Investigation of the physics underlying the principles of design of rare earth - transition metal permanent magnets.

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

Permanent magnets are pervasive in both established and developing technologies. Found in motors and generators, transducers, magnetomechanical devices and magnetic field and imaging systems, there is a multi-billion pound worldwide market for them. They are also both fascinating and challenging in terms of their fundamental materials physics.
With the drive towards more energy efficient technologies, renewable energy supplies and further miniaturisation of devices, there is a growing demand for stronger and cheaper magnetic materials. Most strong magnets are comprised of rare earth (RE) and transition metal (TM) atoms arranged in specific crystal structures. The TM element, such as iron or cobalt, helps the ferromagnetism to persist to high temperatures and the RE component, such as neodymium or samarium, is there to generate a large magnetisation which is hard to reorientate away from an 'easy' direction specified by the crystal structure. Nd2Fe14B-based magnets, originally developed back in the 1980's, are very widely used examples but their magnetic performance deteriorates rapidly above T=100 C and for this reason they are doped with critical rare earth metals like Dy for many applications. This inevitably leads to environmental and geopolitical supply issues. The other well-known RE-TM champion permanent magnet class, Sm-Co5-based, developed in the 1970's, has better high temperature performance but the cost and availability of cobalt can be a problem. There is now a concerted effort worldwide to come up with new permanent magnetic materials with improved magnetic characteristics and reduced dependence on critical elements. However much of this search is being conducted heuristically. There is therefore an excellent opportunity for our proposed ab-initio magnetic materials modelling, applied and tested in parallel with state-of-the-art sample synthesis, characterisation and experimental investigation to have an impact. This work is aimed understanding intrinsic magnetic properties and refining the design principles of RE-TM magnets.
Each RE atom in the magnet has a magnetic moment which is set up by its nearly localised f-electrons. These moments are immersed in a glue of septillions of valence electrons coming from all the RE and TM atoms. Local magnetic moments associated with the TM atoms can also emerge from this complex electron fluid. The magnetic properties stem from how the RE and TM local moments affect and are affected by each other and the electron glue, on how the atoms are arranged and on the overall response to applied fields. We will establish and apply a theory which provides a parameter-free accurate account of the valence electrons and which incorporates the effects of the local moments by averaging over them so that temperature dependent effects can be described. With inclusion of spin-orbit coupling of the electrons, predictive modelling of the temperature, compositional and structural dependence of the magnetic hardness of the RE-TM magnets is feasible. To bring this to fruition, at each stage, we will test and improve the theory by comparison with detailed experimental measurements, both laboratory-based and at central facilities. We will study three relatively simple crystal structures which many RE-TM combinations form and drive each study towards addressing a technologically relevant topic whilst planning the work to best extract insight into the fundamental materials physics. The challenges are: (1) Find out how to improve Sm-Co5-based magnets for use at high temperatures by compositional tuning; (2) Investigate if a Nd-Fe12-based magnet can be designed with better (cheaper) permanent magnet properties than Dy-doped Nd2Fe14B; (3) Find the optimal ranges of temperature and composition for Tb(1-x)Dy(x)-Fe2 intermetallics for the development of new magnetostrictive materials. Suitable high temperature magnets will also be built into trial sensors and tested over a range of temperatures.

