Magnetic vortex state of natural lunar γ-Fe.

Abstract

On the lunar surface α-Fe and Fe-Ni alloys are widespread and the dominant carriers of the lunar remanent magnetization. Metallic γ-Fe, however, is stable only at high temperatures and has not been documented in lunar samples. Here we report the identification of ambient stable γ-Fe preserved as nanometer-scale particles in Chang'e-6 impact glass fragments from the South Pole-Aitken Basin. Electron holography analyses reveal that these γ-Fe particles have a magnetic vortex domain state. Based on these results, we propose that the formation and preservation of γ-Fe at lunar surface temperature requires incorporation of trace γ-phase-stabilizing elements together with extremely rapid quenching during lunar impact processes. Our findings provide a basis for understanding the lunar core dynamo evolution and impact-induced magnetic recording mechanisms.