Enhanced spin-orbit torques in Pt/Co/Ta heterostructures

Abstract

Spin-orbit torques (SOTs) are studied in perpendicularly magnetized ultrathin Co films sandwiched between two heavy metals, Pt and Ta. A significant enhancement of the Slonczewski-like torque is achieved by placing dissimilar metals with opposite spin Hall angles on opposite sides of the ferromagnet. SOTs were characterized through harmonic measurements and the contribution by the Ta overlayer was isolated by systematically varying its thickness. An effective spin Hall angle of up to 34% is observed, along with a sizable field-like torque that increases with increasing Ta layer thickness. Current-induced switching measurements reveal a corresponding increase in switching efficiency, suggesting that by engineering both interfaces in trilayer structures, the SOTs can be significantly improved. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4902529] Current-induced torques in ultrathin ferromagnets sandwiched by a heavy metal and an oxide have been of significant recent interest for highly efficient magnetization switching [1][2][3][4][5][6][7][8] and domain wall motion.9-14 In these systems, strong spinorbit torques (SOTs) can arise through the spin Hall effect (SHE) [2][3][4]13,[15][16][17][18][19] and the Rashba effect 11,[20][21][22][23] at the heavymetal/ferromagnet interface, which can be exploited for lowpower operation of spintronic memory and logic devices. These effects produce both a Slonczewski-like torque [24][25][26] that drives magnetization switching, and a field-like torque 27 whose effective field lies parallel to the interface and orthogonal to the current flow direction. The Slonczewski-like torque is similar to the spin-transfer torque in conventional currentperpendicular-to-plane geometries, and is conveniently parameterized by an effective spin Hall angle h SH , which refers to the ratio of out-of-plane spin current to in-plane charge current. Since the efficiency of SOT switching relates directly to the magnitude of h SH , much effort is currently directed at identifying materials and interfaces for which h SH is large.Pt and Ta have been the most widely examined spin Hall metals due to their relatively high spin Hall angles, $þ0.07 for Pt (Refs. 2, 13, and 17) and $À0.12 to À0.15 for b-Ta (Ref.3) and their simultaneous utility as underlayers that promote perpendicular magnetic anisotropy (PMA) in thin Co and CoFe(B) films. Recent work has aimed to increase the SHE efficiency by seeking other heavy metals and alloys with larger h SH , with a record of þ0.3 having been reported for beta phase W. 4 To date, most work has focused on heavy-metal/ferromagnet/oxide trilayer structures, where the oxide layer plays the role of breaking inversion symmetry, without actively contributing to the SOTs. One could therefore anticipate that by engineering SOTs at both interfaces, the efficiency of current-induced torques could be further increased using already-known spin Hall materials.Here, we examine SOTs in ultrathin Co films sandwiched between two heavy metals, Pt and Ta, whose spin Hall ang...

Current-induced magnetization switching in Pt/Co/Ta with interfacial decoration by insertion of Cr to enhance perpendicular magnetic anisotropy and spin–orbit torques

“…So, considerable efforts have been devoted to obtain a large θ SH . An enhancement of the SOT switching efficiency was reported in a Pt/Co/Ta structure [14], in which the spin Hall angles of Pt and Ta have opposite signs.…”

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

“…Finally, ζ is the ratio of PHE and AHE coefficients ζ = R PHE / R AHE ; the former was obtained by measuring the Hall voltage with the field oriented at ϕ = 45 • as described in Refs. [8,12,13,24]. Additionally, the anomalous Nernst effect (ANE) may give an additional contribution to V ω2 due to current-induced inhomogeneous heating [8].…”

“…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]…”

“…Due to the limited range of our electromagnet, the high-field regime cannot be reached for all temperatures, so we have analyzed the data in the low-field regime where V ω2 is linear, using the following relations [6,13,44]:…”

“…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...…”