Structural and magnetic features of solid-phase transformations in Mn/Bi and Bi/Mn films

Solid-phase transformations at different annealing temperatures in Mn/Bi (Mn on Bi) and Bi/Mn (Bi on Mn) films have been studied using X-ray diffraction, electron microscopy, and magnetic measurements. It has been shown that the synthesis of the $\alpha$-MnBi phase in polycrystalline Mn/Bi films begins at a temperature of $\sim$120$^\circ$C and the Mn and Bi layers react completely at 300$^\circ$C. The resulting $\alpha$-MnBi(001) samples have a large perpendicular magnetic anisotropy ($K_u\simeq1.5\times10^7$ erg/cm$^3$) and a coercive force $H>H_C\sim3$ kOe. In contrast to Mn/Bi, the ferromagnetic $\alpha$-MnBi phase in Bi/Mn films is not formed even at annealing processes up to 400$^\circ$C and Mn clusters are formed in a Bi melt. This asymmetry in phase transformations occurs because chemosorbed oxygen existing on the surface of the Mn film in Bi/Mn films suppresses a solid-phase reaction between Mn and Bi. The analysis of the results obtained implies the existence of new low-temperature ($\sim$120$^\circ$C) structural transformation in the Mn-Bi system.