2-VARIABLES SCALING OF THE MAGNETIC VISCOSITY IN BA-FERRITE NANO-PARTICLES

Careful measurements of the magnetic viscosity S = dM/dln t of non-oriented nano-particles of Ba-ferrite (317 Angstrom as mean diameter) show a remarkable scaling plot vs. two variables: the usual T ln(t/tau(0)) and the applied magnetic field H. This plot takes into account two distributions, the particle size rho(s) and the switching field nu(H-0) distributions. This last plays a dominant role on the magnetic viscosity of Ba-ferrite particles at low temperature. Taking a Lorentzian distribution for nu(H-0), one derives from the measured magnetic viscosity a particles size distribution rho(s) which is exactly the same as the distribution observed directly by transmission electron microscopy. The agreement, obtained without free fitting parameter, allows to reproduce all typical features of the S(H, T) showing that rho(s) and nu(H-0) are the relevant distributions. This approach allows to avoid erroneous interpretations of S(H, T) at low temperatures. In particular bimodal distributions, sometimes derived from magnetic viscosity experiments might, in some cases, be attributed to a convolution of rho(s) and nu(H-0).