Photo-induced variation of optical and dielectric properties of ferrites at room temperature

Photo-induced phenomena have been observed at low temperatures in ferrites doped with a small concentration of certain ions. These effects disappear when the material is heated above some critical temperature Tk. We have observed photo-induced effects which appear and persist for a long time at room temperature in undoped single crystal ferrites (Mn0.91Zn0.09Fe2O4, bulk, and MeFe2O4, films, with Me=Mn, Mg, Ni, and Co) which are irradiated with a low intensity laser beam in a dc magnetic field. At room temperature a photo-induced variation was found in both the infrared reflectivity (in the range 2%–16%) and the optical transmission spectra (in the range 2%–22%) depending on the spectral wavelength and pumping laser beam polarization (linear or circular). This variation exceeds the low-temperature photo-induced change in transmission for semiconductors by 1–3 orders of magnitude. At room temperature we observed a photo-induced shift toward lower wavelengths of both the peak of the infrared reflectivity band (ω2, by 8 cm−1) and optical transmission bands (in the range 2–217 nm). The ω2 band shift and the reflectivity change are mainly reversible for right- and left-handed polarization. The refractive indices n, the absorption factors k, the dielectric constants ε, and the ionic charges were determined from the experimental infrared spectra (250–800 cm−1) of bulk ferrite by the technique of ‘‘successive analysis’’ (dispersion analysis followed by Kramers–Krönig analysis), both before and after the irradiation. The photo-induced changes in the optical and dielectric properties of ferrites are connected with the photo-induced change in the magnetization, which was directly measured with a torque magnetometer. The observed effects might find an application in optical recording.