Beyond spin-magnetism of magnetic nanowires in porous silicon

A ferromagnet-semiconductor (Si-Ni-) nanowire composite, electrochemically prepared in an n-doped (001) silicon wafer, is studied using SQUID magnetometry at magnetic fields up to 7 T parallel and perpendicular to the Ni-wires (diameter 50 nm, length 1 mu m, 4 x 10(8) wires mm(-2)). Apart from the conventional spin-magnetism of Ni which is saturated at low field, an additional giant paramagnetism is observed at fields > 1 T, which is nearly temperature independent, shows strict linear field dependence, strong anisotropy and a lack of saturation. Taking the difference of the orthogonal magnetizations this unconventional paramagnetism becomes obvious. It is based on mesoscopic currents driven by the symmetry breaking at the wire-silicon interface due to the Rashba field (spin-galvanic effect). Spin-polarized carriers from the Ni-wires are captured in low angular momentum quantum states (l = 1, 2,...). An attempt has been made to estimate the observed giant magnetic moment under simplifying assumptions.