High Density Hexagonal Nickel Nanowire Arrays with 65 and 100 nm-PERIOD
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High Density Hexagonal Nickel Nanowire Arrays with 65 and 100 nm-PERIOD Kornelius Nielsch1, Ralf B. Wehrspohn1, Saskia F. Fischer2, Helmut Kronmüller3, Jochen Barthel1, Jürgen Kirschner1, Thomas Schweinböck4, Dieter Weiss4 and Ulrich Gösele1. 1 Max-Planck-Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany. 2 Department of Electronic Materials, Ruhr-Universität-Bochum, Lehrstuhl für Werkstoffe der Elektrotechnik, 44780 Bochum. 3 Max-Planck- Institute of Metal Research, Heisenbergstr. 1, 70569 Stuttgart. 4 Thomas Schweinböck und Dieter Weiss, Institute of Applied Physics, University of Regensburg, Universitätsstr. 31, 93040 Regensburg, Germany.
ABSTRACT Highly ordered alumina pore channel arrays are used as templates for the fabrication of magnetic nanowire arrays. These well-defined templates are based on the approach by Masuda and Fukuda and have an interpore distance of 65 and 100 nm and a monodisperse pore diameter of ≈30 nm. The pore channels are hexagonally arranged in 2D-domains, which extend over more than ten interpore distances. Nearly 100% metal filling of the alumina pore structures is obtained by a novel pulsed electrodeposition technique. Due to the high ordering degree of the nanowires arrays, we detect a squareness of ≈100% and coercive fields of 1200 Oe in the direction of the nanowires. The MFM measurements have been carried out by applying magnetic fields on magnetized and demagnetized samples to study the switching behavior of individual nanowires inside the arrays. Magnetic wires have been locally switched by a strong MFM tip and a variable external magnetic field. The MFM results show a good agreement with the bulk magnetic hysteresis loops.
INTRODUCTION The possibility to obtain tailored nanostructured magnetic materials has stimulated a world wide research effort towards innovative products in the fields of magnetic storage, bio-medical diagnostics and drug delivery. Patterned perpendicular storage media consisting of magnetic nanowire arrays [1-4] in a magnetically insulating matrix allow high storage densities [5]. More than 700 Gbit/in2 areal density has been predicted for these structures. Furthermore, these nanowire arrays are very well suited for the preparation of ferrofluidic solutions containing monodisperse nanowires, which are very promising for effective cancer treatments and diagnostic methods [6]. One promising technique to obtain highly-ordered magnetic nanowire arrays is based on hexagonally arranged porous-alumina templates [5, 7-9]. Since 1981, there have been numerous studies on ferromagnetic nanowire arrays in disordered alumina templates [5,7,8]. These structures had large size distributions of the pore diameter and interpore distance and the filling degree of the pores was not specified. Based on a recent approach by Masuda [10], ordered alumina pore channel arrays can be obtained with a sharply defined pore diameter (7). Recent micromagnetic finite element simulations of Hertel [16,17] show that for nickel nanowires with diameters ≤ 40 nm the magnetic
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