The magnetic structures of ${\mathrm{DyFe}}_4{\mathrm{Al}}_8$ and ${\mathrm{HoFe}}_4{\mathrm{Al}}_8,$ which have been reported to be unusual spin-glass systems, were studied in detail by neutron diffraction, using both unpolarized and polarized beams. In fact these compounds have long-range magnetic order in both the Fe and rare-earth sublattices. The Fe sublattice orders at 175 K with the moments $\left(1.0\mathrm{}{\mu }_{\mathrm{B}}\right)$ in the $\mathrm{ab}$ plane in a cycloid magnetic structure with a propagation vector along [110]. At $\sim 50\hspace{0.5em}\mathrm{K}$ the rare-earth moment starts to order in ${\mathrm{DyFe}}_4{\mathrm{Al}}_8$ and follows the modulation of the Fe sublattice. The ordering of the holmium occurs at a slightly higher temperature $\left(\sim 80\hspace{0.5em}\mathrm{K}\right)$ than the dysprosium. At a lower temperature higher-order harmonics of the modulation develop. The magnetic structure of the rare-earth at low temperature is a bunched elliptical cycloid, following the modulation of the Fe sublattice. Although the antiferromagnetic coupling of the rare-earth magnetic moments has long-range order, giving sharp magnetic satellites in the diffraction patterns, a non-negligible fraction of the $4f$ moment does not contribute to these peaks but appears as diffuse scattering beneath the Bragg peaks. This indicates the presence of short-range ferromagnetic correlations between neighboring rare-earth moments. The magnetic structure of the rare-earth sublattice is very sensitive to a small applied magnetic field, mimicking spin-glass behavior in the bulk magnetic properties. The magnetic structure of ${\mathrm{DyFe}}_4{\mathrm{Al}}_8$ was studied under an applied magnetic field. A field as low as 0.125 T severely distorts the magnetic modulation and 0.75 T in the $\mathrm{ab}$ plane is sufficient to align all the rare-earth moments ferromagnetically. The cycloidal antiferromagnetic coupling of the Fe moments remains unperturbed up to at least 5 T.

• Received 22 September 1999

DOI:https://doi.org/10.1103/PhysRevB.61.6176