Dependence of the efficiency of spin Hall torque on the transparency of Pt/ferromagnetic layer interfaces

Abstract

We report that spin current transport across Pt-ferromagnet (FM) interfaces is strongly dependent on the type and the thickness of the FM layer and on post-deposition processing protocols. By employing both harmonic voltage measurements and spin-torque ferromagnetic resonance measurements, we find that the efficiency of the Pt spin Hall effect in exerting a damping-like spin torque on the FM ranges from < 0.05 to > 0.10 under different interfacial conditions. We also show that the temperature dependence of the spin torque efficiencies for both the dampinglike torque and field-like torque is dependent upon the details of the Pt-FM interface. The "internal" spin Hall angle of the Pt thin films used in this study, after taking the interfacial spin transmission factor into account, is estimated to be ~ 0.20. This suggests that a careful engineering of Pt-FM interfaces can improve the spin-Hall-torque efficiency of Pt-based spintronic devices. The spin Hall effect (SHE) [1, 2] causes an electrical current density e J flowing through a material with strong spin-orbit interactions to generate a transverse spin current density s J . The amplitude of s J is characterized by the spin Hall angle . The most straightforward way to determine a lower bound [3], θ SH LB , on the spin Hall angle in normal metal (NM) systems is to measure the current-dependent torque that is exerted on an adjacent ferromagnet (FM) when spin current flows to the NM-FM interface. Research has shown [4, 5]that there are two different components of torque that can be observed in this case: a "dampinglike" torque , where m is the orientation of the ferromagnetic moment, and a "field-like" torque . The determination via τ DL measurements of a large θ SH LB ≈ 0.07, in Pt-FM thin film bilayers [3, 6, 7], and the subsequent observation of even larger, "giant" spin Hall angles for high resistivity Ta (amorphous or β phase Ta), θ SH LB ≈ 0.12 [8] and β-W, θ SH LB ≈ 0.33 [9, 10], have opened up a very active area for research and technology development.Recent calculations utilizing Boltzmann analysis [11] and the drift-diffusion approximation [12] have noted that if the NM-FM interface is not completely transparent to the flow of the spin current then spin backflow will reduce the torque applied to the FM by the SHE.This reduction can be characterized, as suggested above, by defining a damping-like spin torque efficiency ξ DL , and also a field-like torque efficiency ξ FL , for a particular NM-FM interface, such that ξ DL can be less than or equal to the "internal" spin Hall angle θ SH that quantifies the spin current generated in the absence of an adjacent FM. Within a diffusive model, the effects of spin backflow are expected to modify the spin torque efficiencies in the form [11,12]

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

“…Moreover, as the perpendicular magnetic anisotropy field (H 0 an ) is also one of the key parameters in SOT systems with perpendicular magnetic anisotropy (PMA), so far, considerable research efforts have also been devoted to obtain an enhanced H 0 an to improve the thermal stability of spintronic devices [15,16], such as engineering the interface quality [17,18,19], changing identified SOC materials [16], and utilizing different post processing methods [20]. In a word, all these reports with views are not only to increase the magnitude of θ SH , but also to enhance the PMA.…”

Spin Torque Study of the Spin Hall Conductivity and Spin Diffusion Length in Platinum Thin Films with Varying Resistivity

“…Values of DL j ξ for Pt have been reported spanning the range 0.06 -0.12 [19,[27][28][29], depending on the FM/Pt interface [31], and are usually accompanied by a relatively small field-like (FL) torque efficiency whose magnitude and sign vary with the interface, FM magnetic anisotropy and temperature [29,[32][33][34][35][36]. From an analysis of SBF based on a spin diffusion model [24,25] is a well-known phenomenon due to strong diffusive scattering at a Pt surface [48,[53][54][55][56][57].…”

“…Two common methods to quantify the strength of the SHE are to employ ferromagnet/normal metal (FM/NM) bilayers and either (1) detect the spin transfer torque that the SHE-induced spin current from the NM layer exerts on the magnetization of the adjacent FM layer [19,20], or (2) use spin pumping to inject a spin current from the FM to the NM and detect the electric current in the NM layer that is induced by the inverse SHE (ISHE) [21][22][23]. In the former case due to spin backflow (SBF) at the FM/NM interface [24,25] and/or enhanced spin scattering at the interface (spin memory loss or SML) [26], only a portion NM|FM [19,[27][28][29][30][31], beta-Ta [19] and beta-W [4].…”

“…The DL and FL SH torque efficiencies of these PMA samples were measured by the HR technique [20,29] with the same alternating voltage amplitude (4 V) applied to the Hall bars in all measurements, corresponding to an alternating electric field of constant magnitude 67 kV/m E = . Fig.…”

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

“…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]…”