Magnetization reversal and magnetic anisotropy of Fe, Ni and Co nanowires in nanoporous alumina membranes
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Magnetization reversal and magnetic anisotropy of Fe, Ni and Co nanowires in nanoporous alumina membranes M. Kröll1, L. J. de Jongh2, F. Luis2, P. Paulus2, G. Schmid3 1 Physics Department, Trinity College Dublin, Dublin 2, Ireland 2 Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9506, 2300 RA Leiden, The Netherlands 3 Institut für Anorganische Chemie, Universität GH Essen, Universitätsstr. 5-7, 45117 Essen, Germany ABSTRACT The magnetization reversal and magnetic anisotropy of Fe, Ni and Co nanowires is studied at low temperatures. All nanowires show a strong shape anisotropy with the easy axis being parallel to the long axis of the wires. Co nanowires additionally show a temperature dependent magnetocrystalline anisotropy along the hexagonal c-axis, which is directed nearly perpendicular to the long axis of the wires, as is confirmed by X-Ray diffraction measurements [1] and reported by Strijkers et al. who performed NMR measurements on samples prepared in a similar way [2]. Therefore, at low temperatures and for large wire diameters a competition between magnetocrystalline and shape anisotropies can be observed. Co wires with a small diameter, however, do not show a significant magnetocrystalline anisotropy. Fcc-Co, which is only known as a high-temperature Co modification and which does not have a large magnetocrystalline anisotropy constant, becomes the predominant Co modification here [1,3]. Investigations on the size dependence of the switching field for Fe and Ni nanowires provide information about the magnetization reversal process, which takes place via a nucleation of small magnetic domains probably at the end of the wires, and subsequent propagation of the domain wall along the wire.
INTRODUCTION Nanostructured materials are of considerable interest because of many new possible applications in different fields such as information technology, biotechnology, medicine or environmental engineering. Scientifically they are interesting because the physical properties of a material can change significantly if its lateral dimensions are reduced down to the nanometer scale. These changes can occur if the size of at least one dimension is reduced down to a regime comparable, for example to that of the DeBroglie wavelengths of electrons, the excitation wavelengths of phonons or magnons, or the domain wall width DW as regards ferromagnetic materials. A cylindrical particle with a diameter smaller or comparable to DW and a length that is much larger than DW can be considered to be one-dimensional. One-dimensional nanostructures can be built by performing chemical or electrochemical reactions in the pores of a suitable host or matrix material. As matrix material polycarbonate membranes [4], zeolites [5] or alumina membranes [6] may be used. We chose nanoporous alumina membranes as the template material because they are easily accessible by anodic oxidation of high purity aluminum in polyprotic acids, they are chemically and physically stable and their properties (pore diameter, thickness) can be easily varie
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