Temperature dependences of ferroelectricity and resistive switching behavior of epitaxial BiFeO_3 thin films

We investigate the resistive switching and ferroelectric polarization properties of high-quality epitaxial BiFeO3 thin films in various temperature ranges. The room temperature current–voltage(I–V) curve exhibits a well-established polarization-modulated memristor behavior. At low temperatures( 253 K), the I–V curve shows an open circuit voltage(OCV), which possibly originates from the dielectric relaxation effects, accompanied with a current hump due to the polarization reversal displacement current. While at relative higher temperatures( 253 K), the I–V behaviors are governed by both space-charge-limited conduction(SCLC) and Ohmic behavior. The polarization reversal is able to trigger the conduction switching from Ohmic to SCLC behavior, leading to the observed ferroelectric resistive switching. At a temperature of 298 K, there occurs a new resistive switching hysteresis at high bias voltages, which may be related to defect-mediated effects.     

First-principles study of FeSe epitaxial films on SrTiO_3

The discovery of high temperature superconductivity in Fe Se films on SrTiO_3 substrate has inspired great experimental and theoretical interests. First-principles density functional theory calculations, which have played an important role in the study of bulk iron-based superconductors, also participate in the investigation of interfacial superconductivity. In this article, we review the calculation results on the electronic and magnetic structures of Fe Se epitaxial films, emphasizing on the interplay between different degrees of freedom, such as charge, spin, and lattice vibrations. Furthermore, the comparison between Fe Se monolayer and bilayer films on SrTiO_3 is discussed.     

缓冲层Ta对FePt薄膜L10有序相转变及矫顽力的影响

制备了Ta/FePt/C系列多层膜，研究了样品在不同温度退火后的磁特性和微结构.实验结果表明，不同厚度的Ta缓冲层具有不同的微结构特征，显著影响FePt层的L1₀有序相的形成及相应的矫顽力.当Ta缓冲层较薄，Ta层为非晶态，且较为粗糙，由此使FePt在界面处产生较多的缺陷并导致较高密度的晶界，在退火过程中，受束缚相对较弱的非晶态的Ta原子比较容易沿FePt的缺陷和晶界处向FePt层扩散，使FePt在相变过程中产生的应力比较容易释放，同时，Ta在扩散过程中产生的缺陷，降低了FePt有序 关键词： FePt薄膜 0相')" href="#">L1₀相 原子扩散     

Structural and physical properties of BiFeO3 thin films epitaxially grown on SrTiO3（001） and polar（111） surfaces

The influence of surface polarity on the structural properties of BiFeO3 （BFO） thin films is investigated. BFO thin films are epitaxially grown on SrTiO3 （STO） （100） and polar （111） surfaces by oxygen plasma-assisted molecular beam epitaxy. It is shown that the crystal structure, surface morphology, and defect states of BFO films grown on STO substrates with nonpolar （001） or polar （111） surfaces perform very differently. BFO/STO （001）is a fully strained tetragonal phase with orientation relationship （001）[100]BFOII（001）[100]STO, while BFO/STO （111） is a rhombohedral phase. BFO/STO （111） has rougher surface morphology and less defect states, which results in reduced leakage current and lower dielectric loss. Moreover, BFO films on both STO （001） and STO （111） are direct band oxides with similar band gaps of 2.65 eV and 2.67 eV, respectively.     

Structure and properties of GZO thin films grown on ZnO buffer layers

Thin gallium-doped zinc oxide (in GZO the Ga₂O₃ contents are approximately 3 wt%) films having different ZnO buffer layers were deposited using radio frequency (rf) magnetron sputtering. The use of a grey-based Taguchi method to determine the processing parameters of ZnO buffer layer deposition has been studied by considering multiple performance characteristics. A Taguchi method with an L₉ orthogonal array, signal-to-noise (S/N) ratio, and analysis of variance (ANOVA) is employed to investigate the performance characteristics in the deposition operations. The effect and optimization of ZnO buffer deposition parameters (rf power, sputtering pressure, thickness, and annealing) on the structure, morphology, electrical resistivity, and optical transmittance of the GZO films are studied. Annealing treatment and reduction in thickness resulted in a decrease in root-mean-square (RMS) surface roughness of the ZnO buffer layer. Using the optimal ZnO buffer layer obtained by the application of the grey-based Taguchi method, the electrical resistivity of GZO films was decreased from 2.94×10⁻³ to 9.44×10⁻⁴ Ω cm and the optical transmittance in the visible region was slightly increased from 84.81% to 85.82%.     

