Enhancement of ferromagnetism by Co and Ni substitution in the perovskite LaBiMn₂O_6+δ

Abstract

The substitution of cobalt and nickel for manganese in the perovskite manganate LaBiMn₂O_6+δ has been realized, leading to the perovskites LaBiMn₂₋M_xO_6+δ, with M=Co, Ni and 0≤x≤2/3. In contrast to the literature these phases are found to be orthorhombic with Pnma symmetry. More importantly, it is shown that ferromagnetism is enhanced, T_C being increased from 80 K for the parent compound (x=0) to 97 K for the Ni phase, and to 130 K for the Co phase. Moreover, strong competition between ferromagnetism and a glassy-ferromagnetic state in the case of nickel or 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.