Study On Spin Hall Rectification Effect In Heavy Metal/Ferromagnetic Metal System

The rectification effect depends on the asymmetric charge transport properties of the material system,which can convert the alternating current into a non-zero direct current.Regular microelectronic devices can implement rectification techniques on semiconductor heterojunctions with broken symmetries,such as Schottky heterojunctions and p-n junctions.With the development of spintronics and the exploration of spin-dependent scattering,the concept of spin rectification has attracted the attention of many researchers.Spin-rectification devices have been developed based on anisotropic magnetoresistance(AMR),giant magnetoresistance(GMR),tunneling magnetoresistance(TMR),etc.,where the differences in charge transport are derived from the resistance oscillations generated by the magnetic moment precessions of the ferromagnetic layer or the oscillating Oster field generated by the radio-frequency AC current.For example,the so-called spin-torque diode effect is shown in nanometre-scale Co Fe B/Mg O/Co Fe B magnetic tunnel junction(MTJ),where the rectification effect occurs in the case of a.c.frequency in resonance with spin oscillations that arise from spin-transfer torque.In these devices,however,an external magnetic field is usually required to tilt the magnetization direction of the ferromagnetic metal(FM)layer.In addition,since the current path of the MTJ-based diode passes through the insulating layer of the device,this undoubtedly increases the potential risk of the barrier breaking,reducing the durability of the device.From the application point of view,the realization of spin rectifiers in the absence of external magnetic fields and excited by in-plane currents is important for the further development of modern spin-dependent diode technology.In addition,the use of electron spin to realize information storage is a hot topic in the field of spintronics.Compared with traditional data memory,magnetic random access memory(MRAM)based on spin-orbit torque(SOT),which is widely studied at present,has the characteristics of high efficiency,stability and not easy to lose.The information writing of SOT-MRAM uses the spin Hall effect(SHE)of heavy metal(HM)to convert the charge current into a spin current,and drives the magnetic moment switching by applying the effect of SOT to the FM.However,the threshold current density for magnetic moment switching in SOT drives is typically 10~7A/cm~2,leading to severe device heating problems.Therefore,how to improve the switching efficiency of SOT is the current research topic of interest and difficulty.To address the above two issues,we use the magnetron sputtering technique to prepare heterojunction structures consisting of a heavy metal layer and a ferromagnetic layer.And do the following work around it:1.Double layer heavy metal/ferromagnetic metal films(Pt/Co and Ta/Co)are studied and the existence of spin Hall rectification effect(SHRE)is found.The Pt/Co and Ta/Co bilayer structures were fabricated using magnetron sputtering and the samples were processed into Hall devices using lithography and ion beam etching.Finally,through the physical property measurement system(PPMS),it is found that there is spin Hall rectification effect in the heavy metal/ferromagnetic bilayer films(Pt/Co and Ta/Co).A sinusoidal alternating current is injected along the in-plane current channel of the Hall device to generate a non-zero rectifier voltage signal.Compared with conventional semiconductor diodes,the rectifier mechanism in this structure relies on the spin Hall effect of the heavy metal layer as well as the spin dependent asymmetric scattering,so the symbol of the rectifier voltage exhibits controllable and hysteretic(non-volatile)characteristics.This discovery opens a new way to design spin diode devices using the spin Hall rectification effect.2.Spin-orbit torques in Pt/PtCoTa bilayer structures have been studied.Since the heavy metals Pt and Ta have opposite intrinsic spin Hall angles,we expect to facilitate magnetic moment switching in ferromagnetic layers by constructing compositional gradients in ferromagnetic alloys with opposite spin Hall angles.For this reason,we designed PtCoTa alloy with perpendicular magnetization anisotropy(PMA)and prepared Pt/PtCoTa bilayer structure by magnetron sputtering.The two-layer structure was fabricated as a Hall bar device by lithography and ion beam etching.Finally,the SOT was measured and characterized by a physical property measurement system.The efficiency of SOT switching can be significantly improved and the threshold current density of SOTs can be reduced by synthesizing PtCoTa component gradient ferromagnetic alloys with appropriate proportions of Pt,Co and Ta.This discovery opens new avenues to improve the development of low-power and non-volatile data storage.