Magnetic properties of Fe-Ni and Fe-Si alloys under pressure: Confrontation of SQUID magnetometer and Mössbauer spectroscopy measurements

• Applicant: Dr. Qingguo Wei
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2018 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 410243243

Iron together with nickel and light alloying elements such as silicon and sulfur, is believed to be the major constituent in cores of terrestrial planets. There is very little knowledge on magnetic properties of iron or materials in cores of terrestrial planets under high pressure. Such knowledge is important for understanding generation of magnetic field in planetary cores. Hexagonal closed packed (hcp) state is favored for Fe and Fe-Ni alloys in Earth’s inner core. By far, the magnetic state of hcp-Fe remains contentious. By confront superconducting quantum interference device (SQUID) magnetometer data with Mössbauer spectroscopy, we found that iron at 19.2 GPa still possessed a measurable remanent magnetization, whereas the sextets from hyperfine splitting in the Mössbauer spectra had completely disappeared. Understanding the origin behind the discrepancy between the SQUID magnetometer and Mössbauer spectroscopy results is of fundamental importance for anyone interested in magnetism, and especially those interested in high-pressure research. Fe68Ni32 and Fe64Ni36 are ferromagnetic at ambient condition with Curie temperatures of 128°C and 227°C, respectively. Their Curie points shift by about -45 K/GPa and -35 K/GPa, which means above ~3 GPa and ~6 GPa they should become paramagnetic. In our experiments, a measurable ferromagnetic remanent magnetization persists in Fe64Ni36 up to 16.3 GPa and becomes much more magnetic immediately upon decompression. We would like to confirm this on Fe68Ni32 and confront both Fe68Ni32 and Fe64Ni36 against Mössbauer. In our experiments, we also found that SIRM and sextet peak intensities correlated well under pressures. The mechanism for this is unclear. It is important to verify whether such a relationship is repeatable on other samples. Bcc-Fe91Si09 is a good candidate. It is ferromagnetic at ambient conditions. With Si alloyed in Fe, bcc Fe91Si09 is stable up to 50 GPa at room temperature. Importantly, our preliminary work suggests that the SIRM of Fe91Si09 constantly increases with compression up to 20 GPa and further still upon decompression. Here, we propose a further study on magnetization of Fe-Ni alloys around invar range (Fe68Ni32 and Fe64Ni36) and Fe-Si alloy (Fe91Si09) under high pressures: continue confrontation SQUID magnetometer and Mössbasuer spectroscopy in one non-magnetic diamond anvil cell.

• DFG Programme: Research Grants