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

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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.