Nguyen Van Vuong


The Low Temperature Phase (LTP) content determines the spontaneous magnetization Ms of the rare-earth-free hard magnetic material MnBi. LTP in MnBi samples alloyed by the arc-melting is timely developed when they are annealed at the annealing temperatures Ta < 340 oC. Because of the complexity of the phase diagram of MnxBi(100-x) system, the content hardly reaches the value of 100 wt%. Based on the phase diagram of MnxBi(100-x) system, the upper limit * was calculated and 59.8 wt% is the highest content which can be reached when the alloy is isothermally annealed for a long time. The time-dependent behavior of (t) reveals that the LTP is formed from Mn and Bi phases by the diffusion mechanism. The time-dependent diffusion equation has been used to investigate the diffusion process between Mn and Bi in order to form the LTP. The comparison between the theoretical and experimental data allowed to estimate the mutual diffusion coefficient D 510-12 cm2/s. This small value of D is suggested due to not only the low value of Ta necessary for forming LTP but the high surface tension of Bi melted at Ta as well. The calculated results showed that the size distribution of Mn grains embedded in the Bi matrix affected the dependence (t), enhancing and inhibiting in the samples annealed for short and long times, respectively. To increase over *, the anneal at Ta superimposed by the small temperature gradient of 2 oC/cm has been performed. This temperature-gradient driven annealing technique helped to overcome * and reach the value of 83 wt% which corresponds to the Ms = 60 emu/g measured at the field strength of 4 Tesla.


rare-earth-free MnBi magnets, content of low temperature phase, phase diagram, time-dependent diffusion process, spontaneous magnetization Ms.

DOI: https://doi.org/10.15625/2525-2518/54/1A/11805 Display counter: Abstract : 183 views.


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Published by Vietnam Academy of Science and Technology