Two-Mode Magnetic Switching of a Ferromagnetic Nanowire-Granule-Array Embedded in a Porous Silicon Matrix
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Two-Mode Magnetic Switching of a Ferromagnetic Nanowire-Granule-Array Embedded in a Porous Silicon Matrix K. Rumpf 1, P. Granitzer 1, H. Krenn 1 1 Institute of Physics, Karl-Franzens-University Graz, Universitaetsplatz 5, A-8010 Graz, Austria ABSTRACT A low-cost method has been developed to produce metallic, perpendicularly oriented ferromagnetic nanowires embedded in crystalline silicon. The mesoporous silicon structure consists of channels with a diameter of about 60 nm and about 30 µm in depth. The electrochemically prepared ferromagnetic nanosystem is a composition of granules and wires which leads to a peculiar magnetic behaviour. The interesting magnetic properties of this bimodal system are investigated by SQUID-magnetometry. The hysteresis loop shows two switching fields at different magnetic fields, one in the low field range and the other one in the high field range. The first switching field at about 500 Oe is due to the magnetization of the granules, the second switching field nearby 5 T (at room temperature) is caused by dipolar coupling of the nanowires which become single domain at high fields due to Bloch Wall motions. This promising ferromagnetic nanocomposite system is not only interesting in basic research but gives raise to a lot of silicon based applications like spin-injection devices and magnetic field sensors. INTRODUCTION In nanoscience and nanotechnology there is an increasing interest in the research of ferromagnetic nanowires due to their applications in high-density magnetic storage devices, spintronics and magnetic sensors. One method to produce one dimensional metallic nanowires with high aspect ratio is the filling of hexagonally ordered porous-alumina templates [1]. This nanocomposite arrangement was under great investigation in recent years [2-4]. Because of the suitable use of silicon in applications an inexpensive technique to fabricate self-organized ferromagnetic filaments perpendicular to the surface in a silicon based matrix [5] was developed. Because of the high aspect ratio (1:1000) these wires show a peculiar magnetic behaviour which is investigated in detail. The investigations have been carried out on Ni-nanowires embedded in a porous silicon template. EXPERIMENTAL DETAILS Ni-nanowires incorporated into porous silicon are prepared by a two step electrochemical process arranged in an electrolytic cell. In the first step a mesoporous silicon skeleton is formed during an anodization process in a 10% HF-solution. Using convenient etching parameters (current density, etching time, temperature of the electrolyte) as well as a proper doping density of the wafer, oriented pores with a diameter between 10 nm and 60 nm and a pore-length up to 30 µm can be achieved. Afterward this mesoporous structure is loaded with a ferromagnetic
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material, like Ni, during an electroplating procedure. The electrodeposition is carried out in a NiCl2-solution with pulsed current. Magnetic measurements are performed with a Quantum Design MPMS XL SQUIDmagnetometer in a broad magnetic
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