Current-induced spin–orbit field in permalloy interfaced with ultrathin Ti and Cu

Abstract

How spin-orbit torques emerge from materials with weak spin-orbit coupling (e.g., light metals) is an open question in spintronics. Here, we report on a field-like spin-orbit torque (i.e., in-plane spin-orbit field transverse to the current axis) in SiO2-sandwiched permalloy (Py), with the top Py-SiO2 interface incorporating ultrathin Ti or Cu. In both SiO2/Py/Ti/SiO2 and SiO2/Py/Cu/SiO2, this spin-orbit field opposes the classical Oersted field. While the magnitude of the spin-orbit field is at least a factor of 3 greater than the Oersted field, we do not observe evidence for a significant damping-like torque in SiO2/Py/Ti/SiO2 or SiO2/Py/Cu/SiO2. Our findings point to contributions from a Rashba-Edelstein effect or spin-orbit precession at the (Ti, Cu)-inserted interface.2 An electric current in a material with spin-orbit coupling generally gives rise to a non-equilibrium spin accumulation [1-6], which can then exert torquesi.e., spin-orbit torques (SOTs)on magnetization in an adjacent magnetic medium [7][8][9]. SOTs are often classified into two symmetries: damping-like SOT that either counters or enhances magnetic relaxation, and field-like SOT (or "spin-orbit field") that acts similarly to a magnetic field. Next generations of nanomagnetic computing devices may benefit from an improved understanding of mechanisms for SOTs and the discovery of new thin-film systems enabling large SOTs.While most efforts have focused on conductors known for strong spin-orbit coupling (e.g., 5d transition metals, topological insulators, etc.) [7,8], recent reports have shown SOTs in ferromagnets interfaced with materials that are not expected to exhibit significant spin-orbit coupling [10][11][12][13][14]. For example, a large damping-like SOT has been reported in ferromagnetic Ni80Fe20 (permalloy, Py) interfaced with partially oxidized Cu [10,11]; quantum-interference transport measurements have revealed that Cu with an oxidation gradient can, in fact, exhibit enhanced spin-orbit coupling comparable to that in heavier metals (e.g., Au) [15]. As another example of SOTs that emerge by incorporating seemingly weak spin-orbit materials, Py interfaced with a Ti seed layer and Al2O3 capping layer exhibits a sizable field-like SOT [12]. The key observed features of this spin-orbit field in Ti/Py/Al2O3 [12] are: (1) it points in-plane and transverse to the current axis, irrespective of the magnetization orientation in Py; (2) its magnitude scales inversely with the Py thickness, i.e., it is interfacial in origin; (3) it is modified significantly by the addition of an insertion layer (e.g., Cu) at the Py-Al2O3 interface. Ref. [12] claims that this spin-orbit field is governed by a Rashba-Edelstein effect (REE) [1,5,16,17] at the Py/Al2O3 and Cu/Al2O3 interfaces. However, the complicated stack structures of SiO2(substrate)/Ti/Py/(Cu/)Al2O3 with multiple dissimilar interfaces in Ref.[12] obscure the mechanisms of the spin-orbit field, particularly the roles played by the Ti and Cu layers.Here, by using simpler stack structures, we gain i...

Spin–orbit effects in pentavalent iridates: models and materials

“…This includes, for example, various topological effects [4][5][6][7][8], that stem from the non-trivial geometry of the electronic Bloch bands, providing not only the platform to realize the well known particles in the standard model of relativistic high-energy physics in the low energy excitations of the condensed matter systems, but also leading to the emergence of the field of quantum computation. In addition, there are Rashba and Dresselhaus spin-orbit interactions [9][10][11] due to a gradient of electrostatic potential in non-centrosymmetric systems, which constitute the heart of the present day hot topic of spin-orbitronics research [12][13][14][15][16][17][18][19][20]. The present review, however, focuses on a different aspect of the SOC effects, namely, the interplay between SOC and Coulomb interaction in a crystalline solid, which often leads to unconventional phases of matter including the novel spin-orbit Mott insulating state.…”

Giant interface spin-orbit torque in NiFe/Pt bilayers*

“…[31] Oxygen effect diffusing from the SiO 2 substrate to the NiFe layer may also occur in the sputter deposition, resulting in the enhancement of the interfacial SOC, and it may be another origin of the giant interface SOT in the SiO 2 /NiFe/Pt structure. Moreover, the possible mechanism of the giant interfacial field-like SOT in NiFe/Pt bilayer may also be explained by spin-orbit precession at the interface, [32] where the polarized conduction electrons of NiFe layer are reflected from the NiFe/Pt interface and then precesses about the Rashba effective field, and in turn exert a spin torque on magnetization of NiFe layer.…”

Spin-orbit torques and photocurrents in 2D materials

“…Although the majority of the SOT measurements are performed in a bilayer configuration of a HM and FM, there have been reports of SOTs in systems consisting only of a single FM layer [112][113][114][115]. For some reports, the torques emerging in the single ferromagnetic layer could even switch its own magnetization [116,117].…”