The Changing Shape of Magnetic Refrigeration: an investigation of adaptive magnetic materials

Modern cooling is based almost entirely on a compression/expansion refrigeration cycle - a technology more or less unchanged since its invention over a century ago. It is a high-energy demand industry which consumes billions of kWh every year. Yet, modern refrigeration is close to its fundamental performance limit which is well below what is thermodynamically possible. Furthermore, the liquid chemicals used as refrigerants, which eventually escape into the environment, are ozone layer depletive and global warming gases, or hazardous chemicals.

Recently magnetic refrigeration has emerged as a promising way for a new and environmentally friendly solid state cooling technology. Prototype magnetic fridges have been demonstrated during the last decade. They have been proven to be much more energy efficient than conventional fridges and can span a broad temperature range around room temperature. But most prototypes use expensive rare earth metals such as gadolinium as the refrigerant and alternatives are urgently required. Several families of promising magnetic materials have been discovered but up to now this process has been a heuristic one. In this proposal we intend to establish an ab-initio quantum materials modeling tool to transform this process and to facilitate its application by groups working with magnetic materials. In the most suitable materials the interactions that underpin the magnetic properties have to be delicately poised and our modeling will need to be able to track and indicate their temperature dependence, how they vary with compositional and structural changes and/or when dopants are added.

In a magnetic refrigerant randomly oriented magnetic moments in the material align when a magnetic field is applied making the solid warm up. By removing this heat using a heat transfer fluid, like water or air, and then removing the field allows the magnetic material to lower its temperature. The heat from the object being cooled is then extracted with the heat transfer fluid and the cycle completed. The changes in entropy and temperature that happen when a magnetic field is applied to a material describe the magnetocaloric effect and this proposal will determine it and the magnetic interactions behind it on a quantitative basis. Our results for several classes of materials will be tested against the extensive experimental data available. A particularly novel and ambitious part of the work will be to investigate how to nanostructure a large magnetocaloric effect. To this end we will study some rare earth - transition metal heterostructures and optimise the effect.

This physics which produces a strong warming effect when a magnetic field is applied has another intriguing facet. It can explain how some of the most promising materials also change their shape significantly in the presence of a magnetic field. Such magnetoplastic, 'magnetic shape memory' effects have diverse potential technological applications, such as micropumps, sonars and magnetomechanical sensors. We will adapt our theoretical nanostructural modeling to investigate the strengths and anisotropies of the magnetic interactions across a boundary defect in the material and how they lead to the defect itself moving as a magnetic field is applied. A test case of a Ni-Mn-Ga Heusler alloy will be undertaken and the effect will be optimised as the composition of the alloy is varied.

There will be strong, direct benefits arising from this work principally among the researchers on the project itself and academic and industrial researchers in the fields of solid state cooling, adaptive magnetic materials, spintronics, and magnetism. We expect major long term economical benefits to arise from the exploitation of intellectual property (IP), in particular the computational materials modelling techniques and proposals for promising new adaptive magnetic materials, developed during the project. Benefits will also accrue from our proposed outreach activities in the short term and from useful materials and practical devices developed in the longer term.

A project goal is to provide a fundamental understanding and optimisation of the magnetocaloric effect (MCE) in various intermetallic materials for the prediction of novel magnetocaloric materials. As a result, energy efficient and environmentally friendly magnetic refrigeration technology will have a strong materials foundation, and be rapidly developed to a commercial status. Moreover the closely related study of magnetic shape memory alloys can lead to novel applications not possible with more conventional adaptive materials.

Another source of solid state cooling, the electrocaloric effect (ECE) in relaxor ferroelectrics, also has strong commercial potential. There are natural parallels between what we are proposing for MCE materials and the more phenomenological modelling suited for electrocalorics. A postgraduate research project funded by a Warwick DTA and sponsorship from a local SME will start in October 2011. Both MCE and ECE communities share issues concerning measurement and analysis methods and how best to integrate materials into applications and both can benefit from the increased connections. Our project and the complementary ECE study can encourage these links.

The academic impact of our work will arise from publishing in general readership and top specialist journals. We will facilitate the application of the computational techniques developed for ab-initio modelling of magnetic materials at finite temperatures by other groups researching magnetic materials and advertise more broadly via the European Psi-k Network (http://www.psi-k.org/). Full documentation and a user-friendly interface for the computational code will be prepared in consultation with experimental colleagues and a 'hands on workshop' organised to encourage a wide userbase for the code. An International Conference on Magnetocaloric Materials will be organised bringing together the growing number of research groups, academic and industrial, working in this area and where our theoretical results will be reported, computational code advertised and workshop run.

The project's postdoctoral position will provide an excellent career development opportunity for the scientist involved. In addition to theoretical physics and scientific computing skills, the PDRA will develop useful transferable skills via their role liaising with experimental groups. The present project has a great scope for valuable and engaging summer vacation project work for undergraduate students and we would hope to run 2 projects funded by the Warwick University's Skills Centre. This is a major benefit for the development of high-level research skills for graduates.

The Warwick Physics Department employs a full-time school teacher-fellow who manages proactive schools engagement programmes. The importance of developing new 'green' and energy efficient cooling technology and the ideas underpinning this will encourage school students to study science further. Moreover 'Smart Materials' are part of the core GCSE science curriculum so telling school pupils about magnetically smart alloys will intrigue them about the possibilities of science, as well as helping to make learning more appealing. We will develop resources with the guidance of the teacher-fellow and distribute to local schools.