The doping effects of BiFe1-xCoxO3 (x=0.0-0.8) in layered perovskite Bi4Ti3O12 ceramics

Bi5Fe1-xCoxTi3O15(x = 0.0, 0.2, 0.4, 0.5, 0.6, and 0.8) multiferroic ceramics are synthesized in two steps using the solid state reaction technique. X-ray diffraction patterns show that the samples have four-layer Aurivillius phases. At room temperature (RT), the samples each present a remarkable coexistence of ferromagnetism (FM) and ferroelectricity (FE). The remnant polarization (2P r ) reaches its greatest value of 14 μC/cm 2 at x = 0.6. Remnant magnetization (2M r ) first increases and then decreases, and the greatest 2M r is 7.8 menu/g when x = 0.5. The magnetic properties for x = 0.4 are similar to those for x = 0.6, indicating that the magnetic properties originate mainly from the coupling between Fe 3+ and Co 3+ ions, rather than from their own magnetic moments.     

High temperature ferromagnetism in cubic Mn-doped ZrO2 thin films

Theory has predicted that high temperature ferromagnetism (FM) should be found in cubic fake-diamonds, Mn-doped ZrO₂. Experimentally, it is shown that Mn-doped ZrO₂ ceramics are not ferromagnetic, but the nanosized Mn-doped ZrO₂ thin films grown on LaAlO₃ substrates can be ferromagnets with T_C above 400 K. The largest saturated magnetic moment (M_s) is huge as of about 230 emu/cm³ for the Mn_0.05Zr_0.95O₂ films, and it decreases as the Mn content increases. The intrinsic FM is strongly associated with the cubic structure of Mn-doped ZrO₂, and the Mn–Mn interactions via oxygen intermediates are important. No electrical conductivity is observed. Mn-doped ZrO₂ thin films can be truly considered as excellent candidates for spintronic applications.     

Laser reactivation of Room-Tc ferromagnetism in Mn-doped insulating TiO2 thin films

Room-T _c ferromagnetic insulating Mn-doped TiO₂ thin films were prepared by plasma assisted molecular beam epitaxy (PAMBE). The thin films show obvious decay of ferromagnetism when aged in air at room temperature for one year without any special treatment. A distinct reactivation of ferromagnetism, together with a decrease in coercive force is achieved in these samples under laser irradiation treatment. The possible mechanism for the phenomena is also discussed. It is suggested that the bound magnetic polaron (BMP) forming via photo-generated defect-bound carriers accounts for the laser reactivation of room-T _c ferromagnetism.     

The first-principles study of ferroelectric behaviours of PbTiO3/SrTiO3 and BaTiOn/SrTiO3 superlattices

We have performed the first-principles calculation to investigate the origins of ferroelectricities and different po- larization behaviours of superlattices BaTiO3/SrTiO3 and PbTiO3/SrTiO3. The density of state （DOS） and electronic charge profiles show that there are strong hybridizations between atoms Ti and O and between atoms Pb and O which play very important roles in producing the ferroelectricities of superlattices BaTiO3/SrTiO3 and PbTiO3/SrTiO3. Ow- ing to the decline of internal electric field in SrTiO3 （ST） layer, the tetragonality and polarizations of superlattices decrease with increasing the fraction of SrTiO3 in the superlattices. We find that the polarization of PbTiO3/SrTiO3 is largerthan that of BaTiO3/SrTiO3 at the same ratio of components, because the polarization mismatch between PbTiO3 and SrTiO3 is larger than that between BaTiO3 and SrTiO3. The polarization and tetragonality are en- hanced with respect to those of bulk tetragonal BaTiO3 in the superlattices BaTiO3/SrTiO3, while the polarization and tetragonality are reduced with respect to those of bulk tetragonal PbTiO3 in superlattices PbTiO3/SrTiO3.     

稀释磁性半导体Sn1-xMnxO2的室温铁磁性

采用固相反应法，制备了不同成分的稀释磁性半导体Sn_1-xMn_xO ₂(x=002，004，006)．利用x射线衍射和傅里叶变换红外光谱法证明 了锰均匀地掺杂到二氧化锡中．在室温下研究了掺锰二氧化锡基稀释半导体的磁性，发现它具有明显的铁磁性 ，同时对磁性的强弱与锰的含量和烧结温度的关系作了研究． 关键词： 稀释磁性半导体 掺杂 烧结 铁磁性 1-xMn_x O₂')" href="#">Sn_1-xMn_x O₂     

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.     

RHEED observation of BaTiO3 thin films grown by MBE

Ferroelectric BaTiO₃ thin films with a thickness of 10 monolayers (ML) were epitaxially grown on SrTiO₃(0 0 1) substrates by very slow deposition using molecular beam epitaxy (MBE). The investigations were carried out by two growth methods: (i) codeposition and (ii) alternate deposition of the metal elements in an oxygen atmosphere. In situ observation of reflection high-energy electron diffraction confirmed that an epitaxial cube-on-cube structure was prepared. After the deposition, X-ray diffraction measurements were carried out. The 10-ML-thick BaTiO₃ films were highly c-axis oriented single crystals with good film quality.     

