Pressure dependence on the remanent magnetization of Fe-Ni alloys and Ni metal

Abstract

We measured the acquisition of magnetic remanence of iron-nickel alloys (${\mathrm{Fe}}_{64}{\mathrm{Ni}}_{36}$, ${\mathrm{Fe}}_{58}{\mathrm{Ni}}_{42}$, and ${\mathrm{Fe}}_{50}{\mathrm{Ni}}_{50}$) and pure Ni under pressures up to 23 GPa at room temperature. Magnetization decreases markedly for ${\mathrm{Fe}}_{64}{\mathrm{Ni}}_{36}$ between 5 and 7 GPa yet remains ferromagnetic until at least 16 GPa. Magnetization rises by a factor of 2--3 for the other compositions during compression to the highest applied pressures. Immediately upon decompression, magnetic remanence increases for all Fe-Ni alloys while magnetic coercivity remains fairly constant at relatively low values (5--20 mT). The amount of magnetization gained upon complete decompression correlates with the maximum pressure experienced by the sample. Martensitic effects best explain the increase in remanence rather than grain-size reduction, as the creation of single domain sized grains would raise the coercivity. The magnetic remanence of low Ni Invar alloys increases faster with pressure than for other body-centered-cubic compositions due to the higher magnetostriction of the low Ni Invar metals. Thermal demagnetization spectra of ${\mathrm{Fe}}_{64}{\mathrm{Ni}}_{36}$ measured after pressure release broaden as a function of peak pressure, with a systematic decrease in Curie temperature. Irreversible strain accumulation from the martensitic transition likely explains the broadening of the Curie temperature spectra, consistent with our x-ray diffraction analyses.