Spin-glass behavior of spinel iron oxide particles on iron metal particle recording media

Small (30 Å diameter) spinel iron oxide particles which form the protective layer on acicular iron metal particle recording media are found to exhibit no observable magnetization. The typical experimental decrease in sample magnetization for several media specimens is about 3% from 5 to 300 K. This change is the same as the thermal decrease in metal core magnetization, determined by Mössbauer spectroscopy. The predicted decrease in sample magnetization assuming normal spinel oxide behavior is typically about 20%. Two intraparticle models for reduced magnetization in oxide nanoparticles are found to be incompatible with the magnetization data. An interparticle ‘‘super-spin-glass’’ model is compatible with the magnetization data. The alignment between adjacent oxide nanoparticle moments is hypothesized to depend on the particular sublattices in contact, since the sublattice exchange interactions are all antiferromagnetic. An inverse particle size dependence is thus expected, in agreement with spinel oxide magnetization reduction and other phenomena seen in a wide range of studies on larger oxide particles and films.