Thota, S.; Reehuis, M.; Maljuk, A.; Hoser, A.; Hoffmann, J.-U.; Weise, B.; Waske, A.; Krautz, M; Joshi, D. C.; Nayak, S.; Ghosh, S.; Suresh, P.; Dasari, K.; Wurmehl, S.; Prokhnenko, O; Büchner, B.: Neutron diffraction study of the inverse spinels Co2TiO4 and Co2SnO4. Physical Review B 96 (2017), p. 144104/1-13
10.1103/PhysRevB.96.144104

Abstract:
We report a detailed single-crystal and powder neutron diffraction study of Co2TiO4 and Co2SnO4 between the temperature 1.6 and 80K to probe the spin structure in the ground state. For both compounds the strongest magnetic intensity was observed for the (111)M reflection due to ferrimagnetic ordering, which sets in below TN = 48.6 and 41 K for Co2TiO4 and Co2SnO4, respectively. An additional low intensity magnetic reflection (200)M was noticed in Co2TiO4 due to the presence of an additional weak antiferromagnetic component. Interestingly, from both the powder and single-crystal neutron data of Co2TiO4, we noticed a significant broadening of the magnetic (111)M reflection, which possibly results from the disordered character of the Ti and Co atoms on the B site. Practically, the same peak broadening was found for the neutron powder data of Co2SnO4. On the other hand, from our single-crystal neutron diffraction data of Co2TiO4, we found a spontaneous increase of particular nuclear Bragg reflections below the magnetic ordering temperature. Our data analysis showed that this unusual effect can be ascribed to the presence of anisotropic extinction, which is associated to a change of the mosaicity of the crystal. In this case, it can be expected that competing Jahn-Teller effects acting along different crystallographic axes can induce anisotropic local strain. In fact, for both ions Ti3+ and Co3+, the 2tg levels split into a lower dxy level yielding a higher twofold degenerate dxz/dyz level. As a consequence, one can expect a tetragonal distortion in Co2TiO4 with c/a < 1, which we could not significantly detect in the present work.