Chirality, symmetry-breaking, and chemical substitution in multiferroics

Multiferroic materials attract significant attention due to their potential utility in a broad range of device applications. The inclusion of heavy metal centers in these materials enhances their magnetoelectric properties, yielding fascinating physical phenomena such as the Dzyaloshinskii–Moriya interaction, nonreciprocal directional dichroism, enhancement of spin-phonon coupling, and spin-orbit-entangled ground states. This dissertation provides a comprehensive survey of magnetoelectric multiferroics containing heavy metal centers and explores spectroscopic techniques under extreme conditions. A microscopic examination of phase transitions, symmetry-breaking, and structure-property relationships enhances the fundamental understanding of coupling mechanisms.

In A2Mo3O8 (A = Fe, Zn, Ni, and Mn), we use optical spectroscopy to analyze the electronic properties and compare our findings with first principles electronic structure calculations. We find that Fe2+ ions in the A site create a many-body effect from a valence band due to screening of the local moment – similar to a Zhang-Rice singlet. These findings advance the understanding of unusual hybridization with orbital occupation and the structure-property relationships in various metal-substituted systems.

In a chiral, polar magnet, Ni3TeO6, we explore toroidal geometry to complete the set of configurations and develop structure-property relations by combining magneto-optical spectroscopy and first-principles calculations. The formation of Ni toroidal moments is responsible for the largest effects near 1.1 eV - a tendency that is captured by our microscopic model and computational implementation. Furthermore, we demonstrate deterministic control of nonreciprocal directional dichroism in Ni3TeO6 across the entire telecom wavelength range. This discovery will accelerate the development of photonics applications that take advantage of unusual symmetry characteristics.

In Co4B2O9 (B = Nb, Ta), we combine variable temperature infrared spectroscopy, lattice dynamics calculations, and several different models of spin-phonon coupling to reveal the mechanism of magnetoelectricity. We reveal a spin-phonon coupling in Co2+ shearing mode near 150 cm−1 with coupling constants of 3.4 and 3.4 cm−1 for Co4Nb2O9 and the Ta analog, respectively. These coupling constants derive from interlayer exchange interactions, which contain competing antiferromagnetic and ferromagnetic contributions. Comparison to other contemporary oxides shows that spin-phonon coupling in this family of materials is among the strongest ever reported, suggesting an origin for magnetoelectric coupling.