Analysis of far-infrared optical spectra of multiferroics

Author

Eric C. Standard, New Jersey Institute of Technology

Document Type

Dissertation

Date of Award

Spring 5-31-2014

Degree Name

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

Department

Federated Physics Department

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

Abstract

The optical spectra of multiferroic crystals Dy₃Fe₅O₁₂, Tb₃Fe₅O₁₂, and four hexagonal RMnO₃ (5D= Er, Tm, Yb, Lu) are studied using intense bright synchrotonic light in the infrared spectral range. In regards to the materials analyzed, two rare earth iron garnets and four rare earth manganites in total are examined. These materials have attracted much attention due to their interesting magnetoelectric properties. They could serve as the basis for the next generation of faster and more energy efficient memory and logic devices. To examine these materials, two optical techniques were utilized: transmission spectroscopy and rotating analyzer ellipsometry. Transmission spectroscopy experiments analyze absorption of light passed through a material. Ellipsometry examines the change of polarization state for a reflected beam of light. Work has been done at the Brookhaven National Laboratory - National Synchrotron Light Source. Requisite for this project is a pair of broadband retarders for far- IR spectral range. Several retarders are devised to accomplish this task. They were designed using computer simulations and calculations before they are fabricated. Work towards the development of these retarders is included in this dissertation.

Ferrimagnetic Dy₃Fe₅O₁₂ single crystals are studied in the spectral range between 12 and 700 cm^-1, and in a wide temperature range between 5 K and 300 K using transmission spectroscopy and rotating analyzer ellipsometry (RAE). It is found that in the temperature range below T_C=16 K for the magnetic ordering of Dy³⁺ spins, a number of ligand field (L F) and Kaplan-Kittel (KK) exchange resonance modes emerge. Temperature dependences of their frequencies allowed us to estimate the ratio between the Fe-Dy and Dy-Dy exchange constants.

Tb₃Fe₅O₁₂ single crystals are studied in the range between 15 and 100 cm^-1, in magnetic fields up to 10 T, and for temperatures between 5 and 150 K using only transmission spectroscopy. A number of IR active excitations are attributed to electric- dipole transitions between ligand-field splitting of Tb³⁺. Anticrossing between the magnetic exchange excitation and the ligand-field transition occurs at the temperature between 60 and 80 K. The corresponding coupling energy for this interaction is 6 cm^-1. Temperature-induced softening of the hybrid IR excitation correlates with the increase of the static dielectric constant in Tb₃Fe₅O₁₂.

The hexagonal RMnO₃ (5 = Er, Tm, Yb, Lu) single crystals are studied between 8 and 125 cm^-1, in fields up to 9T and for temperatures from 1.5 to 100 K. It is observed that the magnon frequency increases systematically with a decrease of the rare earth ion radius. The magnetic ordering of R ions (R=Er, Tm, Yb) is seen at low temperatures T<3.5 K and in strong external magnetic fields. The observed effects are analyzed taking into account main magnetic interactions in the system including exchange of the Mn³⁺ spins with R³⁺ paramagnetic moments.

Recommended Citation

Standard, Eric C., "Analysis of far-infrared optical spectra of multiferroics" (2014). Dissertations. 168.
https://digitalcommons.njit.edu/dissertations/168