Planned Impact

There will be strong, direct benefits mainly among academic and industrial researchers of magnetic phenomena and materials, crucial to a wide range of technologies, and among the researchers on the project itself. We expect major long term economic benefits to arise from the exploitation of intellectual property (IP) - in particular, the detailed design rules for permanent magnets, along with the underpinning physics, to be elaborated by our joint theory-experiment studies. 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.
Whilst our proposal is directed at understanding the fundamental materials physics of rare earth - transition metal magnets, a notable strength of our team for delivering impact is applied physicist R.S.Edwards' (RSE) and materials scientist A. Walton's (AW) strong, well-developed industrial links. RSE is a member of the EPSRC-funded Research Centre in Non-Destructive Evaluation (RCNDE) and the Warwick-based (UoW) Centre for Industrial Ultrasonics. RSE will present results from the materials testing and sensor production to industrial members and seek support to take any promising magnetic materials for EMAT technology to the next level. AW leads the Magnetic Materials Group at Birmingham (UoB) which is the only UK research team focussed on processing of permanent magnetic materials. In May 2015 he chairs the UK Magnetics Society which has over 400 industrial and academic members and has very strong direct industrial links with over 20 companies both within the UK and internationally at various points in the supply chain for rare earth magnets. New materials discovered within this project will be exploited though these industrial networks.
This project will describe intrinsic magnetic properties on the nanoscale. To make the most of the results obtained they need to be incorporated into simulations of magnetic materials over longer length scales so that device functionality and manufacturing processing effects can be modelled. A new EPSRC-funded Collaborative Computational Project (CCP) on computational magnetism (EP/M022668/1) will help us deliver this objective. This network serves to develop, maintain, and distribute high quality codes, addressing multi-scale magnetism. It will also organise meetings, workshops and conferences. J.B.Staunton (PI) is to be its inaugural chair and other founding core members include STFC Daresbury project partners. This new CCP will reach beyond the immediate academic community and directly engage with industrial researchers developing new magnetic devices and materials.
The project's 2 PDRAs will have excellent career development opportunities through working with a diverse team of physicists and material scientists and developing important transferable skills. The present project additionally has great scope for valuable and engaging summer work for undergraduate students and we will run several projects each year funded by the UoW's Skills Centre. This is a major benefit to the development of high-level research skills for graduates. UoW Physics' full-time school teacher-fellow manages proactive schools engagement programmes. Strong magnets are excellent for hands-on demos and questions about their behaviour and wide-spread use. RSE is heavily involved in physics outreach and regularly talks at schools about her research. This project will inform and expand the range of research she can present, and she will work with the teacher-fellow to ensure this reaches the local public and school children. AW heads outreach for the School of Metallurgy and Materials (UOB) and magnetic materials form a major part of the activities into which our project will be incorporated. AW regularly runs open days for schools as part of the institute of materials schools programs, alongside two STEM ambassadors in his group.

Publications

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

 
Description We have developed an sub-nanoscale quantitative description of the temperature dependence of magnetism in doped rare earth-Co5 compounds by a theoretical/computational investigation compared with experimental measurements. We have developed a new theory in which we can directly simulate the experiments used to measure the Magnetic Anisotropy of Rare-Earth-Transition-Metal Ferrimagnets and tested it by comparing its results to experimental measurements made on a single crystal of GdCo5 which we grew. A further joint experiment/theory project has investigated the high magnetic field induced canting of the magnetic moments in this ferrimagnet, We have applied the theory to the whole family of the rare earth-Co5 magnets and describe well the variation of their magnetisation with temperature showing that our approach is applicable to a wide range of permanent magnets. Commercial magnets based on samarium and cobalt (Sm-Co) actually consist of several different crystal phases with varying chemical compositions. The bulk of the magnet is made up of Sm2Co17, but these regions are surrounded by a cell boundary phase which has a chemical composition closer to Sm(Co1-x-yFexCuy)5. We studied the prototype cell boundary phase Y(Co1-x-yFexCuy)5 to understand how the chemical composition (iron and copper content) affects the magnetic properties, namely the magnetic anisotropy, of the transition metal. We have developed our torque magnetometry and crystal growth capabilities and are extending the joint theory/experimental work to the magnetostrictive rare earth - Fe2 Laves phase magnets.
A paper describing our new theoretical framework for a parameter-free approach to computational modeling of rare earth/transition metal magnets at nonzero temperature was selected as a highlight in Physical Review Materials as an Editor's Suggestion. The paper demonstrates this capability with a theory/experiment comparison for the rare earth-Co5 permanent magnet class of which the champion magnet SmCo5 is a prominent member. https://doi.org/10.1103/PhysRevMaterials.3.101401
Exploitation Route As guidance for ways to optimise the properties of rare earth - transition metal magnets. Improvements for ab-initio theory modelling of ferrimagnets. Techniques for single crystal growth of rare earth - transition metal magnets.
Sectors Energy,Manufacturing, including Industrial Biotechology

URL https://warwick.ac.uk/fac/sci/physics/research/theory/research/electrstr/pretamag/
 
Description Lecture at European School of Magnetism "Condensed Matter Magnetism : bulk meets nano" 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Like previous editions of ESM, the 2017 School aimed at providing a thorough understanding of magnetism based on a broad series of fundamental lectures, while offering the latest insights into up-to-date aspects of magnetism with lectures focusing on a special topic. The topic covered in 2017 wasmodern aspects overlapping condensed matter in bulk materials on one side, and nanomagnetism/spintronics on the other side. This covered a wide range of fundamental phenomena deeply rooted in condensed matter physics, and opportunities for applications. The detailed topics covered were: basic concepts, magnetism in matter, tools, magnetization textures and dynamics, materials and effects, industry perspectives. The School was an opportunity for young scientists from the two fields, to meet, share their expertise and network. I was one of the organisers and gave a lecture on spin-orbit coupling and magnetic anisotropy.
Year(s) Of Engagement Activity 2017
URL http://magnetism.eu/69-program.htm