Date of Award

Fall 2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Applied Physics - (Ph.D.)

Department

Physics

First Advisor

Andrei Sirenko

Second Advisor

Michael Kotelyanskii

Third Advisor

John Francis Federici

Fourth Advisor

Martin Schaden

Fifth Advisor

Tao Zhou

Sixth Advisor

Ken Keunhyuk Ahn

Seventh Advisor

Vitaly A. Shneidman

Abstract

Multiferroics, materials which possess several ferroic orders, are the focus of research in recent years. Among these materials are oxide crystals, such as, for example, RMnO3, RMn2O5, R3Fe5O12, where R stands for rare earth ions. The most fascinating physics occurs when magnon-lattice coupling reveals itself in the far-IR spectra of multiferroics. The expected optical behavior puts multiferroics into a more general category of bi-anisotropic materials, the properties of which could be only described using anisotropic dielectric ε(ω), magnetic μ(ω), and magnetoelectric α(ω), α'(ω) tensors. In the first part of this thesis, general approaches for investigation of optical spectra of bulk and multilayer bi-anisotropic structures are shown. Analytical solutions for the optical spectra of certain crystal symmetries are derived for multilayer structures with both magnetic and magnetoelectric interactions.

The second part of the thesis contains analysis of the experimental data. Initially, Mueller matrix spectroscopic ellipsometry is reviewed. Analysis for optical spectra measured for isotropic Dy3Fe5O12 is given, for which contributions from magnetic and electric excitations are identified and the absence of strong magneto-electric effects is explained. In comparison, anisotropic orthorhombic perovskite ME- material, TbMnO3, with a spiral spin structure is investigated and contributions from coupling of different ferroic orders are shown in the far-IR Mueller matrix spectra of electromagnon excitations. Detail studies of optical properties of hexagonal multiferroic oxides RMnO3, (R = Ho, Er, Tm, Yb, and Lu), are studied in the far-infrared spectral range between 100 and 2000 cm-1 and temperatures between 1.5 K and 300 K by means of several experimental techniques: Muller matrix spectroscopic ellipsometry, rotating analyzer ellipsometry, and optical transmission spectroscopy. Spectra of the optical phonons are described in terms of the temperature dependencies of their frequency, damping, and oscillator strength. For all studies, oxide materials' clear signatures of the spin-phonon interaction are found below the temperature of the antiferromagnetic phase transition TN due to magnetic ordering of Mn3+ spins. A decrease of the ionic radius for R3+ ions between Ho3+ and Lu3+ in the corresponding RMnO3 compounds result in systematic variation of the frequency for several optical phonons. A magnetic excitation at ~190 cm-1 is observed at low temperatures below TN and interpreted as resulting from two-magnon absorption.

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