Magnetic properties of iron-nickel metals and alloys under high pressure with relevance to planetary cores

This dissertation explores the effects of pressure on the magnetic
remanence of iron-nickel and iron-silicon alloys relevant to the
solid inner cores of the terrestrial planets and Earth’s moon. The
Earth’s inner core likely comprises mostly pure iron in a hexagonal
close packed (hcp) structure. Experiments on pure iron powder and
foil were carried out up to 21 GPa at room temperature. The most
important conclusion from this work is that either hcp-iron is
ferromagnetic or that a poorly understood, intermediate hcp phase
of iron is ferromagnetic. It was also determined that the results
must be corrected for magnetic shape anisotropy, which is related
either to the original sample material (foil) or how the bulk
sample volume changes shape due to increasing oblateness of the
chamber during pressurization. Fe-Ni alloys in the face centered
cubic (fcc) phase with compositions around Fe64Ni36, called Invar,
exhibit near-null thermal expansion, making them useful for
technological applications. Models explaining the Invar effect
evoke magnetovolume effect that compensate for thermal expansion.
Previous work suggested that the Curie temperature of Fe64Ni36
decreases 35 K per GPa, which predicts that around 5 GPa, Fe64Ni36
will turn paramagnetic. Our experiments on Fe64Ni36 found a marked
decrease in magnetization between 5-7 GPa, consistent with former
studies, but that it remains ferromagnetic until 16 GPa. 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. Experimental
results on body centered cubic (bcc) Fe-Ni alloys match well with
those for pure iron-- again leading to the conclusion that either
an intermediate hcp phase, or that the hcp phase itself, is
ferromagnetic. The ubiquitous enhancement in magnetization under
pressure, or during pressure release, of the Fe-Ni and Fe-Si alloys
is associated with strain-induced martensitic effects. Finally, a
defocused laser heating technique was developed to measure the
Curie temperature in diamond or moissanite anvil cells. Preliminary
results on titanomagnetite (Fe2.4Ti0.6O4) are broadly consistent
with previous work.