Structural and magnetic study of Tb1-xCaxMnO3 perovskites

The crystal and magnetic structures of Tb1-xCaxMnO3 series have been studie d by x-ray and neutron-diffraction techniques. Macroscopic magnetic propert ies have been characterized using ac magnetic susceptibility measurements. The relationships between structural and magnetic behaviors are discussed. Our study shows that the magnetism of manganese in this system is very comp lex due to the small size of the rare earth. TbMnO3 does not develop a simp le A-type magnetic order, as does LaMnO3, but an incommensurate sinewave st ructure. As the Ca content increases up to x = 0.33, the magnetization incr eases without, however, reaching a truly ferromagnetic state. Instead, the magnetic ground state is composed of ferromagnetic clusters in a paramagnet ic matrix. Charge ordering is observed at higher values of x and the magnet ic ground state evolves, with increasing x values, from the CE type to a mi xture of different magnetic structures including some noncollinear structur es. For x = 0.85, we observe a phase segregation at low temperatures into a n orthorhombic phase and a monoclinic phase.