

Reference
Z.C. Wen, H. Sukegawa, S. Mitani and K. Inomata, Perpendicular magnetization of Co₂FeAl full-Heusler alloy films induced by MgO interface, Appl. Phys. Lett. 98, 242507 (2011).


- Microwave emission from pseudo spin valve nanopillars with Heusler alloy Co₂Fe(Ge_0.5Ga_0.5) electrodes.
- Large emission amplitude exceeding 150 nV/Hz0.5 and narrow generation linewidth below 10MHz.
- Reduced non-linear phase noise by tuing the applied field magnitude and its angle within the film plane.

Reference
J. Sinha, M. Hayashi, Y. K. Takahashi, T. Taniguchi, M. Drapeko, S. Mitani and K. Hono, Large amplitude microwave emission and reduced nonlinear phase noise in Co₂Fe(Ge_0.5Ga_0.5) Heusler alloy based pseudo spin valve nanopillars, Appl. Phys. Lett. 99, 162508 (2011).


- Fully epitaxial Cr(001)/ultrathin Fe(001)/MgO(001)/Fe(001) tunnel junctions.
- Sharp resonant conductance peaks due to the Fe quantum-well layer.
- Symmetry-selective transport via MgO barrier.

Reference
T. Niizeki, H. Sukegawa, S. Mitani, N. Tezuka and K. Inomata, Correlation between symmetry-selective transport and spin-dependent resonant tunneling in fully epitaxial Cr/ultrathin-Fe/MgO/Fe(001) magnetic tunnel junctions, Appl. Phys. Lett. 99, 182508 (2011).


- Fully epitaxial Fe/MgAl₂O₄/Fe(001) magnetic tunnel junctions.
- MgAl₂O₄: (001)-oriented spinel-type structure.
- A small lattice mismatch (~1%) between Fe and MgAl₂O₄.
- Tunnel magnetoresistance (TMR) ratios up to 117% (165%) obtained at room temperature (15 K).
- Large Vhalf, ranging from 1.0 to 1.3 V at room temperature.

Reference
H. Sukegawa, H. Xiu, T. Ohkubo, T. Furubayashi, T. Niizeki, W. Wang, S. Kasai, S. Mitani, K. Inomata, and K. Hono, Tunnel Magnetoresistance with Improved Bias Voltage Dependence in Lattice-Matched Fe/Spinel MgAl₂O₄/Fe(001) Junctions, Appl. Phys. Lett. 96, 212505 (2010).


- Epitaxial spin-valve nanopillar with a half-metallic full-Heusler Co₂FeAl_0.5Si_0.5 (CFAS) alloy.
- Giant magnetoresistance ratio of 7%-9% in CFAS/Ag/CFAS.
- Spin-transfer switching with a relatively small dc current (~10⁶ A/cm²).
- Statistical analysis based on a thermal activation model.
- Averaged critical current density JC0 of 9.3x10⁶ A/cm² with a thermal stability factor KV/kBT of 40.

Reference
H. Sukegawa, S. Kasai, T. Furubayashi, S. Mitani, and K. Inomata, Spin-transfer switching in an epitaxial spin-valve nanopillar with a full-Heusler Co₂FeAl_0.5Si_0.5 alloy, Appl. Phys. Lett. 96, 042508 (2010).