Exchange Anisotropy in Mixed Manganites with the Hausmannite Structure

Magnetic properties are reported of powder compounds isomorphous to hausmannite, but containing partial to nearly complete substitution of diamagnetic Zn²⁺ or Mg²⁺ ions in the tetrahedral sites of Mn₃{O}₄$. X-ray diffraction of these compounds reveals a single phase, whose structure corresponds to a tetragonally distorted spinel. Unidirectional anisotropy is present and is detected by the observation of hysteresis loops displaced along the field axis when the materials are cooled to low temperatures in magnetic fields of several kilo-oersteds. The unidirectional behavior is stable to reverse field pulses of 140 koe. An exchange anisotropy model is proposed involving interactions between ferrimagnetic and nearly antiferromagnetic regions brought about by the random distribution of the diamagnetic ions among the tetrahedral sites and the consequent magnetic inhomogeneity. This model is similar to that proposed for disordered alloy systems. Magnetic viscosity effects are observed which correspond in part to ordinary magnetic viscosity, but which are influenced by the presence of unidirectional anisotropy. The identity of this anisotropy is not impaired by the ordinary viscosity effects. In addition, observations of certain differences in the hysteresis loops measured dynamically and quasi-statically reveal an "extraordinary" viscosity associated with a gradual breakdown of the unidirectional anisotropy.