Ferromagnetism of iron under pressure to 21.5 GPa

Abstract

Computational, Mössbauer, and synchrotron radiation experiments arrive at disparate conclusions regarding the magnetic state of the high‐pressure, hexagonal closed packed, phase of iron, which likely comprises the bulk composition of Earth's inner core. Using a nonmagnetic, moissanite anvil cell together with a superconducting magnetometer, we measured the remanent magnetization of iron in response to applied magnetic fields under pressure up to 21.5 GPa at room temperature. Two independent experiments 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 phase increases several times over initial conditions while the coercivity of remanence remains mostly invariant with pressure.

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

“…3), as it does for titanomagnetite, pyrrhotite, and pure iron [25,31,32]. A marked difference between the magnetic behavior with high pressure for Fe-Ni metals (ferromagnets) versus iron oxides or iron sulphides (ferrimagnets) is that coercivity decreases or changes little in ferromagnets, yet, with few exceptions, markedly increases in ferrimagnets.…”

“…Because the applied field direction lies along the long axis of the sample, increasing the maximum to minimum axis ratio will have the apparent effect of decreasing Bcr while increasing SIRM [26]. We can correct for the change in demagnetization factor by normalizing the SIRM values for the change in shape (S corr ) via a power function S corr = 8.36 × 10 −4 (h/d) −0.66 [25]. In other words, if h/d is flattened from 0.8 to 0.3, SIRM will increase 1.9 times and Bcr will decrease ß20% in the long axis plane.…”

“…Because the samples' masses are unknown in our experiments, relative values are used for SIRM in order to compare the results. The SIRM data require a shape correction because sample geometry influences magnetization intensity depending on the degree of oblateness and the direction of the applied field relative to the plane of the oblate spheroid [25]. For this reason we measured the horizontal cell dimensions (front and back sides) at each pressure step with a Leica MZ12.5 microscope fitted with a DSC295 digital camera (1 µm resolution) ( Table I).…”

“…(Color online) Normalized SIRM versus Ni concentration during compression to 5 GPa; pure iron data from Wei and Gilder[25].…”

Neutron depolarization imaging of the hydrostatic pressure dependence of inhomogeneous ferromagnets

“…Progress on this question concerns a wide range of scientifically interesting fields like geophysical investigations, fundamental research in correlated electron systems as well as applied research, such as material synthesis and its optimization for tailored applications. For example, investigation of the distribution and development of magnetic properties under pressure in natural minerals like magnetite is of paramount importance in geophysical research to obtain a detailed understanding of the composition and development of the earth's interior 1 . In this context it can be crucial to use methods which allow for nondestructive and in-situ probing of the minerals.…”