Transverse susceptibility of magnetic inks. milling process

Transverse susceptibility (chi(tr)) tests the dispersion quality of magnetic inks by probing the small amplitude, low frequency AC susceptibility in the presence of a perpendicular DC field. The existing theoretical model of this experiment considers only individual particles and fails to predict the changes of the susceptibility observed when aggregates are broken by milling. We have extended the model to describe milling of magnetic inks as a continuous transition from an initial state of completely aggregated, immobile particles to a final stare of monodispersed particles. Each intermediate state of the ink is represented as a combination of the initial and final states. The particles in the aggregates contribute to chi(tr) only by the oscillation of their magnetic moment with the AC field. Monodispersed particles contribute to chi(tr) by displacement of their magnetic moment and by their physical motion. This model describes quantitatively the changes of the susceptibility with milling. The increase of chi(tr) is attributed to the increased number of monodispersed particles which also effectively increases the magnetic interaction between them and accounts for the shift of the maximum susceptibility to higher DC fields. (C) 1999 Elsevier Science B.V. All rights reserved.