Effect of La doping on the structure and cycloidal spin ordering in multiferroic BiFeO3

A series of Bi1-xLaxFeO3 samples with 0.00 <= x <= 0.30 was synthesized by the sol-gel method. The effects of lanthanum concentration on the phase formation, microstructure and cycloidal spin ordering were studied using X-ray diffraction, scanning electron microscopy and Mo ssbauer spectroscopy. The crystal structure of the La-doped bismuth ferrite transformed from rhombohedral R3c (x <= 0.05) to a mixture of R3c and cubic Pm (3) over barm (0.07 <= x <= 0.15) and finally to a mixture of R3c, Pm (3) over barm and orthorhombic Pbam (0.20 <= x <= 0.30). The Pbam phase, with characteristic porous microstructure shown by microscopy images, was observed in Bi1-xLaxFeO3 compounds for the first time. Based on the Mossbauer spectroscopy, it was found that the cycloidal spin ordering started to disappear at x = 0.07. With increasing La concentration the share of the cycloid decreased from 100% at 0.00 <= x <= 0.05 to 0% at x = 0.30. At the beginning, for x <= 0.02, the anharmonicity parameter, m, of the cycloidal spin ordering was about 0.5, which is typical of a pure BiFeO3 compound. In the range 0.05 <= x <= 0.25, the m parameter was of the order of 0.1, which indicated the practically harmonic character of the cycloid. The structural transition at x = 0.07 was accompanied by a substantial increase in magnetization.