Preparation Of Nanocomposite Nd₂Fe₁₄B/α-Fe Ribbons By Melt-Spinning And Theoretical Calculation Of The Effective Anisotropy

As a new type of permanent magnetic material,nanocrystalline composite is composed of soft phase and hard phase with nanoscale microstructure.The magnetic material can have the high remanence of the soft phase and the high coercivity of the hard phase according to the theory of exchange-coupling.It is highly likely to become a new generation of permanent magnet materials.But the energy product of the actual material is much lower than the value predicted by the theory.This is mainly attributed to the great difference of the microstructure between the actual material and the theoretical model.In the first part of this paper,we investigate the influence of preparation parameters on magnetic properties of the nanocomposite material(Nd,Pr,Dy)₂(Fe,Nb)₁₄B/α-Fe prepared by melt-spinning and subsequent annealing technique.The magnetic properties,phase composition,microstructure are studied by vibrating sample magnetometer(VSM),X-ray diffraction(XRD) and scanning probe microscope(SPM).In the second part,the effect of exchange coupling interaction on effective anisotropy of nanocomposite are investigated.The main results are as follows:1.Rapid quenched ribbons with nominal compositions of Nd_4.2Pr_3.6Dy_0.7Fe_85.5Nb₁B₅ were prepared by melt spinning at different surface speed.The magnetic properties change greatly with the wheel speed.The reason is that the ribbons prepared in different surface speed have different cooling rate and leads to different microstructure.SPM micrograph show that the grain size decreases with the wheel speed increasing.It indicates that the exchange coupling between soft and hard grain are enhanced.The magnetic property of optimally quenched sample(30m/s) is H_cj=538.33kA/m,B_r=0.68T,M_r/M_s=0.73,(BH)_max=53.44 kJ/m³.Subsequent annealing is needed to crystalline amorphous phase,which further improves the magnetic properties due to much finer and more homogeneous microstructure.The optimum magnetic properties have been obtained in the sample spun at 35m/s and then annealed at 500℃for 2 min:H_cj=506.6kA/m,B_r=0.87T,M_r/M_s=0.72,(BH)_max=70.38 kJ/m³.2.In nanocomposite permanent magnetic material,the anisotropy at grain boundary is changed due to the exchange-coupling between neighboring grains.The average anisotropy of magnet is called "effective anisotropy".In this paper,we put forward an expression of anisotropy in which the effective anisotropy at grain boundary is not zero..Based on this expression,the effective anisotropy of single soft or hard grain has been investigated in nanocomposite material.The results show that the effective anisotropy of soft grain increases with the reduction of grain size and the increase of soft-hard coupling;the effective anisotropy of hard grain decreases with the decrease of grain size and the increase of hard-soft coupling.In other words,the exchange-coupling interaction between soft and hard grains increases the average anisotropy of the soft grain and decreases the average anisotropy of the hard one.Furthermore,considering the area fraction of different coupling situation,the dependence of effective anisotropy on soft or hard grain size and volume fraction of hard phase were calculated.The calculated results show that in order to obtain higher effective anisotropy in nanocomposite permanent magnetic material,the grain size should be about 25nm.