Thermal hysteresis in the structural, magnetic and transport properties of hard magnetic MnBi films

• The α-MnBi phase was obtained by annealing Mn/Bi films. • Thermal hysteresis was observed in the structure, resistance and magnetism of the α-MnBi films. • Thermally hysteretic processes were observed in two different temperature ranges. • Up to 260 °C, thermally hysteretic processes were related to the presence of unalloyed Bi. • Above 260 °C, thermal hysteresis was due to the α to β-MnBi transition. In this work, the magnetic and transport properties associated with structural changes in MnBi hard magnetic films, with different Bi concentrations, were detailed. Two particular temperature regimes were studied. In the first place, at temperatures close to and below 260 °C (melting temperature of Bi in the samples) the resistance of the MnBi films underwent strong changes, showing thermal hysteresis, and depending on the amount of Bi in the samples. Thus, these resistance changes were attributed to the presence of unalloyed Bi in the samples. The maximum value ​​of the relative change of resistance, ΔR I /R(T2) (where ΔR I =R(T1)-R(T2) being T1 and T2 the temperatures just immediately below and above the structural transition) was around 180 % for the sample with the highest Bi concentration. Second, at temperatures above 260 °C, both the magnetic moment and the resistance of the samples also showed significant changes around the transition from the Low Temperature (α-MnBi) to the High Temperature (β-MnBi) phase. These changes, which were attributed to this structural transition, also presented thermal hysteresis and also depended on the Bi concentration of the samples. However, contrary to the previous situation, the samples with the lowest amount of unalloyed Bi, and consequently with the highest value of the MnBi volume fraction presented a maximum value of the ΔR II /R(T4) ratio, (where ΔR II =R(T3)-R(T4) being T3 and T4 the temperatures immediately above and below the structural transition from the β to the α MnBi phase) this magnitude reaching a maximum value of 25 % for the sample with the lowest Bi content.