CAS: Using Narrow Bands and Competing Exchange Interactions as Design Principles for Magnetocaloric Materials

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Non-technical Abstract:Magnetism is a fascinating physical phenomenon that enables modern sensing, logic, and memory devices. A less-known application for magnetism is in refrigeration technology through a mechanism called the magnetocaloric effect (MCE). A magnetic refrigerator is similar to regular refrigerators, except instead of compressing and decompressing a gas, it magnetizes and demagnetizes a solid-state material to create the low temperatures. Materials used for this application typically contain rare-earth elements, for example gadolinium and erbium. These elements are expansive and not readily available, especially not in the U.S.. With this project, supported by the Solid State and Materials Chemistry program and the Condensed Mater Physics program in the Division of Materials Research, the researchers at Boston College synthesize new materials that exhibit a giant MCE and contain only minimal amounts of rare-earth elements. In this way the project addresses critical aspects of sustainability by eliminating ozone-depleting refrigeration gases and by minimizing the rare-earth content of the magnetocaloric materials. This interdisciplinary project provides a platform for training the next generation of materials scientists with a deep understanding of quantum chemistry and the ability to implement fundamental science to solve the societal problems. Additionally, this project provides career workshops for pre-college students from underrepresented minorities in the Boston area and research opportunities for students from Jackson State University, a HBCU.Technical Abstract:Materials that exhibit a magnetocaloric effect (MCE), the reversible change of temperature as a function of magnetic field, provides a sustainable platform for refrigeration technology. Magnetic refrigerators are considerably less energy consuming than regular refrigerators and do not rely on ozone-depleting gases. The main challenge for commercializing magnetic refrigerators is the lack of materials with a large refrigeration capacity (RC) made from earth-abundant elements. Giant magnetocaloric materials are either made from toxic elements such as P and As, or have a large rare-earth content. As such, the synthesis of giant magnetocaloric materials that are non-toxic and have little to no rare-earth content is a critical aspect of sustainability. With this support researchers from Boston College synthesize such materials by following two chemical design principles, namely narrow bandwidths and competing exchange interactions. The narrow bandwidth leads to a magnetic transition near room temperature via the Stoner mechanism. The critical temperature can be adjusted by doping, which enables the design of multi-stage magnetic refrigerators. Competing exchange interactions broaden the magnetic transition and lead to entropy release over a wide temperature range, thereby enhancing RC. The investigated materials have less than ten per cent rare-earth content. The researchers elucidate novel mechanisms for achieving giant MCE in these materials with minimal rare-earth content. The educational impacts include the engagement of underrepresented students in a multidisciplinary research project between chemistry and physics as well as training students at the national high-magnetic field and synchrotron radiation facilities.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.