Charge and magnetic ordering in the electron-doped magnetoresistive materials CaMnO_3-δ (δ=0.06,0.11)

The magnetoresistive "electron"-doped materials CaMnO_3-δ (δ=0.06,0.11) have been investigated using powder neutron diffraction. The two materials are n-type semiconductors which exhibit antiferromagnetic ordering at T_N∼125 K, but they have different magnetic structures. The CaMnO_2.94 sample orders in a simple G-type antiferromagnetic structure, which is also observed in CaMnO₃. The CaMnO_2.89 sample, on the other hand, exhibits two magnetic features: the G-type reflections as noted above, and a set of reflections that can be indexed on a k = (0,0,1/4) ordering wave vector. A model for the magnetic structure is proposed which involves Mn³⁺/Mn⁴⁺ charge ordering concomitant with the magnetic ordering. The presence of a set of weak, temperature independent structural reflections which can also be indexed on a k = (0,0,1/4) supercell suggests an oxygen vacancy ordering which may play a role in the charge ordering.