  Using a time-resolved magneto-optical Kerr effect (TR-MOKE) microscope, we
observed ultrafast demagnetization of inverse-spinel-type NiCo2O4 (NCO)
epitaxial thin films of the inverse spinel type ferrimagnet NCO 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 MOKE microscopy at 515 nm (the second harmonic). We successfully 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. These results, combined with the results of our
x-ray photoemission spectroscopy study, indicate that this NCO thin film is a
ferrimagnetic metal whose electronic structure deviates from the theoretically
predicted half-metallic one.
