Magnetic order in the fcc symmetry: phase diagram and structure of ReB 12

We present results of studies of the phase diagram and the magnetic structure of the rare-earth dodecaborides ReB12 (Re=Tm,Er,Ho) based on magnetisation measurements and neutron-scattering experiments. For the magnetisation studies single-crystalline samples of TmB12, ErB12 and HoB12 were used. Neutron-scattering experiments were performed on a non-isotopically enriched HoB12 powder sample above and below the Neel temperature and in zero magnetic field and on enriched Ho11B12 at 1.6 K and in magnetic fields of 0 T and 4.5 T. Our investigations show that the compounds TmB12 and ErB12 have a single phase diagram while, in HoB12, a more complex phase diagram is found in an applied field and neutron-diffraction data reveals unconventional antiferromagnetic order at q=(2/3,1/3,1/3) and (0,2/3,2/3). The results are similar to the nuclear ordering of Cu and Ag and in good agreement with first-principles theories developed for magnetic order in the frustrated fcc symmetry. This indicates that the rather complex phase diagram of rare-earth dodecaborides arises from the interplay between dipolar and RKKY interactions.