Magnetization Switching Driven By Spin Orbit Torque In Pt/[Co/Ni]_n Multilayers

In the fast-developing information age,a large amount of information and data needs to be processed and preserved,which makes people's requirements for information storage devices towards the direction of higher density,larger capacity,higher speed,lower cost and miniaturization.Spin Transfer Torque Magnetic Random Access Memory(STT-MRAM)relies on current to realize magnetization switching and then information storage.It has the advantages of fast writing,non-volatile,infinite erasing and writing,etc.It can become the second-generation general-purpose memory.Compared to STT-MRAM,Spin Orbit Torque-Based Magnetic Random Access Memory(SOT-MRAM)due to its faster writing speed and lower writing current,better compatibility,etc,have become the hotspots in the field of spintronics and magnetic information storage.At present,the physical mechanism of SOT-driven magnetization reversal is still not fully clarified,and the critical current required for magnetization reversal needs to be further reduced.In this thesis,the magnetization swtiching process driven by the spin-orbit torque in Pt/(Co/Ni)_n/multilayers is studied.By measuring the interface Dzyaloshinskii-Moriya interaction(DMI)effective field,the key factors affecting the SOT drive magnetization reversal in this system are obtained;and in the(Co/Ni)_n/IrMn structure,the partial flipping behavior under zero field is realized by the exchange bias effect of the antiferromagnetic layer.Finally,The double shifted behavior in hysteresis loops is explained.The details are as follows:(1)The effects of the capping layer(Ta and Pt,respectively)and the thickness of the magnetic layer on the SOT-driven magnetization reversal in the Pt/(Co/Ni)_n structures were investigated.The SOT-driven magnetization reversal is realized by the pulse current,and the spin torque efficiency and the DMI effective field in all structures are measured by the method of measuring anomalous Hall loops.It is found that the magnitude of the DMI effective field decreases with the increases of the number of Co/Ni cycles,and does not change significantly with the change of the thickness of the capping layer Ta,indicating that the DMI in this system mainly originates from the underlying Pt/Co interface.Since the DMI of the upper and lower interfaces cancel each other out,when the capping layer is Pt,the strength of DMI is about zero.In addition,damping-like spin torque efficiency decreases with the increase of the number of Co/Ni cycles,and the spin torque efficiency is larger when the cover layer is Ta,while the spin torque efficiency tends to be zero when Pt is used,which proves that damping-like spin torque is originated from the spin Hall effect of heavy metals Ta and Pt.Finally,critical switching current is positively correlated with the magnitude of spin torque efficiency.These results provide a theoretical basis for optimizing SOT-based information storage devices.(2)According to the macro-spin model,in the SOT-driven magnetization reversal process,an external magnetic field is required to break the symmetry of the system with perpendicular magnetic anisotropy,which is disadvantageous for the application of SOT in MRAM.We add an IrMn layer between the Co/Ni magnetic layers and the top heavy metal layer.Since there is an exchange bias between the magnetic layers and the antiferromagnetic layer,the film structure with the IrMn layer itself has an exchange bias field.Theoretically,magnetization switching without field can be achieved.In this research topic,we change the thickness of the IrMn layer,since the exchange bias field we obtained is small(about a few Oster),only a partial magnetization switching is realized,and it is hoped that the interface exchange bias is performed through subsequent work to achieve current-driven magnetization switching without extra magnetic field.(3)In the(Co/Ni)_n/IrMn structures,we found that when the thickness of the IrMn layer is 5 nm or more,the hysteresis loop of the films will undergo a two-step magnetic reversal.In order to explore the causes for this phenomenon,we measured the magnetic force microscope images of different film structures and found that the two-step switching phenomenon is related to the ferromagnetic layer and the antiferromagnetic layer.After the addition of the IrMn layer,the ferromagnet/antiferromagnet exchange coupling of the magnetic interface eventually produces antiferromagnetic domains in the antiferromagnetic layer.The antiferromagnetic domains of different orientations will produce exchange bias fields in different directions,resulting in a two-step switching phenomenon in this system.