Spin-orbit coupling inactivity of Co2+ ion in geometrically frustrated magnet GeCo2O4

We report single-crystal neutron diffraction studies on a spinel antiferromagnet GeCo2O4, which exhibits magnetic order with a trigonal propagation vector and tetragonal lattice expansion (c/a ≃ 1.001) below TN = 21 K. For this inconsistency between spin and lattice in symmetry, magnetic Bragg reflections with a tetragonal propagation vector were discovered below TN. We discuss spin and orbital states of Co2+ ion underlying the new magnetic component.