Four-fold anisotropy epitaxial magnetite films grown by infrared-pulsed laser deposition

Magnetite (Fe3O4) films grown by pulsed laser deposition (PLD) are usually prepared with ultraviolet lasers. This is presumably done to minimize the presence of particulates on the films which might result in detrimental magnetic properties. However, inertia seems to be the rule followed when choosing the preparation parameters in a vast number of published studies. A systematic and comprehensive study on the influence of the laser wavelength on the microstructure, morphology and magnetic properties of Fe3O4 films is missed. Furthermore, and despite of magnetite being a material extensively studied in the shape of bulk and films, a rigorous study of the angular evolution of the magnetic properties in dependence of the measuring temperature for magnetite films grown on different substrates is still due. In this study high quality stoichiometric magnetite thin films of about 50 nm have been grown by ablation from a self-prepared sintered hematite target using a nanosecond infrared (IR) laser at 1064 nm and a substrate temperature of 475ºC. Single crystal substrates of SrTiO3:Nb, MgAl2O4 and MgO have been used in the preparation of Fe3O4 epitaxial films. By comparison, polycrystalline Fe3O4 films were grown on Al2O3 and Si substrates. The films were characterized using XRD, AFM, Raman and Mössbauer spectroscopy, vectorial magneto-optical Kerr effect microscopy (v-MOKE) and SQUID magnetometry. All films consisted of stochiometric magnetite and presented a Verwey transition at TV=115-118 K. Room temperature (RT) coercivity values (35 – 45 mT) agree well with values reported in the literature for magnetite films in this thickness range. In-plane hysteresis loops were measured in the temperature range 90-300 K by v-MOKE in dependence of the orientation of the applied magnetic field in the 0-360º range (angular step of 4.5º). For all epitaxial films under study, the highest RT coercivity is found at 0, 90, 180 and 270º, thus orthogonal to each other, while the lowest coercivity is found between those. The easy-axis directions rotate by 45º respect to the RT values when lowering temperature below TV. This well-defined four-fold symmetry is indicative of biaxial magnetic anisotropy, highlighting the high quality of the films grown by IR-PLD