Assessment of Arrays of Permanent Nanomagnets Produced By Bi-Metal Deposits within Porous Silicon

Abstract

This work presents the investigation of the magnetic behavior of bi-metal nanostructures within nanostructured silicon with the aim to exploit the magnetic properties of both metals and gain control of the exchange coupling between the two metals especially with respect to their volume ratio. Furthermore a variation of the structure size and the proximity of the metal deposits modify the exchange coupling and thus the energy product. Finally nanocomposite systems with an energy product as high as possible should be achieved to give rise to on-chip applications using permanent nanomagnets, especially arranged in arrays. Magnetic nanostructures of two different metals are deposited within nanostructured silicon to control the magnetic switching behavior of the silicon/metal nanocomposite. Two different templates, porous silicon and porous silicon nanotubes are utilized to achieve such nanocomposites. The morphology (pore diameter, tube diameter) of the two systems is comparable. In the case of the utilization of porous silicon templates a mesoporous morphology with average pore diameters of 50 nm is used and these oriented and separated pores are filled with two different metals, namely Ni and Co. The two metals are deposited alternatingly by electrodeposition. A further approach is the chemical growth of Co nanoparticles within porous silicon nanotubes (SiNTs) and the additional deposition of a Ni layer on the outer surface of the tubes. The inner diameter of the silicon tubes is around 50 nm and the wall thickness is about 10 nm. Since the silicon wall of the tubes offers a porous structure the Co particles, which are localized near the pore surface on the wall of a given nanotube, can touch the Ni layer. An alternative structure involves the deposition of an additional Si layer, after the growth of Co particles inside the tubes, as a spacer before the Ni deposition. The morphology of the deposited bi-metal Ni/Co structures is figured out by SEM and EDX, magnetic measurements are performed by SQUID magnetometry. In the presented work the dependence of the magnetic properties of a nanostructured silicon/bi-metal nanocomposite on the volume ratio of the metals, on the proximity of the nanostructures and also on the size of the metal deposits is figured out. If the distance between the deposited bi-metal structures is small enough magnetic exchange coupling between them [1] is present which can be explained by the obtained magnetic data. By tuning the bi-metal deposition an optimized energy product [2] is achieved which gives rise to self-assembled nanocomposite systems containing permanent nanomagnets and arrays of them, respectively for on-chip applications. [1] R. Skomski, J.M.D. Coey, Permanent Magnetism, 1st Edition, CRC Press, Taylor and Francis Group, 1999. [2] R. Skomski, P. Manchanda, P.Kumar, B. Balamurugan, A. Kashyap, D.J. Sellmyer, IEEE Trans. Mag. 49, 3215, (2013).