Field-free Spin-orbit Torque-induced Switching Of Perpendicular Magnetization At Room Temperature In WTe₂/Ferromagnet Heterostructures

Spin-orbit torque（SOT）provides an efficient way to achieve charge-to-spin conversion and can switch perpendicular magnetization,which is essential for designing novel energy-efficient spintronic devices.In traditional heavy metal（HM）/ferromagnetic（FM）heterojunctions,the charge flow in the HM layer flowing in the in-plane direction（x-direction）generates spin flow in the out of plane direction（z-direction）,and has spin polarization along the y-direction,which can achieve magnetization switching in the in-plane direction.An out-of-plane SOT could deterministically switch perpendicular magnetization.The transition-metal dichalcogenides WTe₂ with a low symmetric crystal structure has strong spin orbit coupling effect and high charge current spin current conversion efficiency,making it a strong competitor as a spin source.Encouragingly,field-free perpendicular magnetization switching of a two-dimensional（2D）material WTe₂/ferromagnet（FM）bilayer has been reported recently,but the working temperature（<200 K）is below room temperature.Here,we report field-free perpendicular magnetization switching on a WTe₂/Pt/Co/Pt multilayer film at room temperature.Controlled experiments confirm that the field-free switching is caused by the in-plane anti-damping SOT generated in the Pt/Co/Pt or[Co/Pt]₂ multilayers and the out-of-plane SOT generated in a FM/WTe₂ a-axis flake.The details are as follows:（1）Fabrication and characterization of WTe₂/FM heterojunction devices.Firstly,Pt/Co/Pt and[Co/Pt]₂ multilayers were prepared by high vacuum Magnetron sputtering;The Hall bar with a linewidth of 5 or 8μm is prepared by using the Electron beam lithography system and an ion etcher;Then,a few layered and rectangular shaped WTe₂ samples were obtained in the glove box using mechanical exfoliation method,and transferred to the etched ferromagnetic Hall bar multilayer film;Finally,wrap the insulation h-BN on the WTe₂ layer to prevent oxidation of the device.The thickness of WTe₂ thin films was characterized by atomic force microscope,and polarization Raman spectroscopy was used to distinguish the a-axis and b-axis of WTe₂ crystal;Test the magnetic characteristics of ferromagnetic samples with vibrating sample magnetometer and SQUID-VSM.Using the magnetoelectric transport system built in the laboratory,the device was tested for anomalous hall resistance,magnetization reversal,and second harmonic resistance.（2）Research on magnetization switching of device WTe₂/FM.Based on the basic characterization of WTe₂/Pt/Co/Pt heterojunction devices,the a-axis and b-axis of the WTe₂ sample were first determined by polarized Raman spectroscopy.Then,current was applied along the a-axis and b-axis of WTe₂ at room temperature to fabricate the WTe₂/FM device and perform electric controlled magnetization switching testing.Subsequently,a combination of second harmonic testing and numerical calculation was used to calculate the effective damping like spin hall angleθ_DLfor the a-axis,b-axis,and only FM layer of the WTe₂/FM device when the current was applied.It was calculated that when the current was applied to the a-axis of device A,the contribution ofθ_DLby WTe₂ was approximately 15 times that of FM.Finally,a thorough exploration was conducted on the switching phenomenon and underlying mechanism of the device.When current is applied to the a-axis of device A,due to the spin polarization current in the z-direction generated by WTe₂ when applying current along its low symmetry axis a-axis,there are anti-damping like torques in the in-plane direction and anti-damping torques in the out of plane direction.The current induced magnetization switching has a deterministic switching direction due to the generation of an out of plane anti-damping torque.（3）Optimizing the field-free magnetization switching of WTe₂/FM device.Based on the basic characterization of WTe₂/Pt/Co/Pt,the conditions of field free magnetization switching of WTe₂/FM are explored.Therefore,firstly,[Co/Pt]₂ multilayers with a thickness of 2 nm were grown by Magnetron sputtering,and WTe₂/[Co/Pt]₂ heterojunction devices were prepared.After coating h-BN,electrically controlled magnetization switching was tested at room temperature.The anomalous Hall effect confirmed that WTe₂/[Co/Pt]₂ heterojunction devices achieved field free magnetization switching.By calculating the switched ratio of WTe₂/[Co/Pt]₂ devices,it was found that the switched ratio under zero magnetic field was increased by about 6%by thinning the FM layer.