Self-assembled Epitaxial Growth And Performance Study In PZT-NFO Magnetoelectric Nanocomposite Film

Not only applied electric field can produce ferroelectric polarization, but also induce magnetic stress anisotropy field due to the magnetoelectric(ME) effect in multiferroic composite materials. Of particular interest is 1-3 dimensional ME nanocomposite film. Very high ME coefficient can be obtained in this film due to the heteroepitaxial growth of the NFO nanopillar and the PZT matrix and the interface over volume ratio far greater than layered ME composite film. As a result, the 1-3D nanocomposite films provide higher degree of freedom for the design of next generation micro-magnetic devices.In order to expand the applications of ME multiferroic composite in the tunable microwave devices, there are two main work considered in my thesis. First, how to prepare high quality ME composite film with large ME effect; Second, how to obtain ME composite film with high operating frequency and low ferromagnetic resonance linewidth required by tunable microwave devices, besides the ME coupling properties.In this thesis, epitaxial Pb(Zr0.52Ti0.48)O3-NiFe2O4(PZT-NFO) nanocomposite films were grown on the(100)-oriented MgAl2O4(MAO) substrates. A series of preparatory work was completed in order to obtain high quality PZT-NFO films, including target sintering, substrate surface processing and bottom electrode sputtering. We have prepared the PZT-NFO target with good performance at the sintering temperature of 1100 oC. MAO(100) and(111) substrates were chemical etched and then heat treated to obtain atomic surface steps. The purpose of substrate surface processing is to improve the ordering of NFO nanopillar. AFM morphology analysis shows that there are atomic steps on the surface of MAO(100) substrate with height of about 0.4 nm. Since MAO substrate is not conductive, we cannot directly test P-E property of the thin film, therefore, bottom electrode was prepared beforehand. We have successfully prepared epitaxial Fe3O4 and SrRuO3(SRO) bottom electrodes on the MAO substrate. However, it was found that the Fe3O4 electrode was oxidized and become power in the subsequent preparation of PZT-NFO thin film. Although SRO is inferior to Fe3O4 in the lattice matching with the MAO substrate, it is quite stable and can serve as bottom electrode to test P-E performance of thin films.Self-assembled nanocomposite Pb(Zr0.52Ti0.48)O3-NiFe2O4 films were prepared on the(100)-oriented MgAl2O4 substrates by a 90° off-axis magnetron sputtering method. The influences of substrate temperature, argon over oxygen ratio and sputtering power on the structure and properties of PZT-NFO nanocomposite films were studied. The optimal growth conditions are substrate temperature of 800 oC, argon over oxygen ratio of 1:1 and sputtering power of 160 W. XRD studies reveal that the PZT-NFO film is epitaxial along both the in-plane and out-of-plane directions, and the vertical lattice mismatch between the PZT phase and the NFO phase is very small. AFM and SEM analysis show that the PZT-NFO films have clear 1-3 dimensional nanocomposite structure, and the diameter of NFO nanorods is 80~150 nm. Further decreasing argon over oxygen ratio is beneficial for the formation of NFO, however, increasing RF power causes a transition from an 1-3 nanocomposite to a 0-3 one. Magnetic measurement shows that the saturation magnetization of NFO phase is 120~160 kA/m, lower than that of bulk NFO phase. P-E measurements show that the saturation polarization is about 6.9 μC/cm2. FMR test showed that the peak-to-peak FMR linewidth(ΔHpp) of the PZT-NFO films is about 130 Oe, thanks to so the very small lattice mismatch between PZT phase and NFO phase.