Model interfaces for redesign of Nd-Fe-B magnets

Rare-earth-element permanent magnets based on Nd-Fe-B are vital for use in electric vehicles and wind turbines, making them central to Europe’s green-energy future. These materials have outstanding magnetic properties, based on the Nd2Fe14B phase. However, the intrinsic properties of the tetragonal Nd2Fe14B phase are massively under-exploited in a magnet and much effort is needed to translate them into better extrinsic magnet properties. This is because Nd-Fe-B magnets are complex, multiphase materials, whose properties do not depend only of the intrinsic properties of the Nd2Fe14B phase but also on the microstructure of the whole material and especially on the nature of the grain boundary phases formed during material processing. The global project in which the PhD program will take place involves creating novel grain boundaries and interfaces based on micromagnetic simulations and computational thermodynamics. An important aspect will be to test these ideas by creating model interfaces using purposely grown Nd2Fe14B single crystals and growing specific interface thin films. The methodology will be further extended to isolated single grains from recycled or fresh streams to develop a new form of Nd-Fe-B magnet by redesigning the magnet microstructure with innovative, single-grain in-situ grain-boundary engineering. The PhD program will focus on the most fundamental aspects of the above global project, by creating model interfaces from which the basic physical mechanisms at play can probably be better understood. For this purpose, the growth of high quality single crystals will be carried out, either by self-flux or by Czochralski methods, with dimensions of a few mm2. Possible improvements in the sample sizes will be tested using alternative methods such as the floating zone crucible-free method. Crystals will be extracted and aligned using back-Laue X-ray diffraction. The low index (001) and (100) surfaces of single crystals will be examined under ultra-high vacuum conditions down to the atomic level. This is challenging as, despite the technological importance of the Nd2Fe14B tetragonal phase, the literature on the surface science of this material is very limited. The first step will consist in the optimization of the surface preparation conditions in order to obtain atomically flat terrace and step surface morphology. The surfaces will be characterized by methods such as low-energy electron diffraction, scanning tunneling microscopy and photoemission spectroscopy. Then a model interface will be created by growing an ultra-thin film of the specific interfacial material on top of the bare Nd2Fe14B substrate. The initial growth mode of the film along with its structure and chemistry will be determined at the surface for various conditions. An interface analysis (cross-sectional observation) will bring crucial information on the chemical distribution and possible phases formed as a function of the growth parameters and post-treatments. The resulting magnetic properties will be investigated at various length scales using different techniques, in collaboration with other partner’s institutions. The detailed understanding of such model system will be crucial to implement the global strategy of Single-Grain Re-Engineered Nd-Fe-B Permanent Magnets at the industrial scale.