Dr. Ioanna Bakaimi
Structural, Magnetic and Magnetodielectric Behavior of Geometrically Frustrated Manganese Oxides
Ioanna Bakaimi
University of Crete, School of Sciences & Engineering, Chemistry Department
and
Foundation for Research & Technology (FORTH), Institute of Electronic Structure & Laser (IESL)
ABSTRACT
The current trend in materials science is the discovery of smart materials which combine different properties and are controlled by external factors. Magnetoelectrics can be characterized as such, since they exhibit a cooperative response in magnetic and electric fields. In practice that means that the magnetic properties are affected by an electric field and the electric properties can be controlled by an external magnetic field. This is the corner stone property in many multifunctional devices such as spintronics and sensors.
The current thesis reports the finding of two new magnetodielectric compounds the α-NaMnO2 and β-NaMnO2. Spin driven magnetodielectric coupling in β-NaMnO2 is reported for the first time. Neutron powder diffraction data of the β-NaMnO2 reveal a noticeably complex crystal structure and moreover, two magnetic orderings, a commensurate state at high temperature and an incommensurate one with temperature lowering. Distinct dielectric anomalies appear at the temperatures where the magnetic transitions occur, pointing out the coupling between the magnetic and electric degrees of freedom. Surprisingly α-NaMnO2 also exhibits a sharp dielectric anomaly at the same temperature region where β-NaMnO2 develops the incommensurate structure. Transmission electron microscopy studies connect the inhomogenuous microstructure in both polymorphs with the observed physical response. Planar defects appear in both phases. In α-NaMnO2 are generated by the coherent intergrowth of β-NaMnO2, and vice versa. However the latter suffers by an increased concentration of such defects. The final outcome of these structure irregularities is the formation of a modulated superstructure which generates the lattice topology of the α-NaMnO2, the β-NaMnO2 and promotes the development of intermediate phases.
The observation of the β-NaMnO2 in the microstructure of α-NaMnO2 relates the dielectric anomaly of the latter with the incommensurate magnetic ordering of the former. The unusual finding of magnetodielectric coupling induced by the microstructure inhomogeneity opens new perspectives in the field of the magnetodielectric systems and the relevant underlying mechanisms. The role of the nanodomains and the microstructure irregularities on the development and strength of the magnetodielectric coupling are intriguing questions which require extensive analysis and relevant future work.
KEYWORDS
Multiferroics, ferroelectrics, geometrical frustration, induced magnetoelectric coupling, inhomogeneities, manganese oxides, neutron scattering, magnetism, dielectric anomaly.
December 2014