Microstructure Modulation And Magnetic Properties Of Cobalt/Platinum Based Multilayer Films

Cobalt/heavy metal based multilayer films have excellent spin properties,and have broad application prospects in MRAM memory,track memory,microwave detector and spin oscillator.The spin properties of cobalt/heavy metal based multilayer films are mainly derived from the FM/HM interface in the films,where the interface effect and atomic coupling directly determine the film properties.Due to the lack of in-film interface characterization technology,few studies have been conducted on the microstructure of FM/HM interface in cobalt/heavy metal-based multilayer films,which seriously affects the regulation of magnetic properties of cobalt/heavy metal-based multilayer films by the interface structure.Magnetron sputtering technology has the advantages of simple and efficient sputtering film regulation is convenient and strong adhesion.Molecular dynamics simulation can provide information about microstructure and dynamics in space and time.In this paper,the FM/HM interface of cobalt/platinum-based multilayer films was taken as the research object,and the Pt/Co/W three-layer films and[Pt/Co]n multilayer films were prepared by magnetron sputtering.The microstructure and atomic composition distribution of Pt/Co,Co/W dual interface and Co/Pt multilayer stacking interface were studied.The surface and interface roughness,atomic agglomeration and atomic diffusion were analyzed by molecular dynamics simulation.The influence of microstructure on magnetic properties of thin films was investigated.Specific research contents and main conclusions are as follows:(1)The effect of Co layer thickness on the crystal structure and composition distribution of Pt/Co/W three-layer films was studied.The interface roughness and atomic diffusion were studied by molecular dynamics simulation,and the effect of microstructure on magnetic properties was analyzed.The effect of high magnetic field heat treatment on the interface microstructure and magnetic properties of Pt/Co/W thin films was investigated.The results show that:(a)The three-layer film was prepared by sputtering as an amorphous structure;The interface characteristics are as follows:the atoms in the bottom Pt layer are stable,the Co layer and the top W layer have mutual diffusion phenomenon,and the W layer diffuses significantly to the Co layer.The magnetic properties of the film can be improved by increasing the thickness of Co layer.The thicker the Co layer is,the magnetization characteristics in the surface of the three-layer film are significant,the saturation magnetization decreases first and then increases,and the coercivity decreases.The molecular dynamics simulation not only demonstrated the Co/W diffusion in Pt/Co/W three-layer films,but also characterized the growth pattern and interfacial structure of the three layers:the Co layer was a layered growth pattern,and the W layer was an island-like growth pattern.The surface roughness of W layer is obviously higher than that of Co layer,and with the increase of Co layer thickness,the surface roughness of Co layer and W layer is obviously increased.The increase of Co/W interface roughness is conducive to the mutual diffusion of atoms between the interface.(b)High magnetic field heat treatment is beneficial to improve the crystallinity of amorphous films.The higher the temperature is,the higher the degree of crystallization is.The strong magnetic field parallel to the film surface is conducive to intralayer diffusion and uniform composition distribution.At 200℃,the strong magnetic field parallel to the film surface can inhibit the interfacial alloying of Co/Pt and Co/W.The strong magnetic field perpendicular to the film surface will enhance the Co atom diffusion to the Pt layer,and the diffusion depth at 300℃ is stronger than 200℃.After improved crystallinity,the saturation magnetization of the film increased and coercivity decreased.(2)With Co layer thickness,substrate temperature and cycle number as variables,the Pt/Co multilayer stacked films were made by magnetron sputtering method,and molecular dynamics simulation was used to explore the effects of each variable on the microstructure and magnetic properties of the film interface.The results are as follows:(a)The higher the thickness of the single layer of Co in the[Pt/Co]n multilayer,the better the crystallization of Co(100).The thicker Co monolayer(2nm)enhances the crystallization of Co(100).Thin Co monolayer(lnm)Co mixes more easily with Pt layer to form CoPt3 alloy.The simulation results show that the surface of Pt layer is smoother than that of Co layer,and the surface roughness of each layer increases with the increase of the number of layers.The thick Co monolayer(2nm)has higher roughness than the thin Co monolayer(2nm),and the Pt monolayer has less lift.The crystallinity of CoPt3 in Co monolayer thin(1nm)multilayers is better than that of Co in thicker multilayers,resulting in slightly higher saturation magnetization.But the coercivity to rectangle ratio is lower.(b)Substrate temperature can enhance the diffusion ability of Co and Pt atoms in the multilayer stacked films.Compared with room temperature,the alloying degree of Co and Pt atoms in the films heated by substrate is higher,and some Co atoms are diffused to the Pt cover layer and oxidized by air.The simulation results show that the relaxation energy enhanced by heating substrate can reduce the surface roughness of the first four layers,and the roughness of the last five layers is similar to that of the room temperature sample.Due to the diffusion of Co and Pt atoms at the Co/Pt interface and the oxidation of Co atoms in the Pt cover layer,the saturation magnetization of the films sputtering on heated substrate decreases.(c)The interfacial anisotropy of the multilayer films can be improved by increasing the number of periods,resulting in the out-of-plane rectangle ratio exceeding in-plane,which is beneficial to promote the vertical magnetization of the multilayer periodic films.The increase of the period number promotes the film alloying and interfacial diffusion,leading to the blurring of the interface,which reduces the magnetic proximity effect and the saturation magnetization.