Abstract
Temperature-driven dewetting of a self-template layer has been exploited to spontaneously grow epitaxial magnetic Fe3O4 nanocrystals by pulsed laser deposition. A 10 unit cell thick Fe3O4 template layer was deposited at 400 °C on a perovskite SrTiO3(001) substrate, followed by heating at 1100 °C, which induced dewetting of the Fe3O4 template layer, forming three-dimensional islands that were uniformly distributed over the substrate surface area. These islands were used as seed crystals for the growth of spatially separated quasi-epitaxial Fe3O4 nanopyramids. Structural analysis by scanning transmission electron microscopy and X-ray diffraction revealed incoherent growth of fully relaxed (001)-oriented Fe3O4 nanocrystals on the SrTiO3 (001) substrate. Higher-order epitaxial matching of the substrate and film lattices appears to be responsible for the well-defined in-plane orientation of the Fe3O4(001) nanopyramids despite the large nominal lattice mismatch of −7.5%. The nanopyramid structure strongly affected the magnetic characteristics of magnetite, dramatically reducing the coercive field compared to conventional magnetite thin films. The coercivity of the Fe3O4 nanopyramids was also strongly size dependent. This is attributed to a transition between poly- and monodomain states. A superparamagnetic phase was detected for the smallest pyramid sizes.
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