Preparation And Properties Of NiFe/BaTiO₃ Magnetoelectric Composite Films

Magnetoelectric composite films have both ferroelectric and ferromagnetic properties,which can be controlled by magnetic field and electric field.In recent years,ferroelectric,ferromagnetic and magnetoelectric composite films have been widely used in electronic devices,especially in the latest field of flexible electronic devices.This paper focuses on the ferroelectric,ferromagnetic and their coupling effects in the multiferroic composite system,and makes a detailed study on the preparation,microstructure and performance characterization and optimization of thin films.High quality magnetoelectric composite films were prepared by using lead-free barium titanate as ferroelectric phase and ferronickel alloy as ferromagnetic phase.The preliminary work mainly focuses on the preparation and performance optimization of ferroelectric thin films,the improvement of ferroelectric properties of thin films by heat treatment,the energy storage characteristics and frequency dependence of ferroelectric materials,and then the preparation of magnetic NiFe thin films and magnetoelectric composite thin films.Finally,the modeling and simulation are carried out by using COMSOL software.The main research work and results are as followsBaTiO₃ thin films with SrRuO₃(SRO)buffer layer were grown by pulsed laser deposition on LaAlO₃ and SrTiO₃ substrates with different orientations and types,such as(111)-oriented LaAlO₃ and(100)-,(110)-and(111)-oriented SrTiO₃.The ferroelectric and dielectric properties were tested by two different testing methods(top-top and top-bottom),and the microstructure and electrical properties were studied by X-ray diffraction,scanning electron microscopy and dielectric tester.The results show that the epitaxial growth quality of BTO thin films is very good,showing good ferroelectric and dielectric properties.The crystal structure of BTO thin films is affected by the substrate type and orientation.The films show strong ferroelectric and dielectric anisotropy.Different electrode configuration will affect the voltage required for polarization,but will not affect the final polarization state.The(110)-oriented BTO films have the best dielectric properties,while the(111)-oriented BTO films have better ferroelectric properties.The residual polarization of the film is about51.74?C/cm² and the dielectric constant is 298.Single crystal BaTiO₃(BTO)films were prepared on(100)-SrRuO₃(SRO)buffered SrTiO₃(STO)substrates by pulsed laser deposition(PLD).The samples were annealed by rapid thermal treatment(RTP)in the temperature range of 600-750?.XRD,ferroelectric test system,Hall effect test station and current voltage(?)characteristics were used to study the microstructure and electrical properties.The results show that RTP temperature affects the microstructure,interface contact and ferroelectric properties of the films.RTP treatment at 650?has smaller leakage current,resistance,barrier height and better ferroelectric properties.This is because the high temperature treatment affects the barrier height at the Pt/BTO interface,which affects the ferroelectric properties.BTO thin films were grown on(100)-oriented single crystal Nb:SrTiO₃(NSTO)substrates by pulsed laser deposition.The results show that the growth quality of BTO thin film is good,Pt/BTO/Nb:NSTO device has better ferroelectric properties,P_r=34.79?C/cm²@7V,and has better energy storage density U_Re=12.30J/cm³ after rapid heat treatment at 600?.Compared with the non annealed films,the rapid thermal processing(RTP)greatly enhances the ferroelectric and energy storage properties of the films,and the ferroelectric and energy storage properties show a strong voltage dependence,which is expected to become the raw materials for the preparation of ferroelectric and energy storage devices.The ferroelectric properties of BTO thin films were measured at different frequency f.The ferroelectric frequency dependence of BTO thin films was observed.The experimental data(loop area,ferroelectric polarization and coercive field)were analyzed based on the existing theoretical models.The results show that the saturation polarization and residual polarization decrease with the increase of test signal frequency,and the polarization decreases with the increase of applied signal frequency,which is due to the influence of frequency on ferroelectric domain.Ni₈₀Fe₂₀(NiFe)thin films with different thickness were grown on SiO₂,STO and NSTO substrates by direct current magnetron sputtering(DC-MS)at different temperatures.Their optical and magnetic properties were studied by PPMS and UV-VIS spectrophotometer.The results show that proper high temperature(300?)environment can significantly improve the electrical and magnetic properties of the films.The films grown on different oriented substrates show magnetic anisotropy,with the maximum saturation magnetization,remanent magnetization and minimum coercivity:Ms=8.4emu/g,Mr=6.625emu/g,Hc=28Oe.The results of integrating sphere UV spectrophotometer show that the film has the maximum absorption(0.37)and the minimum reflectivity(40%)at 230nm,and the maximum transmittance(30%)at336nm.Magnetoelectric composite films were prepared by RF-MS and DC-MS in the same cavity.By changing the deposition sequence of the two layers in NiFe-BTO composite films,the growth of the two layers shows different morphology.In NiFe/BTO structure,as a buffer layer,BTO can release the binding of NiFe layer from the substrate and increase the stress on BTO layer,so it shows stronger magnetoelectric coupling.Based on COMSOL software,NiFe/BTO composite film is established,and the stress,strain and voltage changes of piezoelectric layer(BTO)and magnetostrictive layer(NiFe)under the action of external magnetic field are demonstrated.The preliminary results show that the modeling is reasonable.