Layer-dependent spin-orbit torques generated by the centrosymmetric transition metal dichalcogenide β−MoTe2

Abstract

Single-crystal materials with sufficiently low crystal symmetry and strong spin-orbit interactions can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets, such as the out-of-plane antidamping torque previously observed in WTe 2 /ferromagnet heterostructures.Here, we present measurements of spin-orbit torques produced by the low-symmetry material β-MoTe 2 , which unlike WTe 2 retains bulk inversion symmetry. We measure spin-orbit torques on β-MoTe 2 /Permalloy heterostructures using spin-torque ferromagnetic resonance as a function of crystallographic alignment and MoTe 2 thickness down to the monolayer limit. We observe an outof-plane antidamping torque with a spin torque conductivity as strong as 1/3 of that of WTe 2 , demonstrating that the breaking of bulk inversion symmetry in the spin-generation material is not a necessary requirement for producing an out-of-plane antidamping torque. We also measure an unexpected dependence on the thickness of the β-MoTe 2 -the out-of-plane antidamping torque is present in MoTe 2 /Permalloy heterostructures when the β-MoTe 2 is a monolayer or trilayer thick, but goes to zero for devices with bilayer β-MoTe 2 .1 arXiv:1906.01068v1 [cond-mat.mes-hall] 3 Jun 2019Spin-orbit torques represent one of the most promising methods for manipulating emerging magnetic memory technologies [1]. When a charge current is applied to a material with large spin-orbit coupling, such as a heavy metal [2][3][4][5][6][7], topological insulator [8,9], or transition metal dichalcogenide (TMD) [10-16], a spin current generated through mechanisms such as the spin Hall or Rashba-Edelstein effects can be used to exert a torque on an adjacent ferromagnet. Recent work from several research groups has focused on understanding how a controlled breaking of symmetry in a spin-generating material / ferromagnet heterostructure can be used to tune the direction of the observed spin-orbit torques for optimal switching of magnetic devices [12][13][14][17][18][19][20][21][22][23][24]. For instance, the presence of magnetic order within a spin-generation layer can allow current-generated spin directions that are typically forbidden for highly-symmetric non-magnetic metals [17][18][19][20]. Similarly, our group has shown that by using WTe 2 as the spin-source material, a TMD with a low-symmetry crystal structure, it is possible to generate an out-of-plane antidamping torque [12,13] -the component of torque required for the most efficient mode of switching for magnets with perpendicular magnetic anisotropy, but forbidden in higher-symmetry materials. Only one other material, SrRuO 3 , has been shown to generate an out-of-plane antidamping spin-orbit torque [20], arising from symmetry breaking associated with magnetic order. Many questions remain regarding the mechanism and necessary conditions for generating a strong out-of-plane antidamping torque.In this work, we study the spin-orbit torques generated in TMD/ferromagnet heterostructures with a crystal symmetry that is distinct from WTe 2 i...

Spin‐Orbit Torque Magnetization Switching in MoTe2/Permalloy Heterostructures

“…Previous studies of SOT based on 2D materials have been reported in semiconducting MoS 2 and WSe 2 , [ 23,24 ] metallic NbSe 2 , [ 25 ] semimetallic WTe 2 , [ 26–28 ] and MoTe 2 . [ 29,30 ] Especially, the novel observations of out‐of‐plane spin induced anti‐damping torque in WTe 2 and MoTe 2 hold a great promise for field‐free switching of the perpendicularly magnetized system. However, a direct demonstration of SOT‐induced magnetization switching utilizing a 2D material is still at its infancy and the thickness‐dependence of SOTs in 2D materials is not well understood.…”

“…From experiments, we extract the SHC of ≈250 (ℏ /e ) Ω −1 cm −1 , which is similar to the value of ≈280 (ℏ /e ) Ω −1 cm −1 reported by Safeer et al., [ 30 ] but larger than the previously reported value of ≈29 (ℏ /e ) Ω −1 cm −1 . [ 29 ]…”

“…The inversion symmetry of the 1T′ crystalline structure is broken in our bilayer devices. Combined with the relatively large SOC that one can infer from recent experimental values of ξ SH in MoTe 2 , [ 29,30 ] in addition to the SHE, we inquire a non‐equilibrium spin accumulation arising from the Rashba–Edelstein effect. [ 7 ] Figure 3d shows the thickness‐dependence of spin accumulation collinear with the magnetization in Py, , [ 33 ] computed at charge neutrality as well as μ = ± 60 meV.…”

Spin-Orbit Torques in Transition Metal Dichalcogenide/Ferromagnet Heterostructures

“…Thickness dependent ST-FMR measurements on NbSe 2 (1-10 layers)/Py heterostructures revealed an in-plane DL torque τ y e with a torque conductivity (σ y e 3 × 10 3 (Z/2e)(Ωm) − 1 ) comparable to other TMD/Py heterostructures and observable down to a monolayer of NbSe 2 (Guimarães et al, 2018). Similar to β-MoTe 2 /Py (Stiehl et al, 2019a), τ y e shows only a weak thickness dependence.…”

“…Another interesting semi-metallic TMD is β-MoTe 2 which, different than WTe 2 and similar to TaTe 2 , possess inversion symmetry in its bulk form. Using β-MoTe 2 /Py bilayers Stiehl et al observe the presence of an out-of-plane DL torque τ z e using ST-FMR measurements (Stiehl et al, 2019a). This is allowed by the inversion symmetry breaking at the β-MoTe 2 /Py interface and indicates that inversion asymmetry in the bulk is not a strict requirement for τ z e to be observed.…”