Ferroelectricity and Magnetic Structures in Delafossite Multiferroics AgFeO₂

Abstract

In recent years, interrelation between spin and lattice degrees of freedom in frustrated magnets have been attracting much attention. In such systems, complex magnetic structures stabilized by frustrated exchange interactions between spins break inversion symmetry and induce a ferroelectric polarization. In this study, we have performed dielectric measurements and neutron diffraction experiments on the delafossite AgFeO₂. The neutron diffraction experiment demonstrated successive magneto-structural phase transitions at T_N1 = 15 K and T_N2 = 9 K. The magnetic structure for 9 K ≤ T ≤ 15 K is a spin-density wave with a temperature dependent incommensurate modulation k = ( -1, q, 1/2 ; q ~ 0.384). Below 9 K, the magnetic structure turns into elliptical cycloid with the incommensurate propagation vector k = (-1/2, q, 1/2; q ~ 0.206). Based on the deduced magnetic point-group symmetry m1′ of the low temperature polar phase, we conclude that the ferroelectric polarization in AgFeO₂ is perpendicular to the monoclinic b axis and is driven by the inverse Dzyaloshinskii-Moriya effect with two orthogonal components, p^→₁ ∝ r^→_ij × (S^→_i × S^→_j ) and p^→₂ ∝ S^→_i × S^→_j.