Research On Fabrication And Magnetic Anisotropy Of Co₂FeSi Alloy Thin Films

Magnetic random access memory（MRAM）is expected to be the next generation of ultra-low power general purpose memory chips because of its advantages of high read/write speed,low power consumption and non-volatility.The basic unit is a magnetic tunnel junction composed of ferromagnetic free layer,tunneling oxide layer and ferromagnetic reference layer.Co-base Heulser alloy Co₂FeSi can be used as ferromagnetic free layer of magnetic tunnel junction because of its low coercivity,high spin polarization,high Curie temperature and low damping coefficient.Perpendicular magnetic anisotropy and in-planeuniaxial magnetic anisotropy of Co₂FeSi thin films with thickness ranging from 3 nm to 100nm have been studied.The main results are as follows:On the one hand,monolayer Co₂FeSi material has almost no Perpendicular magnetic anisotropy due to its symmetrical structure,and Perpendicular magnetic anisotropy can be generated by the interface effect of sandwich structure.The Pt/Co₂FeSi（3-9nm）/Mg Al₂O₄dual-interface multilayer films were prepared.The easy axis of the deposited samples was in the plane.After annealing at 350℃,the samples showed Perpendicular magnetic anisotropy within the thickness of 3.5-5.5nm of Co₂FeSi,and the unit saturation magnetization was about908emu/cm³.It is close to the theoretical value of Co₂FeSi bulk（～1104emu/cm³）.The vertical magnetic anisotropy energy density ranges K_μfrom 10⁵ to 10⁶erg/cm³,and the interface vertical magnetic anisotropy energy density K_μ1.32±0.18 erg/cm².By changing the thickness of Pt/Co₂FeSi and Co₂FeSi/Mg Al₂O₄,it is proved that Pt/Co₂FeSi and Co₂FeSi/Mg Al₂O₄interface can provide positive contribution to Perpendicular magnetic anisotropy.In addition,XRD results show that the Pt（111）peak is significantly enhanced after annealing,which is another reason to promote the Perpendicular magnetic anisotropy.On the other hand,the semi-metallic properties of Co₂FeSi alloy films can be better utilized only by forming ordered body-centered cubic structures.In order to obtain ferromagnetic films with high spin polarizability,thicker Co₂FeSi（5-100nm）samples were prepared to investigate the effects of magnetic layer thickness,film structure,annealing temperature and sputtering power on the in-plane magnetic anisotropy of the films.When the thickness of Co₂FeSi is below 25nm,the in-plane uniaxial magnetic anisotropy increases monotonically,and the maximum energy density Ku of in-plane uniaxial magnetic anisotropy is 6.4×10³erg/cm³.Further increasing the thickness of Co₂FeSi will lead to a decrease of Ku.The unit saturation magnetization of the deposited samples is only 715emu/cm³,and the maximum saturation magnetization is close to 846emu/cm³ after annealing at 200℃.The anisotropic energy density of cubic magnetic crystals Kc generated by annealing is approach to10⁴ erg/cm³.XRD results show that the crystal structure of the film can be improved by increasing the film thickness,annealing temperature and sputtering power.