Irregular to Regular Magnetic Nanowires in Porous Silicon

Porous media consisting of nanometric pores and channels and filled with magnetic material can be efficiently functionalized for magnetosensoric and magnetooptic applications. The large surface-to-volume ratio and the variations in geometry and electronic structure have a dramatic effect on transport, magnetic and optic properties. To date, a compromise between regular and disordered arrays, pore size, robustness and device costs has to be achieved. Porous silicon is known since the 19-eigthies as efficient electroluminescent material. Its preparation is cheap with less process steps and porous silicon is also an appropriate matrix to realize oriented, quasi-regular arrays of mesopores perpendicular to the wafer surface. Reversing the bias voltage in the electrochemical cell from anodization to electroplating, the meso-pores are loaded by ferromagnetic nickel to form magnetic nanorods of diameter 10 - 60 nm and of lenght 10 - 30 m. Their extraordinarily high aspect ratio (> 100) and areal density (pitch 100 nm) provides a novel magnetic nanocomposite with peculiar magnetic properties: perpendicular anisotropy and unconventional magnetic switching due to curling magnetization reversal in the high field regime of magnetic hysteresis. The latter behavior is unique to this hybrid porous silicon /ferromagnet system consisting of densily interconnected wires which are strongly coupled by dipolar interaction. It opens an avenue for possible magnetoelectronic and magnetoopic applications based on silicon technology. Using proper electrochemical conditions (current charging, electrolyte concentration, bath temperature and illumination) arrays of monodisperse Ni wires are formed which will be post-treated by rapid thermal annealing to vary the interface composition between silicon and Ni due to formation of silicides and hence to tune the electronic Schottky barriers. In the course of the project technological procedures will be refined to suppress the inherent bimodal precipitation of Ni garanules and wires in favor of coalescing wires during the electrochemical deposition, to clean and smooth the processed surfaces, to thin and metallize the sampels and to tailor a special waveguide geometry for magnetooptic (Faraday and Kerr effect) spectroscopy. Concomitant are structural investigations by electron-micoscopy and non- destructive chemical profiling by scanning X-ray and Auger spectroscopy. Integral magnetic properties are measured by SQUID magnetometry in a wide temperature (2K - 350K) and magnetic field range (0 .. 7T). FTIR- spectroscopy with polarization analysis will be applied to test the special sample structures for a possible enhancement of the magnetooptic response and for possible spin-injection phenomena. The magnetooptic spectra will be modelled by Bruggeman effective medium theory taking into account the gyromagnetic (off-diagonal) terms of the effective dielectric tensor.

Porous media consisting of nanometric pores and channels and filled with magnetic material can be efficiently functionalized for magnetosensoric and magnetooptic applications. The large surface-to-volume ratio and the variations in geometry and electronic structure have a dramatic effect on transport, magnetic and optic properties. To date, a compromise between regular and disordered arrays, pore size, robustness and device costs has to be achieved. Porous silicon is known since the 19-eigthies as efficient electroluminescent material. Its preparation is cheap with less process steps and porous silicon is also an appropriate matrix to realize oriented, quasi-regular arrays of mesopores perpendicular to the wafer surface. Reversing the bias voltage in the electrochemical cell from anodization to electroplating, the meso-pores are loaded by ferromagnetic nickel to form magnetic nanorods of diameter 10 - 60 nm and of lenght 10 - 30 m. Their extraordinarily high aspect ratio (> 100) and areal density (pitch 100 nm) provides a novel magnetic nanocomposite with peculiar magnetic properties: perpendicular anisotropy and unconventional magnetic switching due to curling magnetization reversal in the high field regime of magnetic hysteresis. The latter behavior is unique to this hybrid porous silicon /ferromagnet system consisting of densily interconnected wires which are strongly coupled by dipolar interaction. It opens an avenue for possible magnetoelectronic and magnetoopic applications based on silicon technology. Using proper electrochemical conditions (current charging, electrolyte concentration, bath temperature and illumination) arrays of monodisperse Ni wires are formed which will be post-treated by rapid thermal annealing to vary the interface composition between silicon and Ni due to formation of silicides and hence to tune the electronic Schottky barriers. In the course of the project technological procedures will be refined to suppress the inherent bimodal precipitation of Ni garanules and wires in favor of coalescing wires during the electrochemical deposition, to clean and smooth the processed surfaces, to thin and metallize the sampels and to tailor a special waveguide geometry for magnetooptic (Faraday and Kerr effect) spectroscopy. Concomitant are structural investigations by electron-micoscopy and non- destructive chemical profiling by scanning X-ray and Auger spectroscopy. Integral magnetic properties are measured by SQUID magnetometry in a wide temperature (2K - 350K) and magnetic field range (0 .. 7T). FTIR- spectroscopy with polarization analysis will be applied to test the special sample structures for a possible enhancement of the magnetooptic response and for possible spin-injection phenomena. The magnetooptic spectra will be modelled by Bruggeman effective medium theory taking into account the gyromagnetic (off-diagonal) terms of the effective dielectric tensor.