NiFe2O4 (NFO) 1, is an insulating ferrimagnet with an inverse spinel structure and already has gained a considerable attention of scientific community due to its potential applications in spintronic devices. For NFO thin films, a coherent strain can be introduced by growing them on different substrates: ZnGa2O4 (ZGO) and MgAl2O4 (MAO) with lattice mismatch of 0.06% and 3.1% respectively. This strain greatly affects physical properties such as magnetic damping, where damping for the NFO film grown on ZGO is considerably smaller than the damping of the NFO film grown on MAO film 2. Magnetic damping should also affect ultrafast processes such as the demagnetization and remagnetization time of the system. Koopmans et al. 3 showed that the demagnetization time is inversely proportional to the effective Gilbert damping.

In this work we explore the underlying mechanisms ultrafast demagnetization processes of strained NFO films by using table-top high harmonic generation (HHG) in the extreme ultraviolet (XUV) regime 4, which enables element-specific tracking of spin dynamics. To understand the HHG asymmetry spectra, we compare the discrete HHG spectra to the reflectivity spectra acquired using a continuous synchrotron-based source. The comparison shows good agreement, thereby facilitating assignment of HHG asymmetry features. Time-resolved HHG scans indicate that the demagnetization amplitude is larger for NFO on the MAO substrate while NFO films on both substrates exhibit the similar demagnetization and remagnetization times. In contrast to damping of spin motion at longer timescales, which is highly sensitive to issues such as defect density, the similarity in the magnetization dynamics in the first few ps following a demagnetization pulse reveals the local nature of the ultrafast dynamics. The results display the utility of M-edge spectroscopy for studying spin dynamics in insulating oxide films, which is highly relevant for spintronics based on ferrimagnetic insulators.

References
1 R. Knut et al., J. Phys.: Condens. Matter 33, 225801(2021) 2 A. V. Singh et al., Advanced Materials 29, 1701222 (2017). 3 B. Koopmans et al., Physical Review Letters 85, 844 (2000). 4 S. Jana et al., Phys. Rev. Research 2, 013180 (2020).