Shock-Wave Compression of 30% Ni-70% Fe Alloys: The Pressure-Induced Magnetic Transition

The compressibility of 30% Ni-70% Fe (wt %) in the fcc phase is investigated from atmospheric pressure to 40 kbar with shock-wave loading techniques. The experiments are accomplished utilizing projectile impact techniques with stress profile measurements by the quartz gauge. A sharp change in compressibility indicates a second-order ferromagnetic Curie point transition at a stress of 25 kbar and a volume of 0.9807 V0. The coefficient of Curie temperature change with pressure is found to be −5.8±0.3°C kbar−1. The agreement of this value with previous magnetic measurements, along with the anomalously large compressibility below the transition and the large decrease in compressibility at an elevated temperature, clearly indicates that this transition is a ferromagnetic to paramagnetic transition. Values for the change of thermal expansion and specific heat at the transition are computed from the Ehrenfest relations. These values are consistent with the magnetic character of the transition and give a complete description of the thermodynamic properties of the transition. To provide more compressibility data on various types of ferromagnetic materials, a few additional measurements are reported for Invar and 30% Ni-70% Fe in the bcc phase.