Measurements of magnetization switching in individual nickel nanowires

Studies of the magnetization switching of individual ferromagnetic cylinders at low temperatures (0.1-6 K) were performed using a niobium microbridge-dc superconducting quantum interference device. Cylinders of Ni with diameters ranging from 40 to 100 nm and lengths up to 5 mu m were studied. For diameter values under 50 nm, the switching probability as a function of time can be described by a single exponential function. The mean waiting time tau followed an Arrhenius law as originally proposed by Neel. Temperature and field sweeping rate dependence of the mean switching held could be described by the model of Kurkijarvi which is based on the assumption of Neel of thermally assisted magnetization reversal over a simple potential barrier. Small deviations from this model are evidenced below 1 K. Our measurements allow us to estimate an ''activation volume,'' two orders of magnitude smaller than the wire volume. This confirms the idea of the reversal of the magnetization caused by a nucleation of a reversed fraction of the cylinder, rapidly propagating along the whole sample. A pinning of the propagation of the magnetization reversal occurs for a few samples, where several jumps are observed in the hysteresis curves. The dynamic reversal properties of depinning were quite different from those of nucleation of a domain wall. For example, the probability of depinning as a function of time does not follow a single exponential law. Similar deviations are found for aged samples, revealing the influence of surface oxidation. These deviations from a simple model of thermally assisted magnetization reversal are particularly important when discussing quantum effects in the magnetization reversal at very low temperatures.