TY - GEN
T1 - Unexpected strong spin Hall effect in heavy-element-free paramagnetic CoGa
AU - Lau, Y.
AU - Lee, H.
AU - Hayashi, M.
N1 - Funding Information:
Electrical manipulation of magnetization by spin-orbit torque (SOT) is an attractive alternative for realizing reliable magnetic memories and oscillators, without passing a high current density across an insulating tunnel barrier. To date, the generation of transverse pure spin current and SOT, whether it is of spin Hall effect (SHE), Rashba-Edelstein effect or spin-momentum locking origin, relies primarily on materials or heterostructures containing 5d or 6p heavy elements. It is commonly believed that the spin-orbit coupling (SOC), scaling with Z4 where Z is the atomic number, is an essential ingredient for obtaining giant SOT. Therefore, light elements with small Z should be, in general, inefficient spin current generators. Here, we present experimental evidences of unexpected, appreciable SHE in paramagnetic CoGa compound which does not contain any heavy element. Equiatomic β-Co50Ga50 is known to crystallize in CsCl-type B2 structure (Space group 221), as illustrated in Figure 1(a), with Co and Ga occupying equivalent la (0, 0, 0) and lb (0.5, 0.5, 0.5) sublattice, respectively. Off-stoichiometric Co(Ga)-rich compounds may retain the B2 structure by forming Co antisides (vacancies) in lieu of Ga (Co). Typical stacks of MgO(001) substrate/Co53Ga47(CoGa)(t)/FM/(Mg2Al) Ox(MAO)(2)/Ta(1) (thicknesses in nanometers) are grown by magnetron sputtering. FM denotes either a 2nm-thick ferrimagnetic Mn1.44Ga(MnGa) film with perpendicular magnetic anisotropy (PMA) [1] or 1nm-thick in-plane magnetized ferromagnetic Co20Fe60B20(CoFeB) layer. In a spin Hall metal/magnetic metal bilayer system, the experimental spin Hall efficiency ξ acting on the magnetization is linked to the intrinsic spin Hall angle θ by: ξ = Tθ where typically T < 1 describes the imperfect spin transmission across the interface. We focus on quantifying ξ of CoGa/MnGa and CoGa/CoFeB bilayer systems using three independent techniques: spin Hall magnetoresistance [2], harmonic Hall [3, 4], and DC effective field measurement [5]. Results from the three methods are in reasonable agreement and we obtain ξCoGa/MnGa ~ ξCoGa/CoFeB ~ +0.05, with a CoGa spin diffusion length λ of the order of 2 nm. The sign of ξCoGa/MnGa,CoFeB is the same as that of Pt and is opposite to that of Ta [6]. Combining with the resistivity of CoGa ρ ~ 150μΩ cm, we estimate the spin Hall conductivity of CoGa: σSH ~ 330 (h/(4πe))Ω- 1cm-1, which is about half of that of high-resistivity β-Ta [6]. Note that due to imperfect interface transparency, the intrinsic θ and σSH of CoGa is likely to be even higher. Finally, we show that the appreciable SHE in CoGa can be used to manipulate the magnetization of an adjacent MnGa layer with very high PMA. Typical Hall resistance RH loop of a device with 5nm CoGa seed layer is measured by sweeping an out-of-plane z-field Hz, as shown in Figure 1(b). The anisotropy field of MnGa is determined to be ~20 kOe from magnetometry. The SOT-induced switching experiment is performed by sweeping the current Ix under an applied collinear x-field Hx, while monitoring RH. RH-Ix loops of the same device under opposite Hx are shown in Figure 1(c) and 1(d). The relatively high Hx, necessary for achieving deterministic SOT switching, is an indication of high effective Dzyaloshinskii-Moriya interaction field HDMI of the heterostructure. It is possibly due to the low saturation magnetization of MnGa. The switching occurs at ~12 mA, corresponding to a current density of ~1.7 x 107 A cm-2. We discuss possible mechanisms that may lead to these unexpected results, notably the central role of superparamagnetic Co antisides. These findings not only enrich our understanding on SHE but also possess high technological potential. Owing to the high resonance frequency of ferrimagnetic MnGa [7], the successful demonstration of SOT-induced switching in CoGa/MnGa/Oxide hetero-structures represents a significant step towards the realization of magnetic memories and oscillators operating at sub-THz frequencies. Combining with voltage-controlled magnetic anisotropy at MnGa/Oxide interface, we expect that the competitiveness of this structure can be further enhanced. We would like to thank S. Mitani for providing facilities for thin film deposition. This work was partly supported by JSPS Grant-in-Aids for Specially Promoted Research (15H05702), MEXT R & D Next-Generation Information Technology, and Innovation and Spintronics Research Network of Japan.
Publisher Copyright:
© 2017 IEEE.
PY - 2017/8/10
Y1 - 2017/8/10
N2 - Electrical manipulation of magnetization by spin-orbit torque (SOT) is an attractive alternative for realizing reliable magnetic memories and oscillators, without passing a high current density across an insulating tunnel barrier.
AB - Electrical manipulation of magnetization by spin-orbit torque (SOT) is an attractive alternative for realizing reliable magnetic memories and oscillators, without passing a high current density across an insulating tunnel barrier.
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U2 - 10.1109/INTMAG.2017.8007564
DO - 10.1109/INTMAG.2017.8007564
M3 - Conference contribution
AN - SCOPUS:85034644597
T3 - 2017 IEEE International Magnetics Conference, INTERMAG 2017
BT - 2017 IEEE International Magnetics Conference, INTERMAG 2017
PB - Institute of Electrical and Electronics Engineers Inc.
T2 - 2017 IEEE International Magnetics Conference, INTERMAG 2017
Y2 - 24 April 2017 through 28 April 2017
ER -