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 less than or equal to x less than or equal to 1), temperature (4.2-775 K), and magnetic field strength (1 G-50 kG). The end members of this series namely LaMnO3+delta (with delta greater than or equal to 0) and BaMnO3 crystallize into pseudocubic perovskite and hexagonal lattices, respectively. The perovskite structure is stable for x less than or equal to 0.35 while a phase separation into La0.65Ba0.35MnO3 and BaMnO3 occurs for x > 0.35. The ordered moment on LaMnO3+delta depends sensitively on the processing conditions and the lowest value of 0.006 mu(B) per Mn ion at 4.2 K is realized in the antiferromagnetic compound of delta=0. The complex magnetic behavior of BaMnO3 suggests antiferromagnetic ordering below similar to 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. (C) 2000 American Institute of Physics. [S0021-8979(00)04104-9].