Effect of heavy metal layer thickness on spin-orbit torque and current-induced switching in Hf|CoFeB|MgO structures

Abstract

We study the heavy metal layer thickness dependence of the current-induced spin-orbit torque (SOT) in perpendicularly magnetized Hf|CoFeB|MgO multilayer structures. The damping-like (DL) current-induced SOT is determined by vector anomalous Hall effect measurements. A non-monotonic behavior in the DL-SOT is found as a function of the thickness of the heavy-metal layer. The sign of the DL-SOT changes with increasing the thickness of the Hf layer in the trilayer structure. As a result, in the current-driven magnetization switching, the preferred direction of switching for a given current direction changes when the Hf thickness is increased above ∼7 nm. Although there might be a couple of reasons for this unexpected behavior in DL-SOT, such as the roughness in the interfaces and/or impurity based electric potential in the heavy metal, one can deduce a roughness dependence sign reversal in DL-SOT in our trilayer structure.

Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems

“…et al, 2013b), so that separating the bulk and interface contributions to ξ j DL is not straightforward. Moreover, a change of sign of both ξ j DL and ξ j FL has been reported for Ta/CoFeB/MgO (Allen et al, 2015;Kim et al, 2013a) and Hf/CoFeB/MgO (Akyol et al, 2016;Ramaswamy et al, 2016) at t Ta ≈ 0.5 nm and t Hf ≈ 2 nm, respectively, indicating that there are different mechanisms contributing to the torques that may compete or reinforce each other.…”

“…Such a phenomenological parameter, however, accounts for the transmission of the bulk spin current as much as for the generation of interface spin currents, so that its use to estimate an asymptotic value of the bulk SHE in NMs can be questioned. Moreover, the spacer layer itself can be regarded as a source of spin current, as has been shown in the case of Hf (Akyol et al, 2016;Ramaswamy et al, 2016).…”

Manipulation of Magnetization by Spin–Orbit Torque

“…To increase the efficiency of magnetization switching by SOTs, it is crucial to search for materials with large spin Hall angle (the ratio between spin current density and charge current density). Metals with strong spin–orbit coupling such as Pt, Ta, Pd, W, Hf, etc., have been widely studied in the past several years. It has been reported that the spin Hall angle is significantly related to the crystalline structure and can be varied by changing the thickness or by oxidation …”

Temperature dependence of spin-orbit torques across the magnetic compensation point in a ferrimagnetic TbCo alloy film

“…SOTs with both a fieldlike and dampinglike character can manifest in such systems, with the latter capable of driving magnetization switching [1][2][3][4][5]7,9,[11][12][13]15,21,22,24] and magnetic domain wall motion [26][27][28][29][30][31]. SOTs have been identified and quantified by a variety of techniques including spin-torque ferromagnetic resonance [2,7,14,17,19,25], quasistatic magnetization tilting probed through harmonic voltage measurements [6,[8][9][10][11][12][13]17,[19][20][21][22][23][24][25][26]28,31], and current-induced hysteresis loop shift measurements [18,32,33]. Most studies of SOTs have focused on ultrathin metallic ferromagnet/heavy-metal bilayers with interfacial perpendicular magnetic ani...…”

“…SOTs have been identified and quantified by a variety of techniques including spin-torque ferromagnetic resonance [2,7,14,17,19,25], quasistatic magnetization tilting probed through harmonic voltage measurements [6,[8][9][10][11][12][13]17,[19][20][21][22][23][24][25][26]28,31], and current-induced hysteresis loop shift measurements [18,32,33]. Most studies of SOTs have focused on ultrathin metallic ferromagnet/heavy-metal bilayers with interfacial perpendicular magnetic anisotropy (PMA) [1][2][3][5][6][7][8][9][11][12][13][15][16][17][18][19][20][21][22][23][24][25]…”