Recently, two-dimensional (2D) materials have emerged as a replacement for heavy metals (HM) because of their large spin-charge interconversion efficiency [1]. In this work, we explore a promising and relatively less explored 2D monochalcogenide SnS for spintronics application. Through symmetry arguments, it is proposed that SnS should exhibit unconventional spin-orbit torques (SOTs), which is very important for energy-efficient switching of magnets with perpendicular magnetic anisotropy (PMA) [1]. SnS thin films up to monolayer (ML) thickness have been grown on Si/SiO2 substrates using pulsed laser deposition (PLD) technique at 200 °C and room temperature (RT). Two typical Raman active modes (Ag and B3g) [2] can be distinguished in the acquired Raman spectra [Fig. 1]. We performed spin-torque ferromagnetic resonance (STFMR) measurements to investigate the SOTs in SnS/Ni80Fe20 system. For ST-FMR measurements, we fabricated SnS/Ni80Fe20 rectangular microstrips using the lift-off technique. For STFMR measurements, an in-plane RF current of frequency varying from 4 GHz to 8 GHz was applied to the bilayer in the presence of an external magnetic field (H). We measured the voltage (Vmix) across the device, which originated due to the mixing of AMR and RF current. We determine the in-plane and out-of-plane torque components by fitting the Vmix with a sum of symmetric and antisymmetric Lorentzian. We performed a complete angular dependence of the STFMR signal by varying the magnetization angle φ in the sample plane. In addition to a conventional sin2φcosφ (Vy,s) dependence, we found sin2φ (Vz,s) dependence [Fig. 2], which is a signature of unconventional out-of-plane anti-damping like torque [3]. Thus our results suggest that SnS can be used to achieve efficient switching of magnetic devices with PMA via the strong unconventional out-of-plane anti-damping like torque. 
**References** [1] Y. Liu et. al., ACS Nano 14, 9389 (2020).
[2] A.N. Mehta et. al., J. Microsc. 268, 276 (2017).
[3] D. MacNeill, et al., Nat. Phys. 13, 300 (2017). 

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Evidence of unconventional spin orbit torque in SnS/Ni80Fe20 heterostructures

Transition metal dichalcogenides (TMDs) are potential materials for exerting efficient spin-orbit torque (SOT) on the adjacent ferromagnetic (FM) layer due to their high spin-orbit coupling (SOC) and broken inversion symmetry at the interface [1]. An unconventional SOT is recently observed in WTe2/NiFe heterostructures due to low symmetry in WTe2 [2]. PtSe2 is one of the TMDs having high SOC [3]. In this work, we measured the effective spin Hall efficiency (SHE) in PtSe2/NiFe/Pt heterostructure using spin-torque ferromagnetic resonance (STFMR). PtSe2 was synthesized by salinization of a sputtered Pt (3 nm) film. The growth of PtSe2 was confirmed using Raman measurements, which shows two peaks at 176 cm-1 (Eg) and 206 cm-1 (A1g). Subsequently, a 6 nm thick NiFe (Py) layer followed by Pt (3 nm) capping layer was deposited on PtSe2 using magnetron sputtering. For STFMR measurements, RF current of frequency varying from 3 GHz to 7 GHz was applied to the device in the presence of an external magnetic field (H). The mixing voltage (Vmix) across the device was measured and the symmetric & anti-symmetric components (VS, VA) of Vmix, linewidth (ΔH), and resonance field (Hr) were extracted (Fig.1) from the fitting of STFMR spectra. Effective SHE was calculated using the ratio of VS and VA and found to be increased 33% for PtSe2/Py/Pt compared to reference (Py/Pt). This change in SHE is due to the presence of PtSe2, which was further confirmed by making a stack with Pt/Py/Pt structure. STFMR signal of all three stacks has been compared in Fig. 1 (a). No clear signal was observed for Pt/Py/Pt which confirms the presence of SOT due to PtSe2. Damping parameter was extracted by the fitting of linewidth (ΔH) vs. frequency (f) curve (Fig. 2) and was found to be enhanced by 131% in the case of PtSe2/Py/Pt compared to its reference Py/Pt. Large enhancement in damping parameter indicates large spin pumping into PtSe2. These results indicate the suitability of using PtSe2 as a potential material for spin-orbit torque. 
**References** [1] Q. Shao et al., Nano Letter 16, 7514-7520 (2016)
[2] D. MacNeill et. al., Nature Physics, 13, 300-305 (2017)
[3] Marcin Kurpas and Jaroslav Fabian, Physical Review B, 103, 125409 (2017)
 
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Richa Mudgal

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