Research On Spin-orbit Torque Driven Perpendicular Magnetization Switching In The Splicing Structure

The most promising development in MRAM is to drive the perpendicular magnetization switching by using Spin Orbit Torque(SOT).Due to the strong spin-orbit coupling of heavy metal,when the current flows through the heavy metal layer,the spin flow is generated due to the spin Hall effect or Rashba effect.So that the spinpolarized electrons accumulate at the ferromagnetic layer interface,which results in an exchange of angular momentums between the magnetic moments of the ferromagnetic layer and spin-polarized electrons.Thus,SOT drives the magnetization switching.When the perpendicular magnetization switching is driven by SOT,an in-plane magnetic field is applied to break the symmetry and realize the deterministic magnetization switching.However,the application of magnetic field will increase energy consumption and is not conducive to integration,so it is necessary to use pure electricitic method to realize field-free switching.At present,there are many studies on field-free switching,but most of them are based on specific materials or interfaces.Here,a splicing structure is proposed,which can be flexibly combined with a variety of systems and realize field-free switching without changing its structure.The splicing structure can also be applied to SOT writing wire,and the same ferromagnetic layer can be swtiched in different directions to achieve multi-resistance states by the splicing heavy metal layers.The detailed work is as follows:(1)The Pt/Co/Pt structure is deposited as a ferromagnetic part with perpendicular magnetic anisotropy(PMA),and five kinds of heavy metals are spliced on the side of the ferromagnetic part,which are Ta,W,Hf,Pt and Pd.Among them,Ta,W and Hf have negative spin Hall angle,while Pt and Pd have positive spin Hall angle.After the current is applied along x-direction,Pt as the bottom layer of the ferromagnetic part provides the spin-polarized electrons along the y-direction(σy)to accumulate at the heavy metal/ferromagnetic interface due to the spin Hall effect.While the lateral postspliced heavy metal layer provides spin-polarized electrons along the z-direction(σz).σy and σz simultaneously exert SOT on the ferromagnetic layer to cause its magnetization switching,where σy provides the switching impetus and σz provides the switching direction.With the cooperation of them,deterministic switching is achieved without an in-plane magnetic field.And for five kinds of lateral spliced elements,the positive and negative spin Hall angle devices have opposite direction of the field-free switching curves,demonstrating that the direction of the field-free switching originates from the lateral heavy metal.Based on this principle,a model combined with tunnel junction is proposed,which is expected to be applied to SOT-MRAM.(2)The Pt/Co/Pt structure was deposited as a ferromagnetic part with perpendicular magnetic anisotropy.Above the ferromagnetic part,5 nm Pt and Ta were deposited as the spin sources,and spliced together laterally by electron beam lithography.Because Pt and Ta have opposite spin Hall angle,which make the magnetization switching in opposite directions for the same ferromagnetic layer and show four different resistance states.Therefore,it is expected to be used as a multi-state memory to increase the storage density.It is also found that Pt and Ta modulate the Dzyaloshinskii-Moriya interaction(DMI)of the same ferromagnetic layer,with right-handed chirality on the side of Pt and left-handed chirality on the side of Ta.(3)Field-free switching is achieved in the Pt/Co/Pt wedged film with the bottom Pt layer as lateral wedge along the y-direction.The intrinsic DMI of the Pt/Co/Pt structure is found to be right-handed chirality by magneto-optical Kerr microscopy.When the current is applied,the wedged Pt will produce SOT with uniform intensity on the ferromagnetic layer,forming a chiral spin texture caused by the current.When the dynamic spin chirality is consistent with the intrinsic DMI chirality of Pt/Co/Pt structure,the energy is the lowest.So,the symmetry can be broken to achieve deterministic switching.