Spin-orbit coupling inactivity of Co${}^{2+}$ ion in geometrically frustrated magnet GeCo${}_2$O${}_4$

We report single-crystal neutron diffraction studies on a spinel antiferromagnet GeCo${}_2$O${}_4$, which exhibits magnetic order with a trigonal propagation vector and tetragonal lattice expansion ($c\mathrm{/}a\simeq 1.001$) below $T_{\mathrm{N}}=21$ K. For this inconsistency between spin and lattice in symmetry, magnetic Bragg reflections with a tetragonal propagation vector were discovered below $T_{\mathrm{N}}$. We discuss spin and orbital states of Co${}^{2+}$ ion underlying the new magnetic component.