We have investigated the magnetic, structural, and thermodynamic properties of Ho5Co6Sn18 through x-ray and neutron diffraction, magnetization, and specific heat measurements. Ho5Co6Sn18 displays a magnetic transition at TM=3.4K and can be described with the magnetic space group I41/ac′d′. 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 TM. 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 Ho(2)4 tetrahedron. At 60 mK, the ordered moment of Ho(2) reaches 4.01(8)μB, 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 TM and significantly weakens with the magnetic ordering.
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