Orientation Growth And Magnetoelectric Coupling Effect Of2-2Type CoFe₂O₄/PMN–PT Composite Films

Multiferroic magnetoelectric （ME） materials can not only display ferroelectricand ferromagnetic ordering, but also a novel coupling effect, i.e., ME effect, andthus have significant potential applications in new-type ME memories, spintronicsdevices, etc. Among them, ME composite thin films have been attractingconsiderable attention for their ease of on-chip integration and better electrical,magnetic properties. The present work focused on the preparation, microstructuresand properties of the composite thin films made up of ferroelectricPb (Mg_1/3Nb_2/3)O₃–PbTiO₃（PMN–PT） and ferromagnetic CoFe₂O₄（CFO）. The singlelayered thin films PMN–PT and CoFe₂O₄, and2-2–type bilayered composite thinfilm CoFe₂O₄/PMN–PT and Co_0.9Zn_0.1Fe₂O₄（CZFO）/PMN–PT were prepared bysol-gel spin-coating method.The PMN–PT film with different textures can be obtained by controlling thepyrolysis temperature, annealing in oxygen atmosphere, and film thickness. Highlyoriented PMN–PT films with （100） and （111） textures have been prepared onPt/Ti/SiO₂/Si substrates. The （100）-textured thin films form clustered and islandstructures of fine-grained. The （111）-textured thin films form a densedsubmicrometer-grains microstructure. The （111）-textured PMN–PT films have betterferroelectric and dielectric properties. The CFO films exhibt good ferromagneticresponse at room temperature. The saturation magnetization （Ms） and residualmagnetization （Mr） increases with increasing CFO layer thicknesses, while thethicker films show a lower coercivity （Hc） due to relaxing the clamping stress fromsubstrates. The in plane Hcis slightly smaller than that of the out of plane HcwhileMsand Mrare the reverse, which indicates that the magnetic anisotropy in the CFOthin film is evident.Ferroelectric domain and ferromagnetic domain structures were observed bypiezoresponse force microscopy （PFM） and magnetic force microscopy （MFM）,respectively. The domain wall of PMN–PT films can be seen clearly. It was foundthat domain structure is relevant to the size of the grains. The grains present assingle domain structure. The back-switching process of ferroelectric domain isobvous by appling a dc bias voltage polarization. The center regions of singledomain appear back-switching first, and then back-switching of domain wall. Thedomain structure of CFO thin films are composed of interaction domains thatrandom in direction. The MFM imaging showed the interaction domains as brightand dark areas, which means that they possess opposite magnetization components （out-of-plane and in-plane magnetization） perpendicular to the specimen surface.Utilizing the sol–gel method, CFO/PMN–PT composite thin films withdifferent textures were prepared. Influence of texture on electrical, magnetic, andME coupling properties of bilayered thin films were investigated. Each CFO orPMN–PT layer is relatively uniform in thickness. The composite thin films haveclear interfaces between the top CFO layer and the bottom PMN–PT layer and Pt/Sisubstrate, which exhibit strong ferroelectric, ferromagnetic, and ME responses atroom temperature. The maximal in-plane ME coupling coefficient of the compositethin films reaches up to about65.2mV/cm·Oe when the thickness of CFO layer is400nm. Ferromagnetic thin films with different CFO layer thicknesses werefabricated on the uniform orientation and thickness of ferroelectric PMN–PT layersusing the sol–gel technique. The residual stress of the top CFO layer films wascharacterized by means of grazing incidence X-ray diffraction, and the influencemechanism of residual stress on the ME coupling properties of these CFO/PMN–PTcomposite thin films was explored.Based on the consideration to enhance the ME coupling properties,（100） and（111） textured CZFO/PMN–PT composite thin films with Zn-doping in the CFOlayer were prepared. The effect of Zn-doping on the electrical, magnetic, and MEproperties of composite thin films were investigated. By appling the dc biasmagnetic fiele Hdc=6000Oe, the magnetically induced voltages for （111） and （100）textured CZFO/PMN–PT composite thin films with CZFO layer of300nm thick arefound to be27.42and43.91μV, respectively. The ME coupling properties ofcomposite thin films were improved through the Zn substitute in the CFO layer, andthe texture of bottom PMN–PT layer has an important effect on ME response ofcomposite thin films.