Temperature- and field-induced first-order ferromagnetic transitions in MnFe(P_1-xGe_x)

We have found that the hexagonal ZrNiAl-type compounds MnFe(P _1-xGe_x) with x = 0.15 and 0.17 show temperature- and field-induced first-order paramagnetic-to-ferromagnetic transitions with remarkable hysteresis loops at T_C ≃ 250 K. The hysteresis widths in both temperature and field are much larger than those for a similar system of MnFe(P,As). On cooling below T_C, the hexagonal lattice of MnFe(P_1-xGe_x) contracts by 1.5% along the a-axis and elongates by 2.8% along the c-axis, keeping the crystal symmetry unchanged. The very small volume expansion of less than 0.1% suggests that the first-order transition is related to the anisotropic lattice distortion rather than the volume change.