The Effect of Geometric Constraints in Multiferroic Heterostructures

Ferroelectric materials are characterised by spontaneous polarization that is switchable by an electric field. Although they have been studied for nearly one hundred years, there is a growing research community learning more about these complex materials and their potential applications - many of which have only come to light in recent times. Two reoccurring themes in the community are i) the influence of dimensionality reduction on the physics of ferroelectrics (and multiferroics - which simultaneously exhibit ferroelectric and magnetic order), and ii) the functional response in the GHz regime and faster. These themes tie together when considering future device applications, where densely packed ferroic elements would interface with industry standard high frequency electronics. This thesis attempts to further define the prerequisites needed for such devices. The first topic of the thesis concerns the effect of geometric constraints on the crystallographic and magnetic structures in a model multiferroic. Neutron diffraction experiments were carried out on a series of bismuth ferrite (BFO) films to determine the influence of thickness on the stability of the spin structure. The results demonstrated a thickness-dependent expansion of the period of the spin cycloid in BFO, suggesting size effects can alter the magnetic order. Reduced dimensionality is equally important to the growth of new phases of thin film materials. Hence, strain engineering of BFO and the fine thermodynamic energy balance in mixed phase films was explored. An empirical evaluation of the energetics involved in forming strain-relieving secondary phases in BFO grown on lanthanum aluminate (LAO) substrates enabled the extraction of explicit energy costs for forming inter-phase boundaries. The second topic of the thesis evaluated the local behaviour of ferroelectrics in the high frequency regime. Scanning microwave impedance microscopy (sMIM) was used to probe the dielectric response of mixed-phase BFO thin films, as well as the high frequency AC domain wall conductivity of lead zirconate titanate (PZT) films. The results implied quasi-continuous tunable domain wall conduction in the microwave regime. The results herein offer new insights into the coupling of ferroelectric thin films with magnetism, self-strain, and externally applied high frequency electric fields