Magnetic-Field-Induced Transitions in Spinel GeCo2O4

The spinel GeCo 2 O 4 has the pyrochlore lattice of Co 2+ ions, in which the kagomé and triangular planes stack alternately along the [111] direction. It shows antiferromagnetic ordering below $T_{\text{N}}\sim 21$ K with a characteristic wave vector of $\mathbf{Q}_{M}=(1/2,1/2,1/2)$. The spin arrangement is ferromagnetic in the kagomé and triangular planes and antiferromagnetic between the kagomé planes as well as between the triangular planes. GeCo 2 O 4 exhibits a magnetic phase transition at $H\sim 4$ T. Powder neutron diffraction measurements were performed on GeCo 2 O 4 in ambient and magnetic field. Our results are consistent with the magnetic structure model in which the kagomé and triangular planes behave independently in magnetic field, that is, the spin arrangement between the triangular planes becomes ferromagnetic above ${\sim}4$ T while retaining the antiferromagnetic arrangement between the kagomé planes.