Ferroelectricity and lattice distortion associated with spin orderings in a multiferroic delafossite AgFeO₂

Abstract

Abstract. Spin-lattice coupling and ferroelectric polarization associated with the magnetic ordering in a geometrically frustrated delafossite AgFeO 2 have been studied by high resolution neutron powder diffraction and dielectric measurements. The cycloidal magnetic ground state found in this material and the character of the lattice distortions are unprecedented in the family of the delafossite compounds, implying a peculiar mechanism lifting the geometrical frustration.

57Fe Mössbauer study of unusual magnetic structure of multiferroic 3R-AgFeO2

“…A similar conclusion was recently drawn in the comparative analysis of the lattice distortion in these ferrites [13]. In particular, it has been shown that the b m axis in the monoclinic basis contracts at T < T N1 in AgFeO 2 and elongates in CuFeO 2 .…”

“…The experimental evidence that the b m axis elongates in CuFeO 2 [13] implies that direct AFM exchange (J dir > 0) diminishes and FM superexchange (J sup < 0) will dominate. This mutual cancelation leads to a weakening of the nearest interactions (J 1 > 0).…”

“…At the same time, according to recent neutron diffraction experiments [9,12], the magnetic ordering in 3R-AgFeO 2 at T £ T N2 is in the form of an elliptical cycloid with an incommensurate propagation wave vector Q = 2p(-½, q, ½) m , with q » 0.2026. Such an essential difference in the nonpolar commensurate state of CuFeO 2 and the polar magnetic structure of 3R-AgFeO 2 clearly underlines that the nonmagnetic A (= Cu + , Ag + ) ions play a crucial role in the magnetic exchange interactions and multiferroic behavior of the delafossite AFeO 2 compounds [9,13]. The ground state spin structure and its temperature variation in the quasi-2D systems AFeO 2 is far from trivial, due to a competition between several interactions.…”

Difference in magnetic and ferroelectric properties between rhombohedral and hexagonal polytypes of AgFeO2 : A single-crystal study

“…The nearest neighbor exchange interaction, J1, is known as sum of Fe-O-Fe ∼ 90 • antiferromagnetic superexchange and Fe-Fe direct ferromagnetic exchange interactions in delafossite system. 33 Considering the Fe-O-Fe bond angle and Fe-Fe distance are nearly the same values, 96.54 • and 3.039Å in 2H-AgFeO 2 , 28 and 96.5 • and 3.033Å in 3R-AgFeO 2 , 24,51 we can expect the almost the same J1 value. However, for the next nearest neigh- bor, J2, the third nearest neighbor interactions, J3, in the triangular lattice plane, and the inter-layer exchange interactions, these exchange paths are unknown and expected to be much more complex than the case of J1.…”

“…Because, since magnetic orderings lower the symmetries down to monoclinic in 3R-AgFeO 2 and orthorhombic in 2H-AgFeO 2 below magnetic phase transition temperatures, it is convenient to use these low symmetry notations. Actually, in previous powder neutron diffraction experiments,24,25,28 , nuclear peak splittings and broadenings indicating losing the three fold and six fold rotational symmetry elements of R3m and P 6…”