Ultrafast Optical Spin Switching in Ferrimagnetic Nickel Ferrite (NiFe₂O₄) Studied by XUV Reflection–Absorption Spectroscopy

The ability to optically manipulate spin states has potential to enable ultrafast magnetization switching at rates that are several orders faster than magnetic precession. However, controlling these processes requires understanding of the site-specific charge transfer and spin dynamics during optical excitation and subsequent hot carrier relaxation. Nickel ferrite (NFO) is a ferrimagnetic semiconductor with potential for ultrafast switching. Because of the partial degree of inversion, 12 possible charge transfer excitations exist in NFO. Using extreme ultraviolet reflection–absorption (XUV-RA) spectroscopy to measure the Fe M-edge, Ni M-edge, and O L-edge spectra with femtosecond time resolution reveals that 400 nm light excites an electron transfer from O 2p valence band states to Fe 3d conduction band states. Kinetic analysis shows that this charge transfer state undergoes fast electron and hole polaron formation, where electrons localize to O h Fe centers and holes localize to O h Ni centers. Hole polaron formation increases the crystal field splitting around Ni which drives a spin-state transition leading to a low-spin O h Ni 3+ final state within 0.326 ± 0.14 ps. This study reveals the mechanism of ultrafast optical spin switching in NFO and highlights the capability of XUV spectroscopy to elucidate these dynamics with unprecedented temporal and site-specific resolution.