Spin-reorientational transitions in low-doped Nd1 xCaxMnO3manganites: the evidence of an inhomogeneous magnetic state

A study of crystal structure, elastic, and magnetic properties of low-doped Nd 1− x Ca x MnO 3 ( ) perovskites has been carried out. The ferromagnetic component is shown to increase under hole doping and, simultaneously, the temperature of the orbital order–disorder phase transition decreases. The mechanism of the concentrational transition from a weak ferromagnetic state ( x = 0) to a ferromagnetic one ( x >0.15) is discussed using a two-phase model, according to which the samples consist of weak ferromagnetic and ferromagnetic phases exchange coupled at their boundary. It is found that interaction between different magnetic phases leads to spin reorientation which takes place for compounds around K. In the temperature range from 5 to 20 K, metamagnetic behaviour is revealed for the Nd 0.92 Ca 0.08 MnO 2.98 sample. H versus T as well as T versus x magnetic phase diagrams, which are characterized by the missing of a canted phase, are proposed. The appearance of orientational transitions is explained on the basis of a magnetic analogue of the Jahn–Teller effect taking into account that the magnetic moments of Nd ions are ordered parallel to the moments of Mn ions in the ferromagnetic phase, and opposite to the direction of the weak ferromagnetic vector at T > T eff in the weak ferromagnetic phase.