Magnetic viscosity in Ba-ferrite

Results are presented on magnetic viscosity measurements performed at room temperature on Ba-ferrite pressed powder samples of different particle sizes. Maximum values of viscosity coefficients ( S ) occur around coercive fields, except for a sample with an intrinsic coercive force of 4639 Oe, in which the maximum is at a much higher field (5706 Oe). Activation volumes are of the order of 10 −17 cm 3 and decrease along with particle size (except, again, for the high coercivity sample). Experimental values of coercive fields and viscosity parameters ( kT / q ), used in connection with Gaunt's theory, suggest that the viscosity mechanism in Ba-ferrite, for particle sizes from 7 to 0.7 > m is the strong pinning of domain walls.