Magnetic properties of CuFe_1-xCr_xO₂ nanoparticles surrounded by amorphous SiO₂

CuFe_1-xCr_xO₂ (0-≤-x-≤-1.0) nanoparticles surrounded by amorphous SiO₂ with an average diameter of 30-50-nm were synthesized using a wet chemical method. The annealing temperatures were controlled to yield various sizes of single-phase CuFe_1-xCr_xO₂ nanoparticles. CuFeO₂ bulk crystal is known to have a multiferroic delafossite structure with two Néel temperatures of 11 and 14-K; however, the transition temperature shifted higher as the Cr-ion doping level increased. In addition, the lattice constants decreased in accordance with increased Cr-ion doping, which was confirmed by X-ray diffraction measurements. The magnetization curves showed weak ferromagnetic behavior and no coercivity was observed. Hence, frustration in the triangular lattice of the delafossite structure can be released by Cr-ion doping and higher magnetization can be expected. A fine structure analysis through X-ray absorption fine structure measurements was also conducted. It was found that the structure of the Cu ion is similar to that of Cu₂O, and the c axis of the CuFe_1-xCr_xO₂ should be shortened by the Cr-ion doping.