抄録
Approaches suitable for analyses of superparamagnetism are studied for a well-isolated ferromagnetic nanoparticle assembly. In order to reduce the effects of the particle size distribution, we prepared iron-nitride particles with a narrow size distribution. In the case of static behavior, it is clarified that the conventional approach based on the Langevin function gives spurious information on the distribution of magnetic moments below the temperature that is several times as high as its blocking temperature. The valid approach for analyzing the magnetization curves requires taking into account the anisotropy of each particle in the temperature range. In the case of dynamic behavior, the conventional approach has used the temperature Tmax at which the susceptibility shows a maximum when the temperature is scanned. If the relaxation time in finite fields is estimated using Tmax, we obtain the spurious result that the relaxation slows down with the applied field. The numerical calculations show that this spurious result is affected by the variation of static behavior. Hence, this study proposes an original approach whose results are independent of such effects. The obtained relaxation time in finite fields is quantitatively consistent with the predictions of a classical theory of superparamagnetism.
