Thermal expansion of single-crystal Gd5(Si1.95Ge2.05) showing unusual first-order transformation

Measurements of thermal expansion of single-crystal Gd₅(Si_1.95Ge_2.05) during cooling and heating were conducted for the first time. A very steep change in strain with temperature was observed when the material underwent a phase transformation. This was an unusual simultaneous magnetic and structural phase transformation from a ferromagnet with an orthorhombic crystal structure below the transition temperature T_c to a paramagnet with a monoclinic crystal structure above T_c. This transition temperature T_c was found to depend on the magnetic field, and to exhibit hysteresis depending on whether the material was being cooled or heated. In the absence of a magnetic field, T_c was 267 K on cooling and 269 K on heating. However, when the material was subjected to a magnetic induction B in the range 0-2.5 tesla (T), the transition temperatures, on both cooling and heating, were found to increase linearly with temperature by about 4.8 K/T. This rate of change of transition temperature with magnetic field was in good agreement with calculations based on the assumption that the additional energy due to the magnetic field can suppress the thermal vibration of Gd atoms and that the additional thermal energy per Gd atom needed to cause the phase transition to occur is equal to the additional magnetic energy of each Gd atom caused by the magnetic field.