Magnetic ordering and granularity effects in La1−xBaxMnO3

We report detailed measurements of the magnetic behavior of La1−xBaxMnO3 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 LaMnO3+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 La0.65Ba0.35MnO3 and BaMnO3 occurs for x>0.35. The ordered moment on LaMnO3+δ depends sensitively on the processing conditions and the lowest value of 0.006 μB 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 TC and saturation magnetization Ms with x<0.35. In the two-phase region, Ms 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.