Magnetic relaxation of nanowires: Beyond the Néel-Brown activation process

Magnetic relaxation in Ni nanowires was measured both on single nanowires and on an array of about 10⁶ nanowires. The logarithmic relaxation rate S = dM/dln(t) deduced from after-effect experiments is presented as a function of both the applied field and the temperature. The field dependence S(H) is very similar to the distribution of the switching fields of the nanowires in the array, whereas the field dependence of the relaxation time of one single nanowire is linear in the time window of the experiment. The temperature dependence S(T) of arrays decreases with increasing temperature. The measurement on the macroscopic array is used to infer the thermal behavior of the relaxation time of single nanowires. This temperature dependence is in conflict with the Néel-Brown activation of magnetic monodomains.