Study On Mechanism Of Influence Of Exchange Coupling And Grain Boundary Control On Magnetic Properties Of Permanent Magnetic Materials

As a functional material featuring magnetism,permanent magnet materials are widely used in various fields.NdFeB material and L10FePt thin film material are two typical representatives of permanent magnet materials.The former is mainly used in the field of permanent magnet motor because of its large magnetic energy product and low price;the latter has the advantages of strong magnetocrystalline anisotropy,small critical grain size and good corrosion resistance,and it is an excellent choice for ultra-high density magnetic recording media materials.The operating temperature of permanent magnet motor above 150?determines that NdFeB material needs to have high room temperature coercivity.How to raise the room temperature coercivity of magnet is a key problem.The use of low melting point rare earth alloy for grain boundary diffusion of magnet is considered to be an effective method to increase the room temperature coercivity of a magnet.For L10FePt magnetic recording media materials,the extremely high coercivity will result in high writing field and it will be difficult to write information,the soft magnetic layer and the hard magnetic layer are combined to form a hard/soft composite permanent magnetic material,which can be solved perfectly by the exchange coupling between the hard and soft magnetic layers.In this paper,NdFeB and L10FePt permanent magnet materials are studied,and the magnetic properties of magnets are controlled based on the principle of grain boundary diffusion and exchange coupling.FePt(10nm)mutilayer film and FePt(10nm)/Pt(t)/Fe(3nm)/Pt(3nm)(t=0 nm,0.5nm,1nm,2nm)were prepared by magnetron sputtering.The composite film was characterized by magnetic properties,structure,and morphology.When there was no spacer layer,the composite film had strong exchange coupling effect,the coercivity was reduced,and the residual magnetization is obviously enhanced.With the thickness of the non-magnetic spacer layer Pt becoming thicker,the exchange coupling gradually became weakened.After adjusting the thickness of FePt layer,the composite film of FePt(8nm)/Pt(0.5nm)/Fe(3 nm)/Pt(3nm)was prepared,the coercive force of the film was 0.87T,the maximum magnetic energy product was 370KJ/m3,which had reached 95%of the theoretical maximum magnetic energy product of the L10FePt phase.Ta(20nm)/Nd14Fe77B9(100nm)/diffusionlayer(10nm)/Ta(20nm)(diffusion-layer=Nd-Al)thin films were prepared by magnetron sputter.After the diffusion of Nd85Al15,Nd70Al30 and Nd55Al45,the coercivity of the thin films increased from 1.45T to 2.06T,2.26T,2.24T,respectively.When Nd70Al30 diffusion was used,it was observed that the content of Nd-rich grain boundary phase in the magnet increased under transmission electron microscope,and the coercive temperature coefficient of the magnet increased from-0.557%/K to-0.507%/K.Using MQU-F magnetic powder as raw material,an isotropic magnet with a coercive force of 2.0T was prepared by hot pressing at 650? and 75MPa,and an anisotropy magnet with a coercivity of 1.45T was obtained by hot deformation at 850? and 270MPa.The Nd80Cu20 alloy was used for grain boundary diffusion treatment at 550?,600? and 650?,respectively.The coercivity of the magnet increased with increasing temperature,reaching a maximum value of 2.05T at 650?.The BSE-SEM results showed that the Nd-rich grain boundary phase content of the magnet was significantly increased,and the ferromagnetic elements Fe and Co content in the grain boundary phase were significantly reduced.