Enhancement of ferromagnetism by Co and Ni substitution in the perovskite LaBiMn2O6+d

The substitution of cobalt and nickel for manganese in the perovskite manganate LaBiMn2O6+d; has been realized, leading to the perovskites LaBiMn2-x(M)xO6+d, with M = Co, Ni and x =0-2/3. In contrast to the literature those phases are found to be orthorhombic with Pnma symmetry. More importantly, it is shown that ferromagnetism is enhanced, TC being increased from 80 K for the parent compound (x = 0) to 97 K for Ni-phase, and to 130 K for the Co-phase. Moreover, a strong competition between ferromagnetism and a glassy-ferromagnetic state in the case of nickel or a spin-glass behaviour in the case of cobalt is observed. These phenomena are interpreted in the frame of a phase separation scenario, where the ferromagnetic Mn4+/Ni2+ and Mn4+/Co2+ interactions reinforce the Mn3+/Mn4+ interactions. These compounds are found to be insulating with a relatively large positive value of thermoelectric power.