Growth and characterization of magnetoelectric and multiferroic oxide thin films

Abstract

Multiferroic oxides and magnetoelectric materials have attracted a great deal of attention due to their potential applications in many multifunctional devices such as sensors, and non-volatile memory. Magnetoelectric effect in solids manifests a polarization change under the magnetic field or a magnetization change under an electric field. Such materials include single-phase crystalline multiferroics and artificially assembled compounds containing ferromagnetic and ferroelectric phases. Single phase multiferroic materials possess both electric and magnetic orders and magnetoelectric effect originates from the coupling of ferroelectric and magnetic dipoles. Magnetoelectric composites consist of piezoelectric and ferro/ferrimagnetic components. The magnetoelectric effect happens because of the mechanical stress mediation. Multiferroic nanostructure materials start to draw considerable interest because of the demand of on-chip integration, which cannot be realized in their bulk counterpart. In this present work, there are two main sections concerning about magnetoelectric nanocomposite and single phase multiferroic BiFeO₃-based materials: (1) We systematically study the growth and characterization of ferroelectric/magnetic nanocomposite thin films with 2-2, 1-3 and superlattice structure by pulsed-laser deposition. Microstructure and properties in the nanostructured thin film with different compositions have been studied. Magnetic-field-dependent capacitance measurement reveals an increase in capacitance of about 1.46% with an applied magnetic field. The influence of perovskite/spinel interface and residual strain on the electric and magnetic properties has been investigated in Pb(Zr,Ti)O₃/CoFe₂O₄heterostructures. Temperature-dependent dielectric behaviors indicate the strong Maxwell-Wagner space charge effect in the multilayered composite, which is in agreement with theoretical evaluations. This interfacial engineering in perovskite/spinel heterostructure will provide an understanding and an additional degree of freedom to control the ferroelectric, magnetic and interfacial coupling in epitaxial multilayered and superlattice multiferroics.(2) Study of ferroelectric domain in multiferroic BiFeO₃-based thin films by piezoresponse force microscopy: the switching mechanism in monodomain BiFeO₃ epitaxial thin films on (001) and (110) SrTiO₃substrates has been studied. Various scanning with different relative positions of the cantilever and the switched area re-construct the in-plane and out-of-plane polarization directions before and after switching, then the domain switching dynamics in monodomain BiFeO₃thin films. Switching size, substrate mismatch strain, electric field, and time-dependent relaxation have been systematically explored; BiFeO₃films near its morphotropic phase boundary has been deposited on LaAlO₃substrates. Study of the ferroelectric domain configuration and structural analysis reveals that the material is composed of tetragonal and rhombohedral phases which can be transformed into each other back and forth under an electric field. The switching mechanism and the physics behind have been studied. Large electric-field-induced strain happens during the phase transition, which shows its potential application in lead-free micro-actuator and probe-based data storage devices.