Theory of Co2+ exchange isolation in ferrimagnetic spinels and garnets

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Abstract
The previously reported mechanism of exchange isolation that was proposed to explain the absence of strong positive magnetocrystalline anisotropy effects from Co2+ ions in LiTi spinel ferrite is approximated by a three‐cation superexchange model. This simplified system is then analyzed in terms of the dependence of the single‐ion Co2+ anisotropy contribution on magnetic dilution. To account for the reduced anisotropy effects in the presence of strong spin‐lattice relaxation, Co2+ is considered to act as a paramagnetic ion through decoupling from the exchange fields of the iron sublattices by magnetic dilution. The exchange‐isolation concept is based on selective clustering of Li1+ and Ti4+ dilutant ions that results from the minimization of the total energy comprised of (i) magnetic exchange energy, (ii) lattice electrostatic energy, and (iii) crystal‐field stabilization energy. Each of these contributions is discussed in terms specific to the spinel lattice and, in particular, to the ionic distributions surrounding the octahedral site occupied by the Co2+ ion. The model is also tested qualitatively by comparing its predictions with published experimental results for the ferrimagnetic CaV garnet system.

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