Rare-earth based alloys for hard-magnetic applications: Temperature and pressure dependent phase stabilities

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Hard-magnetic materials play a central role in current European efforts to develop sustainable energy concepts that allow climate protection. The functionality of most devices that contain electric engines (such as electrical or hybrid cars) or generators (such as wind turbines) depends crucially on the performance of the permanent magnets. The current trend goes towards systems with a high magnetic energy density to increase efficiency and to reduce size, weight and production costs of devices. For this Nd2Fe14B is currently the system of choice. The main reason for intensive world-wide efforts to replace this material system is, however, the high price for the involved rare-earth elements such as Dy or Tb, but also Nd. In contrast to them, Ce is a rather abundant rare-earth element, which when used in compounds could lead to so-called rare earth balance magnets. Ce-based hard-magnetic materials are, however, difficult to produce, since the magnetically interesting ternary phases are often competing with binary phases such as CeFe2 in the resulting microstructures.A main goal of the current research project is therefore to achieve a fundamental understanding for the physical reasons behind the thermodynamic stability of RE-based alloys, with RE=Ce, Pr, and Nd and to develop from there efficient routes to synthesise Ce-containing intermetallics. Using a variety of theoretical and experimental methods, we will derive phase diagrams for these materials. Three degrees of freedom will be in the focus of the investigations: (i) the temperature, T, is a key control parameter in the production process, but also for many applications; (ii) the pressure, P, can have a strong impact on meta-stabilities as well as magnetic properties, and (iii) the chemical composition, X, can be varied such that physical trends for phase stabilities become more apparent and that intrinsic and extrinsic magnetic properties are further optimized. These questions will be addressed in a joint effort of three partner institutions: Density-functional theory-based techniques will be used at Max-Planck-Institut für Eisenforschung to derive free energies and all relevant entropy contributions for the full parameter space, P-T-X, from first principles. The theoretical challenge of strong electronic correlations will be addressed by Ecole Polytechnique with a particular focus of chemical trends using dynamic mean-field theory (DMFT). Advanced synthesis and comprehensive characterization in wide H-, P- and T-ranges will be performed at the Technische Universität Darmstadt and the obtained experimental data will be used as benchmark for theory. Synthesis will also benefit from the theoretical input to develop a thermomechanical synthesis route in the P-T-X diagram that allows stabilizing the desired magnetic phases.