Effects of BaTiO<Subscript>3</Subscript> and SrTiO<Subscript>3</Subscript> as the buffer layers of epitaxial BiFeO<Subscript>3</Subscript> thin films

BiFeO₃ (BFO) thin films with BaTiO₃ (BTO) or SrTiO₃ (STO) as buffer layer were epitaxially grown on SrRuO₃-covered SrTiO₃ substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

Optical properties of BiFeO3 epitaxial thin films

The properties of neodymium-doped BiFeO₃ nanosized films on magnesium oxide single-crystal substrates are studied. The films are obtained using high-frequency sputtering with the aid of layered growth. The structural perfection of the films is analyzed using the X-ray diffraction. The transmission of the films with different thicknesses is studied in the wavelength interval 200–1100 nm. The spectra are processed with the aid of a dispersion formula for permittivity of a sum of oscillators with allowance for damping, so that direct and indirect transitions can be revealed. The absorption edges are estimated to be 2.81 and 2.78 eV for the direct transitions of the films with thicknesses of 14 and 60 nm, respectively.     

Optical properties of epitaxial BiFeO3 thin films

Optical properties of epitaxial BiFeO₃ thin films grown via pulsed-laser deposition on (110) DyScO₃ substrates have been investigated. Their near-normal spectroscopic reflectivity was measured in the spectral range 2 to 14 eV at room temperature, while spectroscopic ellipsometry in the spectral range 1–6 eV was measured in the temperature range from 300 to 775 K. The optical response functions have been calculated and a direct optical gap was determined varying from 2.75 to 2.70 eV in this temperature range.     

Magnetoconductivity of thin epitaxial silver films

The magnetoconductance (MC) of thin epitaxial Ag films on Si(111) surfaces is studied as a function of film thickness (1–125 monolayers (ML)) at 20 K under ultra high vacuum (UHV) conditions. Three different regimes of magnetoconductance are observed depending on the degree of disorder in the films which is controlled by film thickness and annealing procedures. Thick films (d>3 ML) with diffuse electron transport show in the case of large elastic scattering times ₀ a classical, negative MC B ² and in the case of small ₀ a positive MC due to weak localization effects. The MC of thin films (d<2 ML) which have a conductance smaller than e ²/h, i.e. localized electron states, is negative again.     

Hard magnetic properties of rapidly annealed NdFeB thin films on Nb and V buffer layers

NdFeB thin films of the form A (20 nm)/NdFeB(d nm)/A(20 nm), where d ranges from 54 to 540 nm and the buffer layer A is Nb or V were prepared on a Si(1 0 0) substrate by magnetron sputtering. The hard Nd₂Fe₁₄B phase is formed by a 30 s rapid anneal or a 20 min anneal. Average crystallite size ranged from 20 to 35 nm with the rapidly annealed samples having the smaller crystallite size. These samples also exhibited a larger coercivity and energy product than those treated by a 20 min vacuum anneal. A maximum coercivity of 26.3 kOe at room temperature was obtained for a Nb/NdFeB (180 nm)/Nb film after a rapid anneal at 725°C. Initial magnetization curves indicate magnetization rotation rather than nucleation of reverse domains is important in the magnetization process. A Brown's equation analysis of the coercivity as a function of temperature allowed us to compare the rapidly annealed and 20 min annealed samples. This analysis suggests that rapid annealing gives higher quality crystalline grains than the 20 min annealed sample leading to the observed large coercivity in the rapidly annealed samples.     

Improved photovoltaic effects in Mn-doped BiFeO_3 ferroelectric thin films through band gap engineering

As a low-bandgap ferroelectric material, BiFeO_3 has gained wide attention for the potential photovoltaic applications,since its photovoltaic effect in visible light range was reported in 2009. In the present work, Bi(Fe, Mn)O_3thin films are fabricated by pulsed laser deposition method, and the effects of Mn doping on the microstructure, optical, leakage,ferroelectric and photovoltaic characteristics of Bi(Fe, Mn)O_3 thin films are systematically investigated. The x-ray diffraction data indicate that Bi(Fe, Mn)O_3 thin films each have a rhombohedrally distorted perovskite structure. From the light absorption results, it follows that the band gap of Bi(Fe, Mn)O_3 thin films can be tuned by doping different amounts of Mn content. More importantly, photovoltaic measurement demonstrates that the short-circuit photocurrent density and the open-circuit voltage can both be remarkably improved through doping an appropriate amount of Mn content, leading to the fascinating fact that the maximum power output of ITO/BiFe_(0.7)Mn_(0.3)O_3/Nb-STO capacitor is about 175 times higher than that of ITO/BiFeO_3/Nb-STO capacitor. The improvement of photovoltaic response in Bi(Fe, Mn)O_3 thin film can be reasonably explained as being due to absorbing more visible light through bandgap engineering and maintaining the ferroelectric property at the same time.     

