Low-temperature magnetic phase transitions in the Nd_0.9Dy_0.1Fe₃ (BO₃)₄ multiferroic. Part 1. Transitions induced by the magnetic field directed along the axis of the trigonal symmetry. Spontaneous transitions taking place with the changes of temperature

The behavior of elastic and magnetic properties of the single crystal Nd_0.9Dy_0.1Fe₃ (BO₃)₄ at low temperatures in the absence of a magnetic field and in the external field H||C₃ have been studied. The transition to the magnetic ordered state of the magnetic subsystem and two following one after another spontaneous spin-reorientation phase transitions manifested themselves in the temperature behavior of the velocity of acoustic modes and the magnetic susceptibility. The possibility of realization of the spontaneous transition to an incommensurate (spiral) magnetic phase has discussed. It was shown that an external magnetic field directed along the trigonal axis of the crystal has induced the sequence of spin-reorientation phase transitions. The phase H-T diagram (H||C₃) was constructed.