Interface effects in epitaxial multiferroic complex oxide heterostructures

Abstract

Multiferroic oxide heterostructures exhibit superior functional properties as a result of the interplay between the lattice, charge, orbital and electronic degrees of freedom. In particular these interactions can lead to emergent functionalities such as ferromagnetism and ferroelectricity at the interfaces. This offers excellent prospects for advanced devices based on the manipulation of the spin and charge degrees of freedom. However, the understanding of the underlying physics at the interface of these complex structures is still incomplete. This thesis investigates, (i) the effect of structural, electronic and compositional dependencies on the ferroelectric domain switching properties of bismuth ferrite (BiFeO3) multiferroic thin-films and, (ii) the emergent magnetism at the interface of perovskite manganite superlattices comprised of lanthanum strontium manganese oxide (LaMnO3) and calcium manganese oxide (CaMnO3) layers. Pulsed laser deposition (PLD) equipped with in-situ reflection high energy electron diffraction (RHEED) technique is employed for the deposition of the samples to ensure atomically precise layer by layer growth. The deposited structures are characterized using an array of characterization techniques such as atomic force microscopy (AFM), X-ray diffraction techniques, piezoresponse force microscopy (PFM) and physical property measurement systems (PPMS) to investigate the effect of internal and external influences upon the strong electron correlations in multiferroics with exciting potential for future spintronic applications. The thesis is made of three key studies; the first part focuses on the fabrication of BiFeO3 (BFO) thin-films of varying thickness on a SrTiO3 (STO) substrate of two different orientations, namely (100) and (110) with and without a bottom electrode. Through unique epitaxial constraints of a (110)-oriented STO substrate and an intermediate layer of SrRuO3 (SRO), single domain BFO thin-films are realized. Further, the samples revealed that the crystallographic orientation and electronic boundary conditions play a vital role in determining the structural and electronic properties in nanoscale BFO thin-films. Presence of the bottom electrode and the thickness of the film played a vital role in imposing considerable strain in BFO thin-film. It is shown that in the absence of the SRO layer when the thickness is increased from 50 to 100 and 150 nm, BFO films relaxes into a two-domain state. A significant reduction in the out-of-plane lattice parameter has also been observed with the increase in the thickness of the film. The results demonstrate that oxide heteroepitaxy of BFO can be carefully tuned to achieve various ferroic ground states. As the second stage, thin BFO films (~12 nm) have been successfully realised on LaAlO3 (100) substrates with LaSrMnO3 (LSMO) buffer layer in between. AFM topography scans show the absence of the characteristic stripe-like morphology. It is demonstrated that the surface roughness increases with the increase in bottom electrode layer thickness. XRD and RSM reveal the evolution of the phase with the electrode thickness. BFO films with 5nm and 10nm LSMO Buffer layers revealed a typical rhombohedral like R’ phase whereas with the increase LSMO layer thickness to 20nm; the phase evolved to tilted S’ like a phase. The PFM studies revealed that the domain size increases with the increase in thickness of the bottom electrode and sample exhibited both in-plane and out of plane domain signals. As the final stage of this study and the groundwork for future possibilities of coupling perovskite manganites and multiferroic BFO, LMO/CMO superlattices are fabricated on STO (100) substrate. The structures are then characterized using AFM, XRD and PPMS techniques. It is observed that varying the periodicity of the SL layers; we can vary the presence of manganese (Mn) ions with different valence states (Mn2+ and Mn3+) and thus introduce an antiferromagnetic state that co-exists with a robust ferromagnetic state. In summary, the study shows that it is possible to engineer novel functional performances via the interfacial engineering of mechanical and electrical boundary conditions in complex metal oxide heterostructures.