Convergent beam electron diffraction study on ge-based oxide spinels

Transition metal oxides with spinel crystal structure exhibit intriguing and non trivial magnetic phenomena owing to magnetic frustration between spins having antiferromagnetic coupling interaction on triangle or kagome lattice. GeCo2O4(GCO) and GeNi2O4(GNO), which belong to above category, are very rare normal spinels containing Ge ion. Both reveal antiferromagnetic-like phase transitions at 20 K and 12 K, respectively. According to previous neutron and x-ray diffraction measurements, GNO keeps its cubic structural symmetry down to 2 K which is not natural because such a magnetic transition tends to associate with symmetry breaking structural transitions. In order to know whether the structural transition or symmetry change occur or not at the magnetic transition in detail, convergent beam electron diffraction measurements is employed for the compounds.