Orbital order and magnetoelectric coupling in manganites and jarosites

PhD ceremony: Mr. A.J.C. Buurma, 11.00 uur, Academiegebouw, Broerstraat 5, Groningen

Thesis: Orbital order and magnetoelectric coupling in manganites and jarosites

Promotor(s): prof. T.T.M. Palstra

Faculty: Mathematics and Natural Sciences

Transition metal oxides show a wealth of physical properties that emanate from complex physics. Subtle lattice deformations can be reflected in distinct changes in physical properties. This thesis focuses on subtle interplay between the crystal lattice and physical properties. We aim to couple different physical phenomena to create multifunctional materials.

The first group of materials studied in this thesis encompasses orthorhombic NdMnO3. The Mn orbital degeneracy in manganates is lifted by a Jahn-Teller distortion to give a long range alternating pattern of short and long Mn-O bond lengths known as antiferrodistortive orbital order in the ab-plane. Both divalent Ca doping on the rare earth site as well as gallium doping on the octahedral Mn site suppresses orbital order in NdMnO3. X-ray diffraction experiments at elevated temperatures show that the first-order nature of the orbital order-disorder phase transition eventually gives way to a second-order phase transition. Magnetic measurements at low temperature show an enhancement in the ferromagnetic saturation moment with isovalent Ga-doping.

The second part of this thesis focuses on magneto-electric materials that allow the control of magnetic (electric) dipole moments by electric (magnetic) field. We investigate the possibility of a new magneto-electric coupling mechanism driven by symmetric superexchange interactions in iron jarosite. Finally, we compare the magnetic phase diagrams of the hexagonal manganates YbMnO3 and ErMnO3 as a function of temperature and applied magnetic field. We observe differences in the resistivity of the samples, which we attribute to different domain wall densities that result in different in critical fields.