Magnetic and electric response in multiferroic manganites

Multiferroics are materials that display spontaneous ferroelectric and magnetic ordering at the same time. Magnetoelectrics are materials in which an electric polarization can be induced by an applied magnetic field. The cross-coupling between the magnetism and ferroelectricity can potentially be exploited in the construction of novel, multifunctional spintronic devices. However, there are still rather few multiferroics known and the magneto(di)electric coupling in many of them is too weak to be useful for applications. Therefore, a better understanding of the mechanisms of magneto(di)electric coupling is required, as well as finding parameters by which the coupling may be controlled. There are several typical mechanisms by which multiferroic behaviour can be generated, including ferroelectricity that is induced by spiral magnetic ordering. In this thesis, we mainly focus on investigating the magnetoelectric coupling in spin-spiral systems because the polarization is often highly tuneable using applied magnetic fields, a useful property for future applications. We choose two systems in this class of materials; the orthorhombic rare-earth manganites RMnO3 and the chromate spinels MCr2O4. In the RMnO3 system, we have investigated both the effect of substituting the R3+ cation by divalent Ca2+, and the effect of mixing a non-magnetic rare-earth (Eu3+) with a strongly magnetic rare-earth (Ho3+). The aim of these investigations was to find parameters that enable systematic control of the electric and magnetic properties and the coupling between them. In the spinel MCr2O4, the investigation was focused on understanding the mechanism of the magnetodielectric coupling present in this system. Another of our investigations focused on better understanding the magnetodielectric coupling in linear magnetoelectric materials. Based on crystallographic and magnetic symmetry arguments we identified MnTiO3 as a magnetoelectric material. We have used Landau theory to model the experimentally measured magnetodielectric phenomena in this compound.