Magnetic and Optical Properties of Uncompensated Calcium Substituted Yttrium Iron Garnet Films

The photoinduced absorption in semiconducting ferrimagnetic uncompensated calcium substituted yttrium iron garnet (Ca:YIG) films has been investigated for the first time. The photoinduced absorption effects show a broad region of induced absorption with two peaks near 2.6 and 1.9 eV and a strong photoinduced transmission (bleaching) in the region near the gap. The pump light intensity dependence of the photoinduced absorption signal indicates that the peak at low energy (1.9 eV) recombines bimolecularly while the peak at high energy (2.6 eV) recombines monomolecularly. The lifetime of the photoinduced carriers has been measured from the frequency dependence of the photoinduced absorption signal. The temperature dependence of the photoinduced absorption peak observed near 1.9 eV has the same temperature dependence as the low-temperature anomalies observed in the magnetizations. A model to explain the low-temperature magnetic anomaly assumes that a non-S state iron ion, Fe ^{4+}, is created in the tetrahedral site that is ferromagnetically coupled to the octahedral Fe^{3+}. The broad nature of the absorption near 1.9 eV is characteristic of a resonant transition of photocarriers and is understood in terms of a polaron hopping model. The heavy doping of Ca produces a band-tail in the density of states. The pumping light removes the electrons from the tail to the conduction band hence lowering the Fermi level. This effect leads to a strong photoinduced transmission observed near the band-edge.