RUI: Optical Studies Of Magnetic, Charge And Orbital Ordering In Lone-Pair Compounds And Magnetite

NON-TECHNICAL ABSTRACT

The idea of ordering is an important one in physics. Charge, magnetic and orbital order are good examples of the ordering processes that may take place in materials. The basic idea is that when conditions are right the electric charge in a compound will organize itself in a certain fashion. The electron 'spin' is another electronic property that may result in ordering. One can think of an electron spin as a small permanent magnet carried by an electron. The magnetic strength and orientation of this magnet is called its magnetic moment. Sometimes these magnetic moments order in a particular fashion resulting in magnetic (spin) ordering. Yet another type of ordering is associated with the way electrons orbit the nucleus of the atom. There are different types of electron 'orbitals' that can be distinguished by their shapes (spherical versus dumbbell like for example). It is believed that in some compounds orientation of the orbitals may result in orbital ordering. Any type of ordering results in new properties of the compound. For instance charge ordering my lead to a compound with less electrical conductivity. Spin and orbital ordering may result in a compound that is a stronger magnet. Understanding how ordering happens therefore will lead to better materials engineering. This project uses optical spectroscopy to study and understand different types of ordering in two types of materials. Magnetite is the first magnetic material known to mankind and the physics of charge ordering in magnetite has been a puzzle since its discovery in 1939. Transition metal tellurite halides Co5(TeO3)4Br2, Co7(TeO3)4Br6 display a rich magnetic phase diagram indicating intricate magnetic and possibly orbital ordering and therefore are good candidates to study spin and orbital ordering. Students will be actively involved in this project and benefit significantly from the state-of-the-art equipment and from the collaboration with some of the nation's leading scientific laboratories. High school students will have a chance to work on some aspects of this project.

TECHNICAL ABSTRACT

Correlated electron systems are known to display a number of different types of ordering resulting in a rich phase diagram. Understanding the complex nature of these ordering processes can be achieved by optical spectroscopy. This individual investigator award supports a systematic infrared and Raman spectroscopic study of the evolution of the electronic, orbital, spin and lattice excitations as the magnetite, and lone-pair transition metal tellurite halides, undergo structural and magnetic transitions. After more than six decades of research the nature of the structural transition (Verwey transition) in magnetite (Fe3O4) is still an open question. A number of magnetite samples with different Verwey transition temperature provide a basis for systematic studies of this compound. Lone-pair transition metal tellurite halides Co5(TeO3)4Br2, Co7(TeO3)4Br6 are novel materials with low dimensional arrangement of the Te4+ cations and magnetic properties controlled by the unfilled d-orbitals of the Co2+ ion with the spin 3/2. Both Co7(TeO3)4Br6 and Co5(TeO3)4Br2 possess a rich magnetic phase diagram indicating intricate magnetic and possibly orbital ordering. Spectroscopic measurements in these compounds will provide a critical experimental insight into the physics of correlated electron systems. The students employed in this research program will benefit significantly by working in a modern research environment and by developing vital problem-solving skills.