Volume 96, Issue 8, 15 October 2004
Index of content:
- MAGNETISM AND SUPERCONDUCTIVITY (PACS 74-76)
96(2004); http://dx.doi.org/10.1063/1.1790060View Description Hide Description
Magnetostatic interactions in different polycrystalline permalloy dot patterns for dots with diameter and thickness are investigated through magnetic force microscopy imaging, magneto-optic Kerr effect measurements, and micromagnetic simulations.Magnetization reversal occurs through vortex nucleation∕annihilation. Vortexnucleation and annihilation fields for different interacting arrays were observed to vary with pattern type and with the applied field direction and were shown to follow the prediction of a simple point-dipole model. The vortexchirality was established by imaging vortices in a small applied field. The vortexchirality distribution was determined for different arrays and a well-defined ordering phenomenon of the vortexchirality was observed in two-dot and zigzag-dot chain patterns as well as in a honeycomb array of dots. Micromagnetic simulations well account for the type of vortexchirality ordering observed and attribute it to magnetostatic interactions that induce correlated -state magnetization configurations among nearest neighbor dots prior to vortexnucleation.
96(2004); http://dx.doi.org/10.1063/1.1790069View Description Hide Description
High-quality bulk exhibits a structure of voids and agglomeration of crystals on different length scales. Because of this, the superconducting currents percolate between the voids in the ensuing structure. Magnetic measurements reveal that the superconducting currents circulate on at least three different length scales, of , , and whole of the sample (∼millimeter). Each of these screenings contributes to the measured irreversible magnetic moment . The analysis of the field dependence of for samples of subsequently decreasing size showed that the critical current obtained using the simple critical state model is erroneous. This leads to the artifact of the sample size-dependent critical current density and irreversibility field. Our data analysis enables the separation of the contribution of each of the screening currents to . The field dependence of each of the currents follows a stretched exponential form. The currents flowing around whole of the sample give a dominant contribution to in the intermediate fields ( at ) and they can be used to obtain the value of from critical state model, which corresponds to the transport . The stretched exponential field dependence of these currents is similar to the one obtained for high-temperature superconductors, and it seems to be connected with the percolation of the currents.
96(2004); http://dx.doi.org/10.1063/1.1786376View Description Hide Description
The electronic structure and magnetic properties of semi-infinite Cu (001) multilayer systems with , Co, or Ni have been studied by means of a first-principle Green’s function technique based on the tight-binding linear muffin-tin orbital method. The results show that at the ( or Co) interfaces there is a net charge transfer from iron and cobalt atoms to the neighboring paramagnetic copper atoms and an enhancement of the ironmagnetic moment while at the interface, the nickelmagnetic moment is reduced compared with its bulk value. This behavior is explained in terms of the relative positions of the Fermi levels of the magnetic material and copper. The conductance is determined in the ballistic limit, in the current-perpendicular-to-the-plane geometry by means of transmission matrix formulation of Kubo-Landauer formalism. It is shown that the oscillations of the giant magnetoresistance ratio are mainly the result of the oscillations of the minority-spin conductance.
Effects of compressive epitaxial strain on the magnetotransport properties of single-phase electron-doped films96(2004); http://dx.doi.org/10.1063/1.1792808View Description Hide Description
Single-phase electron-doped manganite thin films with nominal composition of have been prepared on substrates by pulsed laser deposition. The conditions for obtaining purely single-phase films lie within a very narrow window of substrate temperature and laser energy density during deposition. In situ postdeposition annealing, mainly to relax the possible epitaxial in-plane tensile strain between the film and the substrate, leads to an increasing c-axis lattice constant accompanied by the formation of secondary phase and higher metal-insulator transition temperature. This is indicative of a strong coupling between the electron and lattice degree of freedom.
Magnetic switching depending on as-patterned magnetization state in Pac-man shaped submicron elements96(2004); http://dx.doi.org/10.1063/1.1793358View Description Hide Description
Magnetization configuration and magnetic switching of submiron “Pac-man” shaped elements with conventional and modified (i.e., elongated) geometry have been studied using magnetic force microscopy,magneto-optical Kerr effect, and micromagnetic simulations. Experimental measurements and modeling analysis clearly demonstrate that the sensitive interplay between exchange, demagnetizing, and shape anisotropy energies leads to distinct magnetization configurations and magnetic switching mechanisms, even though the geometric shape of elements is only slightly modified. In a particular case, such as 40% elongated element, vortex-driven magnetic switching is replaced by well-defined coherent reversal.