Preparation And Magnetic Properties Optimization Of Co-Based (Co-Hf,Co-Zr) Alloy Thin Films

Magnetic MEMS and flexible electronic devices have very broad application prospects in fields such as sensing and communication,among which permanent magnet thin films are an important component.Therefore,research and exploration on permanent magnet thin films are of great significance.Among numerous rare earth free permanent magnet materials,Co-based（Co-Hf,Co-Zr）permanent magnet alloys have attracted much attention due to their excellent properties such as good temperature stability,high Curie temperature,corrosion resistance,and high strength.At present,there are few reports on Co-based permanent magnet alloy thin films,and the basic research needs to be explored.Therefore,Ta（buffer layer 20 nm）/Co₇Hf（t nm）/Ta（protective layer 5 nm）,Ta（20 nm）/Co₁₁Zr₂（t nm）/Ta（5 nm）and Ta（20 nm）/Co-Hf（PR:pseudoperitectic reaction）（t nm）/Ta（5 nm）,Ta（20 nm）/Co-Zr（PR）（t nm）/Ta（5 nm）alloy films were prepared by magnetron sputtering.Studied the effects of sputtering power,heat treatment process,film structure and composition on the morphology,structure,and magnetic properties of thin films.The purpose of this paper is to determine the hard magnetic phase in Co-Hf and CoZr alloy thin films,reveal their coercivity mechanism and determine the best process conditions to optimize their magnetic properties.The specific research conclusions are as follows:（1）The optimal sputtering power for Co₇Hf and Co-Hf（PR）thin films is 80 W.The higher the sputtering power,the faster the deposition rate of target atoms,and higher power promotes the generation of large clusters.The hard magnetic phase in Co-Hfalloy thin films is Co₇Hf phase.The optimal heat treatment temperature for both films is 700℃,and the holding time is 30 minutes.After heat treatment under this process,the Curie temperature of Co-Hf（PR）films is lower than that of Co₇Hf films.After heat treatment at different temperatures,the Co element in the Co-Hf（PR）film will segregate to the surface,forming a Co-rich region and generating a Co phase.With the extension of holding time,the cluster size of the films increases.The optimal magnetic layer thickness of the two films is 300 nm,and H_c first increases and then decreases with the increase of the magnetic layer thickness.The coercivity mechanism is mainly nucleation mechanism when the magnetic layer is thin（50-100 nm）,and it is a mixture of nucleation and pinning mechanism when the magnetic layer thickness is increased.Under the above optimum technological conditions,the H_c=440 Oe,B_r=956 Gs,（BH）_max=85637 GOe of Co₇Hf thin films;the H_c=284 Oe,B_r=822 Gs and（BH）_max=43566 GOe of Co-Hf（PR）thin films.The magnetic properties of Co-Hf（PR）thin films were optimized by changing the number of cycles of the structural units and adjusting the composition of the thin films near the peritectic point to reduce the segregation of Co elements.The H_c and（BH）_max of the thin films were improved after two cycles.When the mass ratio of Co₈₈Hf₁₂ layer to Co₂₃Hf₆ layer was 86.5%:13.5%,the magnetic properties were greatly improved:H_c=435 Oe,B_r=928 Gs,（BH）_max=79112 GOe,which was similar to the（BH）_max of the Co₇Hf film.（2）The optimal sputtering power of Co₁₁Zr₂ and Co-Zr（PR）thin films is 35 W,and the deposition rate of target atoms increases with the increase of power.The hard magnetic phase in Co-Zr alloy films is rhombic Co₁₁Zr₂(R-Co₁₁Zr₂)phase.The optimal heat treatment temperature of Co₁₁Zr₂ films is 750℃,while Co-Zr（PR）films is 800℃,and the optimal holding time of both films is 30 min.Under the above process,the Curie temperature of Co-Zr（PR）films after heat treatment is lower than that of Co₁₁Zr₂ films,and the Zr element in Co-Zr（PR）films after heat treatment will segregate towards the near surface,while pores will appear on the surface of both films when the insulation time is extended.The optimal magnetic layer thickness of Co₁₁Zr₂ thin films is 150 nm and Co-Zr（PR）thin films is 300 nm.The coercivity mechanism of Co₁₁Zr₂ thin films is mainly nucleation mechanism,while the coercivity mechanism of Co-Zr（PR）thin films is mainly nucleation mechanism when the magnetic layer is thin（50-150 nm）,and it is a mixture of nucleation and pinning mechanism when the magnetic layer thickness is increased.Co₁₁Zr₂ and Co-Zr（PR）thin films have polycrystalline structures,with R-Co₁₁Zr₂ and Co phases coexisting.Under the above optimum technological conditions,the H_c=540 Oe,B_r=1510 Gs,（BH）_max=153265 GOe of the Co₁₁Zr₂ thin films;the H_c=945 Oe,B_r=1567 Gs and（BH）_max=278605 GOe of Co-Zr（PR）thin films.The（BH）_max value of Co-Zr（PR）thin films are about 1.8 times that of Co₁₁Zr₂ thin films.