Speaker
Description
The coexistence of ferroelectricity and magnetism being considered for a long time mutually exclusive phenomena represent now a flourishing field of magnetoelectric multiferroicity. In particular, the magnetoelectric coupling is an intrinsic feature of the improper multiferroics where the electric polarization arises from the spiral magnetic ordering. The spirals are the result of competition of nearest neighbor and higher order superexchange interactions. Here, we report on spin-order-induced ferroelectricity and magnetoelectric effect in LiCuFe2(VO4)3 which is isostructural to howardevansite mineral NaCuFe2(VO4)3. In many respects, the howardevansites differ from any magnetoelectric material studied so far. Their magnetic subsystem is represented by two types of 3d-transition metal, Cu and Fe, with third 3d-transition metal, V, being magnetically silent. The Li+ ions occupy the channels of the crystal structure and one of two alkali ions position is half-filled. The temperature dependences of specific heat and magnetic susceptibility in LiCuFe2(VO4)3 evidence the formation of the long range antiferromagnetic order at low temperatures accompanied by distinct anomalies in both properties. As compared to Weiss temperature, the ordering temperature in LiCuFe2(VO4)3 is rather low pointing to reduced dimensionality of the magnetic subsystem. At low temperatures, the specific heat evidences two peaks in close vicinity to each other. This two peak structure of magnetic origin is further confirmed in measurements of both dc- and ac-susceptibility. The upper and lower peak temperatures are TN2 = 9.8K and TN1 = 8.2 K. At lowering temperature, dielectric permittivity in LiCuFe2(VO4)3 exhibit broad frequency-dependent relaxation-type anomaly at T ~ 30K, sharp peak at TN2 and step-like feature at TN1. The broad anomaly at T can be ascribed tentatively to cooperative hopping of Li+ ions within the channels of crystal structure. Its characteristic temperature shifts upward with frequency following the Arrhenius plot with activation energy Dact ~ 60 meV. The broad maximum in dielectric permittivity is insensitive to external magnetic field, but the field rapidly suppresses the singularities at TN2 and TN1. The Mössbauer spectroscopy reveals wide distribution of hyperfine field in between TN2 and TN1 consistent with incommensurate-commensurate scenario suggested by dielectric permittivity data, while the first principles calculations provide values of magnetic exchange interaction parameters. The magnetic subsystem in LiCuFe2(VO4)3 is quite peculiar being constituted by weakly coupled chains of mixed spins. Within chains these spins are coupled by multiple antiferromagnetic and ferromagnetic exchange interactions which open way to formation of non-collinear structures, either commensurate or incommensurate. The latter one breaks the spatial inversion symmetry of the compound.
