Dielectric enhancement in pulsed-laser-deposited (Pb,Sr)TiO3/(Pb,La)TiO3 multilayered films

Multilayered (Pb_0.5Sr_0.5)TiO₃(PST)/Pb_0.8La_0.2TiO₃(PLT)/PST and PLT/PST/PLT films were prepared on platinum coated silicon substrates by pulsed laser deposition. Significant enhancement of the dielectric constant over the whole temperature region from 30 °C up to 220 °C has been observed for both multilayers with respect to the identically prepared single-component PST and PLT films. However, larger frequency dispersion of dielectric properties has also been obtained in the multilayers. This effect is attributed to the compositional fluctuation revealed by the atomic interdiffusion in the interface regions of the multilayers. Thermally and electric-field driven space charges at relative low temperature and low electric field were found in the PLT/PST/PLT and PLT films, attributed to the substitution of La³⁺ non-isovalent for Pb²⁺ in these films. PACS 81.15.Fg; 42.70.Mp; 79.60.Jv     

Material constants of (Ba,Sr)TiO3 solid solutions

The phenomenological theory of solid solution is used to calculate the material constants (elastic compliances, susceptibilities, piezoelectric moduli, pyroelectric constants) of Ba _x Sr_{1 ? x} TiO₃ solid solutions over the entire range of concentrations x at room temperature. The phenomenological potential constants and the numerical values of the material constants are given for specific concentrations.     

Scaling effect of flexoelectric (Ba,Sr)TiO3 microcantilevers

The flexoelectric microcantilever offers an alternative approach for the development of micro/nano‐sensors. The transverse flexoelectric coefficients µ₁₂ of barium strontium titanate microcantilevers were measured at room temperature, and found to keep the same value of 8.5 µC/m for microcantilevers with thickness ranging from 30 µm to 1.4 mm. The calculated effective piezoelectric coefficient and electrical energy density of flexoelectric cantilevers are superior to those of their piezoelectric counterparts, suggesting that the flexoelectricity‐induced polarization can be significantly increased as structures are scaled down due to the scaling effect of strain gradient, holding promise for flexoelectric micro/nano cantilever sensing applications. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multiferroic and magnetoelectric properties of MnFe2O4/(Pb0.8Sr0.2)TiO3 composite films

Abstract

MnFe₂O₄/(Pb_0.8Sr_0.2)TiO₃ (MFO/PST20) heterostructured composite films with three different structures have been grown on Pt/TiO₂/SiO₂/Si substrates by metal–organic decomposition processing via spin coating technique. The structural analysis revealed that the crystal axes of the MnFe₂O₄ are aligned with those of the PST20 ferroelectric matrix with obvious interfaces and no diffusions exist in all the three composite films. These composite films exhibit simultaneously multiferroic and magnetoelectric responses at room temperature. The growth structure of MFO and PST20 layers has an effect on multiferroic and magnetoelectric coupling behaviours of the composite films. The bi- and four-layered MFO/PST20 composite films exhibit superior ferroelectric properties compared to the tri-layered film. The increasing MFO and PST20 layers in the composite films enhance ferromagnetic properties and are closely related to the strain release in MnFe₂O₄ phase. The MFO/PST20 bi-layered composite film shows a high magnetoelectric voltage co-efficient α_E ~ 194 mVcm^?1Oe^?1 at a dc magnetic field H_dc ~ 2.5 kOe. A significant decrease in α_E value has been observed for tri- and four- layered composite films. A close correlation between phase selective residual stress and magnetoelectric properties has been emerged. The results are reasonably encouraging for employing MnFe₂O₄ for growing multiferroic–magnetoelectric composite films.     

