Orbital Hall Effect In Transition Metal Cr And Spin Hall Effect In Topological Semimetal PtSe₂

In modern life,the development of data computing,analysis,and storage fields has brought full convenience to people’s lives.Among them,information storage is a key field based on and serving data,and its development needs to be based on optimization and innovation of storage media.Through the spin Hall effect of heavy metals,people have been able to use spin-orbit torque（SOT）generated by spin current to manipulate the magnetization switching to write information,and have developed magnetic random-access memory（MRAM）with high storage density,good storage stability,and low energy consumption.In order to further reduce device power consumption,how to improve spin-orbit torque efficiency has become the goal of in-depth research by scientists.In the subsequent research process,scientists discovered that due to the atomic spin-orbit interaction（SOI）,the orbital Hall effect（OHE）and orbital current are likely the origin of the large spin Hall effect（SHE）in transition metals.After comparing the orbital Hall conductivity（OHC）and spin Hall conductivity（SHC）of different transition metals,it was found that OHC are generally higher than SHC,and even can exceed an order of magnitude,indicating the great potential of orbital current and orbital torque（OT）in controlling magnetic moments.In order to verify,quantify,and analyze the effects of OHE and OT on magnetic moments in different magnetic devices,we focused on the orbital Hall effect of transition metal Cr.In addition,we have also studied the spin Hall effect and SOT from the topological semimetal PtSe₂ system,which could induce magnetization switching of in-plane magnetized moments.The surface morphology,structure,and magnetism of the sample were characterized by atomic force microscopy（AFM）,X-ray diffraction（XRD）,and magneto-optical Kerr microscope（MOKE）,respectively.Then,the efficiency of OT/SOT was characterized by harmonic voltage signals.Finally,we realized the magnetization switching of samples by using OT/SOT.The obtained research results are as follows:（1）Orbital Hall effect of transition metal Cr.Using magnetron sputtering technology,we first prepared a Pt/Co/Pt symmetric structure.By changing the thickness of the Pt layer,we determined the minimum thickness of Pt layer that can make the structure have stable perpendicular magnetic anisotropy.Based on this,we prepared a Pt/Co/Pt/Cr magnetic heterostructures,and carefully studied the effect of changing the thickness of the Cr layer on the perpendicular magnetic anisotropy（PMA）and OT of the magnetic heterostructures.First,we found that the change of the Cr layer thickness has no significant effect on the PMA of the heterostructures,because the insertion of the Cr layer does not destroy the flat Co/Pt interface,so there is no significant difference in the switching process of the sample magnetization under the external magnetic field.Then,we tested the anomalous Hall and harmonic voltage signals on the Pt/Co/Pt/Cr heterostructures,the results showed that the damping like effective field and field like effective field in the heterostructures increased,as well as the corresponding torque efficiency with the increase of Cr layer thickness in the heterostructures.With further analysis of the trend of damping-like and field-like torque efficiency with Cr layer thickness,we can determine that there is a phase transition in the range of Cr layer thickness from 4 to 6 nm,which leads to a slow increase in damping-like torque efficiency within this thickness range,as well as a jump in field-like torque efficiency within this range.Finally,by comparing the current-induced magnetization switching loops of heterostructures with Cr layer thickness of 4 nm and 1 nm under the same test conditions,it is found that heterostructures with thicker Cr layers have smaller critical switching current density and larger switching ratio.This result further confirms that the OT generated by the Cr layer can manipulate the magnetization in this heterostructures,and achive a lower switching current density.（2）Spin Hall effect in topological semimetal PtSe₂.We first characterized the surface morphology and crystal structure of PtSe₂ substrates with different thicknesses by AFM and XRD.The results showed that PtSe₂ grown on sapphire by chemical vapor deposition has polycrystalline structures with（001）preferred orientation,and the average surface roughness of PtSe₂ with a thickness of 2 nm is greater than that of PtSe₂with a thickness of 10 nm.The main reason may be related to the island like growth structure of Pt at the initial growth stage.Then,PtSe₂ with different thickness were made into magnetic heterostructures with the structure of PtSe₂/FeNi/Ta by magnetron sputtering.The MOKE measurement results show that two samples have a difference in the magnetization switching process at the in-plane easy magnetization direction,which is due to the difference in the flatness of the PtSe₂/FeNi interface.Subsequently,in order to quantify the spin current generated inside the system,we measured the angular dependence between the harmonic Hall voltage and the external magnetic field.According to the theoretical approach based on the dependence of the spin current which has different polarization directions on the harmonic Hall voltage,we determined that the spin current inside the system is generated jointly by the bulk SHE of PtSe₂ and the interface effect between PtSe₂ and FeNi,where the effective spin Hall angle_SH of PtSe₂（10 nm）/FeNi/Ta is 0.15,and the_SH of PtSe₂（2 nm）/FeNi/Ta is 0.07.Finally,we conduct the current-induced magnetization switching test on PtSe₂（10 nm）/FeNi/Ta,and the critical switching current density obtained from the test is 7.7×10⁶ A/cm².The obtained switching loop exhibits significant asymmetry.It maybe explained by the non-single domain states of FeNi at the Hall cross.However,the large spin Hall angle and low critical switching current density still indicate that PtSe₂,as a new two-dimensional material,has great potential in the field of spintronics.