Study On Magnetic Properties And Exchange Bias Effect Of MnBi Alloys

MnBi permanent magnets can be potentially used in medium and high temperature and strong demagnetizing environments,due to high coercivity,moderate saturation magnetization and magnetic energy product.In this paper,MnBi alloys were prepared by melt-spinning,high-energy ball milling and spark plasma sintering（SPS）technique.The relationship between the microstructure and magnetic properties of MnBi permanent magnets was studied,and the exchange bias phenomenon of MnBi alloys after the addition of Pr and Cu elements was investigated.Based on the melt-spinning technique,the effects of Pr and Cu doping on the microstructure and magnetic properties for MnBi alloy were studied.The studies found that with the increase of the content of Pr element,the coercivity increases first and then decreases.Among them,the(Mn₅₅Bi₄₅)₉₅Pr₅ alloy possesses the highest coercivity of7.846 k Oe,while the antiferromagnetic coupling between Mn-Mn and the ferrimagnetic coupling between Pr and Mn are the reasons for its monotonic decrease in saturation magnetization.Cu doping studies show that the coercivity increases first,and then decreases,while the saturation magnetization decreases monotonically with the increase of Cu doping content,which is consistent with the results of Pr doping.The largest coercivity,up to 8.905 k Oe can be obtained for(Mn₅₅Bi₄₅)₉₇Cu₃ alloy.Co-doping studies of Pr Cu show that,upon the increase of Pr Cu content,the coercivity raises monotonically,and the maximum coercivity can reach 17.277 k Oe.The sintered MnBi Pr Cu magnet with high density and excellent magnetic properties was prepared by the spark plasma sintering technique,which the microstructure consists of coarse-grained and fine-grained regions.After annealing at 593 K/0.5 h,the coercivity of 6.74k Oe and the remanence of 15.63 emu/g can be achieved.Research on the coercivity mechanism indicates that the coercivity of the SPS magnet is controlled by the pinning mechanism.Subsequently,the exchange bias phenomenon of Pr and Cu doped systems was studied in depth.The research found that a significant exchange bias phenomenon was observed in Pr-doped MnBi alloys.With the increase of Pr doping content,the exchange bias field enhances first and then decreases.The highest exchange bias field of 9.372k Oe and the maximum coercivity of 17.594 k Oe can be obtained,respectively.The strong coupling effect between the ferromagnetic MnBi phase and the amorphous antiferromagnetic cluster is the reason for the formation of exchange bias.The study of temperature field reveals that with increasing the temperature,the exchange bias field degrades monotonously due to the decrease in interface coupling strength and the weakening of antiferromagnetic anisotropy.The cooling field study demonstrates that as the cooling field increases,the exchange bias field enhances first,then decreases and then remains almost unchanged.The measurement field study shows that the exchange bias field decreases with the increase of measurement magnetic field,which is attributed to the weakening pinning effect of antiferromagnetic spin to ferromagnetic spin.Training effect study found that the spin on the interface tends to be balanced with the increase in the number of tests,that is,the net magnetization of the antiferromagnetic layer interface gradually decreases and approaches equilibrium,which leads to a reduction of the exchange bias field and it tends to be stable,which accords with the formula H_E（n+1）-H_E（n）=-[（n）-_∞]³.Finally,Cu doping studies show that the strong coupling effect of ferromagnetic LTP MnBi and amorphous antiferromagnetic cluster phase is also the reason for the exchange bias effect of MnBi Cu.The Bi₄Cu_0.38Mn_3.41 phase can produce a high coercivity of 7.9 k Oe at 5 K.In addition,a higher exchange bias effect was also observed in Pr Cu co-doped MnBi alloy,and the exchange bias field at 5 K can reach 5.835 k Oe.The exchange bias effect of MnBi Pr Cu alloy can be reduced by annealing.It is found that SPS MnBi Pr Cu sintered magnets possesses weak exchange bias effect.