Appearance of magnetic blocking temperature in zinc magnesium ferrite thin films

Abstract Epitaxially oriented thin films of zinc magnesium ferrite, ZnxMg1−xFe2O4, have been fabricated on a sapphire (0001) substrate by the pulsed-laser-deposition (PLD) method. Changing the ratio of zinc ion to magnesium ion, the ferrimagnetic transition temperatures systematically decrease from 650 to 350 K just as in the bulk, indicating that the thin films maintain an ion configuration similar to the bulk compound. However, the temperature dependence of magnetization includes a magnetic blocking point (TB) between zero-field-cooling and the field-cooling process. The appearance of TB results from a small-size effect of magnetic grains. Scanning electron microscope (SEM) and atomic force microscope (AFM) study reveals that thin films consist of aggregates of tens-of-nanometer-scale grains while the bulk compound includes far larger particles (several-micrometer scale). The TB is systematically controlled with the change of the composition ratio of zinc to magnesium ions.