Microstructure And Magnetic Characteristics Of Mn Bi Permanent Alloys

Manganese bismuth(Mn Bi)compound,as a rare-earth-free permanent magnetic material,is a potential candidate for permanent magnets used in high-temperature applications due to its high magnetocrystalline anisotropy(1.6×106 J/m3,at 300 K),moderate saturation magnetization,and unique positive temperature coefficient of coercivity.In this thesis,based on the melt spinning tenchnique,surfactant assisted ballmilling(SABM),and spark plasma sintering technique(SPS),Mn Bi ribbons,ball milling powders,and SPSed magnets were prepared,respectively.The correlation of composition,microstructure,magnetic characteristics were investigated in detail.Also,magnetic hardening mechanism and related physical mechanism were calrified.Using annealed melt spinning ribbons as precursors,Mn Bi powders were prepared by surfactant assisted ball-milling(SABM).Effects of SABM processes on the phase constituent,microstructure,and magnetic characteristics were investigated.It was shown that the LTP Mn Bi was easily decomposed into Mn phase and Bi phase during the process of ball milling.The fine particle(grain)size and increased magnetic isolation of intergranular phase jointly contribute to high coercivity,and a maximum coercivity value of 1.5 T could be obtained at room temperature.From the studies about magnetic hardening mechanism,it was demonstrated that the pinning model should be responsible to the variation of coercivity.In addition,the recoil loops characteristics and the relation to the microstructure were studied.It was indicated that the non-uniform distribution of the microstructure,could produce a non-negligible local demagnetization field with non-uniform distribution,and resulted in open recoil loops.By improving the non-uniform distribution of the microstructure through ball milling,nearly closed recoil loops could be obtained.Isotropic bulk Mn Bi magnets with high density,were prepared by spark plasma sintering(SPS)technique using ball milling powders as precursors.Effects of microstructure and content of intergranular phase on magnetic properties phase were studied systematically.It was suggested that phase constituent of ball milling powders and SPS mechanism should be responsible to the formation of intergranular phase.Improved coercivity was attributed to the enhanced magnetic isolation effects and fine grain size,jointly.The SPSed magnets with remanence of 26.0 emu/g and coercivity of 7.11 k Oe at room temperature,can be obtained,and still exhibits ferromagnetism at 650 K.From the investigation of thermal magnetic curves,it was shown that the fine particle size(grain size)was favorable to the decrease of spin reorientation temperature.In addition,high applied magnetic filed is helpful to increase the phase transition temperature of ferr-to-paramagnetic for Mn Bi SPS magnets.It was observed that the model of nucleation field should be responsible to the magnetic hardening mechanism.The studies about Henkel plots suggested that the weakened exchange coupling effect originated from the inhomogeneity of particle size and microstructure distribution.At last,to further increase magnetic properties,Pr-Cu doped Mn Bi ribbons were prepared.The dependence of magnetic characteristics on microstructure was investigated by regulating the annealing time and the doping content of Pr and Cu.It was suggested that reduced annealing time or increased doping amount were helpful to the enhancement of coercivity,however,saturation magnetizations were all decreased.The optimum magnetic properties with the coercivity of 18.10 k Oe and the remanence of 23.6 emu/g can be got for Mn53Bi45(Pr0.68Cu0.32)2 ribbons annealed at 593 K/6 h.The fine grain size,induced by the doping of Pr and Cu,was the reason of improved coercivity.The decreased remanence magnetization was attributed to the decrease in the weight fraction of the ferromagnetic LTP Mn Bi phase and enhanced antiferromagnetic exchange interaction existed Mn atoms.In addition,the studies for ZFC-FC curves suggested that,the splitting temperature and the extent of splinning can be regulated by changing the doping amount of Pr-Cu and annealing time.The magnetic harden mechanism could well explained by the pinning model.