Strong Anisotropy Of Magneto-optical Effect In Quasi-three-dimensional Magnetoplasmonic Crystals

Recently, arrays of magnetic particles have been studied intensely, owing to fundamental physics and the potential applications in high density recording media. By controlling the size and the shape of the individual particle, the interactions among the particles, it is possible to manipulate the magnetic properties, for instance, tuning the magnetic anisotropy and magnetic domain configurations. While the properties of magnetic particles are studied intensely, the influence of the array ordering has not yet been explored. In this paper, we fabricate highly-ordered hexagonal magnetic arrays of cobalt half shell on polystyrene micro spheres, and study the magneto-optic signal of the array under different applied magnetic field directions. By controlling the ordering of the array, we discuss the magneto-optic effect in short-range ordered and disordered samples. The main results of this work are as follows:1. By combining capillary force induced self-organization and wedge-shaped capillary channels, we developed a novel method to organize monodispersed colloidal spheres into single domain monolayer colloidal crystals. Based on the facility of two-dimensional colloidal crystal, we easily prepared highly-ordered hexagonal magneoplasmonic arrays in large singlc domain by depositing cobalt using magnetron sputtering method. The high quality of the samples is demonstrated by SEM, laser diffraction beam as well as optical photo. 2. We study Magneto-optic Kerr effect (MOKE) in our quasi-three-dimensional megnetoplasmonic crystal samples. It is found that this system shows significantly MOKE anisotropy, i.e., the arrays of high-ordering show different Kerr loops under different magnetic field directions. This phenomenon has not been reported before in similar systems. While the hysteresis loop from the vibrating sample magnetometer (VSM) measurements, however, did not show any variations upon changing the relative orientation between the magnetic field and the array. It is widely believed that a Kerr rotation is totally derived from magnetization-induced polarization rotation of a light beam. But in our quasi-three-dimensional surface megnetoplasmonic system, the magnetization does not correspond with the Kerr signal.3. It is further observed that such a MOKE anisotropy has a strong dependence on the ordering of the magnetic arrays. The anisotropy disappears for a randomly corrugated Co layer when randomly distributed colloidal spheres are used as the substrate. Even in the sample with short-range ordering, no MOKE anisotropy is detected (the light spot is much larger than the size of short-ranged single domain area). This indicates the ordering of the whole array can have a significant influence on the magneto-optic property. Considering the complicate topography of the quasi-3D magnetoplasmonic array on the micro spheres, more studies are needed to give deep understanding on this phenomenon.