Structural and crystallization behavior of (Ba,Sr)TiO3 borosilicate glasses

Various glass samples were prepared by melt quench technique in the glass system [(Ba_{1? x} Sr _x ) TiO₃]–[2SiO₂–B₂O₃]–[K₂O] doped with 1?mole% of La₂O₃. Infrared spectra show the number of absorption peaks with different spliting in the wave number range from 450 to 4000?cm^?1. Absorption peaks occurs due to asymetric vibrational streching of borate by relaxation of the bond B–O of trigonal BO₃. Raman spectra show the Raman bands due to ring-type metaborate anions, symmetric breathing vibrations BO₃ triangles replaced by BO₄ tetrahedra, and symmetric breathing vibrations of six-member rings. The differential thermal analysis of a glass sample corresponding to composition x?=?0.0 shows crystallization temperature at 847°C and glass transition temperature at 688°C. X-ray diffraction (XRD) pattern of glass ceramic samples shows the major crystalline phase of BaTiO₃ whereas pyrochlore phases of barium titanium silicate. Scanning electron micrographs confirm the results of XRD as barium titanate is major crystalline phase along with pyrochlore phase of barium titanium silicate.     

Co-doped (La, Sr)TiO(3-delta) : a high Curie temperature diluted magnetic system with large spin polarization

We report on tunneling magnetoresistance (TMR) experiments that demonstrate the existence of a significant spin polarization in Co-doped (La, Sr)TiO(3-delta) (Co-LSTO), a ferromagnetic diluted magnetic oxide system (DMOS) with high Curie temperature. These TMR experiments have been performed on magnetic tunnel junctions associating Co-LSTO and Co electrodes. Extensive structural analysis of Co-LSTO combining high-resolution transmission electron microscopy and Auger electron spectroscopy excluded the presence of Co clusters in the Co-LSTO layer and thus, the measured ferromagnetism and high spin polarization are intrinsic properties of this DMOS. Our results argue for the DMOS approach with complex oxide materials in spintronics.     

Raman spectroscopy of BaTiO3/(Ba,Sr)TiO3 superlattices

A series of BaTiO₃/Ba_{1 ? x} Sr _x TiO₃ (BT/BST) superlattices were prepared by pulsed laser deposition on MgO substrates with a constant period of 80 Å (40 Å BT and 40 Å BST) and varying compositions of the BST layer so that the Ba/Sr concentration ratios were 0/100, 30/70, 40/60, 50/50, 60/40, 70/30, 100/0. The soft mode E(1TO) of the polarized Raman spectra transformed depending on the Ba/Sr ratio in the BST layer. As the Sr concentration in the BST layers increased from 0 to 100%, the E(1TO) soft mode half-width varied from 171 to 103 cm^?1 and its frequency increased from 31 to 109 cm^?1 due to the interaction between the epitaxial layers forming the superlattices.     

Dielectric response of (Ba,Sr)TiO3 thin films in a terahertz and IR ranges

Dependences of the dielectric response spectra on the thickness of the films based on the (Ba,Sr)TiO₃ (BST) solid solutions, which were deposited on single-crystal MgO substrates, have been analyzed. Using the Lorentz and Lyddane-Sachs-Teller models, the mutual correspondence of longitudinal optical (LO) and transverse optical (TO) phonon frequencies has established. The giant LO-TO splitting of the lowest-frequency vibration and inversion of LO and TO frequencies of other phonons in the IR spectrum of BST films has been found for the first time.     

X-ray diffraction investigation of BaTiO3/(Ba,Sr)TiO3 superlattices

Samples of BaTiO₃/(Ba_{1 ? x} Sr _x )TiO₃ (BT/BST-x) superlattices have been studied using X-ray diffraction analysis. A complete parallel orientation of all the studied films and the substrate has been revealed, and the modulation periods ?? of the superlattices, the parameters of the unit cell averaged over the ?? period, and the parameters of individual components of the superlattices have been determined.     

Measurement of superhigh-frequency dielectric permittivity for paraelectrics (Ba,Sr) TiO3 in a transverse bias field

An improved apparatus for measuring dielectric permittivity in the shf range by the dielectric-resonance method is described. A cylindrical sample is placed at the antinode of a waveguide magnetic field. A measurement-line probe, placed at the minimum of the standing wave, is used to indicate resonance. Dielectric resonances are traversed by varying the sample temperature and dielectric permittivity. Results of measuring temperature dependences of and tan are given for BaTiO₃ ceramics and BaTiO₃ SrTiO₃ solid solutions in the paraelectric phase. The dependences of on a bias electric field perpendicular to the measuring field are found. It is shown that in a transverse bias field decreases though to a smaller extent than in a longitudinal field. It is concluded that the basic mechanism of paraelectric nonlinearity is the saturation of electron-ion polarization.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 9, pp. 20–25, September, 1971.     

