Modulated magnetic and structural states, Dynamics and functionalities

The aim of this thesis is to describe a number of materials that have attracted interest due to their magnetic, electric or structural properties with the use of generalized models. In hexagonal multiferroic LuFeO3 the study of the magnetic excitations in the material reveals the presence of elusive topological objects (i.e. effective magnetic monopoles and toroidal moments) not previously reported as well ad the non-reciprocal character of particular magnons in the material. The model and the results can be generalized to the class of hexagonal multiferroics. In the case of TbMnO3 we analyze the excitation spectrum of a cycloidal spin system that exhibit improper ferroelectricity. The results reveal that soft electromagnons have a large contribution, due to domain wall-localized modes, to the static dielectric constant of the material and, in general, of the cycloidal systems. A study of the response of GaV4S8 to photoexcitation is performed through atomistic spin dynamics simulations. Thermal properties of GaV4S8 are discovered to be related to the magnetic phases of the material due to the changing magnon transport properties of different magnetic phases. A change of the order of 2 in the thermal conductivity is observed in the transition from cycloidal to ferromagnetic phases A study of improper ferroelectricity in hybrid organic-inorganic perovskites is performed by using a Landau type symmetry breaking theory. The theory shows a rich phase diagram and the presence of nested domain walls. The theory describes a molecular ordering mechanism as a possible explanation for weak improper ferroelectricity observed in NH2(CH3)2Fe(COOH)3. The decade-long problem of the stripe ordering in IrTe2 is addressed. The existing observations of different striped phases in the material are reproduced in this study through a combination of first principle calculations and a phenomenological model. The same model predicts a phase diagram for the dimerized phases in the shape of a Devil's Staircase. This result partially fills a gap in the knowledge about IrTe, up to date no exhaustive explanation is known for the dimerization mechanism, and no other information on the dimerized phase diagram was available aside of the results reproduced in this work. The essence of this work is that minimalistic models and general considerations about energy and symmetries are powerful tools to describe quite diverse problems. This thesis address the importance of the excitations in the properties of certain materials and points out features that have not been previously reported in literature.