Preparation And Properties Of Rare-earth-free Permanent Magnet Of LTP-MnBi

The rare-earth permanent magnet materials are widely used in various fields due to the excellent magnetic properties.With the development of the technology and the further development of the rare-earth performance in recent years,the increasing demand for rare earth leads to the increase in the price of rare earth raw materials.Meanwhile,rare earth materials belong to non-renewable resources like coal and petroleum.So developing new low-rare-earth or rare-earth-free permanent magnetic materials with low cost and high performance has gradually become the focus of researchers.Among the rare-earth-free permanent magnetic materials,MnBi has the unique magnetic performances.The low temperature phase MnBi（LTP-MnBi）has hexagonal Ni-As type structure,in addition to the large magnetocrystalline anisotropy and moderate saturation magnetization,the LTP-MnBi exhibits a very attractive feature,i.e.the coercivity plays a positive temperature coefficient from 150 K to 550 K,which means that the coercivity will increase with temperature.So LTP-MnBi is favorable for the high temperature applications.Based on the advantages of simple operation,low price of electrochemical deposition and good controllability,high purity of magnetron sputtering,LTP-MnBi thin films were prepared by these two methods in this paper.This paper studies from the following aspects:（1）LTP-MnBi thin films were prepared on Cu substrates by electrochemical deposition and subsequently the high temperature annealed treatment.Because that the difference between the reduction potential of Mn²⁺and Bi³⁺is very big,and that the deposition rate of bismuth is very large,so the ratio of Mn and Bi is not easy to control.In order to solve this problem,the change of reduction potential of Mn²⁺and Bi³⁺in mixed solution containing MnSO₄ and Bi（NO₃）₃ with different concentration of EDTA was investigated used the cyclic voltammetry.The results showed that the reduction potential of Mn²⁺shifted to a more positive value while the reduction potential of Bi³⁺shifted to a more negative when the addition of appropriate concentration of EDTA,which reduced the large difference between of them.On the basis,the MnBi films with different ratios of Mn and Bi were prepared using the different deposited potentials by the potentiostatic method.After annealed treatment at 380 ^oC,the low temperature phase was found for the samples in which the ratio of Mn and Bi is 55:46,and the corresponding coercivity reaches 1450Oe.In our knowledge this is the first time to obtain LTP-MnBi thin films by electrochemical method so far.（2）The formation of LTP-MnBi must undergo a high temperature annealed process accoding to the phase diagram and at the same time the melting point of metal Bi is only271 ^oC which deciding the Bi is affected deeply by the annealed temperatre.Bi films were deposited onto the glass substrate by magnetron sputtering.We investigated the influence of the different substrate temperatures on the structure,morphology and growth mode of Bi films.The results showed that the morphology and structure of Bi films are related to the temperature.160 ^oC（T_s/T_m=0.8）is a boundary for the morphology and structure of the filmsfrom room temperature to near the melting point of the metal.On the basis,the films of MnBi with different ratio of Mn and Bi（at%）were deposited on glass substrates by magnetron sputtering.The effects of manganese-bismuth ratio,annealing temperature and holding time on the formation of LTP-MnBi were investigated.The results showed that the content of Mn in the range of 1.06¹.23 is beneficial to the formation of LTP-MnBi.There not exists the LTP-MnBi when the annealing temperature is too low to reach the phase transition temperature and while the magnetic performances of MnBi decrease due to exceeding phase transition temperature when the annealing temperature is too high.,and prolongation of the holding time properly is also beneficial to the formation of LTP-MnBi.