Magnetically disordered phase in epitaxial iron-deficientFe3O4thin films

Abstract

We report on the transport and magnetic properties of iron-deficient $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}\phantom{\rule{0.28em}{0ex}}(\mathrm{F}{\mathrm{e}}_{3\ensuremath{-}\ensuremath{\delta}}{\mathrm{O}}_{4})$ thin films grown with pulsed-laser deposition, where the stoichiometry and amount of cation vacancies are precisely controlled through changes in the oxygen partial pressure during growth. As the stoichiometry evolves from $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}$ to $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$, three distinct structural and magnetic regimes emerge: a $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}$-like regime, a $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$-like regime, and a transition regime. While reflection high-energy electron diffraction measurements reveal that films in all three regimes grow epitaxially cube-on-cube on MgO substrates, the transition-regime films are characterized by an absence of long-range, out-of-plane ordering in the film. Selected area electron diffraction measurements reveal the transition-regime films are well ordered on a local level, but not throughout the entire film. The structural disorder of the transition-regime films does not manifest itself in the transport properties, where a systematic change in resistivity, due primarily to variations in the $\mathrm{F}{\mathrm{e}}^{2+}:\mathrm{F}{\mathrm{e}}^{3+}$ cation ratio, occurs continuously throughout all three regimes. Large differences are observed, however, in the magnetic properties of the transition-regime films, which are reminiscent of magnetically disordered systems. We attribute this unique magnetically disordered state to magnetic frustration arising at the boundaries between the different locally ordered regions.