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 ($c/a\simeq1.001$) below $T_{\rm N}=21$ K. For this inconsistency between spin and lattice in symmetry, magnetic Bragg reflections with a tetragonal propagation vector were discovered below $T_{\rm N}$. We discuss spin and orbital states of Co$^{2+}$ ion underlying the new magnetic component. 
Keisuke Tomiyasu, Ayaka Tominaga, Shigeo Hara, Hirohiko Sato, Tadataka   Watanabe, Shin-Ichi Ikeda, Kazuaki Iwasa, and Kazuy...
cond-mat.str-el