Tunable Optical and Multiferroic Properties of Zirconium and Dysprosium Substituted Bismuth Ferrite Thin Films.

This work presents optical and multiferroic properties of bismuth ferrite thin films that are affected by zirconium and dysprosium substitution. Non-centrosymmetric BiFeO3,Bi0.95Zr0.05FeO3, and Bi0.95Dy0.05FeO3 thin films were coated on Pt/TiO2/SiO2/Si substrates using the spin coating method. The crystal structure, optical properties, microstructural, ferromagnetic, and ferroelectric properties of doped bismuth ferrite thin films were systematically investigated. From the XRD patterns, all the prepared thin films matched well with the rhombohedral structure with R3c space group with no observed impurity phases. The average crystallite size of the bismuth ferrite thin films were between 35 and 47 nm, and the size depended on the type of dopant. The determined energy band gap values of BiFeO3, Bi0.95Dy0.05FeO3, and Bi0.95Zr0.05FeO3 thin films were 2.32 eV, 2.3 eV, and 2 eV, respectively. Doping of Dy and Zr at the Bi site led to reduced surface roughness. The prepared thin films exhibited enhanced ferromagnetic and ferroelectric properties. The remnant magnetization of Zr-doped BiFeO3 was greater than that of the BiFeO3 and Dy-doped BiFeO3 thin films. From the obtained results, it was concluded that Zr-doped BiFeO3 thin films are suitable for solar cell fabrication.

Superparamagnetic state in La0.7Sr0.3MnO3 thin films obtained by rf-sputtering.

A novel superparamagnetic state has been observed in high quality La0.7Sr0.3MnO3 (LSMO) thin films directly grown by rf-sputtering on SiOx/Si(100) substrates. The films are nanostructured without grain boundaries, constituted by locally epitaxial nanoregions grown layer-by-layer with out-of-plane (012) preferential orientation, induced by the constrain of the native silicon oxide. Low magnetic field ZFC-FC magnetization curves show a cross-over from superparamagnetic to ferromagnetic state dependent of the thickness. The thicker film (140 nm) exhibits typical ferromagnetic order. The thinner films (40 and 60 nm) exhibit superparamagnetic behavior attributed to interacting ferromagnetic monodomain nanoregions with critical size, random in-plane oriented, where the inter-monodomain boundaries with surface spin-glass structure regulate the blocking of magnetization depending on the magnetic field intensity. M(H) hysteresis loops showed noticeable coercive fields in all samples, larger than those reported for LSMO. Such properties of half-metal LSMO film foresee potential integration in new Si-technology nanodevices in Spintronics.

Atomic force acoustic microscopy: Influence of the lateral contact stiffness on the elastic measurements.

Atomic force acoustic microscopy is a dynamic technique where the resonances of a cantilever, that has its tip in contact with the sample, are used to quantify local elastic properties of surfaces. Since the contact resonance frequencies (CRFs) monotonically increase with the tip-sample contact stiffness, they are used to evaluate the local elastic properties of the surfaces through a suitable contact mechanical model. The CRFs depends on both, normal and lateral contact stiffness, kN and kS respectively, where the last one is taken either as constant (kS<1), or as zero, leading to uncertainty in the estimation of the elastic properties of composite materials. In this work, resonance spectra for free and contact vibration were used in a finite element analysis of cantilevers to show the influence of kS in the resonance curves due to changes in the kS/kN ratio. These curves have regions for the different vibrational modes that are both, strongly and weakly dependent on kS, and they can be used in a selective manner to obtain a precise mapping of elastic properties.

Nanocomposite YCrO3/Al2O3: characterization of the core-shell, magnetic properties, and enhancement of dielectric properties.

Multifuncionality in polycrystalline multiferroic ceramics can be improved using an advanced synthesis process. In this work, core-shell design is being proposed to enhance the transport properties of biferroic YCrO3. The atomic layer deposition (ALD) thin-film growth technique was used for the YCrO3/Al2O3 (Y@Al) nanocomposite fabrication. A continuous, amorphous, and uniform Al2O3 shell, a few nanometers thick, was obtained and characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy. The transport properties of biferroic YCrO3 coated with 50, 500, and 1000 ALD cycles of insulating Al2O3 were investigated using magnetization and AC conductivity measurements. It is observed that the values of the magnetic coercive field and the magnetization are affected by the amorphous and partially crystallized Al2O3 shell. Additionally, the Y@Al nanocomposite experiments show a notorious decreasing in the loss tangent and the electrical conductivity. Accordingly, hysteresis loops in the polarization versus electric energy data confirm the decrease of the leakage current as a consequence of the Al2O3 shell acting as a barrier layer. The results shown here confirm that the core-shell architecture is a promising alternative for improvement of the magnetic and ferroelectric properties in bulk multiferroics.

Nano and micro reoriented domains and their relation with the crystal structure in the New Fe1.5Zn1.5B7O13Cl boracite.

