Spontaneous Growth of Strain-Free Magnetite Nanocrystals via Temperature-Driven Dewetting

Temperature-driven dewetting of a self-template layer has been exploited to spontaneously grow epitaxial magnetic Fe 3 O 4 nanocrystals by pulsed laser deposition. A 10 unit cell thick Fe 3 O 4 template layer was deposited at 400 °C on a perovskite SrTiO 3 (001) substrate, followed by heating at 1100 °C, which induced dewetting of the Fe 3 O 4 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 Fe 3 O 4 nanopyramids. Structural analysis by scanning transmission electron microscopy and X-ray diffraction revealed incoherent growth of fully relaxed (001)-oriented Fe 3 O 4 nanocrystals on the SrTiO 3 (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 Fe 3 O 4 (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 Fe 3 O 4 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.