Current-induced Magnetization Switching In Perpendicularly Magnetized Heterostructures

With the development of the data era,massive amounts of data and information need to be processed and stored,which places higher and higher requirements on data storage devices.Compared with traditional semiconductor memory,magnetic random access memory(MRAM)has attracted increasing attention,because of its excellent performance such as fast read and write speed,high storage density,non-volatility,low power consumption and high stability.Among various kinds of MRAM,spin-orbit torque based MRAM(SOT-MRAM)is of particular interest.In SOT-MRAM,current can be used to control the magnetization switching.In a heavy metal/ferromagnetic metal bilayer,when a charge current passes through the heavy metal with strong spin-orbit coupling,a spin current is generated in the direction perpendicular to the charge current by the spin Hall effect.And this spin current could exert a torque on the magnetic moment of the adjacent magnetic layer.Under the application of an in-plane magnetic field along the current direction,a deterministic SOT induced magnetization switching can be achieved.To control the magnetization states fully by electrical field is one of the ultimate goals of spintronics.On one hand,spin-orbit torque induced magnetization switching has been realized in(Ga,Mn)As,CuMnAs,and Mn2Au single-layer films with locally broken central symmetry.And it still remains a great challenge to produce a high efficient SOT within a common single-layer ferromagnetic film itself with central symmetric microstructure.On the other hand,an external magnetic field is required to obtain the deterministic magnetization switching for the well studied heavy metal/ferromagnet bilayers.The requirement of additional magnetic field not only makes the device structure more complicate but also reduces the stability of ferromagnetic sublayer,hindering its practical applications.In order to achieve magnetic field free SOT switching,various approaches have been explored,including lateral wedged structure design,interlayer exchange coupling assistance,tilted magnetic anisotropy preparation.In addition,it has been reported that magnetic field-free current-driven magnetization switching can be achieved by using the competing spin currents generated by the double-layer heavy metals with opposite spin Hall angle.In the meantime,other reports found that oblique sputtering of heavy metal layers should be responsible for the field-free SOT switching in these double-layer systems.Hence,controversy remains about the influence of competing spin current on the magnetization switching.The focus of our studies can be divided into three parts.First,perpendicularly magnetized PtCo alloy single layer has been synthesized by alternating sputtering several atom thick Co and Pt sublayers.By introducing a composition gradient in the sample thickness direction,the inversion symmetry of PtCo has been broken,which enables bulk SOT to arise directly.Under the assistance of a small magnetic field of Hx=100 Oe,a remarkable SOT switching is observed.The MOKE measurements indicate that the SOT induced magnetization switching is realized through the domain walls displacement.Using the first and second harmonic measurements,the effective fields act on the magnetic domains in the Ru(2.0nm)/PtCo(3.3nm)/MgO(2.0nm)sample have been measured to be HL?5.0 Oe per 107 A/cm2,HT?3.0 Oe per 107 A/cm2,and the spin Hall angle is about 0.011.The realization of SOT-switching in single-layer ferromagnetic film has one obvious advantage that without the need of introducing any extra heavy metal layer to produce SOT effect,the SOT-switching of the single-layer magnetic film can be directly added into the existing devices based on magnetic tunneling junctions,magnetic spin valves and exchange biased systems to realize a new class of memory and logic functions by SOT effect.Second,we have prepared perpendicularly magnetized Ru/Ta/PtCo/MgO heterostructures.Here,Ta has a negative spin Hall angle,and PtCo alloy has a positive spin Hall angle.We have studied the effect of competing spin currents on the magnetization switching by changing the thickness of Ta.When the spin currents from Ta and PtCo cancel each other,no magnetization switching driven by the current can be detected.In addition,the first and second harmonic measurements have been performed in the Ru/Ta/PtCo/MgO sample with 2.0 nm thick Ta sublayer.And the effective field act on the magnetic domain has been calculated to be HL?8.1 Oe per 107 A/cm2,HT?-1.9 Oe per 107 A/cm2,and the spin Hall angle of the Ru(2.0nm)/Ta(2.0nm)/PtCo(3.3nm)/MgO(2.0nm)sample is about-0.021.Third,oblique sputtering has been used to prepare the Ta sublayer in the Ru/Ta/PtCo/MgO heterojunction.And field-free SOT induced magnetization switching has been achieved because of the additional structural symmetry broken due to the oblique sputtering.The magnetic hysteresis loop and anomalous Hall effect measurements confirm the perpendicular magnetic anisotropy of the studied sample.The SOT switching measured under various in-plane magnetic field indicate that the amplitude of SOT switching first increase then decrease with increasing applied magnetic field.