New iron-zinc chlorine single crystals of Fe1.5Zn1.5B7O13Cl boracite were grown by chemical transport reactions in closed quartz ampoules, at a temperature of 1173 K. The crystal structure was characterized by X-ray powder diffraction (XRD) using the Rietveld refinement method and belongs to the trigonal/rombohedral system with space group R3c (No. 161). The cell parameters were a = 8.5726(1) angstroms, c = 21.0116(4) angstroms, V = 1337.26(3) angstroms3 and Z = 6. The refinement successfully proceeded and ended with sound merit figure values chi2 = 2.25, R(B) = 6.12%. Chemical analysis was performed with X-ray energy dispersive spectroscopy (EDS) and X-ray fluorescence (XRF). Ferroelectric nano and micro reoriented domains were found in this material using polarizing optical microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The examination by TEM showed that in the trigonal/rombohedral system of Fe1.5Zn1.5B7O13Cl nanodomain structures exist. Thin (50-100 nm) mostly planar domains parallel to (100) were frequently observed in Fe1.5Zn1.5B7O13Cl boracite.

Crystal structure and relaxor-type transition in SrBi(2)Ta(2)O(9) doped with praseodymium.

The effects of Pr substitution in the structure and ferroelectric response for the Sr(1-x)Pr(x)Bi(2)Ta(2)O(9) (SBT-Pr) compound have been studied. Rietveld refinement of the x-ray diffraction patterns indicates that the Pr ion progressively replaces the Sr site in the A 2(1)am space group structure. The solubility of Pr in solid solution is around 15%. The replacement induces a change in the crystal structure and, as a consequence, the dielectric properties are affected. The ferroelectric transition at T(m)∼558 K is shifted to lower temperatures, T(m)∼413 K for x = 0.15 composition. Apparently, the tilt angle (α) associated with the c-axis does not play an important role since it remains essentially constant. However, the rotation in the ab-plane (ß) as well as the octahedral distortion observed are strongly related to the coupling between T(m) and x. A relaxor-type transition is observed as Pr is increased, leading to polar microregions above the nominal ferroelectric transition. The local disorder induced by the Pr ion is confirmed by the continuous increase in the diffuseness coefficient according to Isupov's model. These facts hinder the displacement of the TaO(6) octahedra with respect to Bi(2)O(2) along the polarization axis, decreasing the polarization values.

Admittance of rugate filters derived from a 2 × 2 inhomogeneous matrix.

Admittance diagrams are used to analyze qualitatively and quantitatively the behavior of inhomogeneous thin films with an arbitrary refractive-index function. From this study it follows that one can treat the behavior of rugate filters by using the concept of effective refractive indices, which are associated with the phase integral in a simple way. These results are applied to the study of periodic systems, and, as expected, one can consider high and low effective refractive indices to determine the important parameters of these stop bands. With these ideas it is possible that one can deal with rugate filters more closely as homogeneous periodic systems by taking advantage of the existing theory.

Optical properties of parylene and its use as substrate in beam splitters.

The optical properties of a commercial parylene pellicle are obtained using a classical oscillator model with a single oscillator. Using a standard design antireflection stack and beam-splitter multilayer films coated on this pellicle, its performance is greatly improved, flattening the curve and increasing its useful spectral range.

Attenuated-total-reflection technique for the determination of optical constants.

In a glass-metal-dielectric system, it is normally impossible to determine simultaneously the complex dielectric constant, the thickness of the metal, and the corresponding parameters of a dielectric overlayer. We propose the use of the pseudo-Brewster or Abelés angle as an additional parameter to characterize simultaneously a dielectric thin-film overcoating and the metal surface parameters. We use a Kretschmann attenuated-total-reflection configuration. An admittance diagram is used to illustrate graphically the role of an absentee layer at this angle. A study of the limitations of the method is also presented.

Determination of the optical properties of amorphous selenium films by a classical damped oscillator model.

Amorphous selenium films were produced by vacuum evaporation onto glass and aluminum substrates. The chemical composition of the films and interfaces were studied using scanning Auger electron spectroscopy. To gain some insight into the optical properties of the selenium films, we used the classical oscillator model to fit the transmittance spectrum.

Determination of the optical constants of MgF(2) and ZnS from spectrophotometric measurements and the classical oscillator method.

The values of the optical constants of magnesium fluoride (MgF(2)) and zinc sulfide (ZnS) thin films are obtained using a classical oscillator model and the experimental values of their spectral transmittance. Auger electron spectroscopy was performed on the samples to determine the chemical composition of the films. These materials are important in the design of filters, mirrors, and antireflection coatings for optical instrumentation. Unfortunately their properties strongly depend on evaporation conditions. The procedure described here allows direct measurement of the dispersive refractive index of the film after deposition.

Determination of (n,k) for absorbing thin films using reflectance measurements.

We propose a method for determination of the complex refractive index of absorbing materials either in bulk or film geometry by measuring its reflectivity when coated with a well-characterized transparent dielectric at two specific optical thicknesses: n(1)d(1) = lambda(0)/4 and n(1)d(1) = lambda(0)/2. The complex refractive index of the sample ñ = (n,k) is calculated for the monitoring wavelength lambda(0). The selected optical thicknesses of the coating allow the calculation of its geometrical thickness, therefore the variation of ñ with wavelength in the region where the reflectivity is measured can be determined.