| GdMnO3 and TbMnO3 belong to a class of oxide materials which have become interesting in recent years due to indications that an ordered magnetic ground state coexists with an ordered ferroelectric ground state in the same crystalline phase. A striking aspect, or "magnetoelectric property," in some of these systems is that ferroelectric ordering (resulting from ordered dipole moments in the system, and measured by a change in dielectric property) occurs at low temperature, below the Neel temperature at which Mn spins order. This is interesting because, whereas conventional ferroelectricity often shows up at high temperature, together with a change in crystal structure, these systems transition into ferroelectric order at low temperature. Might the appearance of magnetic order drive ferroelectric order? Before jumping to conclusions about the fundamental mechanism of these fascinating phenomena, it is important to search for -- and organize -- clues from careful measurements on high-quality single crystals. This dissertation attempts to tease out some of the many complex relationships between physical properties in these systems. |
