Using a time-resolved magneto-optical Kerr effect microscope, we observed ultrafast demagnetization of inverse-spinel-type ferrimagnet NiCo2O4 (NCO) epitaxial thin films with perpendicular magnetic anisotropy. This microscope uses a pump-probe method, where the sample is pumped at 1030 nm, and magnetic domain images are acquired via magneto-optical Kerr effect microscopy at 515 nm (the second harmonic). We observed the dynamics of the magnetic domain of the NCO thin film via laser irradiation and obtained a demagnetization time constant of approximately 0.4 ps. This time constant was significantly smaller than the large time constants reported for other half-metallic oxides. This timescale of ∼0.4 ps agrees with the spin polarization of ∼0.7 determined by tunnel magnetoresistance [Shen et al., Appl. Phys. Lett. 117, 042408 (2020)].

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All-optical magnetization control has recently been garnering considerable attention to realize next-generation ultrafast magnetic information devices. Here, employing a magneto-optical Kerr effect (MOKE) microscope, we observed the laser-induced magnetization switching of ferrimagnetic oxide NiCo2O4 (NCO) epitaxial thin films with perpendicular magnetic anisotropy, where the sample was pumped by 1030 nm laser pulses, and magnetic domain images were acquired via the MOKE microscope with a white light emitting diode. Laser pulses irradiated the NCO thin film at various temperatures from 300 to 400 K while altering the parameters of pulse interval, fluency, and the number of pulses in the absence of the external magnetic field. We observed accumulative all-optical helicity-dependent switching above 380 K. Our observation of oxide NCO thin films facilitates the realization of magnetization switching using ultrafast lasers without applying a magnetic field.

The relationship between skewness and kurtosis of surface elevation and slope of irregular waves was investigated using laboratory and field observations. The squared values of skewness were found to be proportional to kurtosis as estimated by using the Stokes wave model. The joint probability density functions of surface elevation and slopes were evaluated by applying the principle of maximum entropy. The effects of reflected waves on the joint distribution of the surface elevation and slopes were also investigated. The theoretical conditional distributions of p(h, h x = 0) based on a simple nonlinear wave model reproduced the dependence on the representative wave slope and were in good agreement with the laboratory observations.

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