Effects of Isovalent Substitution in the Manganese Sublattice on Magnetic, Thermal, and Structural Properties of BiMnO₃:  BiMn_1-<i>_x</i>M<i>_x</i>O₃ (M = Al, Sc, Cr, Fe, Ga; 0 ≤ <i>x</i> ≤ 0.2)

Abstract

Solid solutions BiMn1-xMxO3 with M = Al, Sc, Cr, Fe, and Ga and 0 ≤ x ≤ 0.2 were prepared at a high pressure of 6 GPa and 1333−1453 K, and their magnetic, thermal, and structural properties were investigated. The orbital-ordered monoclinic phase of BiMnO3 (phase I) is destroyed by a small percentage of substitution. The M elements can be classified by their ability to destroy phase I in the sequence Ga (x ≈ 0.08) ≈ Fe (x ≈ 0.08) x ≈ 0.04) ≈ Al (x ≈ 0.04) x ≈ 0.02), where phase I is most stable for Ga substitution (up to x ≈ 0.08) and less stable for Sc substitution (up to x ≈ 0.02). The orbital-disordered high-temperature monoclinic phase of BiMnO3 (phase II) is stabilized with larger x. In all cases, a compositional range was found where phases I and II coexist at room temperature. In phase I, the effect of substitution on the ferromagnetic transition temperature is weak (e.g., TC = 102 K for BiMnO3 and TC = 99 K for BiMn0.95Ga0.05O3), but there is a drastic effect on the orbital ordering temperature (e.g., TOO = 474 K for BiMnO3 and TOO = 412 K for BiMn0.95Ga0.05O3). Magnetic susceptibilities of phase I are typical for ferromagnets while, in phase II, ferromagnetic cluster-glass-like behavior is observed. The magnetic transition temperature of phase II (e.g., TC = 70 K for BiMn0.8Ga0.2O3) exhibits a sudden drop compared with that of phase I. The effect of substitution on the structural monoclinic-to-orthorhombic transition is different depending on M (e.g., Tstr = 768 K for BiMnO3, Tstr = 800 K for BiMn0.95Ga0.05O3, and Tstr = 738 K for BiMn0.85Cr0.15O3)