Rare-earth-element (REE) 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 on 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 Nd2Fe14B phase itself has a high saturation magnetization and a large monocrystalline anisotropy, providing a high remanence (Br), reflected in the magnet “strength”, and a high intrinsic coercivity (Hci), making it resistant to demagnetization. The overall performance of the magnet is described by the maximum energy product ((BH)max), which depends both on Br and Hci (see a chart of for various materials).
However, in conventionally sintered magnets, the Nd2Fe14B grains are microscopic. When exposed to a demagnetizing force, demagnetization begins at weaker regions normally at the grain interfaces with the grain-boundary phase before rapidly spreading, i.e. the nucleation model, with magnet coercivity being influenced by the chemical, structural, and magnetic properties of the grain surfaces and the grain boundaries. Despite all the efforts, the coercivities of Nd-Fe-B permanent magnets are only 20% of what is theoretically possible (i.e., the Ha), meaning that significant improvements are possible. With this in line, an idea is to develop Single-Grain Re-Engineered Nd-Fe-B Permanent Magnets by stripping Nd2Fe14B grains from connected impurities of the existing Nd-rich phase and creating a new Nd2Fe14B/grain-boundary interface in-situ on fresh and recycled feedstocks. This process could allow for customizable REE content in the grain boundary and could reduce or even eliminate the need for heavy additional REE. 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 post-doctoral 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. An optimized grain boundary phase of the Nd2Fe14B magnet will be suggested based on a computational thermodynamics approach for the prediction of intrinsic properties (saturation magnetization - Ms, Curie temperature - TC, anisotropy constant - K1 and/or anisotropy field - Ha) to obtain the highest possible magnetocrystalline anisotropy and highest TC with alloying elements. The proof-of concept of novel Nd-Fe-B permanent magnet with new grain boundary will be tested on model interfaces. 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 mm3. 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 (LEED), scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS). 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.

Methods to be employed involve the synthesis of single crystals of the pure Nd2Fe14B phase with large dimensions that are suitable for surface science experiments requiring ultra-high vacuum (UHV) conditions. Single crystals can be grown by the self-flux or by the Czochralski methods, with dimensions of a few mm2. Improvements in the sample dimensions are foreseen 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 UHV. Contamination-free surfaces will be prepared by cycles of Ar+ sputtering followed by optimized annealing cycles at specific temperature in order to obtain atomically flat terraces separated by steps. The surface cleanliness and the near surface chemical composition (first few nanometers) will be determined by in-situ XPS. The surface atomic structures will be investigated by LEED and STM down to the atomic level. Such atomically clean surfaces will serve as platforms to construct model interfaces. Thin films replicating the specific grain boundary phase will be deposited under UHV conditions by physical vapor deposition methods. Different growth parameters will be evaluated, including deposition flux, substrate temperature, post-treatments, etc.. Cross-section lamellae will be prepared and investigated by transmission electron microscopy in various modes. Physical properties of the model interfaces will be analyzed using different experimental methods, both in-house and at partner's institutions.

The Institute Jean Lamour (IJL) is a joint research unit of CNRS and Université de Lorraine.
Focused on materials and processes science and engineering, it covers: materials, metallurgy, plasmas, surfaces, nanomaterials and electronics.
The IJL has 263 permanent staff (30 researchers, 134 teacher-researchers, 99 IT-BIATSS) and 394 non-permanent staff (182 doctoral students, 62 post-doctoral students / contractual researchers and more than 150 trainees), of 45 different nationalities.
Partnerships exist with 150 companies and our research groups collaborate with more than 30 countries throughout the world.
Its exceptional instrumental platforms are spread over 4 sites ; the main one is located on Artem campus in Nancy.