Magnetic phase transitions in the lightly doped Nd1−xCaxMnO3−δ manganites

The lightly doped Nd1−xCaxMnO3−δ (x=0.08, 0.12; δ⩽0.06) manganites have been investigated by means of X-ray, neutron powder diffraction, magnetization and AC magnetic susceptibility measurements. It has been shown that complex magnetic behavior of the samples observed at low temperatures can be adequately described using a magnetic phase separation model. In the framework of this model, the antiferromagnetic and ferromagnetic phases have different sign of the f-d exchange. The neodymium magnetic moments start to be ordered near the Neel temperature (Curie temperature). They are directed parallel to the moments of the manganese ions in the ferromagnetic phase, and opposite to this direction in the antiferromagnetic phase. The first-order magnetic phase transition associated with reorientation of the magnetic moments of the ferromagnetic phase along the Nd magnetic moments in the antiferromagnetic phase is observed for the samples from an intermediate range of hole concentration near Teff ∼ 9 K.