Structural, magnetic, and transport properties of the two electron-doped Ruddlesden-Popper manganites Ca_3-x-Th_xMn₂O₇

New Ca_3-xTh_xMn₂O₇ (x = 0-0.2) compounds with the Ruddlesden-Popper structure have been synthesized. Their crystal structures were refined from X-ray powder diffraction data, taking into account a small orthorhombic (space group Cmcm or Cmc2₁) distortion of the parent tetragonal I4/mmm structure. Doping of Ca₃Mn₂O₇ with tetravalent Th produces a significant ferromagnetic (FM) component in the magnetization at x = 0.05, which diminishes with a further increase of x and completely disappears at x = 0.2. This behavior is compared with single-electron La doping and shows universal scaling of magnetic properties with electron concentration as observed for three-dimensional perovskites as well. A model for the magnetic structure evolution has been proposed on the basis of symmetry conditions for weak ferromagnetism and the structural changes observed. A large negative magnetoresistance (MR) was observed with a maximal value of -66% for the x = 0.05 composition. The temperature and field dependence of MR suggests its tunneling origin. The MR and the weak ferromagnetic component disappear simultaneously at x = 0.2, and a two-dimensional variable range hopping conduction due to confinement of charge carriers within perovskite bilayers is observed.