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The interaction between ferromagnetic and ferroelectric films, the magnetoelectric effect, is a fascinating fundamental research area as well as having potential applications in magnetic data storage devices. We have investigated magnetoelectric coupling effects in thin film heterostructures, consists of metallic ferromagnet, cobalt, and the polymer ferroelectric [P(VDF-TrFE) 70:30]. The work described here encompasses changes in ferroelectric polarization with magnetic field as well as changes in the magnetic anisotropy with ferroelectric polarization.
In samples of Co overlayers on P(VDF-TrFE), in which the Co is not constrained by the substrate, the polarization shows a large change on application of a perpendicular magnetic field. This magnetoelectric effect is reversible, repeatable and possesses odd symmetry with respect to positive and negative magnetic field. Magnetic saturation destroys the effect, implying the presence of multiple magnetic domains is essential for the effect. The flexoelectric effect, the change in polarization due to strain gradients in the ferroelectric film, is a possible candidate for the cause of this effect.
In samples consisting of Co layers overlaid with P(VDF-TrFE), large changes in the magnetic coercivity with changes in ferroelectric polarization are observed. The out-of-plane coercivity is significantly larger for up polarization (i.e. polarization pointing away from the Co layer), whereas the opposite is true for the in-plane coercivity. The magnetic anisotropy, calculated using the areas of magnetization hysteresis loops, is shown to change by as much as 50% as the ferroelectric polarization is switched from up to down. For the thinnest films, the easy axis switches from out-of-plane to in-plane as the ferroelectric polarization is switched. The change in coercivity is proportional to the ferroelectric polarization, as confirmed by taking magnetization loops at intermediate polarization values. Rotation of the magnetization through a large angle, using only electric fields is demonstrated. These large changes in the anisotropy arise from the large electric field at the surface of the Co layer.