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Research Project

"Hunting for rare-earth-free materials"

Rare earth elements can carry a local magnetic moment which is composed of spin as well as orbital contributions of its electrons. Such magnetic moments can provide the pronounced magnetic anisotropy which is of fundamental importance for many applications and basic model systems. Examples include hard permanent magnets in electric motors and wind turbines as well as novel materials for data storage or realizations of the Ising and Heisenberg model. However, rare earth elements are expensive to purify in an ecologically safe manner and at risk of supply shortages. Replacing them by more abundant materials is highly desirable. Besides these practical considerations, the crystal electric field in rare earth elements acts only as a weak perturbation when compared to the effect of spin-orbit coupling. Combining the strong crystal electric field effects observed for transition metals with the large single-ion magnetic anisotropies of rare earth elements may open the way for novel model systems showing a unique interplay of both effects.

Iron-doped lithium nitride represents a novel magnetic material with properties typically associated with those of rare-earth elements. In particular, a pronounced magnetic anisotropy has been observed. At the origin of the unusual behavior of iron-doped lithium nitride is the contribution of the orbital momentum of iron. It is not quenched by the crystal electric field, in contrast to what is usually observed in solids. The resulting magnetic anisotropy and coercivity outperform even well-established hard permanent magnets.

The project aims on establishing iron-doped lithium nitride as a model system. This shall be realized by growing high-quality single crystals, performing a profound characterization and investigating the compound in collaboration with specialized groups. The obtained results are going to support and guide the search for further novel materials with unquenched orbital moments. The special geometry that the iron finds itself in provides a first guiding priciple: one iron atom positioned between two next-neighbor nitrogen atoms, that is a linear two-fold coordination. Known compounds with a corresponding crystal structure and related systems are going to be synthesized and characterized.

An important goal is the investigation and manipulation of the magnetic ordering of such local moments. This can be achieved by doping or applying pressure and can be first tested on lithium nitride. Based on the obtained results, the search for magnetically ordered orbital moments is going to be extented, conceivalbly, beyond the structural motif of the linear, two-fold coordination.

Transition metal compounds with unquenched orbital moments are of technological relevance as well as interesting for basic research. The former is apparent from the rare-earth price trend, showing up to ten-fold increases over the past years. The discovery of a hard, rare-earth-free permanent magnet with sufficiently high ordering temperature and energy product would be of significant commercial relevance. Besides these practical considerations, it might be possible to realize novel magnetic model systems based on the exceptional interplay of spin-orbit coupling and crystal electric field effects. For rare-earth elements, the latter acts only as a weak perturbation whereas both effects are of comparable strength for transition metals. This is at the origin of the unique magnetic properties of iron-doped lithium nitride that shall be understood in detail over the course of the research project.