Probing magnetism of individual nano-structures using Nb μ-SQUIDS in hysteresis free mode

The magnetization reversal of an individual magnetic nano-structure has been best studied using a mu-SQUID. An optimum inductive shunt helps in eliminating the hysteresis in p-SQUID's current voltage characteristics, giving thus a direct voltage readout of magnetic flux with high bandwidth and sensitivity. Here we report on a new mu-SQUID magnetometry setup for studying the magnetism of individual magnetic nano-structures. The switching field anisotropy of Permalloy nano-needles was studied using both hysteretic and non-hysteretic mu-SQUID modes. The observed anisotropy fits well the magnetization reversal through curling mode under the Neel-Brown model as well as micromagnetic simulations. The latter show a reversal through vortex nucleation and annihilation whose signatures we observe through minor hysteresis loops in experiments. Some preliminary magnetization reversal studies on Fe3O4 nano-particles using a non-hysteretic mu-SQUID mode are also briefly discussed.