High Temperature and FORC Investigations of Semiconducting/Ferromagnetic Nanocomposites

Abstract

In the frame of this work the temperature-dependency of the magnetization of various composite systems consisting of porous silicon (PSi) and silicon nanotubes (SiNTs) with embedded Ni, Co, NiCo, FePt, and Fe3O4 structures is discussed. The temperature is varied between 80 and 1273 K which allows to determine the Curie Temperature of the composites which is higher than for the corresponding bulk materials. High temperatures result in an alteration of the chemical sample-composition. Depending on the temperature specific metal-silicide formation occurs usually beginning around 600 K. This leads to modified magnetic behavior of the composites which can be observed by repeated measurements. Magnetic cross-talk between metal deposits can be controlled by the morphology of the porous silicon (distance between the pores) or silicon nanotubes (wall thickness) and also by the distribution and size of the deposits within the pores. To get a clear knowledge about the magnetic interactions single hysteresis curves are not sufficient and thus first order reversal curves (FORC) are performed.The employed PSi is produced by anodization of a highly n-doped silicon wafer. The structures offer an average pore-diameter of 50 nm and a mean distance between the pores of 50 nm which ensures a clear separation of the pores, a crucial point to achieve the appropriate magnetic properties. The SiNTs are produced by ZnO seed deposition on a silicon wafer, growing of the ZnO nanowire arrays (NWA), followed by Si deposition and finally etching off the ZnO. Ni and Co are deposited electrochemically in using for Ni formation a Watts electrolyte, consisting of NiCl2, NiSO4 and H3BO3 and for Co deposition a CoSO4 solution is utilized. A further approach is to grow Co particles inside the pores/tubes in using a CoCl2 6H2O and NaBH4 solution. Fe3O4 are synthesized by high temperature decomposition in the presence of an organic precursor (1). The synthesized particles are infiltrated into the pores/tubes with the help of a magnet beneath the samples. FePt nanocrystals are formed within the SiNTs and PSi by a multistep process, using a solution containing Citric acid, H2PtCl6 6H2O, and Fe(NO3)3 9H2O.Considering Ni loaded porous silicon samples, the magnetic response is recorded in dependence on the temperature to figure out the Curie temperature (TC) of the composite system which is determined from hysteresis curves measured at various temperatures between 80 and 950 K in field steps of 50 K. TC is determined at 700 K, which is about 70 K higher than for bulk Ni. Co loaded porous silicon has been measured up to 1200 K which was still too low to determine TC. TC of bulk magnetite is at 858 K. The investigated porous silicon sample with 8 nm Fe3O4 NPs infiltrated into the pores shows TC around 900 K, 42 K higher than for the bulk material. In the case of FePt loading, the system with PSi as template shows a lower TC compared to SiNTs. The investigations show that the TC of the investigated nanostructured composite systems is always higher than for the according bulk materials. FORC diagrams are used to detremine magnetostatic interactions between the metal deposits. Magnetic coupling is present for Ni, Co and NiCo deposits, wheras in the case of FePt NP loading no magnetic cross-talk between the particles is observed.(1) L. Gutierrez, R. Costo, C. Grüttner, F. Westphal, N. Gehrke, D. Heinke, A. Fornata, Q.A. Pankhurst, C, Johansson, S. Veintemillas-Verdaguer, M.P. Morales,Dalton Transactions, 44, 2943, (2015).