Advanced MOKE investigations

The remagnetisation processes of lateral magnetic micro- and nanostructures are of fundamental as well as technological importance. Without research in this subject the innovation and development of applications such as magnetoresistive sensors, magneto-electronics or high-density non-volatile random access memory are not possible. There are two approaches to investigate the remagnetisation processes of artificially produced magnetic micro- and nanostructures. First, one can use lateral resolving microscopic methods to image domains in single magnetic elements, e. g., Kerr microscopy or magnetic force microscopy, and second, integrating methods such as magneto-optical Kerreffect (MOKE) or superconducting quantum interference device magnetometry (SQUID) can be used to record hysteresis loops in order to gain information about the remagnetisation process. The subject of this thesis is to explore the potential of the magneto-optical techniques vector and Bragg MOKE in combination with micromagnetic simulations. Therefore, the remagnetisation process was investigated and analysed for different lateral magnetic, mainly Fe microsized structures: rectangular islands of different widths, triangular rings of different size and shape, rectangular islands arranged on kagomé lattices or on square lattices, spiral structures in different arrangements, elliptic rings, and rectangular islands structures with out-of-plane magnetisation. The samples were mainly prepared using electron beam lithography in combination with sputtering as standard thin film preparation technique. The main tool in this thesis was the magneto-optical Kerr effect (MOKE) in the longitudinal configuration. The MOKE technique can be used to obtain vectorial information of the magnetisation process (vector MOKE). The lateral microsized magnetic structures are arranged periodically and work as diffraction gratings: illuminating the structure with the laser beam leads to an interference pattern. The Kerr signals measured at the diffraction spots gain information about the domain configuration in the elements (Bragg MOKE). At the higher order diffraction spots the Kerr signal obtains information of the magnetic form factor. The nth order diffraction spot is particularly sensitive to the nth order Fourier component of the magnetisation distribution. Vector and Bragg MOKE allow a deeper insight in the remagnetisation process of micro- and nanostructured ferromagnetic elements than the measurement of a standard hysteresis loop in the usually used MOKE technique. For the interpretation of the measured data micromagnetic simulations were performed. The micromagnetic simulation yields magnetisation profiles of a lateral structure for different field values and, thus, information on the domain configuration. With the magnetisation profiles it is possible to calculate the magnetic form factor and to reconstruct the measured Bragg MOKE hysteresis loops. Therefore, the results of the micromagnetic simulation reflect the remagnetisation process of the lateral structure. Depending on the size and shape of lateral magnetic micro- and nanostructures different kinds of remagnetisation processes take place. In most cases the remagnetisation process is dominated by domain formation where the domain configuration is influenced by the structure under investigation. Rotation processes of the magnetisation play only an underpart. The results in this thesis show that it is possible to tailor hysteresis loops by the right choice of shape, size and distance between the single elements of the structures.