Magnetic phase diagrams of the manganites Ln(1-x)Ba(x)MnOs (Ln = Nd, Sm)

The magnetization and magnetoresistance of Ln(1-x)Ba(x)MnO(3) have been stu died for a single crystal (Ln = Nd, x = 0.23) and for ceramics (Ln = Nd, Sm ; 0 less than or equal to x less than or equal to 0.44). It was shown that Nd1-xBaxMnO3 compositions in the range 0 less than or equal to x less than or equal to 0.05 are inhomogeneous antiferromagnets; a mixed antiferro-ferr omagnet state appears in the range 0.05 < x < 0.10; a pure ferromagnetic st ate develops in the range 0.2 less than or equal to x < 0.4 and in the rang e x greater than or equal to 0.4 an inhomogeneous ferromagnetic state is re alized. Metal-like behaviour below T-C is found in the concentration 0.3 < x less than or equal to 0.44. The concentrational transition from inhomogen eous antiferromagnetic state into spin-glass state in Sm1-xBaxMnO3 was foun d at x > 0.12. Both Nd and Sm sublattices are coupled antiferromagnetically with manganese ions. All the ferromagnetic Nd1-xBaxMnO3 samples show simil ar magnetotransport properties despite of a strong difference in the resist ivity behaviour. Our results could be understood in terms of Mn-O-Mn supere xchange and 3d orbital ordering as well as variation of band structure due to the ionic size effect, Mn4+ ion appearance and magnetic ordering. It is supposed that the gap between the narrow 3d manganese band and wide 2p oxyg en band decreases strongly below T-C.