Mössbauer spectroscopy study on the magnetic transition in Mn_1.1Fe_0.9P_0.8Ge_0.2

The magnetic transition has been studied in Mn_1.1Fe_0.9P_0.8Ge_0.2 by magnetic measurements and ⁵⁷Fe Mössbauer spectroscopy. The alloy crystallizes in the hexagonal Fe₂P-type structure with lattice constants of a=6.0476(4) Å and c=3.4766(7) Å. Both bulk magnetization measurements and Mössbauer spectroscopy show that the as-prepared sample has a significantly lower transition temperature on first cooling (T_C¹≈200 K) than after it has undergone thermal cycling to 20 K (T_C^20 K=240 K). The behavior of the material stabilizes after the first cooling/heating cycle and no further changes are observed in T_C. Working with a stabilized sample, we find that the temperature dependence of the hyperfine field, B_hf(T), is more rapid than that predicted by a simple mean field Brillouin function, and in addition, B_hf(T) shows a thermal hysteresis of 10 K upon cooling versus heating. These results show that the magnetic transition at T_C is definitely first order and suggest that there is an additional irreversible magnetostructural change during the first cooling process of the as-prepared sample.