Optical properties of carbon nanotubes and BaTiO3 composite thin films

Multiwalled carbon nanotubes and BaTiO₃ composite films have been prepared by pulsed-laser deposition technique at room temperature and high temperature of 600℃, separately. The structures of the composite films are investigated by using scanning electron microscopy and x-ray diffraction. The optical behaviours of the samples produced at different temperatures are compared with Raman spectroscopy, and UV-visible absorption. And the observation by Z-scan technique reveals that the composite films have a larger optical nonlinearity, and the samples prepared at high temperatures have better transmittance and opposite sign imaginary part of optical third-order nonlinearity.     

钛酸钡纳米颗粒铁电性临界尺寸的理论分析

从铁电体的Eular-Lagrange方程出发, 取贝塞尔方程级数解的形式, 得到了钛酸钡陶瓷颗粒的总极化强度表达式, 分析了各系数对总极化强度的影响. 根据总极化强度表达式, 采用MATLAB软件对尺寸在100 nm以下的钛酸钡纳米颗粒的铁电性进行了仿真分析. 结合实际数据探讨了尺寸效应对陶瓷颗粒铁电性的影响, 获得了与实验数据相符的数值解和极小值, 从而预测了钛酸钡纳米颗粒铁电性存在的临界尺寸为6 nm.     

Nitrogen doping-mediated room-temperature ferromagnetism in insulating Co-doped SnO2 films

Co-doped SnO₂ films codoped with nitrogen (N) have been prepared by magnetron sputtering to investigate the effect of p-type defects on magnetic properties. The incorporation of N modifies the preferential growth orientation of the films. Multiple characterization techniques reveal that the incorporated Co²⁺ and N³⁻ ions substitute for Sn⁴⁺ and O²⁻ sites in SnO₂ lattice, respectively. As N concentration increases, the band gap of the films decreases because of the formation of Sn-N bond. Room-temperature ferromagnetism is observed in (Co, N)-codoped SnO₂ films, and the saturated magnetic moment is sensitive to the incorporated N concentration. The variations in the magnetic properties as a function of N concentration are discussed on the basis of bound magnetic polaron model.     

Film thickness dependence of electro-optic effect in epitaxial BaTiO3 thin films

Based on the phenomenological Landau-Devonshire theory, we investigate the film thickness dependence of ferroelectric and electro-optic properties of epitaxial BaTiO₃ thin films grown on SrTiO₃ and MgO substrates. By using the effective substrate lattice parameter concept, the film thickness dependence of misfit strain is incorporated into the theory. Therefore, the film thickness dependence of ferroelectric and electro-optic properties in epitaxial BaTiO₃ thin films can be explained. Moreover, a large quadratic electro-optic effect was obtained in the BaTiO₃ thin films, which is in good agreement with the experimental result of BaTiO₃ thin films on the MgO substrate.     

Dielectric properties of BaTiO3-CoFe2O4 nanocomposite thin films

We report on the dielectric characterization of CoFe₂O₄-BaTiO₃ nanocomposites grown by rf sputtering. Dielectric properties have been analyzed for samples grown at different deposition temperatures and with different thicknesses. Impedance spectroscopy data has been analyzed by fitting to an equivalent circuit and different contributions have been identified. Correlations between dielectric properties and deposition temperature and thickness have been established.     

Possible ferrimagnetism and ferroelectricity of half-substituted rare-earth titanate: A first-principles study on Y0.5La0.5TiO3

Titanates with the perovskite structure, including ferroelectrics (e.g., BaTiO₃) and ferromagnetic ones (e.g., YTiO₃), are important functional materials. Recent theoretical studies predicted multiferroic states in strained EuTiO₃ and titanate superlattices, the former of which has already been experimental confirmed. Here, a first-principles calculation is performed to investigate the structural, magnetic, and electronic properties of Y half-substituted LaTiO₃. Our results reveal that the magnetism of Y_0.5La_0.5TiO₃ sensitively depends on its structural details because of the inherent phase competition. The lowest energy state is the ferromagnetic state, resulting in 0.25 μB/Ti. Furthermore, some configurations of Y_0.5La_0.5TiO₃ exhibit hybrid improper polarizations, which can be significantly affected by magnetism, resulting in the multiferroic properties. Because of the quenching disorder of substitution, the real Y_0.5La_0.5TiO₃ material with random A-site ions may exhibit interesting relaxor behaviors.     

