Resumen de Ferroelectricity and magnetoelectric coupling in magnetic ferroelectrics and artificial multiferroic heterostructures
Ignasi Fina Martínez
Multiferroic materials are those materials in which more than one ferroic order coexist. The most technologically appealing multiferroic materials are those showing ferromagnetism and ferroelectricity. Coupling between the mentioned ferroic orders, called magnetoelectric coupling, can yield to new interesting functional applications. In spintronics this coupling would result in the possibility of building magnetic memories controlled by electric field, or transistors where charge is contact-less controlled by a magnetic field. The ultimate goal of the present thesis is to explore the control of the ferroelectric polarization and dielectric properties by magnetic field in thin films. To that purpose dielectric, ferroelectric and magnetoelectric characterization methods of different multiferroic materials have been developed and used. Two big groups of multiferroic materials can be found. On one hand, single-phase multiferroics are those that intrinsically display multiferroicity. On the other hand, multiferroic composites are those where multiferroicity results from the mixture of two different materials that display ferroelectric and ferromagnetic order separately. Single-phase multiferroics can be divided in two subgroups: those where ferroic orders have different sources and those, called magnetic ferroelectrics, where magnetic order induces ferroelectricity and, consequently, larger magnetoelectric coupling is expected. The single-phase multiferroic material studied in the present thesis is YMnO3 in its orthorhombic phase, and it belongs to the magnetic ferroelectrics family. Even though it shows collinear magnetic order in bulk, we will show that cycloidal order in thin film form can be stabilized, giving rise to the capability of controlling the ferroelectric polarization by magnetic field in a reversible manner. Multiferroic composite thin films can be built mainly in two different architectures: vertical (ferromagnetic/ferroelectric columns embedded in a ferroelectric/ferromagnetic matrix) and horizontal (multilayered structures alternating ferromagnetic and ferroelectric materials). Here we compare both, using a ferroelectric perovskite (BaTiO3) and a ferromagnetic spinel (CoFe2O4). We will show that horizontal heterostructures display better ferroelectric properties and larger magnetoelectric coupling, compared to vertical heterostructures, where leakage current is a limiting parameter. The control of dielectric/ferroelectric properties under appropriate heterostructure configuration (in horizontal heterostructures) or deposition conditions (in vertical heterostructures) has been also achieved.
