Interface Stress Effects In Ferroelectric / Ferromagnetic Heterojunction Composite Film

Magnetoeletirc composite material has drawn wide interest of researchers for its potential uses. In magnetoeletirc material, ferroelectrics and ferromagnetics coexist, and a dielectric polarization can be induced by an external magnetic field or conversely, a phenomenon known as the magnetoeletric (ME) effect. Materials systems with strong ME effect have potential applications as magnetic field and stress sensors, data storage and switching devices, actuators and etc. The ME effect in multiferroic composites comes from the direct strain-coupling between ferroelectric phase and ferromagnetic phase. Unfortunately, the physics mechanisms by which the different order parameters can be coupled are generally poorly understood, and in most of the composite systems, including ceramic composites, laminated composites and etc, the situation of residual strain is very complex. In epitaxial thin-film heterostructure, however, the studies of magnetoelectric composite could be permited precisely as crystallographic orientation, layer thickness and interfacial roughness may be controlled accurately, which makes the study of stress interaction in ME composites possible.In this paper, we firstly studied the growth process of BaTiO₃/CoFe₂O₄ bilayer films. Reflection high-energy electron diffraction (RHEED) and high-resolution x-ray diffraction (HRXRD) were used to analyze situation of the interfacial stress. Changes of the ferroelectric and ferromagnetic qualities of the composites films induced by the interfacial stress were also studied. We achieved the control of the interfacial strain by adjusting the thickness of each layer and the number of layers in the composite films.In the study of BaTiO₃/CoFe₂O₄ bilayer films, we found that the BaTiO₃ layer was grown on SrTiO3 substrate in a layer-by-layer growth mode. Within 40 nm, the CFO layer was grown in Stransky-krastonov growth mode, and when the thickness exceeded 40nm, the growth mode changed into island growth mode. The relaxation of the interfacial stress mainly happened during the period of growth mode transformation. The remnant polarization of the composite films decreases with the increasing of CFO thickness. This should be induced by the strengthened tensile stress which weakens the ferroelectric tetragonal phase of BTO. When the thickness of CFO attains to 120 nm, the 2Pr of the composite films decreased to 3μC/cm².The polarization had a response to the static magnetic field, which was strengthened with the increasing magnetic field. The response attained to maximum when the electric field was 5V.In the study of the multilayer films of Pb(Zr_0.52Ti_0.48)O₃/NiFe₂O₄, we found that the interfacial strain between the Pb(Zr_0.52Ti_0.48)O₃ layer and the NiFe₂O₄ layer would be strengthened as the number of layers in the films increased in a certain range, which leading to the increases of remnant polarization of the films. When the number of layers was 7, the 2Pr attains to maximum of 65μC/cm². When the number of layer exceeded the range, the interfacial stress would be relaxed, causing the falling of the remnant polarization. The changes of the interfacial stress had a weak effect on the ferromagnetic qualities of the films.