Study on magnetic reversals an d magnetooptical properties in single- and bilayer two-dimensional micropatterned magnetic anti-dot lattices

Abstract

The current advances in various lithographical methods for fabricating patterned arrays of magnetic anti-dot lattices (MALs) at both micrometer and nanometer scale have received greater attention due to their potential for practical applications. To be more specific, the main motivation for these studies is that the realization of magnetism at such length scales will enable a better performance of the advanced devices such as spintronic devices, high-density magnetic storage, etc. This imposes new challenges for the experimental analysis of their micromagnetic properties. Hence, considering the potential applications and the new challenges led down, I tried to study the basic aspects of such MAL systems. In particular, I tried to study the effect of lattice dimension (anti-dot periodicity) and lattice symmetry (anti-dot arrangement) on the magnetic properties, i.e., magnetic anisotropy, enhancement in switching field and magnetic-domain configuration of single-layer cobalt (Co) MAL systems. Further, I modified the Co MAL system by introducing an additional thin nickel (Ni) layer beneath and studied the changes in magnetic anisotropy, reversal, etc. and even compared them with the single-layer Co MALs to get in-depth understanding. In this study, I have successfully fabricated different sets of MAL samples, viz., single-layer as well as bi-layer of Co and Ni (as basic ferromagnetic materials) by using photolithography and controlled wet-etching processes. Hence in this dissertation, the obtained important results have been discussed in three parts as follows. In single-layer MALs: Co anti-dot square-lattice arrays of different periodicities were fabricated and the magnetic properties were studied using the magneto-optical Kerr effect and magnetic-force microscopy (MFM). Such samples showed uniaxial two-fold anisotropy with the hard and the easy axes along the nearest-neighbor rows/columns and the diagonal, making an angle of 450 with respect to the nearest-neighbor row/column, respectively. The coercive field was strongly influenced by change in the lattice periodicity. The remanent state revealed well-defined domain structures, which varied periodically with the lattice geometry. To elucidate the domain configuration, I also performed the micromagnetic simulations. Another set of single layer MALs: Co anti-dot arrays with different lattice symmetries, square and rhomboid structures, were fabricated and their magnetic reversal properties were studied. Different lattice symmetries induced the corresponding anisotropies with changing easy and hard axes. The nearest-neighbor rule is not applicable in case of the rhomboid anti-dot lattice, while the inclusion theory is. It should be noted that these results were found to be different from those reported by others. The MFM images in the remanent state showed well-defined domain networks, which are periodic in nature according to the lattice geometry. The formation of such a domain configuration was a direct consequence that the lattice symmetry guides the domains according to the anti-dot arrangements. In bi-layer MALs: In this work, I present the results of study on the magnetization-reversal properties in a bi-layered magnetic anti-dot lattice (BMAL) system consisting of upper perforated thick Co layer of 40 nm and lower continuous thin Ni layer of 5 nm, probed by using a superconducting-quantum-interference-device magnetometer and by MFM. A systematic study on the in-plane anisotropy, and the switching-field properties was carried out. The anisotropy in this case was found to be a combinatorial anisotropy having both uni-directionality and uniaxial characteristics. To get the comprehensive knowledge about the domain configuration, we performed the MFM imaging, which was compared with the micromagnetic simulations. The MFM images revealed well-defined periodic-domain structures which can be ascribed to the anisotropies such as magnetic uniaxial anisotropy, configurational anisotropy, etc. I observed that the magnetization reversal of such a BMAL system proceeds through the formation and the annihilation of domains, with a collective switching of them according to the field history and the applied magnetic-field direction. The observed changes in the magnetic properties were closely related to the patterning, which hinders the domain-wall motion, as well as to the magneto-anisotropic BMAL structure.