NERSC Initiative for Scientific Exploration (NISE) 2011 Awards

First-Principles Calculations of Multiferroic Oxides

Yi Wang, Pennsylvania State University

Associated NERSC Project: Spin-lattice Coupling in Magnetic Phase Transition (m891)

As both the magnetization and the polarization can be manipulated by electric fields, magnetic fields, or elastic deformations, multiferroics are of importance for many applications, such as data storage, spintronics, and microelectronic devices. However, even though many published experimental and theoretical works, the complete theoretical framework of multiferroic materials are still not clear.

The main objective of the proposed research is to explore the fundamental nature of multiferroic materials using first-principles approach. Multiferroics are those materials that in a single phase simultaneously exhibit more than one primary ferroic order parameter, typically, ferromagnetism, ferroelectricity, and ferroelasticity. The factors that we plan to consider in our formulation of free energy are:

1. The continuous phase transition among multi dipolar states. In this connection, the magnetocaloric effect is an analogous, but better-known and understood, phenomenon. We can extend our recent works on the thermodynamics of magnetic system.
2. The effects of the long range dipole–dipole interactions on the vibrational contribution to the free energy. As we will use the direct approach to calculate the phonon properties, we need to accurately handle the long range dipole–dipole interactions for the general purpose of calculating phonon density-of-states.
3. The effects of an external field on the free energy.
4. The possible imaginary phonon issue for certain dipolar states. We plan to solve the problem using our recently developed procedure which can calculate the phonon frequencies of a state with high symmetry, using the force constants calculated from a structure with relatively lower symmetry.
5. Thin film materials. The strain effects will be considered by replacing the volume with the lattice vectors in the expression of the free energy formulated in the recent work.