Doping effect on the charge ordering in LuFe2O4

We have investigated change of the charge ordered (CO) structure by substituting Cu2+ by Fe2+ in LuFe2O4 by means of the transmission electron microscopy (TEM), combining the conventional magnetic measurement. It is found that the CO structure in LuFe2O4 is characterized by the formation of the superlattice structure with the wave vector q=13[110] and the size of the CO domains can be estimated to be about 10-20nm on average. On the other hand, the Cu2+ substitution in LuFe2O4 suppressed the formation of the CO structure and induced characteristic local lattice distortion, which gives rise to unique diffuse scattering in the reciprocal space. It is found that LuFeCuO4 exhibits an anti-ferromagnetic transition around 50K, which is lower than that in LuFe2O4. The present results suggest that the substituted Cu2+ ions would serve as quenched charge/spin impurities in the two-dimensional triangular lattice