Spin-charge-lattice coupling in multiferroics and strained ferroelectrics

Multiferroic materials that are simultaneously ferroelectric and magnetic are gaining more and more attention within the scientific community due to the promise of new device applications arising as a result of coupling between their dual order parameters. The magnetoelectric multiferroics are an example of the broader class of multifunctional or smart materials that combine several useful properties in the same substance to produce new phenomena that are more than just the sum of the individual parts. Strain can stabilize ferroelectricity in otherwise nonferrolectric materials as well as modify magnetic transition in magnetic materials. Epitaxial strain is currently being pursued to design "ferro-electromagnetic" films which can demonstrate coupling between polarization and magnetism.The central focus of this thesis work is to develop an understanding of coupling between spin, charge and lattice strain in multiferroics and strained ferroelectrics. For this study, we utilize two techniques: nonlinear optics and scanning probe microscopy. Optical second harmonic generation, a nonlinear optical technique, is shown to be an excellent tool to probe polar and magnetic symmetries in multiferroics while also allowing us to probe the nature of coupling between the two order parameters. Nonlinear optical spectroscopy has been employed to reveal spin-charge coupling in multiferroic BiFeO 3 through the study of magnon sidebands which are otherwise difficult to probe using linear optical spectroscopy. A combination of nonlinear optics and scanning probe microscopy have been utilized to reveal domain dynamics and explore the nature of ferroelectricity in strained ferroelectrics. Second harmonic generation has been utilized to demonstrate that the symmetry of pure elastic lattice distortion in the antiferrodistortive phase in SrTiO 3 is isomorphic to a magnetic point group and phase transitions involving octahedral tilt rotations can be probed successfully using this technique. Confocal SHG microscopy has been used to visualize domain structures in the well known ferroelectric BaTiO3 and evidence of monoclinic distortion is observed near strained fields in the tetragonal phase.