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

Abstract

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 Ni3+ 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.

Visible Light Absorption and Hot Carrier Trapping in Anatase TiO2: The Role of Surface Oxygen Vacancies

“…This localized nuclear distortion increases the XUV transition energy as a result of reduced screening of the metal center’s core-hole excited state by the surrounding O ligands. This effect has been consistently observed for small polarons in α-Fe 2 O 3 , , α-FeOOH, β-FeOOH, CuFeO 2 , CoFe 2 O 4 , NiFe 2 O 4 , and NiO . Although n-type doping introduces only electrons compared to photoexcitation, which generates both electrons and holes, here we probe the Ti M 2,3 -edge, which selectively samples the Ti 3 d conduction band.…”

“…53−56 One example of final state effects has recently been observed in XUV spectroscopy of metal oxides, which shows that small polarons are systematically characterized by a blue shift of the XUV spectrum. 57 This is 63 and NiO. 64 Although n-type doping introduces only electrons compared to photoexcitation, which generates both electrons and holes, here we probe the Ti M 2,3edge, which selectively samples the Ti 3d conduction band.…”

Extreme Ultraviolet Reflection–Absorption Spectroscopy: Probing Dynamics at Surfaces from a Molecular Perspective

“…In a related study on nickel ferrite, we similarly observed optical spin state switching with element specific resolution. 28…”

“…22 XUV-RA measurements of ferrimagnetic spinel metal oxides also demonstrate the sensitivity of this method to spin state, oxidation state, and coordination geometry. 4,28 It should be noted that the advantages of XUV spectroscopy described in this section are general phenomena of core-level spectroscopy ranging from XUV spectral domain to soft and hard X-ray regime. However, the ability to measure core-hole spectra using a tabletop instrument with ultrafast time resolution combined with the surface specificity of XUV light make XUV-RA spectroscopy uniquely suited to provide a detailed understanding of dynamics at surfaces.…”

Femtosecond extreme ultraviolet spectroscopy of an iridium photocatalyst reveals oxidation state and ligand field specific dynamics

“…Table-top extreme ultraviolet (XUV) spectroscopy in the 40-100 eV energy range has been shown to be a powerful technique for probing the ultrafast photophysics of 1 st row transition metal coordination complexes. [1][2][3][4][5][6][7][8][9][10] The M 2,3 -edge, corresponding to 3p → 3d transitions, is sensitive to oxidation state, ligand field, and spin-state, providing direct insight into key changes at the metal center. 9 This technique has been used to examine excited state dynamics at various 1 st row transition metal centers, such as nickel, 2,4,6,11 iron, 2,7,8,[12][13][14] & cobalt, 4,5,15,16 for photocatalysts and other molecular systems.…”