Complex antiferromagnetic order in the garnet Co 3 Al 2 Si 3 O 12

We studied the magnetic properties of the garnet Co(3A)l(2)Si(3)O(12) synthesized under 5.5 GPa and 1250 degrees 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 T-N = 11K without showing magnetic frustration, i.e., the frustration index f = vertical bar theta(CW)/T-N vertical bar 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 mu H-0(c) approximate to 4T was also observed below TN.