The Fabrication And Properties Of Bi_3.15Nd_0.85Ti_3-xHf_xO₁₂/La_0.67Sr_0.33MnO₃ Magnetoelectric Composite Films On Si Substrates

Bismuth titanate(Bi₄Ti₃O₁₂,BIT)material was a kind of excellent lead-free ferroelectric material.The co-substitutedits of BIT at A(Bi-bit)and B(Ti-bit)double doping can effectively modicare the remnant polarization(Pr),coercive field(Ec)and leakage current density,and result in improving its ferroelectric properties.The fabrication of the ferromagnetic-ferroelectric composite films on Si substrates can not only effectively reduce the cost of substrate material,but also can be beneficial to integrate the ferromagnetic-ferroelectric composite films with the semiconductor materials,thus promoting the development of the new functional device.In this paper,the La0.67Sr0.33MnO3(LSMO)ferromagnetic layer and the Nd3+/Hf4+ co-substituted Bi₄Ti₃O₁₂(Bi3.15Nd0.85Ti3-xHfxO12(BNTHX,x=0,0.025,0.05,0.1 and 0.15))ferroelectric layer were successively deposited onto the(00l)-oriented LaNiO3(LNO)layer buffered Si substrates via the Sol-gel method.As a result,BNTHx/LSMO/LNO/Si ferromagnetic-ferroelectric composite films with the 2-2 laminated structure exhibited highly c-axis orientation.The structure,surface morphology,ferroelectric,ferromagnetic and magnetoelectric coupling properties of the composite films were characterized by XRD,SEM,TEM,TFA,VSM and multi-iron material magnetoelectric measurement system,and the specific contents and analysis results were as follows:(1)Three kinds of LNO precursor sols were prepared using 2-methoxyethanaol or anhydrous methanol as solvent,2-aminoethanol or acetylacetone as chelating agent.The effects of precursor sols and rapid heat treatment on the films quality and c-axis orientation of the LNO films were investigated.The results indicated that the surface quality of the LNO films was mainly related to the heating rate during the rapid heat treatment.The LNO films prepared from the anhydrous methanol-acetylacetone precursor sols and by the rapid heat treatment rate at 40?/s exhibited the best film surface quality,the highest degree of c-axis orientation(99.57%)and the lowest resistivity(0.935×10-3 ?·cm).(2)The LSMO ferromagnetic layers and Nd-doped BIT(Bi3.15Nd0.85Ti3O12(BNT))ferroelectric layers were successively deposited onto the LNO/Si substrate by Sol-gel method.As the result,the BNT/LSMO/LNO/Si ferromagnetic-ferroelectric composite films with c-axis orientation were fabricated.The effects of crystallization temperature on the crystal structure,surface morphology and ferroelectric properties were investigated.The results demonstrated that when the crystallization temperature of BNT thin films was 730?,the composite films had the maximum remnant polarization(Pr=20.8 ?C/cm2),the minimum coercive field(Ec=240 kV/cm)and the lowest leakage current density(J=5.4×10-6 A/cm2).(3)The BNTHx/LSMO/LNO/Si magnetoelectric composite films with the different Hf doping amount were fabricated.The effects of the different Hf doping amount on the ferroelectric,ferromagnetic and magnetoelectric coupling properties of the composite films were investigated.The results showed that when the x=0.05,the ferroelectric properties of the composite films were preferable,and exhibited Pr=27.3 ?C/cm2,Ec=198 kV/cm,and the leakage current density was 2.5×10-7 A/cm2.The composite films had soft magnetic properties with the saturation magnetization of 650 emu/cm3 at 50 K,and the coercive field of 150 Oe.The composite films also showed a very excellent magnetoelectric coupling effect.The magnetic coupling coefficient ?E increased first and then decreased with the increasing of the DC bias field Hbias,When the AC magnetic field frequency was 20 kHz,the maximum value of?E was 2.903 V/cm Oe at the Hbias=61.4 Oe.The ?E increased linearly with the increasing of the AC magnetic field frequency under the fixed DC bias magnetic field,the aE value eventually reached about 20 V/cm·Oe at the AC magnetic field frequency of 100 kHz.