Influence Of Mn And Ni Doping On Optical,Magnetic Properties Of BiFeO₃ Films

Multiferroic materials refer to materials with two or more characteristics of ferroelectricity,ferromagnetism and ferroelasticity.As a multiferromagnetic materials,BiFeO₃(BFO)thin film possesses both ferroelectricity with a Curie temperature(T_C=1103 K),ferromagnetism with a antiferromagnetic Néel temperature of(T_N=643 K),and high ferroelectric polarization of(P_r=100?C/cm²)at room temperature.The special properties make them have important application prospects in high density,low power consumption,non-volatile sensors,polymorphic magnetoelectric memories,spintronics and photoelectric devices.At present,it is also found that BFO has a small optical band gap(2.2-2.9 e V)and good chemical stability.However,pure BFO has the disadvantages of charge defect,second phase and high leakage current,which hinders its practical application.Therefore,it is necessary to further study the optical,magnetic and structural characteristics of BFO thin films to improve their properties.Although a lot of research has been carried on the doping effect on the ferroelectric and ferromagnetic properties of BFO films.But,the influence of transition metals Mn and Ni doping on the optical and magnetic properties of BFO films needs to further study.The paper,investigates the effects of Mn and Ni doping and annealing atmosphere on the microstructure and properties of BFO films through experimental research and first-principles simulation calculations.The contents of the research work as following:(1)First of all,BiFe_1-xMn_xO₃(x=0.00,0.03,0.05 and 0.07)(BFMO)films were prepared by sol-gel method,and influence of Mn doping on the crystal structure,magnetic and optical properties of BFO films was studied.The results show that with the increase of Mn doping amount,the BFO thin films exhibit better crystallinity.Raman spectrum and XRD analysis shows that Mn doping leads to the unit cell is slightly disturbed in BFO film.The doping of Mn enhances the saturation magnetization of the BFO film,which may be attributed to the unit cell is slightly disturbed,thereby suppressing the cycloid spin structure in the BFMO film.Meanwhile,the optical band gap of BFMO thin films also decreases obviously,especially when the doping amount is x=0.07,the band gap gets a small value of 2.20 e V.Secondly,the electrical,magnetic and optical properties of BFMO(doping amount is 0.25 and 0.50)investigated by the first-principles method.The results show that the band gap is decreased with the increase of Mn doping content.The spin density results show that,there is obvious spin polarization around the Fe atoms in BFMO.The absorption coefficient results show that,after Mn doping,the absorption coefficient increases.(2)The electrical,magnetic and optical properties of Ni-doped BFO with different proportions are investigated by the first-principles method.The calculation results show that the band gap of BiFe_1-yNi_yO₃(y=0.00,0.125,0.25 and 0.50)are 1.37e V,0.86e V,0.82 e V and 0.65 e V,respectively.The band gap widths decrease with the increase of Ni doping content.The spin density results show that,when the doping amount is25%,there is obvious spin polarization around Ni and Fe,as the doping amount reaches to 50%,there is a spin polarization only around Ni.The absorption coefficient results show that,after Ni doping,the absorption coefficient first decreases and then increases as the amount of Ni doping increases.(3)The effects of annealing atmosphere(air,nitrogen and oxygen)on the crystal structure,magnetic,optical and photocatalytic properties of the thin film are investigated.The XRD and Raman spectra results indicate that,contrast to annealing in the air and nitrogen,annealing in oxygen atmosphere is beneficial to fabricate single phase BFO,the thin film annealed in a nitrogen atmosphere has Bi₂O₃ heterogeneous phase.The SEM and AFM results indicate that,BFO films annealed in oxygen environment have larger grain size and a smoother surface.The measured magnetism results indicate that the BFO films reflect the weak ferromagnetism by the post annealing in the three different kinds of gases.The BFO films annealed in the air and oxygen show relatively better phase purity and exhibit relatively higher saturation magnetization.The optical band gap value of BFO annealed in the oxygen is smaller than BFO annealed in the air and nitrogen respectively.Finally,the photocatalytic efficiency of BFO films which annealed in three different annealing atmospheres was compared.The photocatalytic activity of BFO films annealed in the oxygen atmosphere is favorable.In conclusion,oxygen annealing is beneficial to improve the crystallinity,magnetic,optical and photocatalytic properties of BFO films.