Mössbauer study of magnetically ordered states in Heusler compounds Fe\(_{3-x}\)Mn\(_x\)Si

It has been known that the Heusler compound Fe \(_{3-x}\) ${}_{3-x}$ Mn \(_x\) ${}_x$ Si has a complex magnetic phase diagram. For 0.75 \( \le x \le \) $\le x\le$ 1.75, it exhibits a ferromagnetic transition at \(T_\textrm{C }\) $T_{\text{C}}$ , and at a lower temperature \(T_\textrm{A}\) $T_{\text{A}}$ , another transition to a phase with both ferromagnetic and antiferromagnetic components. Recently, however, magnetization and specific heat measurements revealed that for \(x=\) $x=$ 1.7, yet another transition occurs at a temperature lower than \(T_\textrm{A}\) $T_{\text{A}}$ , i.e., there are three transitions at zero magnetic field. In our previous study, the nature of the ordered phases and transitions for \(x =\) $x=$ 1.7 and 1.68 were investigated using Mössbauer spectroscopy, and the existence of a hyperfine field \(B_\textrm{hf}\) $B_{\text{hf}}$ 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 \(_{3-x}\) ${}_{3-x}$ Mn \(_x\) ${}_x$ 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 \(T_\textrm{A2}'\) $T_{\text{A2}}^{\prime }$ . The spectra show that \(B_\textrm{hf}\) $B_{\text{hf}}$ is present for \(x=\) $x=$ 1.8 and 1.9 at low temperatures, and the values of \(B_\textrm{hf}\) $B_{\text{hf}}$ are comparable with that for x = 1.7. With increasing temperature T, \(B_\textrm{hf}\) $B_{\text{hf}}$ decreases smoothly, and at \(T>T_\textrm{A2}'\) $T>T_{\text{A2}}^{\prime }$ the spectrum becomes a singlet and \(B_\textrm{hf}\) $B_{\text{hf}}$ is zero. These results confirm the existence of magnetic order for x = 1.8 and 1.9.