The study of crystal structure, elastic and magnetic properties of Nd_1-xCa_xMnO₃ (x<=0.15) perovskites has been
carried out. The ferromagnetic component is shown to increase under hole doping and, simultaneously, the temperature of
orbital order-disorder phase transition decreases. The mechanism of concentrational transition from weak ferromagnetic
state (x=0) to ferromagnetic one (x>0.15) is discussed using 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 0.06<=x<=0.1 compounds around T_eff~9 K. For the
Nd_0.92Ca_0.08MnO_2.98 sample, the metamagnetic behaviour is revealed in the temperature range from 5 to 20 K. H vs.
T as well as T vs. x magnetic phase diagrams are proposed. The appearance of orientational transitions is explained on the
basis of magnetic !
analogue of Jahn-Teller effect taking into account that the magnetic moments of Nd ions are ordered parallel to the moments
of Mn ions in ferromagnetic phase, and opposite to the direction of weak ferromagnetics vector at T>T_eff in weak
ferromagnetic phase.