Linear-response theory applied to the dynamics of submicronic magnetic particles

We first present magnetic aftereffect measurements of an array of submicronic amorphous noninteracting Co particles (300 nm \ifmmode\times\else\texttimes\fi{} 200 nm \ifmmode\times\else\texttimes\fi{}30 nm). At low temperatures, the thermal dependence of the magnetic viscosity S=dM/d(ln(t)) exhibits an unusual behavior. Comparison with measurements performed on a single particle allows the effects of the averaging over the statistical ensemble of the array of 2\ifmmode\times\else\texttimes\fi{}${10}^{7}$ Co particles to be studied experimentally. The link between both experiments is then performed by using the linear-response theory. This formalism allows the noise, D(T), which accounts for the thermal dependence of the coupling of switching spins to their environment, to be defined. The consistency of this description is checked by comparing the noise measured in this way on the array of particle with the noise measured on the single particle through the Arrhenius law. In both cases, the thermal dependence of the noise D(T) has a coth(${\mathit{T}}_{0}$/T) form.