Magnetoelectric Properties of Epitaxial Spinel Oxide Thin Films
Non-collinear spinel multiferroics, which have an AB2O4 structure, have attracted a great deal of attention in the past few years. Amongst them, the one that has drawn great attention is spinel vanadate, FeV2O4 (FVO), which has transition metal elements at both A and B sites. The interplay among the spin, orbital and lattice degrees of freedom makes FVO a very complex system which exhibits interesting properties. This thesis focuses on the growth and functional characterization of high-quality FVO epitaxial thin films on perovskite SrTiO3 substrates. The magnetic and electrical properties of FVO films are presented as well as the characterization of magnetoelectric coupling of the material. Secondly, another unique spinel, Cobalt Chromite, CoCr2O4 (CCO), which has a canted conical magnetic ordering and shows spontaneous polarization, is also studied in this thesis. The polarization and resistivity-switching behaviour of CCO/LSMO (CoCr2O4/La6.67Sr3.33MnO3) heterostructure is also investigated. The thesis is organized into 7 chapters to give a systematic discussion. In Chapter 1, the structure and physical properties of spinel FVO and CCO are introduced. The background theory and experimental methods, which are used in the following chapters, are presented in Chapter 2. The general theories of ferromagnetic, ferroelectric and multiferroics are introduced in this chapter, followed by physical models for dielectric constant and resistivity. The basics of the methods employed are also discussed. In Chapter 3, the growth method and the structural characterization of epitaxial FVO films on STO substrates are presented. The effects of pulsed laser deposition (PLD) growth conditions on the FVO films are discussed and structural characterization by X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), etc. are presented. In Chapter 4, the electron and spin states of FVO films, studied by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) are presented, followed by the magnetic properties as measured by superconducting quantum interference device (SQUID) and magnetic force microscopy (MFM). In Chapter 5, the dielectric properties of FVO films are investigated by measuring the capacitance of a Pd/FVO/Nb:STO junction, and the magnetocapacitance effect of the material is also presented. The fabrication of CCO/LSMO heterostructure is described in Chapter 6 and its polarization and resistivity-switching behaviour are discussed. The conclusions of this thesis, and the directions of possible future related work, are provided in Chapter 7.
Magneto-electric effect, Iron Vanadate, Cobalt Chromite, Epitaxial thin films
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