Pressure Dependence on the Remanent Magnetization of Fe-Ni Alloys

Thursday, 18 December 2014
Stuart A Gilder, Qingguo Wei and Bernd Maier, Ludwig Maximilians Universit├Ąt, Geosciences, Munich, Germany
We measured the acquisition of magnetic remanence of iron-nickel alloys under pressures up to 23 GPa at room temperature. Experiments on pure iron using different pressure transmission media reveal a higher remanent magnetization at 21.5 GPa than at initial conditions, which could be attributed to a distorted hexagonal closed packed phase grown during the martensitic transition. Upon both compression and decompression, the remanent magnetization of the body centered cubic phases increase several times over initial conditions while the coercivity of remanence remains mostly invariant with pressure. Similar behavior is observed for the face centered cubic phases, where magnetization rises by a factor of 2-3 during compression to the highest applied pressures. Immediately upon decompression, magnetic remanence increases while magnetic coercivity remains fairly constant at relatively low values (5-20 mT). One exception is for the invar composition Fe64Ni36, where magnetization decreases markedly between 5 and 7 GPa. 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 Fe64Ni36 measured after pressure cycling 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.