The crystal and magnetic structures of ${\mathrm{Tb}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ series have been studied by x-ray and neutron-diffraction techniques. Macroscopic magnetic properties 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 complex due to the small size of the rare earth. ${\mathrm{TbMnO}}_3$ does not develop a simple A-type magnetic order, as does ${\mathrm{LaMnO}}_3,$ but an incommensurate sinewave structure. As the Ca content increases up to $x=0.33,$ the magnetization increases without, however, reaching a truly ferromagnetic state. Instead, the magnetic ground state is composed of ferromagnetic clusters in a paramagnetic matrix. Charge ordering is observed at higher values of x and the magnetic ground state evolves, with increasing x values, from the CE type to a mixture of different magnetic structures including some noncollinear structures. For $x=0.85,$ we observe a phase segregation at low temperatures into an orthorhombic phase and a monoclinic phase.

• Received 7 February 2000

DOI:https://doi.org/10.1103/PhysRevB.62.5609