LINEAR-RESPONSE THEORY APPLIED TO THE DYNAMICS OF SUBMICRONIC MAGNETIC PARTICLES

We first present magnetic aftereffect measurements of an array of subm icronic amorphous noninteracting Co particles (300 nm x 200 nm x 30 nm ). At low temperatures, the thermal dependence of the magnetic viscosi ty S = dM/d(ln(t)) exhibits an unusual behavior. Comparison with measu rements performed on a single particle allows the effects of the avera ging over the statistical ensemble of the array of 2 x 10(7) Co partic les to be studied experimentally. The link between both experiments is then performed by using the linear-response theory. This formalism al lows 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 meas ured in this way on the array of particle with the noise measured on t he single particle through the Arrhenius law. In both cases, the therm al dependence of the noise D(T) has a coth(T-0/T) form.