Resumen de Preparation and characterization of biferroic nanostructures with magneto-electric coupling
Complex oxides present a broad spectrum of functional properties. In the last decade special attention was directed to materials with a possible coexistence of two or more ferroic orders (i.e. ferroelectric and ferromagnetic order). Such coexisting orders may be strongly coupled and thus lead to large magneto-electric responses. Appealing for application are materials that show these features well above room temperature, but single phase materials are scarce. Artificially combining materials with desired bulk properties is an alternative route to achieve coexistence of multiple ferroic orders above room temperature. In such systems magneto-electric coupling can arise via elastically coupled magnetostrictive and piezostrictive phases. In this work we have studied two promising model systems: layered horizontal heterostructures and self-organized column-matrix heterostructures. The ferroelectric perovskites BaTiO3 (BTO) and BiFeO3 (BFO) and ferrimagnetic spinel CoFe2O4 (CFO) were used, all have critical temperatures well above room temperature.
First, we describe the growth of horizontal heterostructures by pulsed laser deposition, optimizing the deposition conditions of single ferroelectric (BTO) and ferromagnetic (CFO) films and then integrate them in bilayered structures studying the effect of stacking order on the structural and functional properties. It is found that in spite of the structural dissimilarity of CFO spinel and BTO perovskite, high quality (00l)-oriented epitaxial bilayered heterostructures can be grown, independent of the stacking order. We have used reflection high energy electron diffraction to monitor the lattice relaxation in real time. BTO slowly relaxes when grown on low-mismatched perovskite substrates while it instantaneously relaxes on highly mismatched CFO layer. The films are ferroelectric and ferromagnetic above room temperature, and the BTO layer undergoes structural transitions at temperatures close to bulk transition temperatures. At these transitions a large change in the dielectric permitivity is observed under magnetic field, indicating magneto-electric coupling.
Second, the growth of self-organized two-phase nanocomposite heterostructures will be described. Phase separation at the nanoscale can lead to materials with extremely large interface area, i.e. by forming columns with a few nanometers in diameter embedded in a continuous matrix. Thus it may be an alternative route to combine ferroelectric and ferromagnetic phases and reduce the inuence of the rigid substrate. Here, we have investigated 65%BiFeO3-35%CoFe2O4 columnar nanocomposites prepared by pulsed laser deposition on (001) and (111) SrTiO3 (STO) and other (001) substrates. We determined a narrow window of growth conditions that permits stoichiometric growth of the nanocomposite at expense of limited size control of the columnar features. Exploring different mismatched (001) substrates showed that depending on the induced stress, BFO stabilized in the tetragonal T-BFO or rhombohedral R-BFO phase while CFO is growing as columns. The stabilization of different BFO phases allows to modify the ferroelectric polarization direction which can be rotated from [111] close to [001] substrate direction. The magnetization easy axis can be directed either out-of-plane or in-plane depending on the strain state of the magnetic columns. We confirmed magneto-electric coupling at the nanoscale by scanning probe techniques, measuring the local magnetic response before and after electric poling in CFO/R-BFO composites. We also investigated a columnar nanocomposite system 65%BaTiO3-35%CoFe2O4 grown by rf-sputtering on SrTiO3(001). Optimal growth conditions were found to produce epitaxial nanocomposite films with phase separation, (00l)-texture, column-matrix topology, as well as being ferromagnetic and ferroelectric at room temperature.
