Synthesis and characterization of multiferroic thin films
Multiferroic materials and multiferroic materials systems which simultaneously exhibit ferroelectricity and magnetism have attracted great attention because of their exotic physical properties and their potential applications which utilize coupling of magnetism and ferroelectricity. The goal of this thesis was to study multiferroic materials systems in thin film and multilayer forms in order to explore the possibility of fabricating room temperature thin film devices.
In particular, we have focused on two types of multiferroic materials systems: (1) intrinsic multiferroic/magnetoelectric thin film materials and (2) magnetostrictive/piezoelectric bilayer systems for investigation of the strain-mediated magnetoelectric (ME) effect.
BiFeO3 is an intrinsic multiferroic which displays ferroelectricity and antiferromagnetism at room temperature, and thus of strong interest for ambient device applications. In this thesis, we have extensively investigated the role of microstructure on the properties of BiFeO3 thin films. We studied multiphase formation in Bi-Fe-O thin films, and found that formation of secondary phases such as α-Fe2O3, γ-Fe 2O3, and Fe3O4 increased overall saturation magnetization and released the misfit strain of the BiFeO3 grains in the films. We also observed large polarization in Bi-Fe-O thin films containing secondary phases that have almost fully relaxed the misfit strain.
We have studied several aspects of the ME effect which are directly relevant to possible novel device applications. Electric field tunable spintronic devices using the ME effect have been proposed. In one such device configuration, the desired effect is electric field tuning of giant magnetoresistance or tunnel magnetoresistance through control of exchange bias via the ME effect. We have investigated the feasibility of such a device using exchange-biased Co/Pt multilayers on Cr2O3 thin films.
The strain-mediated ME effect at the interface of magnetostrictive/ piezoelectric bilayers has been widely used to demonstrate magnetic field detection with extremely high sensitivity. Although the overall mechanism of such an effect is known, the details of the bilayer interfaces and how they affect the coupling is not understood. In order to directly observe the strain-mediated ME coupling effect, we fabricated bilayer thin film structures and performed in-situ dynamic observation of magnetic domains while an electric-field was being applied using Lorentz transmission electron microscopy. Electric-field induced motion of magnetic domain boundaries in the magnetostrictive layer was observed for the first time.