Influence of Sn addition on the phase formation, microstructure and magnetic properties of Mn-Bi alloy ribbons

In this work, the phase formation, microstructure and magnetic properties of Mn ₅₄Bi _46−xSn _x (x = 0, 1, 2, 3 and 4) alloy ribbons fabricated by melt-spinning method combined with annealing were investigated. The fraction of the low-temperature MnBi phase (LTP-MnBi) depends not only on the Sn concentration but also on the annealing regime. In the as-quenched ribbons, the fraction of the LTP-MnBi decreases monotonically from 38.1 to 21.1 wt% when the Sn concentration increases from 0 to 4 at%. Meanwhile, the fraction of this phase increases significantly from 69.0 to 83.2 wt% when the Sn concentration changes from 0 to 2 at% and tends to decrease upon further increasing the Sn concentration to 4 at% for the ribbons annealed at 250 °C for 16 h. Saturation magnetization, M _s, of the annealed samples changes consistently with the fraction of LPT-MnBi and reaches a maximum value of 70.4 emu/g in the sample with Sn concentration of 2 at%. The addition of Sn also aids in refining the grains, leading to an improvement in the hard magnetic properties of the material. The coercivity, H _c, of the alloy ribbons tends to increase as the Sn concentration increases and changes non-monotonically with annealing time. The maximum energy product, (BH) _max, reached 5.5 MGOe in the sample with x = 2, demonstrating the promise of Mn-Bi based alloys in practical applications. The density functional theory calculations were also performed to clarify the effect of Sn addition. The calculated results confirmed the decrease in the lattice parameters with Sn substitution in the experimental data. The total magnetic moment per formula unit and the evaluated saturation magnetization are 3.62 µ _B and 78.28 emu/g, respectively, for Sn substitution of ∼3.1 at%. (Although the total magnetic moment decrease with doping, the saturation magnetization per unit mass could be improved.)