Absence of relaxor-like ferroelectric phase transition in (Pb,Sr)TiO3 thin films

We have performed dielectric and micro-Raman spectroscopy measurements in the 298–673 K temperature range in polycrystalline Pb_0.50Sr_0.50TiO₃ thin films prepared by a soft chemical method. The phase transition have been investigated by dielectric measurements at various frequencies during the heating cycle. It was found that the temperature corresponding to the peak value of the dielectric constant is frequency-independent, indicating a non-relaxor ferroelectric behavior. However, the dielectric constant versus temperature curves associated with the ferroelectric to paraelectric phase transition showed a broad maximum peak at around 433 K. The observed behavior is explained in terms of a diffuse phase transition. The obtained Raman spectra indicate the presence of a local symmetry disorder, due to a higher strontium concentration in the host lattice. The monitoring of some modes, conducted in the Pb_0.50Sr_0.50TiO₃ thin films, showed that the ferroelectric tetragonal phase undergoes a transition to the paraelectric cubic phase at around 423 K. However, the Raman activity did not disappear, as would be expected from a transition to the cubic paraelectric phase. The strong Raman spectrum observed for this cubic phase is indicative that a diffuse-type phase transition is taking place. This behavior is attributed to distortions of the perovskite structure, allowing the persistence of low-symmetry phase features in cubic phase high above the transition temperature. This result is in contrast to the forbidden first-order Raman spectrum, which would be expected from a cubic paraelectric phase, such as the one observed at high temperature in pure PbTiO₃ perovskite. PACS 78.30.-j; 77.80.Bh; 64.70.Kb; 68.55.-a; 77.22.-a; 77.55.+f     

两种不同(Ba,Sr)TiO3薄膜介电-温度特性的研究

研究并比较了两种不同(Ba0.5,Sr0.5)TiO3(BSTO)薄膜介电-温度特性.采用脉冲激光沉积技术在Pt/Ti/SiO2/Si(100)衬底上制备BSTO薄膜,发现制备条件的不同,可以得到介电性质完全不同的BSTO薄膜.在550℃和氮气氛下制备的BSTO薄膜在常温下具有很高的介电常数,在10kHz下,超过2500,并在200K温度以上介电常数基本不变.它的一些电学性质不同于在正常条件(650℃和氧气氛下)制得的BSTO薄膜,而类似于目前广泛报道的巨介电常数材料如CaCu3Ti4O12.两种薄膜介电性质测试结果表明: 氧气氛下制备的BSTO薄膜呈现铁电-顺电相变,符合居里-外斯定律;低温氮气氛下制备的BSTO薄膜,介电弛豫时间和温度的关系符合德拜模型,是热激发弛豫.文中给出了产生这种介电特性的初步解释.     

Relaxor properties of Nb-modified (Ba0.8Sr0.2)TiO3 ceramics

Nb-doped (Ba_0.8Sr_0.2)TiO₃ ceramics were prepared using conventional mixed-oxide processing technique. Permittivity and loss factor were investigated as a function of temperature for various frequencies of the measuring field. The obtained results confirmed the relaxor ferroelectric behaviour of the studied ceramics, i.e. a strong frequency dispersion of the permittivity maximum and a visible shift of its temperature with frequency. Analysis of real part of permittivity allowed us to determine the value of the freezing temperature characterising the relaxor ferroelectrics. The physical processes, responsible for the relaxor behaviour of the studied ceramics are discussed.     

Enhanced magnetic and optical properties of pure and (Mn,Sr) doped BiFeO3 nanocrystals

Pure and (Mn, Sr) doped BiFeO₃ nanoparticles with the diameter in the range of 30–100 nm were controllably synthesized by a soft chemical method at a relatively low temperature. The diameter of the product was well controlled by adjusting the concentration of tartaric acid. Vibrating sample magnetometer (VSM) measurements verified that great enhancement of ferromagnetic property could be achieved either by decreasing their diameter or adding doping ions (Mn, Sr). Having suppressed the spiral spin structure, BiFeO₃ nanoparticles with perfect ferroelectric and ferromagnetic properties were excellent in ferroelectromagnet applications. The photoluminescent property of BiFeO₃ nanoparticles was also investigated in order to exploiting their possible application in nanoscale optoelectronic devices.     

