MEASUREMENTS OF MAGNETIZATION SWITCHING IN INDIVIDUAL NICKEL NANOWIRES

Studies of the magnetization switching of individual ferromagnetic cyl inders at low temperatures (0.1-6 K) were performed using a niobium mi crobridge-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 probabili ty as a function of time can be described by a single exponential func tion. The mean waiting time tau followed an Arrhenius law as originall y 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 magneti zation reversal over a simple potential barrier. Small deviations from this model are evidenced below 1 K. Our measurements allow us to esti mate an ''activation volume,'' two orders of magnitude smaller than th e wire volume. This confirms the idea of the reversal of the magnetiza tion caused by a nucleation of a reversed fraction of the cylinder, ra pidly 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 pro perties 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 oxidati on. These deviations from a simple model of thermally assisted magneti zation reversal are particularly important when discussing quantum eff ects in the magnetization reversal at very low temperatures.