Magnetic ordering and granularity effects in La_1-xBa_xMnO₃

We report detailed measurements of the magnetic behavior of La_1-xBa_xMnO₃ bulk samples over a wide range of composition (0≤x≤1), temperature (4.2-775 K), and magnetic field strength (1 G-50 kG). The end members of this series namely LaMnO_3+δ (with δ≥0) and BaMnO₃ crystallize into pseudocubic perovskite and hexagonal lattices, respectively. The perovskite structure is stable for x≤0.35 while a phase separation into La_0.65Ba_0.35MnO₃ and BaMnO₃ occurs for x>0.35. The ordered moment on LaMnO3+15 depends sensitively on the processing conditions and the lowest value of 0.006 μ_B per Mn ion at 4.2 K is realized in the antiferromagnetic compound of δ=0. The complex magnetic behavior of BaMnO₃ suggests antiferromagnetic ordering below ∼ 150 K. The Ba-substituted compounds order ferromagnetically with a monotonic increase of T_C and saturation magnetization M_s with x<0.35. In the two-phase region, M_s scales with the fraction of the ferromagnetic phase present in the sample. Magnetic granularity, superparamagnetism, and an effective medium approach for susceptibility of the two-phase mixture are discussed.