Phase transitions and spin excitations in new multiferroics with modulated magnetic structure

Abstract

Coexistence of an antiferromagnetic (modulated) structure and electric polarization has been revealed in single crystals of Eu_{1 − x} Y_xMnO₃ (0.2 ≤ x ≤ 0.5) and Gd_{1 − x} Y_xMnO₃ (0 ≤ x ≤ 0.2) manganites. Hence, these compounds can be considered as a new family of multiferroics. Various phase transitions, both spontaneous and induced by magnetic fields up to 250 kOe, accompanied by anomalies in magnetization, magnetostriction, permittivity, and electric polarization, have been found, and phase T-x diagrams have been constructed. In the submillimeter range (8–40 cm⁻¹), new spin excitations—electromagnons—have been revealed; they are excited by an electric field. It is established that suppression of the modulated structure by a magnetic field leads to the disappearance of electromagnons; this process is accompanied by significant changes in the permittivity in a wide frequency range.