Hiroki Wadati

Rare-earth-free ferrimagnetic oxides are emerging as attractive platforms for investigating ultrafast spin dynamics. Here, we study the photoinduced magnetization dynamics of epitaxial NiCo2O4 (NCO) thin films by the time-resolved magneto-optical Faraday effect using two independent pump–probe configurations: 1030/515 and 800/400 nm. In both measurements, photoexcitation induces an immediate reduction of the magneto-optical signal within the experimental time resolution, followed by a reproducible slower demagnetization component with a characteristic timescale of approximately 5–6 ps and a subsequent recovery on the ∼100 ps timescale. Importantly, this picosecond demagnetization component is observed consistently across the two experimental configurations and excitation wavelengths, demonstrating that it is an intrinsic feature of the ultrafast magnetic response of NCO thin films. Because the earliest-time dip may contain a transient optical contribution, we describe the overall response as type-II-like, rather than assigning a definitive textbook type-II classification solely on the basis of the sub-resolution signal. These results establish a robust two-step ultrafast demagnetization behavior in NCO and highlight rare-earth-free oxide ferrimagnets as promising systems for exploring multisublattice spin dynamics on ultrafast timescales.

ABSTRACT X‐ray absorption spectroscopy and X‐ray magnetic circular dichroism have long served as indispensable tools for probing the electronic and magnetic properties of transition‐metal compounds with elemental selectivity. In recent years, the emergence of femtosecond lasers has opened a new avenue for studying nonequilibrium dynamics in condensed matter. However, conventional optical techniques lack elemental and orbital specificity, making it difficult to disentangle the coupled charge, spin, and lattice responses in complex materials. The development of X‐ray free‐electron lasers (XFEL) and laboratory high‐harmonic generation (HHG) sources has enabled the extension of X‐ray absorption and scattering techniques into the femtosecond time domain. Time‐resolved X‐ray absorption spectroscopy, X‐ray magnetic circular dichroism, and resonant soft X‐ray scattering now provide direct, complementary access to element‐ and momentum‐resolved ultrafast dynamics. This review summarizes recent progress in these techniques, focusing on pump‐probe measurements of laser‐induced demagnetization, spin‐state transitions, and valence and structural changes in transition‐metal compounds. We also discuss advances in tabletop HHG‐based X‐ray spectroscopy and its integration with large‐scale XFEL facilities. These developments provide powerful routes for visualizing the nonequilibrium evolution of charge, spin, orbital, and lattice degrees of freedom, offering new insights into the ultrafast control of quantum materials.