| Magnetic material is a favorable medium for storing information,and has the advantages of durability,non-volatile and anti-radiation.Hard disks using magnetic materials as storage media have been widely used in production and life of people.At present,the hard disk information is written using a magnetic field.However,due to the divergence of magnetic field,the storage density of hard disks is restricted.Changing the writing method and using a current or an electric field to switch the magnetization is a candidate to further increase the storage density of magnetic storage.Among them,how to use the current to switch the magnetization in magnetic materials has become one of the most important topics in the field of spintronics.In this thesis,we mainly study the spin-orbit torque,an important method of switching magnetization by current.The basic structure of spin orbit torque is a heavy metal layer/ferromagnetic layer structure.The heavy metal layer such as Pt,Ta can generate spin current flowing into the adjacent ferromagnetic layer by spin Hall effect.At the same time,the current will also induce a Rashba field at the interface.Besides writing information by currents,multistate memory is another way to increase density of storage.In a multistate memory one current channel can control multiple states,reducing the space occupation by current channels.In this thesis,a four-state memory cell is studied.A TaOx buffer layer is used to make the vertical magnetic anisotropy of the two magnetic regions different.Under a fixed in-plane magnetic field,a current is applied to the device in the same direction as the magnetic field.Due to the spin orbit torque caused by the current,the magnetization orientations of both regions can be switched with different critical switching currents.By changing the form of the current pulse through the conductive channel of the device,the magnetization state of the device can be switched among four states.In addition,we also showed a four-state memory structure of Co/Pt/Co multilayer.Under a fixed in-plane magnetic field in the direction of the current,opposite spin orbit torque caused by the current are applied to the top and bottom Co layers,respectively.Therefore,the magnetization of the top and bottom Co layers can be switched at the same time,and have different switching direction and different critical current.By applying different types of current pulses to the device,we have achieved the switching of the magnetization among four states.In order to meet the application requirements of magnetic storage based on the spin-orbit torque,it must also be able to achieve current-induced magnetization switching in absence of external magnetic field.For the first time,using only the ferromagnetic materials without antiferromagnetic materials,we achieved current induced magnetization switching in Ta(3)/Pt(3)/Co(1.4)/Ta(tTa)/Co(6)/Pt(2 nm)without external magnetic field by utilizing spin-obit torques.The in-plane magnetic field was replaced by the effective field caused by interlayer exchanged coupling of two Co layers through spacer Ta layer.The current induced switching behavior can be tuned by changing pre-magnetization the in-plane Co layer.Ta not only has very large spin Hall angle,but also its sign is opposite to that of the bottom Pt layer.Thus both Pt and Ta layer contribute to current induced magnetization switching,resulting 2/3 of threshold current density reduction in current induced magnetization switching.Besides,we proved exchange coupling between the two Co layers is anti-ferromagnetic,where the exchange field is 635 Oe for the sample with the thickness of 1.5 nm for Ta.Furthermore,the magnitude of the current-induced perpendicular effective magnetic field from spin orbit torque was obtained to be 9.2 Oe/(107Acm-2). |
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