Permeability measurements in cobalt ferrite and carbonyl iron powders and suspensions

Magnetic permeability data of cobalt ferrite and carbonyl iron suspensions are discussed. Using an induction method, the relative differential permeability, mr;dif ; was measured as a function of the internal magnetic field for different volume fractions of the solid phase. In the case of cobalt ferrite suspensions, the mr;dif H curve was obtained for a first increasing ramp of magnetic field (data series ‘‘1’’), a decreasing ramp (data series ‘‘2’’), and the second increasing one (data series ‘‘3’’). Series ‘‘1’’ showed a maximum in the mr;dif H trend that did not appear in series ‘‘2’’ and ‘‘3’’. Furthermore, the data in series ‘‘2’’ and ‘‘3’’ are always below those in series ‘‘1’’. The latter behavior could be ascribed to the presence of hysteresis, and in fact it was not observed in carbonyl iron suspensions, where hysteresis is absent. The presence of the maximum in permeability is common for both types of suspensions. It is found that it only disappears if the particle motions are restricted by dispersing them in a rigid (epoxy) matrix, or if the suspensions are previously structured by applying a magnetic field to a sample prepared in an elastomer matrix. We conclude that the maxima in mr;dif H curves are associated to the motion or orientation of the dispersed particles during application of the first field ramp. The comparison with predictions of models allows to deduce some quantitative information on the structures formed by the particles. r 2002 Elsevier Science B.V. All rights reserved.