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

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

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.

Planned Impact

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.

Publications

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Boldrin D (2018) Multisite Exchange-Enhanced Barocaloric Response in Mn 3 NiN in Physical Review X

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Mankovsky S (2017) Temperature-dependent transport properties of FeRh in Physical Review B

 
Description Ab-initio materials modelling is based on well-established electronic density functional theory (DFT) and is typically confined to treatments of materials at low temperatures. Standard DFT is therefore not suited to investigating some technologically important materials' properties at higher temperatures. An extension of DFT,the disordered local moment (DLM) picture, addresses this shortcoming for magnetic materials by incorporating the effects of magnetic excitations. In this project we developed this DFT-DLM method to explore field and temperature induced metamagnetic transitions and their applications for magnetic refrigeration. We demonstrated this materials modelling
to be predictive via the close collaborations with project partners in experimental groups at Ames Lab., USA on rare earth materials and Imperial College on transitions between magnetic states.

Highlights include the following:
(i)We explained experimental observations of Gd-Mg, Gd-Zn and Gd-Cd compounds which are at odds with the conventional free electron RKKY model of rare earth magnetic interactions. We found the origin of the ferromagnetic - antiferromagnetic competition in Gd-Mg manifested by non-collinear, canted magnetic order at low temperatures, and
explained why its isoelectronic cousins, Gd-Zn and Gd-Cd, are instead simple ferromagnets with much higher Curie temperatures. Moreover our predicted pressure dependence of the Curie temperature for Gd-Cd, was confirmed by our Ames experimental collaborators. The insight from this work, exposing a prominent role for d-electrons in a complex valence electron glue for the magnetic ordering of the 4f moments, provides strong direction for further joint theory-experiment work on permanent magnets.

(ii) In another study on CoMnSi-based materials we showed how the metamagnetism of an antiferromagnetic metal can be tuned and tricriticality found even in the absence of anisotropy. Experiment confirmed the magnitudes of the calculated critical magnetic fields and also the predicted trend produced by adding (removing) electrons by Ni (Cr) doping. The theory also explained the sensitivity of the temperature dependence of the metamagnetic critical fields to Mn-Mn separations. Tuning a magnetic refrigerant material close to a tricritical point is desirable for applications.

(iii) Another study on FeRh alloys found that a large fraction of their record sized magnetocaloric effect that occurs around a ferromagnetic-antiferromagnetic transition is electronic and that the transition itself results from a fine balance of competing electronic effects which is disturbed by small compositional changes and defects.

(iv) In an collaboration with an experimental group at Brookhaven Lab.(USA) the magnetic interactions in a transition metal oxide insulator were revealed.
Exploitation Route The materials modelling developed in this project has been shown to be predictive and can be applied to a wide variety of magnetic materials to aid the design of materials with desired magnetic properties over chosen temperature ranges. It can used in conjunction with magnetic characterisation and spectroscopy experiments. Several studies are on-going and future studies being planned. Further developments are also being directed at the physics underlying the properties of rare earth - transition metal permanent magnets which are technologically important for many applications. The modelling is being applied to magnetic materials in which stimuli other than magnetic fields can provoke a heating or cooling effect. This coupling between different caloric effects has the potential to lead to new solid state cooling devices.
Sectors Energy,Manufacturing, including Industrial Biotechology

URL http://www2.warwick.ac.uk/fac/sci/physics/research/theory/research/electrstr
 
Description Article in International Innovation magazine, 2013 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Media article describing some work on magnetic materials published in "International Innovation" magazine produced by Research Media Ltd. (http://www.researchmedia.eu/) whi claim that the magazine communicates research to the "broadest spectrum of stakeholders across research, policy and practice via a range of online and offline dissemination methods that are simple and free to access."

Increase in requests for further information about this research activity.
Year(s) Of Engagement Activity 2013
URL http://www2.warwick.ac.uk/fac/sci/physics/research/theory/research/electrstr
 
Description Co-organiser and speaker at Critical Rare Materials one day meeting at University of Birmingham, 21st Feb. 2014 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact My own talk sparked questions and discussion afterwards

The meeting as a whole prompted a call for a broad based network of people from academia, industry who are interested in critical rare earth materials, their applications and supply to be set up.
Year(s) Of Engagement Activity 2014
URL http://www.birmingham.ac.uk/university/colleges/eps/news/college/UK-Magnetic-Society-gathers-at-Birm...
 
Description Magnetism workshop(Chester) 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Adaptive magnetic materials, such as magnetic shape memory alloys or magneto-caloric intermetallics are characterized by the changes in structural and magnetic properties when subjected to an external influence.

The goal of this workshop was focussed on the understanding of magnetic materials beyond the ground state, and their response to external stimuli. In particular, the aim was to build solutions that will bridge the current gap in our fundamental understanding of the magnetic states of rare earth materials when compared with what is available for d-electron based materials. The workshop aimed to merge and enhance insights from the different computational approaches currently under development for the study of magnetic order and magnetic excitations. The predictions of the simulations and possible further improvements of the existing computational methodologies to f-electron systems were debated by the computational materials modellers and experts in both experimental and applied magnetism.
Year(s) Of Engagement Activity 2015
URL https://eventbooking.stfc.ac.uk/news-events/ab-initio-modelling-workshop
 
Description Organisation and running of workshop Computation meets Experiment, Warwick, July 2013 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Participants in your research and patient groups
Results and Impact The workshop comprised a 4-day 'hands-on' activity introducing largely early career researchers to the use of an electronic structure code that is being used in my research on magnetic materials followed by a weekend conference which focussed on experimental-computational modelling collaboration.

The feedback from participants was very favourable.
Year(s) Of Engagement Activity 2013
URL http://www2.warwick.ac.uk/fac/sci/physics/news/events/kkr_greens