Complex antiferromagnetic order in the garnet Co3Al2Si3O12

We studied the magnetic properties of the garnet Co3Al2Si3O12 synthesized under 5.5 GPa and 1250 °C by means of dc magnetic susceptibility, neutron powder diffraction, and specific heat. Although the magnetic Co2+ ions form a geometrically frustrated hyper-Kagome lattice, we find that Co3Al2Si3O12 exhibits a long-range antiferromagnetic order below TN=11K without showing magnetic frustration, i.e., the frustration index f≡|θCW/TN| is close to unity. Refinements from neutron-diffraction data show that Co3Al2Si3O12 forms a complex antiferromagnetic structure characterized by the coexistence of collinear and commensurate spiral magnetic ordering of Co2+ sublattices. Magnetic field induced spin-flop transition at μ0Hc≈4T was also observed below TN.