Asymmetric reversible diode-like resistive switching behaviors in ferroelectric BaTiO3 thin films

In this work, the resistive switching behaviors of ferroelectrictric BaTiO3/La0.67Sr0.33MnO3 .heterostructures de- posited by pulsed laser deposition are investigated. The BaTiO3 films show both well-established P-E hysteresis loops, and asymmetric reversible diode-like resistive switching behaviors, involving no forming process. It is found that both the ON/OFF ratio and the stability of resistive switching are substantially dependent on operation voltage （Vmax）. At a Vmax of 15 V, a large ON/OFF resistance ratio above 1000 is obtained at a Vmax of 15 V, which is able to maintain stability up to 70-switching cycles. The above resistive switching behaviors can be understood by modulating interface Schottky barriers as demonstrated by I-V curve fitting.     

横场-伊辛模型中BaTiO3的铁电行为

采用横场-伊辛模型描述了BaTiO₃中量子效应对铁电行为可能的贡献.计算显示量子效应可以明显地减少铁电体的自发极化.模型给出的理论结果与BaTiO₃的介电及极化实验结果符合较好. 关键词： 铁电性 量子效应 3')" href="#">BaTiO₃     

Enhanced ferromagnetism in Mn-doped TiO2 films during the structural phase transition

The crystal structures and magnetic properties of Ti_0.9747Mn_0.0253O₂ films prepared by sol–gel dip coating have been investigated. Room temperature ferromagnetism was observed both in the films of pure anatase phase and of mixed anatase and rutile phases. For the first time, enhancement of the ferromagnetism was revealed as the phase transition from anatase to rutile occurs: from 0.7±0.01μ_B/Mn to 1.1±0.05μ_B/Mn. The possible mechanism for the observed magnetism enhancement is discussed.     

Ion-beam-induced ferromagnetism in Ca-doped LaMnO3 thin films grown on Si (100)

The ion-bean-induced room temperature ferromagnetic ordering in pulsed laser deposited Ca-doped LaMnO₃ thin films grown on Si (100) are presented in the present study. In addition to this, changes bought by the ion beam in structural, morphological and electrical properties are presented. Dense electronic excitation produced by high energy 120?MeV Ag⁹⁺ ion irradiation causes change in surface roughness, crystallinity and strain. It is also evident that these excitations induce the magnetic ordering in this system. The observed modifications are due to the large electronic energy deposited by swift heavy ion irradiation. The appearance of ferromagnetism at 300?K in these samples after irradiation may be attributed to the canting of the antiferromagnetically ordered spins due to the structural distortion. It is observed that the irradiated films show higher resistance than unirradiated films for all the compositions.     

Orientation-dependent ferroelectricity of strained PbTiO3 films

PbTiO₃ is a simple but very important ferroelectric oxide that has been extensively studied and widely used in various technological applications. However, most previous studies and applications were based on the bulk material or the conventional [001]-orientated films. There are few studies on PbTiO₃ films grown along other crystalline axes. In this study, a first-principles calculation was performed to compute the polarization of PbTiO₃ films strained by SrTiO₃ and LaAlO₃ substrates. Our results show that the polarization of PbTiO₃ films strongly depends on the growth orientation as well as the monoclinic angles. Further, it is suggested that the ferroelectricity of PbTiO₃ mainly depends on the tetragonality of the lattice, instead of the simple strain.     

Surface segregation in Nb-doped BaTiO3 films

We have used in situ photoemission spectroscopy to investigate Niobium doping in polycristalline BaTiO₃. The valence band maximum position progressively shifts from 2.5 eV for undoped to 2.84 eV for Nb-doped films. Ceramics and single crystal have been investigated for comparison with thin films. Nb-doped BaTiO₃ ceramics and Nb-doped SrTiO₃ single crystal show higher Fermi level position indicating that our doped films are less conducting regarding their bulk parents. This was confirmed by impedance spectroscopy under variable temperature. Large amount of niobium is clearly observable at surface but the amount of dopant is drastically reduced below the near-surface region, as evidenced by depth profile. Therefore, we provide evidence of surface segregation which would explain the contrasted resistivity values reported in literature for such donor-doped films.     

The effect of annealing on the room temperature ferromagnetism in co-sputtered In2O3: C thin films

In this paper, we report investigation of room temperature (RT) ferromagnetism in In₂O₃ (InO) thin films doped with carbon prepared by the co-sputtering method. InO thin films both undoped and C doped with varied thicknesses in the range of 45 to 80 nm were synthesized on Si substrates with varied C concentrations. The carbon concentration was varied from 1.6 to 9.3 at%. The undoped InO films showed no trace of ferromagnetism. Carbon doped films (InO:C) exhibited ferromagnetism at RT, which was of the orders of 10⁻⁵ emu and varied strongly with C concentrations. It is observed that the magnetization reached a maximum value of 5.7 emu/cm³ at 4 at% C. Annealing of the InO:C films in an oxygen environment resulted in a decrease in the magnetization, indicating the crucial role of oxygen vacancies in the films. It is concluded that the oxygen vacancies were important and compete with C substitution for the RT ferromagnetism.