  High quality stoichiometric magnetite (Fe3O4) films grown by infrared pulsed
laser deposition (IR-PLD) on different surfaces were investigated in order to
study the influence of the substrate, orientation, and thickness on their
magnetic behavior. Different single crystal (001)-oriented substrates, i.e.,
SrTiO3(001), MgAl2O4(001) and MgO(001), have been used for the preparation of
epitaxial Fe3O4(001) films. By comparison, polycrystalline magnetite films were
obtained on both single crystal Al2O3(0001) and amorphous Si/SiO2 substrates.
The thickness has been varied between 50 - 400 nm. All films consist of
nanocrystalline stoichiometric magnetite with very small strain (<1\%) and
present the Verwey transition (Tv) between 110-120 K, i.e., close to bulk
magnetite (122 K). In general, Tv depends on both microstructure and thickness,
increasing mainly as the thickness increases. Room temperature
angular-dependent measurements reveal an in-plane fourfold symmetry magnetic
behavior for all films grown on (001)-oriented surfaces, and with the easy axes
lying along the Fe3O4 [010] and [100] directions. Remarkably, the fourfold
magnetic symmetry shows up to 400 nm thick films. In turn, the films grown on
single crystal Al2O3 (0001) and on amorphous Si/SiO2 surfaces display an
isotropic magnetic behavior. Coercive field (Hc) depends on microstructure and
film thickness. The largest (lowest) Hc value corresponds to the thinner film
grown on a single crystal SrTiO3(001) (amorphous Si/SiO2) surface, which
present the largest (lowest) strain (crystallinity). Moreover, the coercivity
follows an inverse law with film thickness. Our results demonstrate that we can
artificially control the magnetic behavior of stoichiometric IR-PLD grown Fe3O4
films by exploiting substrate-induced anisotropy and thickness-controlled
coercivity, that might be relevant to incorporate magnetite in future
spintronic devices.
