New materials for magneto-optical applications.

Abstract

In this doctoral thesis, structural, optical, and magneto-optical properties of Heusler compounds, ferrites, and Perovskites were investigated. The aim of the work was to determine their properties and possible suitability for the applications in magneto-optical devices, such as the non-reciprocal optical insulators, or in spintronic devices. Another goal was to compare their measured properties with ab-initio results to improve the accuracy of the first principle calculations and then analyze the preferred transitions responsible for magneto-optical activity.

The Heusler compound materials studied contained a bulk sample of Co2FeAl0.5Si0.5 prepared by arc melting in protective atmosphere and a thin film of Rh2MnGe and Rh2MnAl prepared using pulsed laser deposition and magnetron sputtering, respectively, on MgO substrate. Next studied were the thin films of magnetite (Fe3O4) prepared by molecular beam epitaxy at different reactive oxygen pressures again on MgO substrate. Co-ferrite layers prepared by dip-coating from a solution of dissolved compounds and subsequent annealing prepared on glass, quartz glass, and silicon were also characterized. The last type of the studied compounds were the Perovskites made by floating zone remelting of sintered powders of source oxides of neodymium, praseodymium, iron, and manganese. These samples were made in stoichiometries of NdMn(1-x)Fe(x)O3 and PrMn(1-x)Fe(x)O3, where x is between 0 and 1. Of the produced volume orthorhombic Perovskites, however, we included the results only for NdFeO3, which so far has been the only one with measurable magneto-optical effect at room temperature.

For all presented samples their optical properties were determined by spectroscopic ellipsometry. Magneto-optical properties were obtained using apparatuses for measuring the spectroscopic Kerr effect in reflection either in a polar or longitudinal configuration. For the first time we also present the results of spectroscopic quadratic Kerr effect on magnetite samples.

A detailed comparison of the measured properties with ab-initio calculations is presented for Heusler's compound Co2FeAl0.5Si0.5. This material showed the highest magneto-optical effect from the ones we have measured and is one of the materials of particular interest in spintronics thanks to its use as a half-metallic ferromagnet in spintronic filters and tunneling magnetic junctions, in which it shows abnormally high spin polarization even at room temperature. However, our results indicate that it is not a true half-metal at room temperature.