Research Of Microstructure And Magnetic Properties Of MnBiM (M=Ti,Zr,Ga And Gd)Magnets

Neodymium-iron-boron(Nd-Fe-B) permanent magnets are widely appliedto electromagnetic devices. However, because of the high cost and limitedavailability of rare-earth elements, there is growing interest in the development ofnew magnetic materials that are free of these elements. MnBi is an ideal candidatefor new permanent-magnetic materials free of rare earths. It shows outstandingmagnetic properties, and exhibits a positive temperature coefficient of coercivity atcertain temperature range. MnBi has received widely attention due to those obviousadvantages. The main study of this thesis includes:(1)Research on preparation and magnetic properties of MnBi.It is difficult to obtain single-phase MnBi because MnBi exists in severalclosely related forms. The segregation of Mn at the peritectic temperature makes itmore difficult to do that. Three different methods were used to preparedMnxBi1-x(46≥x≥62) samples for achieving higher percentage of MnBi-LTP(lowtemperature phase). Magnetic properties of these samples were determined. Resultshows melt-spinning followed by heat treatment is the best method to prepare MnBialloy. The primary phase of the annealed ribbons is MnBi-LTP. The energy product is0.75MGOe and is75times larger than it of the sample prepared by induction melting.The Curie temperature is approximate615K. Coercivity of MnBi is deeplydependent on temperature and exhibited a positive temperature coefficient during100~550K. The temperature coefficient of coercivity β reached2.89%/K during300~550K. The coercivity is found to increase with the increase in temperaturereaching a maximum of1.132T at550K and then decrease as the temperatureincrease further. The coercivity at550K is26times larger than it at100K. Theenergy product is found to increase with the increase in temperature reaching amaximum of1.103MGOe at450K and then decrease as the temperature increasefurther. (2)Effect of partial substitution of Bi with non-rare-earth elements on the magneticproperties of MnBiMnBi1-xMx(M=Ti，Zr，Ga) samples were prepared by melt-spinning followed byheat treatment. Magnetic properties of these samples were determined.The coercivity of MnBi sample increase sharply and the grain become smalleras Ti substituted in. The coercivity of MnBi0.3Ti0.7sample is0.853T and is6timeslarger than it of the MnBi sample. The increase in coercivity when Ti substituted inmay be in part due to the refinement of grain size and in part due to the increase ofgrain boundaries to pinning the domain-wall. The remanence is found to increasewith the increase in temperature during100~350K as Ti substituted in.The grain of MnBi sample become smaller as Zr substituted in. There is no newphase emerge in XRD pattern while Zr substituted in. The coercivity is found toincrease with the increase in x(Zr content) reaching a maximum of0.705T whenx=0.3and then decrease as x increase further. The Curie temperature increaseobviously when Zr substituted in. The Curie temperature of MnBi0.5Zr0.5is foundbeyond670K. MnBi1-xZrxsamples have better magnetic properties than MnBi andMnBi1-xTixsamples in high temperature. The coercivity, remanence and energyproduct of x=0.3sample are all increase with the increase in temperature during300~600K.The coercivity of MnBi sample increase sharply as Ga substituted in. Thecoercivity of MnBi0.5Ti0.5sample at300K is0.664T and is5times larger than it ofthe MnBi sample. The increase in coercivity when Ga substituted in may be in partdue to the increase of crystal field anisotropy and in part due to the increase of grainboundaries to pinning the domain-wall. The magnetic properties of MnBi1-xGaxsamples remain a high level only when the temperature is below400K. The Curietemperature of MnBi1-xGaxis found to decrease with the increase in Ga content.(3)Effect of partial substitution of Bi with rare-earth element on the magneticproperties of MnBi. MnBi1-xGdxsamples were prepared by melt-spinning followed by heat treatment.Magnetic properties of these samples were determined.The grain of MnBi sample become smaller as Gd substituted in. The phasecomposition of MnBi1-xGdxare complicated. MnBi0.5Gd0.5sample contains4phases.MnBi-LTP is the only ferromagnetic phase. The coercivity is found to increase withthe increase in x(Gd content) reaching a maximum of0.654T at x=0.3and thendecrease as x increase further. The magnetic properties in high temperature increaseobviously when x≤0.3and decrease obviously when x>0.3.