Mössbauer study of magnetically ordered states in Heusler compounds Fe _3-x Mn _x Si

It has been known that the Heusler compound Fe _3-x Mn _x Si has a complex magnetic phase diagram. For 0.75 ≤ x ≤ 1.75, it exhibits a ferromagnetic transition at T_C , and at a lower temperature T_A , another transition to a phase with both ferromagnetic and antiferromagnetic components. Recently, however, magnetization and specific heat measurements revealed that for x= 1.7, yet another transition occurs at a temperature lower than T_A , i.e., there are three transitions at zero magnetic field. In our previous study, the nature of the ordered phases and transitions for x = 1.7 and 1.68 were investigated using Mössbauer spectroscopy, and the existence of a hyperfine field B_hf was confirmed. This demonstrates that the low temperature phases are magnetically ordered phases. In this study, to further investigate the ordered phases and transitions in Fe _3-x Mn _x Si, we measure Mössbauer spectra for x = 1.8 and 1.9, where only a single antiferromagnetic transition appears to exist as judged from magnetization measurements, and the transition temperature is defined as T_A2' . The spectra show that B_hf is present for x= 1.8 and 1.9 at low temperatures, and the values of B_hf are comparable with that for x = 1.7. With increasing temperature T, B_hf decreases smoothly, and at T>T_A2' the spectrum becomes a singlet and B_hf is zero. These results confirm the existence of magnetic order for x = 1.8 and 1.9.