Temperature-dependent magnetic anisotropy in Ni nanowires

Magnetic properties of Ni nanowire arrays, prepared by oblique evaporation of Ni onto V-groove InP substrates, were investigated between 5 and 300 K using magnetoresistance and SQUID magnetization measurements. The results show that as-prepared wires, which range from 70-130 nm in width, have an easy axis of magnetization parallel to the wire axis at room temperature, but transverse to the wire axis at low temperature. The crossover of the easy axis direction from transverse to parallel as a function of temperature is more pronounced for the narrower wires. We interpret our results in terms of a competition between a temperature-dependent magnetic anisotropy (K-perpendicular to), which tends to align the magnetization transverse to the wire axis, and the shape anisotropy of the wires which tends to orient it along the wire axis. Several mechanisms are proposed (e.g., oblique evaporation, stress, and surface oxidation) from which K-perpendicular to could originate. Based upon the stress values deduced from K-perpendicular to, and the thermal expansion mismatch between Ni and InP, the stress mechanism appears to dominate. (C) 1998 American Institute of Physics. [S0021-8979(98)02814-X].