Observation of spontaneous spin reorientation in Nd1 − xDyxFe3(BO3)4 ferroborates with a competitive R-Fe exchange

The magnetic, magnetoelectric, and magnetoelastic properties of DyFe3(BO3)4 single crystals and the Nd0.75Dy0.25Fe3(BO3)4 substituted compound with the competitive Nd-Fe and Dy-Fe exchange interactions have been investigated in magnetic fields up to 210 kOe. It has been shown that the antiferromagnetic state in Nd0.75Dy0.25Fe3(BO3)4 below the Néel temperature is easy-plane and a spontaneous spin-reorientation transition to a uniaxial state occurs near 25 K. Anomalies are observed in various physical characteristics in a spin-flop transition induced by the magnetic field along the trigonal axis. The H-T phase diagrams are constructed; they are in good agreement with a simple model taking into account the anisotropy of the exchange splitting and g factors of the ground doublets of the Dy3+ and Nd3+ ions. The features of the magnetoelectric effect in various magnetic phases have been studied. Spontaneous electric polarization has been revealed in the region of the existence of an easy-plane antiferromagnetic state in the substituted compound. A qualitative theoretical justification of the possibility of the appearance of the spontaneous spin-reorientation transition in the Nd0.75Dy0.25Fe3(BO3)4 substituted compound is given.