Magnetostructural Coupling in Itinerant Magnets

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Part 1: Non Technical Certain materials show large changes in their temperature when they are brought close to strong magnets. In recent years, there has been a recognition that such effects can be employed in cooling, for example, in air-conditioning or refrigeration, and that these magnetic cooling technologies can potentially even replace more conventional ways of cooling involving compression/expansion cycles. This fundamental study of the structure of known and new materials as they are exposed to changes in the temperature and the strength of the magnetic field is carried out with funding from the Solid State and Materials Chemistry program. The goal of this project is to identify how known materials can be expected to perform in useful devices, as well as inform the search for new and improved materials that could lead to further improvements in efficient cooling. Estimates are that cooling technologies consume as much as a fifth of the total energy used. Better materials and a better understanding of processes for cooling, both of which are target goals of this project, are therefore of national interest in terms of the US staying at the forefront of this important emerging technology. Part 2: Technical Changes in the temperature of magnetic materials due to applied magnetic fields were noted as long ago as 1881, the huge interest in the utility of this effect in employing magnetocaloric materials for cooling near room temperature is however more recent. While paramagnetic materials can be employed for cooling at very low temperatures, for applications near room temperatures and for applications of modest magnetic fields, the strongest effects are observed in materials close to their ferromagnetic Curie temperatures. The systems of interest in this project are 3d transition-metal based ferromagnets with Curie temperatures near room temperature. The main focus of the proposed work is on materials with structure types derived from the aluminum diboride structural aristotype, and are characterized by their ability to host two magnetic atoms. Typically, one of the two magnetic atoms is manganese. In many magnetocalorics, the magnetic transition appears to be associated with a structural change. To investigate this further, this research is guided by answering the following two questions: (i) How do the crystal structures change as ferromagnetic transitions are traversed with magnetic field or by cooling through the transition temperature, and what are the implications for magnetocaloric cooling? and (ii) Can metamagnetic transitions (i.e. spin-state transitions) be designed into materials in such a way that the entropy change associated with turning on a magnetic field is enhanced? The experimental aspect of the research focuses on known magnetocaloric materials as well as the discovery of new ones. Density functional theory-based methods are employed to co-design material compositions.