Magnetic properties and noncollinear spin structure of the tin-rich stannide Ho5Co6Sn18

We have investigated the magnetic, structural, and thermodynamic properties of ${\mathrm{Ho}}_{5}{\mathrm{Co}}_{6}{\mathrm{Sn}}_{18}$ through x-ray and neutron diffraction, magnetization, and specific heat measurements. ${\mathrm{Ho}}_{5}{\mathrm{Co}}_{6}{\mathrm{Sn}}_{18}$ displays a magnetic transition at ${T}_{M}=3.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and can be described with the magnetic space group $I{4}_{1}/a{c}^{\ensuremath{'}}{d}^{\ensuremath{'}}$. The two holmium sublattices Ho(1) and Ho(2) exhibit different magnetic behaviors. Ferromagnetic order on the Ho(1) site develops into a net magnetization along the $c$ axis below ${T}_{M}$. In contrast, the ordering of the Ho(2) site is determined by geometric magnetic frustrations. The Ho(2) spins order into the noncollinear, two-in-two-out antiferromagnetic pattern on the $\mathrm{Ho}{(2)}_{4}$ tetrahedron. At 60 mK, the ordered moment of Ho(2) reaches $4.01\phantom{\rule{0.16em}{0ex}}(8)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, which is about half of the Ho(1) moment. Upon the application of a magnetic field, ferromagnetic components along the $c$ axis are induced on Ho(2), along with the in-plane antiferromagnetic components, indicating the XY-like spin anisotropy of the Ho(2) spins. A magnetic quasielastic neutron scattering signal is observed above ${T}_{M}$ and significantly weakens with the magnetic ordering.