Spin-orbit Torque Induced Magnetization Switching And Its Application

With the development of information technology,the information age is coming,in which energy-efficient and intelligent information processing are vitally important.Taking electrons’spin degree of freedom into account,spintronic devices break the limitation of high-power consumption and high cost in microelectronics.Hence,spintronics has become one of the hottest research areas.In particular,the emergence of magnetic random access memory(MRAM)has attracted extensive attention due to its excellent characteristics such as nonvolatility,fast read and write speed,low power consumption,and so on.In recent years,based on the spin-orbit torque(SOT),high speed and low power manipulation of magnetization switching has been achieved,which provides an alternative way for the development of ultrafast and energy-efficient MRAMs.In addition,SOT-based devices have lots of advantages,including non-volatility,anti-radiation,remarkable endurance and stability,and good compatibility with current CMOS technology,which makes the SOT-based devices good candidates for the application in the fields of in-memory processing,neural network computing,microwave detectors,and spin nano-oscillators.Electrical manipulation of magnetism is also a research hotspot of spintronics.Combining spin-orbit coupling with inverse symmetry breaking,electrical manipulation of perpendicular magnetization switching can be achieved via SOT in nonvolatile memories and logic devices.SOT-induced magnetization switching has been achieved in heavy metal/ferromagnetic heterojunctions and non-centrosymmetric magnetic single-layer films such as CuMnAs,(Ga,Mn)As,and Mn2Au.However current-induced perpendicular magnetization switching in common ferromagnetic single-layer films with central inversion symmetry is still a great challenge.In addition,the realization of spin logic functions within a single nonvolatile memory cell provides a promising way for achieving logic-in-memory,which could break the von Neumann bottleneck and improve the data processing efficiency.Especially,programmable spin logic devices based on SOT have drawn attention because of their CMOS compatibility,high computing capability,low power consumption and good scalability.Moreover,SOTinduced magnetization switching without a magnetic field has been experimentally demonstrated by using wedged structure,exchange bias,interlayer exchange coupling,and creating a spin current gradient,which provides a promising way to realize fully electrical controllable spin logic operations and lays the foundation for the realistic application of programmable spin logic devices.Although remarkable progress has been made in field-free magnetization switching and reconfigurable spin logic devices,it is difficult to control the functionality of the prepared memory unit at-will.And it remains a great challenge to achieve flexible manipulation of the multifunctional spintronic devices.In this thesis,we mainly focus on the spin-orbit torque controllable magnetization switching and its application.Three main achievements of our study are given below.1.Perpendicular magnetization switching in CoPt alloy composition gradient filmsWe prepared single-layer CoPt alloy films with perpendicular magnetic anisotropy by magnetron sputtering.It is demonstrated that current-induced magnetization switching in the CoPt gradient film itself has been realized without an additional heavy metal layer.The composition gradient is introduced in the thickness direction to break the structure inversion symmetry by alternately sputtering Co and Pt thin layers.To unveil the mechanism of SOTinduced magnetization switching in the CoPt single-layer alloy,we observed domain wall nucleation and expansion by magneto-optical Kerr microscopy and explained the currentinduced magnetization switching based on the domain wall displacement model.Moreover,we quantitatively characterized the current induced z-direction effective field acting on the domain walls.In the meanwhile,the SOT-induced longitudinal effective field(ΔHL≈5.0 Oe per 107 A/cm2)and transverse effective field(ΔHT≈3.0 Oe per 107 A/cm2)on magnetic domains are quantitatively characterized by harmonic Hall voltage measurements.And,the calculated effective spin Hall angle of the studied CoPt is θSH≈+0.011.It is demonstrated that currentinduced Oersted field,spin-transfer torques,the spin Hall effect of Pt,and the interface Rashba effect could not explain the observed results.It is concluded that the introduced artificial component gradient breaks the inversion symmetry,and produces the bulk spin-orbit coupling,which results in the non-equilibrium spin to induce the perpendicular magnetization switching.The SOT-induced magnetization switching in a ferromagnetic single-layer film without additional heavy metal layers provides a promising solution to directly introducing SOT functionality into existing spintronic devices,such as spin valves and magnetic tunneling junctions.This is benifical for the development of electrical controllable novel memory and logic devices.2.Complete spin logic in IrMn/Co/Ru/CoPt magnetic heterojunctionsThe IrMn/Co/Ru/CoPt magnetic heterojunction with the composition gradient CoPt layer has been fabricated.In the IrMn/Co/Ru/CoPt heterojunctions,the top CoPt alloy layer has perpendicular magnetic anisotropy,and the bottom Co layer has in-plane magnetic anisotropy.The in-plane exchange bias between IrMn/Co and the magnetization direction of the in-plane Co layer can be controlled by simultaneous application of magnetic field and current pulse.Moreover,a controllable field-free magnetization switching of the CoPt alloy is achieved relying on the exchange bias and interlayer exchange coupling effect,where four different magnetization configurations are achieved.By controlling the four variables,i.e.,the initial control current pulse,the initial control magnetic field,and the input voltages of two terminals,all 16 Boolean logic functions can be realized in a single four-state nonvolatile memory.The realization of complete spin logic operations in a single multi-state nonvolatile memory unit provides a new candidate for the development of new electronic devices beyond the von Neumann computing architecture.3.Room temperature manipulation of double exchange bias in IrMn/Co/Pt/CoPt/IrMn heterojunctionsWe prepared the magnetic heterojunction with a core structure of IrMn/Co/Pt/CoPt/IrMn and conducted a systematical study on the manipulation of both in-plane exchange bias between IrMn and Co layers and out-of-plane exchange bias between CoPt and IrMn layers.The inplane exchange bias can be reversibly controlled by simultaneously applying colinear in-plane magnetic field and current pulse along x-direction.And,the out-of-plane exchange bias can be controlled by applying out-of-plane magnetic field along the z-direction or colinear in-plane magnetic field and current pulse along the x-direction.Thanks to the interlayer exchange coupling between Co/Pt/CoPt,the out-of-plane exchange bias can also be manipulated by purely electrical induced SOT switching.Based on this purely electrical manipulation of outof-plane exchange bias,four-state nonvolatile memory has been demonstrated.Our work not only expands the design flexibility of future multifunctional devices,but also opens a new path for the development of multidimensional room temperature reconfigurable spintronic devices.