PUB - Publikationen an der Universität Bielefeld

The magnetooptic Kerr effect (MOKE) is a well known and long used tool for ferro-, ferri- and antiferromagnetic material characterization. MOKE is also utilized in optical communication e.g. in optical isolators and circulators or in planar waveguide structures. Many of the MOKE techniques rely solely on linear MOKE (LinMOKE), i.e. magnetooptic (MO) effects linear in magnetization M and neglect the contribution of higher orders. Nevertheless, a higher-order term being proportional to M2 and called quadratic MOKE (QMOKE) can be strong enough to additionally contribute to the overall MOKE signal. A technique known as 8-directional method separates LinMOKE and two constituent QMOKE contributions and is well known for (001) oriented cubic crystal structures, but is not yet developed for other cubic crystal orientations. The origin of (Q)MOKE can be described as the perturbation of the permittivity tensor by M of a magnetized sample. The permittivity tensor for a cubic crystal is fully described up to the second order in M by linear MO parameter K and two quadratic MO parameters Gs and 2G44. Although those parameters have been studied in the past, the systematic experimental technique that would effectively yield the spectral dependence of quadratic MO parameters is lacking. This work extends the 8-directional method to the crystal orientations (011) and (111). The theoretical equations that are derived through an approximative analytical description relating MOKE with the elements of the permittivity tensor (e.g. K, Gs and 2G44) are compared to numerical simulations using Yeh’s 4×4 matrix formalism and to the experimental measurements. We further present a novel approach to QMOKE spectroscopy of (001) oriented cubic crystal structures that is based on the classical 8-directional method, but using the combination of only four magnetization directions together with a sample rotation of 45◦. By reproducing the measurement procedure numerically the spectral dependencies of the quadratic MO parameters Gs and 2G44 are extracted from QMOKE spectroscopy in addition to the linear MO parameter K that is extracted from LinMOKE spectroscopy. This new procedure is applied to prototypical ferromagnetic samples of Fe(001) thin films with various thicknesses grown on MgO(001) substrates and to the thin film samples of Co2MnSi(001) Heusler compounds with different annealing temperatures promoting different degree of L21 ordering. Gs and 2G44 are experimentally determined in spectral range of 0.8–5.5eV for both materials. In case of the bcc Fe we show that the dependence on Fe layer thickness is small, indicating small contribution of the interface. In case of the Co2MnSi Heusler compounds we found that higher amount of L21 ordering promotes interband contribution to the MOKE spectra.

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Thesis under joint supervision in the Bielefeld University and VˇSB–Technical University of Ostrava