Catalytic precursors: (Fe,Mn)3O4 and γ-(Fe,Mn)2O3

Iron-manganese oxide catalysts are known to influence the production of short chain hydrocarbons in the Fischer-Tropsch-synthesis process. XRD, XRF, gravity measurements and Mössbauer-effect spectroscopy has been used to study a catalytic precursor made by coprecipitation of iron and manganese oxides. The sample is characterized as a mixture of two compounds with the same iron to manganese ratio. The two compounds are cubic spinels with the same crystallographic parameters, however, one compound is a defect spinel while the other is not. In the Mössbauer studies the defect spinel shows superparamagnetic relaxation behavior—fluctuations of the magnetization vector along two opposite easy directions, while the non-defect spinel shows evidence of collective excitations—fluctuations of the magnetization vector in directions close to the easy direction. These properties are related to the particle sizes.     

Temperature effect on the trapped-charge relaxation in (Ba,Sr)TiO3 films on silicon

The trapped-charge relaxation has been calculated for a two-layer insulator with negligible conductivity in one of the layers the conductivity of the other layer obeying the Pool-Frenkel mechanism. The calculated values are compared with the experimental data on the trapped-charge relaxation in (Ba,Sr)TiO₃ films on silicon substrates. On the basis of this comparison, the depth of location and concentration of Frenkel emission centers are estimated. The results obtained can be used in the analysis of relaxation processes in ferroelectrics, which have strong electric fields in surface layers.     

Electrical transport, magnetism, and magnetoresistance in La0.7Sr0.3(Mn1-xCox)O3

The crystal structure (rhombohedral) is not changed on the replacement of Mn by Co. The lattice parameters a and c decrease with the increasing Co concentration. In this paper we have systematically investigated the effect of Co doping on the Mn site in the La_0.7Sr_0.3MnO₃ and find that Co substitution leads to a drastic suppression of ferromagnetic long-range order and metallic state. However, we observed substantial enhancement of MR ratio over the entire temperature studied for x=0.1 and a large increase of one at the low temperature for the compositions of x=0.2 and 0.3. The largest MR ratio as high as ≈78% is observed at 15 K in x=0.2 and 0.3. As x is further increased toward 1, conduction and ferromagnetism strongly recover but the MR ratio decreases obviously. The end member with x=1.0 has metallic and cluster spin-glass ferromagnetic behavior (T<T_C) and a smaller value of MR (≈2–8%) in T<300 K with a maximum MR ratio of 8% at around T_C (=238 K). Received: 11 September 1998 / Accepted: 19 November 1998 / Published online: 17 March 1999     

High-Pressure Mössbauer Measurements on Nanocrystalline Perovskite (La,Sr)(Mn,Fe)O3

We report here the Mössbauer measurements on nanocrystalline perovskite structured manganite La_0.8Sr_0.2Mn_0.8Fe_0.19 ⁵⁷Fe_0.01O₃ as a function of pressure up to 10 GPa at room temperature. The nanocrystalline sample, prepared by sol–gel technique found to have crystallite sizes of ∼138 ± 10 Å. Zero-field electrical resistivity measurements with temperature support the nanocrystalline nature. At ambient pressure, Fe³⁺ as well as Fe⁴⁺ ions are distributed in two different environments – Fe³⁺ in low symmetric site surrounded by Mn³⁺ ions only while Fe⁴⁺ in high symmetric site with at least one Mn³⁺ ion. Pressure seems to affect the higher symmetric site. A sudden increase in isomer shift at 0.52 GPa indicates the first order phase transition representing the transformation of Fe⁴⁺ to Fe³⁺. Another transition at 3.7 GPa, represents the presence of Fe³⁺ in single kind of environment. Pressure dependence of electrical resistivity measurements verifies the transitions attributing the first order transition to the cross over of localized-electron to band magnetism.