Multiferroic composites an overview

Abstract

Multiferroics, i.e. materials with magnetic and electric order coexisting, have been attracting much of interest the latest years. The interaction of magnetic and electric subsystems manifests itself as magnetoelectric (ME) effect that is interesting for practical applications such as the sensor techniques, microelectronics and magnetic memory systems. One of the most attractive substances for creation of new ME materials is the bismuth ferrite BiFeO3 due to its record high temperatures of electric (Tc ¼ 1083 K) and magnetic (TN ¼ 643 K) ordering. Noteworthy that the Giant ME effect at room temperature has been obtained for the first time in thin films of this material. In the bulk BiFeO3 samples the spatially modulated spin structure exists in which the magnetization vectors of antiferromagnetic sublattices change periodically from point to point with a period 620A ˚, incommensurate to the crystal lattice period (spin cycloid). The presence of spatially modulated spin structure results in zero value of the volume-averaged ME effect. A necessary condition for ME effect observation is the suppression of spin-modulated structure, that takes place in strong magnetic fields when the system undergoes the incommensurate–commensurate (IC–C) phase transition between spin-modulated and homogenous antiferromagnetic states. It has been noted in Ref. that there is a profound analogy between spatially modulated spin structures in multiferroics and spatially modulated structures in nematic liquid crystal (director vector waves). This periodic director vector structures in nematic liquid crystal arise in external electric field (flexoelectric effect ) and can be controlled with the electric field. The question arises whether the electric field can control spatially modulated structures in multiferroics in the same way as in liquid crystal.