Effect Of Annealing Temperature On The Structures And Properties Of Bi_4.15Nd_0.85Ti₃FeO₁₅ Thin Films

Multiferroic materials present magnetic order and ferroelectricity in the same phase and a coupling between them over a certain range of temperatures simultaneously. These materials have potential applications in both magnetic and ferroelectric devices. These materials have the coupling of electric and magnetic polarization which provides an additional degree of freedom in device design. For decades, the fundamental physics of multiferroic materials have been concentrated by physicists. So far, Bi Mn O3 and Bi Fe O3 are the most frequently investigated multiferroic materials, which present ferroelectric order and spin order at a relatively high temperature. For example, perovskite Bi Fe O3（BFO） has antiferromagnetic behavior with a Néel temperature of about 370℃ and ferroelectric behavior with a high Curie temperature of about 810℃. Unfortunately, BFO crystals, ceramics and thin films have high leakage current at room temperature due to oxygen nonstoichiometry and valence fluctuation of Fe ions. Some single-phase magnetoelectric layered bismuth oxides are taken as alternatives of BFO, such as Bi5Ti3 Fe O15（BTF）. BTF is composed of four perovskite layers sandwiched by two（Bi2O2）2+ layers, prepared by inserting Bi Fe O3 unit into compound Bi4Ti3O12. The（Bi2O2）2+ layers play important roles of both space-charge compensation and insulation blocks. However, a small polarization and poor fatigue endurance is a problem of polycrystalline Bi4Ti3O12, and most of BTF material’s reports show poor ferroelectric properties with small polarization. La-modified or Nd-modified Bi4Ti3O12 material has been reported as good fatigue resistance with larger remanent polarization. On the other hand, Nd dopant can improve BFO’s the multiferroic and electric transport properties. Particularly, Bi3.15Nd0.85Ti3O12（BNT） films have large polarization and fatigue-free characteristics.In this dissertation, we report the structural and electric properties of Bi₄.15Nd₀.85Ti₃FeO₁5（BNTF） thin films, which are prepared by the intergrowth of Bi Fe O3 and Bi3.15Nd0.85Ti3O12 followed by annealing at 700℃,750℃ and 780℃ respectively. The metal-organic decomposition method was employed to fabricate the film on（111）Pt/Ti/Si O2/Si substrates. BNTF films exhibit single-phase perovskite structure. We investigate the effect of annealing temperature on the structures and properties of BNTF thin films, the main results are as followings:1. By increasing annealing temperature the range of 700℃⁷80℃, the remnant electric polarization increases, imply an enhancement of the ferroelectric properties of BNTF thin films. The leakage current density varies with the annealing temperature. The samples’ surface roughness, grain size and surface electric reaction are found to be risen with the increasing annealing temperature. Particularly, microstructures of 780℃ annealing sample surface shows obvious difference form others, which probably promotes the thin film’s ferroelectricity.2. BNTF thin films exhibit fatigue phenomenon. Compared to other ferroelectric materials, the fatigue endurance of BNTF thin films is improved, with a fatigue loss less than 10%. We found the pyroelectricity of 700 BNTF and 750 BNTF is similar, but 780BNTF’s pyroelectricity changes a lot at the temperature from RT to 200℃.3. The magnetic polarization is not observed in BNTF films at room temperature in VSM measurements. In the measurements of electric transportation, BNTF films present metallic character from 10 K to RT.