We have investigated the magnetic structure of the fcc antiferromagnet $\mathrm{Ho}{\mathrm{B}}_{12}$ by magnetization and specific heat measurements on small single crystals prepared from natural elements and by neutron diffraction on isotopically enriched powder samples. Magnetization measurements up to $9\mathrm{T}$ show up to three magnetic phases in the $B$ vs $T$ phase diagram, depending on the orientation of the applied field. The specific heat in zero field exhibits a very steep increase at $T_N=7.4\mathrm{K}$, but its maximum is reached only at a lower temperature. In applied magnetic field up to $8\mathrm{T}$ additional $\lambda$-like anomalies are observed which confirm the phase boundaries from the magnetization measurements. Powder neutron diffraction in zero magnetic field reveals an antiferromagnetic structure below $T_N$. The basic reflections can be indexed with $(1∕2\pm \delta 1∕2\pm \delta 1∕2\pm \delta )$, where $\delta =0.035$, pointing to an incommensurate magnetic structure. In a field below $2\mathrm{T}$ (in the lowest-field magnetic phase) the principal reflections remain; in a higher magnetic field they become suppressed. Moreover, the magnetic background strongly decreases with applied field. The analysis of results shows that an amplitude-modulated, incommensurate structure likely represents the magnetic order of $\mathrm{Ho}{\mathrm{B}}_{12}$. The very complex phase diagram of this compound can arise from the interplay between the RKKY and dipole-dipole interaction and/or from frustration effects in the fcc-symmetry lattice.

• Received 5 November 2003

DOI:https://doi.org/10.1103/PhysRevB.70.224416