Synthesis and Magnetic Characterization of (Porous Silicon/"Hard-Soft" Magnetic) Nanocomposites

Abstract

The purpose of this work is to combine magnetic nanostructures of two different metals within porous silicon (PS) to influence the magnetic switching behavior of this porous silicon/metal nanocomposite. These “hard/soft” magnetic nanocomposites have been achieved by two different routes. Porous silicon templates in the mesoporous regime have been employed for the electrochemical incorporation of different types of magnetic nanostructures. Two kinds of PS templates have been used, on the one hand double sided porous silicon which has been achieved by etching of an ultrathin wafer in a double tank cell and on the other hand a silicon wafer has been porosified on one side only. The double-sided porous silicon has been filled with Ni on one side and with Co on the other side, also using a double tank cell. In the case of the one-sided sample both metals have been electrochemically deposited alternatingly inside the pores. Magnetic characterization of the samples has been performed by SQUID magnetometry (field range ± 6 T, temperature range 4 – 300 K). Considering the magnetic measurements of the porous silicon containing “hard and soft” magnetic nanostructures, one sees that the magnetization curves show a first term due to the softer magnetic material, Ni, which exhibits a saturation magnetization of 0.6 T (bulk Ni) and a second one due to the harder magnetic material, Co, which exhibits a saturation magnetization of 1.7 T (bulk Co). Thus the hysteresis curves of the investigated nanocomposite systems show two different slopes corresponding to the two different materials. In any case samples offer a clear kink corresponding to the different magnetic switching behavior of Ni and Co, whereas the slope is a little steeper for double sided samples. The gained results indicate that in the case of filling both materials in one porous layer only poor exchange coupling between Co and Ni is present which could be due to the formation of a small amount of NiCo alloy at the interface. The aim is to control the magnetic behavior of the samples by the ratio between the two magnetic materials. The presented two-metal/PS magnetic nanocomposite is fabricated during a low-cost two-step electrochemical process whereas double sided as well as one sides PS acts as template. Because the magnetic properties are determined by the geometry of the deposits and their spatial arrangement the coercivity, remanence and magnetic anisotropy of the samples (both types) can be modified by varying the size and shape of the Ni and Co deposits. Thus the magnetic characteristics of the specimens can be broadly tailored, as desired. Nevertheless the magnetic properties of the resulting nanocomposites are not only correlated with the size, shape and spatial distribution of the deposited metal structures but also strongly depend on the filling ratio between the softer (Ni) and harder (Co) magnetic materials. In fabricating bi-metal nanocomposites which offer specific magnetic characteristics the magnetic coupling between the nanostructures plays an important role, especially exchange coupling between the deposits of two different metals.