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=frac(1, 3)[1 1 0] and the size of the CO domains can be estimated to be about 10-20 nm 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 50 K, 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.