Superparamagnetic behavior of structural domains in epitaxial ultrathin magnetite films

We provide evidence that thin films of magnetite ${\mathrm{Fe}}_3{\mathrm{O}}_4$ epitaxially grown on single-crystalline MgO(100), form domains with unique magnetic properties. Strong 180° Fe-O-Fe superexchange paths at antiphase domain boundaries result in a frustration of the interdomain interactions. Probe-layer Mössbauer spectroscopy shows superparamagnetic behavior in the entire film, in spite of the nearly perfect layer-by-layer growth. Superconducting quantum interference device magnetometry is used to determine the temperature and thickness dependence of the superparamagnetic blocking temperatures.