| Ferroelectric/ferromagnetic composite materials, simultaneously exhibiting ferroelectricity and ferromagnetism, which are composed with ferroelectric phase and ferromagnetic phase materials, and magnetoelectric effect (ME) due to the interfacial coupling effects between the ferroelectric and ferromagnetic phases. Kinds of new devices based on ME have bee made, such as magnetic sensors, ME current sensors, transformers, gyrators, phase filters, resonators, ME magnetic read/write devices. Compared to the complex stress/strain situations in the magnetoelectric composite bulk materials, the strain/stress in the heteroepitaxy magnetoelectric composite films can be adjusted by controlling the films'growth conditions at nanoscale easily and will induce the changes of the ferroelectric and erromagnetic properties, which is more propitious to study the physical mechanism of the magnetoelectric coupling effect. The studies of the stress/strain in the heteroepitaxy magnetoelectric composite films not only make the study of the stress/strain coupling interaction mechanism in magnetoelectric composites possible, but also take an extremely important step to produce integrated magnetoelectric devices which response to magnetic, electric and mechanical.In this paper, the PZT/NZF magnetoelectric composite thin films were deposited on MgO (001) and SrTiO3 (001) crystal substrates by the pulsed laser deposition, respectively. The microstructures and cross section images of the thin films were analysed by X-ray diffraction and scanning electronic microscope, respectively. The results showed that the composite thin films are epitaxial c-axis orientation and have good crystal qualities. The results obtained by ferroelectric test analyzer and vibrating sample magnetometer showed that the composite thin films have good ferroelectric and ferromagnetic properties. Finally, in order to analyze the stress/strain in the heteroepitaxy magnetoelectric composite films, we used the high-resolution X-ray diffraction to test the in-plane and out-plane lattice parameters of the composite films deposited on the STO substrate.We firstly studied the PZT/NZF composite films deposited on the MgO substrate, the ferromagnetic property of the composite films is decreased by the PZT layer, especially the out-plane saturation magnetization is decreased about one time. In the PZT/NZF/MgO composite structure, the films have the best ferromagnetism when the PZT layer is about 100 nm, the in-plane and out-plane saturation magnetization and the coercive magnetic field are 224.65 emu/cm3, 110.51emu/cm3, 278.03 Oe and 924.59 Oe, respectively. But the ferroelectric property of the composite films is enhanced by the NZF layer, especially the films have the best ferroelectricity when the NZF layer is about 200 nm in the NZF/PZT/SRO/MgO composite structure. The saturation and remanent polarization and the coercive electric field are 55.564μC/cm2, 87.559μC/cm2 and 196.150 kV/cm。There are similar variational phenomena in the PZT/NZF composite films deposited on the STO substrate like the films deposited on the MgO substrate. The ferromagnetic property of the composite films is decreased by the PZT layer, especially the out-plane saturation magnetization changed intensely, the films have the best ferromagnetism when the PZT layer is about 100 nm in the PZT/NZF/STO composite structure, the in-plane and out-plane saturation magnetization and the coercive magnetic field are 238.50 emu/cm3, 129.53 emu/cm3, 354.27 Oe and 449.97 Oe. The ferroelectric property of the composite films is enhanced by the NZF layer, and the ferroelectricity arrived the maximum when the NZF layer is about 100 nm in the NZF/PZT/SRO/STO composite structure. The saturation and remanent polarization and the coercive electric field are 60.337μC/cm2, 93.097μC/cm2 and 165.625 kV/cm. The results gotten by the high resolution X-ray diffraction analysis of the composite films indicated that the best composite structure is NZF/PZT/SRO/STO, which have the best crystal qualities, the fewest crystal defects and the minimal stress/strain.In the end, comparisons of the microstructures and magnetoelectric properties of the composite thin films deposited on the MgO and STO substrates, we obtained that the NZF/PZT/SRO/STO composite structure is the best. In this structure, the composite films have the best crystal qualities, the fewest crystal defects and the minimal stress/strain which indicated that the maximal interfacial coupling interaction and the strongest magnetoelectric coupling effect. |
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