Rare-earth nitrides: mixed valence, strongly correlated heavy fermions

Full text: Most mononitrides of the lanthanides (LN) are intrinsic ferromagnetic semiconductors; they form the only epitaxy-compatible series to populate this exceedingly rare material class. We have grown them as films for fifteen years, with the aim of developing devices, particularly MRAM for cloud computing centres. Along the way we have unravelled much concerning their outrageous coupled electronic/magnetic behaviour: strongly correlated electrons, strong spin-orbit interactions, orbital contributions to their ferromagnetic states, a zero-magnetisation ferromagnetic material. They offer exciting opportunities to investigate strongly correlated electrons and their heavy Fermion states. The LN adopt a strongly ionic (L⁺³ and N^-3) rock-salt structure. The 5d conduction and 2p valence bands are the full story in the end members (LaN, LuN) and in GdN, with its half-filled 4f band, but the rest of the fourteen LN have very narrow 4f bands threading through and hybridising with the more mobile 5d and 2p bands, exactly the heavy Fermion states that have attracted a great deal of mostly theoretical interest. They permit exploration of strong correlation and heavy Fermion physics, with in addition the potential, in these semiconductors, to dope into and across the hybridisation gap. We have now advanced to be able to identify several among the light rare-earth nitrides that show exactly heavy-Fermion promise. I will discuss the experiments that have opened the heavy-Fermion box, with a focus on EuN, SmN and NdN. (author)