Structural, Magnetic, and Transport Properties of the Two Electron-Doped Ruddlesden−Popper Manganites Ca3-xThxMn2O7

New Ca 3 - x Th x Mn 2 O 7 ( 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 Cmc 2 1 ) distortion of the parent tetragonal I 4/ mmm structure. Doping of Ca 3 Mn 2 O 7 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.