Assessment of Novel Magnetic/Semiconducting Composites - FePt Nanoparticles Grown within Porous Silicon and Silicon Nanotubes

Abstract

Porous silicon (PSi) and silicon nanotubes (SiNTs) are presented as a platform for filling with FePt nanostructures offering a hard magnetic behavior and resulting in novel magnetic/semiconducting composite systems. The difference of the magnetic characteristics between the two systems is figured out. Furthermore the FePt filled templates are compared with the same template materials filled with Ni/Co by electrodeposition.The porous silicon is produced by anodization of a highly doped n-type silicon wafer in a 10 wt% HF solution [1]. The morphology of the porous structure is achieved by the doping density of the silicon wafer and the choice of the electrochemical parameters such as electrolyte concentration, applied current density and temperature. In this case the morphology offers separated pores of about 60 nm and a mean distance between the pores of 50 nm. The SiNTs are fabricated in using an array of ZO wires as template, subsequent silicon deposition and finally etching off the ZO. The inner diameter of the tubes and also the wall thickness can be tuned by the fabrication process. In this work SiNTs with comparable inner diameter to the PSi structure and a wall of about 10 nm are used [2]. FePt nanoparticles (NPs) are deposited electroless inside the pores and the tubes, respectively whereat the molar ratio of Fe is varied (Pt:Fe 1:1, 1:3 and 1:6). For this purpose a 3 component solution consisting of H2PtCl6, Fe(NO3)3 and citric acid is used, whereas the ratio of the components is modified.Ni and Co, both metals are electrodeposited within the nanostructured silicon. In the case of depositing Ni and Co from separate electrolytes, aqueous NiSO4 and CoSO4 solutions are used. The composition of the Ni-salt solution is 170g/l NiSO4 and 40 g/l H3BO3. For the Co deposition the electrolyte contains 120 g/l CoSO4 and 30 g/l H3BO3. An applied current density of 15 mA/cm2 and a frequency of 0.1 Hz are applied for 15 min. In the case of depositing Ni and Co from a single electrolyte, a solution in the ratio 1:1 of the Ni-salt to Co-salt is employed. Ni is deposited by applying a potential of 1.1 V and Co is deposited by changing to 1.8 V; the necessary deposition time of Co was three times longer than for Ni.Magnetic characterization of the samples is performed by VSM (Vibrating Sample Magnetometer), the structure is analyzed by SEM, TEM and EDX.The varying magnetic response of the different composite systems, porous silicon and silicon nanotubes is investigated. PSi/FePt shows a higher coercivity and remanence than SiNTs/FePt and thus a higher hard magnetic performance. The variation of the coercivities between SiNTs/FePt and PSi/FePt is about 57%.Considering the FePt deposits with different molar ratio of Fe the coercivities vary in a range of 5% in the case of both template types. Comparing FePt loaded samples with Co loaded samples in all cases an increase of the coercivity and of the remanence is observed for FePt, whereat in the case of PSi as template material the increase is significantly stronger than in the case of SiNTs samples. Figure 1 shows the comparison of the hysteresis of PSi and SiNTs filled with FePt NPs.Porous silicon loaded with Ni/Co offers a different magnetic behavior, especially in the case of stacked deposition from two separate electrolytes. The achieved composite shows a two-slope hysteresis, which will be discussed in contrast to the single electrolyte deposition.By enabling an accurate control of the size and of the volume ratio of different metal structures such materials are promising for the fabrication of three-dimensional arrays of permanent nanomagnets.[1] K. Rumpf, P. Granitzer, H. Michor, NRL 11, 398 (2016).[2] K. Rumpf, P. Granitzer, R. Gonzalez-Rodriguez, J. Coffer, M. Reisser, Phys. Stat. Sol. A 217, 1901040 (2020). Figure 1