Electrical properties and Pockels effect in BaTiO_{3}/SrTiO_{3} superlattices

The electrical and linear electro-optical properties of BaTiO $_{3}$ (BT)/SrTiO $_{3}$ (ST) superlattices epitaxially grown on ST substrate are theoretically investigated using a microscopic quantum mechanical model based on the orbital approximation in correlation with the dipole–dipole interaction. Both the first-, second-, and third-order electronic polarizabilities were considered in this calculation in order to obtain accurate results for both the spontaneous local electric field, spontaneous polarization, relative dielectric constant, and linear electro-optic coefficients (Pockels constants) of BT/ST superlattices. The calculations show that the spontaneous local electric field, spontaneous polarization and linear electro-optic coefficients of BT/ST superlattices increase with their content in BT while the relative dielectric constant increases with their content in ST. Moreover, we have shown that significant enhancement of the linear electro-optic coefficients can be achieved in BT/ST superlattices. This enhancement, which concerns the ST as well as the BT layers, arises from the combined effects of strain, induced in the BT layers by the epitaxial growth, and internal polarizing electric field originating in the BT layers and acting on the electronic clouds of the polarizable constituent ions of the system.     

Mechanisms of exchange bias with multiferroic BiFeO3 epitaxial thin films

We have combined neutron scattering and piezoresponse force microscopy to show that the exchange field in CoFeB/BiFeO_{3} heterostructures scales with the inverse of the ferroelectric and antiferromagnetic domain size of the BiFeO3 films, as expected from Malozemoff's model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the BiFeO3 film at the interface with CoFeB. In view of these results, we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using BiFeO3.     

Strain-induced properties of epitaxial VOx thin films

We have grown VOx thin films on different substrates in order to investigate the influence of epitaxial strain on the transport properties. We found that the electric conductivity is much larger for films grown under compressive strain on SrTiO3 substrates, as compared to bulk material and VOx films grown under tensile strain on MgO substrates. A clear crossover from metallic to semiconducting behavior is observed when increasing the oxygen content x. Apparently, the application of strain induces a Mott-Hubbard insulator-to-metal transition in VOx<1. The VOx/SrTiO3 films show an unexpected large positive magnetoresistance effect at low temperatures, which is not found in the VOx films grown under tensile strain on MgO or on a substrate with a similar lattice parameter.     

Comparison of the effect of PLT and PZT buffer layers on PZT thin films for ferroelectric materials applications

In order to study the effect of different buffer layers on the Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films, 10-nm thick (Pb_0.72La_0.28)Ti_0.93O₃ (PLT) and Pb(Zr_0.52Ti_0.48)O₃ buffer layers have been deposited on the Pt(1 1 1)/Ti/SiO₂/Si substrates by pulsed laser deposition, respectively. The top buffer layers were also deposited on PZT thin films with the same thickness of the seed layers in order to enhance the fatigue characteristics of PZT thin films. We compared the results of dielectric constant, hysteresis loops and fatigue resistance characteristics. It was found that the dielectric properties of PZT thin films with PLT buffer layers were improved by comparing with PZT thin films with PZT buffer layers. The polarization characteristics of PZT thin films with PLT buffer layers were observed to be superior to those of PZT thin films using PZT buffer layers. The remanent polarization of PZT thin films showed 36.3 μC/cm² and 2.6 μC/cm² each in the case of use PLT and PZT buffer layers. For the switching polarization endurance analysis, PZT thin films with PLT buffer layers showed more excellent result than that of PZT thin films with PZT buffer layers.     

TbMnO3 epitaxial thin films by pulsed-laser deposition

Epitaxial TbMnO₃ films have been fabricated on SrTiO₃(001) and LaAlO₃(001) substrates by pulsed laser deposition (PLD), the structure and surface morphology of the films were characterized by X-ray diffraction with Cu K_α radiation and atomic force microscopy. The electrical transport and magnetic properties of the TbMnO₃ films and bulk were examined, the resistivity and the forbidden band width E_g change with epitaxial orientation, semiconductor transport properties are found in the films and bulk, the average of the E_g of the films on SrTiO₃ and on LaAlO₃ is equal to the E_g of the bulk. The two TMO films have different magnetization mode, the magnetization of the film on SrTiO₃ have an analogy to that of TbMnO₃ single crystal.     

Ferromagnetism in epitaxial orthorhombic YMnO3 thin films

Epitaxial orthorhombic YMnO₃ thin films, (0 0 1) oriented, have been grown by pulsed laser deposition on (0 0 1)SrTiO₃ substrates. Their crystal structure and magnetic response have been studied in detail. Although bulk o-YMnO₃ is antiferromagnetic, our magnetic measurements reveal intriguing thermal hysteresis between the zero-field-cooled and field-cooled curves below the onset of the antiferromagnetic ordering temperature, thus signaling a more complex magnetic structure with net ferromagnetic moments. We discuss on the possible origin of this net magnetization and we have found a correlation of the magnetic response with the strain state of the films. We propose that substrate-induced strain modifies the subtle competition of magnetic interactions and leads to a non-collinear magnetic state that can thus be tuned by strain engineering.