Oxide electronics

Abstract:

The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH4)2[FeCl5(H2O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroicity. From the crystal structure analysis, we observed an order-disorder phase transition related to the ordering of the ammonium counterion. We have determined the magnetic structure below TN, at 2 K and zero magnetic field, which corresponds to a cycloidal spin arrangement with magnetic moments contained in the ac-plane, propagating parallel to the c-axis. The observed ferroelectricity can be explained, from the obtained magnetic structure, via the inverse Dzyaloshinskii-Moriya mechanism.

Abstract:

Perovskite materials provide a large variety of interesting physical properties and applications. Here, we report on unique properties of a fully ordered magnetodielectric double-perovskite, Sc2NiMnO6 (space group P21/n, a = 4.99860 Å, b = 5.35281 Å, c = 7.34496 Å, and β = 90.7915°), exhibiting sequential magnetic transitions at T1 = 35 K and T2 = 17 K. The transition at T1 corresponds to a single-k antiferromagnetic phase with propagation vector k1 = (1/2, 0, 1/2), while the second transition at T2 corresponds to a 2-k magnetic structure with propagation vectors k1 = (1/2, 0, 1/2) and k2 = (0, 1/2, 1/2). Symmetry analysis suggests that the two ordering wave vectors are independent, and calculations imply that k1 is associated with the Mn sublattice and k2 with the Ni sublattice, suggesting that Mn-Ni coupling is very small or absent. A magnetodielectric anomaly at T2 likely arises from an antiferroelectric ordering that results from the exchange-striction between the two magnetic sublattices belonging to k1 and k2. The behavior of Sc2NiMnO6 demonstrates 3d double-perovskites with small A-site cations as a promising avenue in which to search for magnetoelectric materials.

Abstract:

We develop a theory of electromagnons in CuO by combining a symmetry analysis based on irreducible corepresentations, ab initio calculations, and simulations of spin dynamics using a model Hamiltonian and the Landau-Lifshitz-Gilbert equation. We show that the electromagnon measured in [Jones et al., Nat. Commun. 5, 3787 (2014)] with the electric field along the [101] direction originates from a magnetoelectric coupling mediated by Dzyaloshinskii-Moriya interactions and consists of a rigid rotation of the Cu spins around the axis defined by the electric field. Furthermore we predict the existence of a second electromagnon originating from exchange striction and coupled to electric fields along the [010] direction in the AF2 phase.