Spin Orbit Torque-Driven Chiral Magnetic Domain Wall Displacement And Field-Free Perpendicular Magnetization Switching

Spin-orbit torque(SOT)effect is an important research branch in the emerging field of spintronics.Its properties and applications have been widely studied in recent years,and a large number of related results have been produced.For example,it is found that SOT effect can be effectively used in switching magnetization of ferromagnets,magnetic tunneling junctions,antiferromagnetic metals and compensated ferrimagnets,manipulating chiral-spin rotation,modulating exchange bias,sensing magnetic field,inducing domain nucleation,driving spin wave propagation,and stimulating nano-oscillator.In chiral magnetic domain wall(DW)system,the combination of SOT,Dzyaloshinsky-Moriya exchange interaction(DMI),and magnetization damping can generate rich and novel physical phenomena.Therefore,in this thesis,by using the single-layer CoPt ferromagnetic films and the synthetic antiferromagnets(SAF)with CoPt as the magnetic layer,we systematically studied the SOT-driven chiral DW motion and perpendicular magnetization switching.By combining the dynamics calculation in DW model,we deepened our understanding of the damping-like SOT effective field,clarified the physical significance of critical magnetic field and critical current in magnetization switching,and realized field-free perpendicular magnetization switching in strong coupling SAF.This provides a new approach for the practical application of SOT effect in low-power information storage and processor devices.Specifically,it includes the following three aspects:(1)Analysis and characterization of the effective field induced by dynamic magnetization damping in chiral DWs.In conventional experiments,in order to quantify the SOT effective magnetic field in a perpendicular magnetization film,one of the common methods is to measure the shift of out-of-plane anomalous Hall effect curves by varying the magnetic field along perpendicular direction under the fixed inplane external field and applied electric current.The shift of the Hall curve is the magnitude of the perpendicular SOT effective field.In chiral ferromagnetic DWs,the SOT effective field depends on the in-plane external field and applied electric current used in the measurements.In this case,it is usually believed that the effective field measured by the shift of Hall curve is the damping-like SOT effective field,but the contribution of magnetization damping under the total static magnetic field has not been considered.In fact,magnetization damping as an effective magnetic field is usually ignored in the magnetization switching measured by magnetic hysteresis loops due to its dynamical and instantaneous feature.In our work,by the experimental measurements of anomalous Hall effect and the theoretical calculations of DW dynamics,we demonstrated that the effective field measured by the shift of Hall curves includes not only the damping-like SOT field,but also other perpendicular effective fields in the form of dynamical damping,which are caused by the external magnetic field,pinning field,DMI effective field,and in-plane demagnetization field.These findings provide key insights into the generation of damping-like SOT effective field in chiral DW,as well as new ideas for the mechanism and application of magnetization damping.(2)Study on critical magnetic field and critical current for SOT induced magnetization switching.In the traditional perpendicular magnetization switching by SOT,a considerable in-plane external magnetic field along the current direction is needed.Although some work employed interlayer exchange coupling,exchange bias,broken structural symmetry and other methods to replace the external in-plane magnetic field with another form of the inner effective field,and realized external magnetic fieldfree magnetization switching,the role of external in-plane magnetic field in magnetization switching and the critical magnetic field for the magnetization switching have not been fully studied and demonstrated.On the other hand,some efforts have attempted to reduce the current density of SOT induced magnetization switching,and have defined the critical current density,but the definitions given by macrospin model and DW model are completely different.In our work,through the measurements of SOT induced magnetization switching,we demonstrated that the role of the critical current is to overcome the coercivity of the ferromagnetic film,thereby depin the DWs.By the calculations of DW dynamics,we clarified that the essential function of the in-plane critical magnetic field is to break the degeneracy of the two DWs(up-to-down and down-to-up)which have been depinned by the current,and produce the significant relative velocity between them to realize perpendicular magnetization switching.On this foundation,we proposed a proof-ofconcept that a field-free magnetization switching may be realized by low current density in magnetic films with small coercivity and multiple inversion asymmetry,which could be prepared by oblique sputtering.This work provides the possibility of developing spintronic devices with low power consumption and high integration.(3)Realization of field-free SOT induced magnetization switching in strong coupling SAF.Perpendicularly magnetized SAF with strong interlayer antiferromagnetic exchange coupling has attracted a lot of research in recent years due to its high thermal stability,excellent anti-interference ability,and ultrafast spin dynamics.Although the field-free magnetization switching by SOT has been realized recently in the SAF with weak interlayer antiferromagnetic coupling,a large in-plane external magnetic field of 500 to 4000 Oe is still needed in the SAF with strong coupling to obtain deterministic perpendicular magnetization switching by SOT,which seriously limits the practical applications of SAF.In our work,we experimentally demonstrated that the critical magnetic field for breaking the degeneracy of chiral DWs to realize the SOT induced magnetization switching can be varied in a wide range from field-free to 3000 Oe in perpendicularly magnetized SAF with the structure of CoPt/Ru/CoPt and strong interlayer antiferromagnetic coupling,which strongly depends on the interfacial DMI and SOT at bottom and top interfaces with multiple inversion asymmetries.Our DW dynamical calculations further revealed that the field-free magnetization switching is caused by SOT induced the displacement of Neel-type DW with left-handed chirality under the condition that the interfacial DMI of the upper magnetic layer has significantly smaller value than that of the lower magnetic layer.This physical mechanism provides a profound understanding of the role of SOT and DMI in magnetization switching of magnetically coupled chiral system,meanwhile,the realization of SOT induced fieldfree perpendicular magnetization switching in SAF with strong interlayer antiferromagnetic coupling paves the way to its practical applications.