Modulated phases of ferroic oxides: CaFe2O4 and BaTiO3 thin films

The aim of this thesis is to investigate the coexistence and evolution of modulated structures in ferroic materials. Periodic structures and patterns are ubiquitous in nature in systems displaying competing interactions and evolve similarly under external perturbations, suggesting an underlying universal mechanism. A better understanding and control of such systems is, thus, not only interesting from a fundamental point of view, but can also allow for designing materials with enhanced response to external stimuli and novel functionalities. In this work we investigate two exemplary systems displaying modulated phases in thin film form: modulated spin arrangements in the antiferromagnet CaFe2O4 and periodic ferroelectric/ferroelastic domain structures in BaTiO3. Our results provide new insight in the complex magnetic phase diagram of CaFe2O4, characterized by the competition between antiferromagnetic and ferromagnetic couplings. Here we find that only one antiferromagnetic phase is thermodynamically stable, but there is also a net magnetization over a large temperature range, indicating the presence of other frozen spin structures. In addition, we show that the phase transition between different periodic domain structures in BaTiO3 occurs through period doubling cascades, reminiscent of the route to chaos in dynamical systems.