Magnetic ordering and ferroelectricity in multiferroic2H−AgFeO2: Comparison between hexagonal and rhombohedral polytypes

Abstract

Magnetic and dielectric properties of the hexagonal triangular lattice antiferromagnet 2H -AgFeO 2 have been studied by neutron diffraction, magnetic susceptibility, specific heat, pyroelectric current, and dielectric constant measurements. The ferroelectric polarization, P ≃ 5 µC/m 2 , has been found to appear below 11 K due to a polar nature of the magnetic ground state of the system. In the temperature range of 11 K T 18 K, an incommensurate spin density wave (ICM1) with the nonpolar magnetic point group mmm1 ′ and the k 1 = (0,q 1 b ,0; q 1 b = 0.390-0.405) propagation vector takes place. Below 14 K, a proper screw ordering (ICM2) and k 2 = (0,q 2 b ,0; q 2 b = 0.385-0.396) appears as a minor phase which coexists with ICM1 and the ground state down to the lowest measured temperature 5.5 K. No ferroelectric polarization associated with the ICM2 phase was observed in agreement with its nonpolar point group 2221 ′ . Finally, a spiral order with cycloid and proper screw components (ICM3), and k 3 = (q 3 a ,q 3 b ,0; q 3 a = 0.0467,q 3 b = 0.349) emerges below 11 K as the ground state of the system. Based on the deduced magnetic point group 21 ′ , we conclude that the ferroelectric polarization in ICM3 is parallel to the c axis and is caused by the inverse Dzyloshinskii-Moriya effect with p 1 ∝ r ij × (S i × S j ). Unlike the rhombohedral 3R-AgFeO 2 polytype, the additional contribution to the macroscopic polarization p 2 ∝ S i × S j is not allowed in the present case due to the symmetry constraints imposed by the hexagonal lattice of 2H -AgFeO 2 .

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

“…The previous powder study for 2H-AgFeO 2 argued that the difference in the parent space group, rhombohedral or hexagonal, affects the ferroelectric polarization at low temperature in of AgFeO 2 significantly. 28 In 2H-AgFeO 2 , there exist several magnetic phases which are a SDW (11 K ≤ T ≤ 18 K), a proper screw (T ≤ 14 K), and a general spiral phases below T = 5.5 K, which are coexistent with each other even at the lowest temperature. The emergence of ferroelectric polarization is concomitant with onset of the general spiral order.…”

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

“…Magnetoelastic parameters (b E ,b T2 ) that determine the contribution of the distortions such as elongation-contraction of the cation-anion bonds and deviations of the anion-cation-anion bonding angles from π/2, respectively, were estimated earlier for FeO 6 clusters in RFeO 3 : b E » 24 cm −1 ; b T2 ≈ 6 cm −1 [16]. The positive sign of the b T2 produces an important result that the trigonal distortion along C 3 axis with the Fe 3+ O 6 octahedron contraction along the axis (θ ij > π/2) makes the axis to be the easy one, while the octahedron elongation along the C 3 axis makes the respective perpendicular plane (111) to be the easy plane.…”

“…AMO 2 delafossite-like oxides (A = Cu + , Ag + and M = Cr 3+ , Fe 3+ , Co 3+ , Ni 3+ ) with triangular, geometrically frustrated spin structures attract much attention as magnetoelectric multiferroic materials [1][2][3][4][5][6][7]. The ferroelectricity in this class of multiferroics appears as a result of the phase transition, inducing an unusual magnetic structure that breaks the crystal symmetry.…”

Synthesis and characterization of AgFeO2 delafossite with non-stoichiometric silver concentration

“…However, most of these methods allowed obtaining the silver ferrite as a mixture of 3R and 2H polytypes in various proportions. Both polytypes of silver ferrite could be prepared separately only under special conditions (high-pressure 3 GPa and 6 GPa), as shown in [17,18].…”

“…Among methods of producing silver delafossite materials, it may be distinguished as: (1) low-temperature methods (e.g., hydrothermal synthesis, metathetical reaction, oxidizing fl ux reaction, sol-gel) [10][11][12][13][14], (2) high-temperature synthesis (e.g., solid-state reaction [15][16][17][18][19]) and (3) thin fi lm techniques (e.g., atomic layer deposition, pulsed laser deposition (PLD), chemical vapour deposition (CVD), RF sputtering [6,[20][21][22]). However, they do not always allow obtaining compounds of high purity.…”