CEMS characterisation of Fe/high-κ oxide interfaces

The interfaces between Fe and different high-κ oxides are investigated by means of conversion electron Mössbauer spectroscopy (CEMS). Information on the magnetic ordering at the interface is obtained from the magnetic hyperfine splitting of the Mössbauer spectra. The reactivity of the Fe atoms at the interface (intermixing) is also estimated by CEMS. X-ray diffraction (XRD) and X-ray reflectivity (XRR) provide additional information on the intermixing and different phases present at the interface. CEM-spectra show the presence of both ferromagnetic and paramagnetic phases. CEMS and XRD results show that the Fe/HfO₂ and Fe/Al₂O₃ interfaces are the least reactive. The degree of intermixing between Fe and the high-κ oxide is determined by the oxide surface roughness.     

Growth,electronic properties and thermal stability of the Fe/Al2O3 interface

Soft X-ray photoelectron spectroscopy and resonant photoemission have been used to study the growth and electronic properties of Fe ultrathin films deposited on Al₂O₃ substrates. A simultaneous multilayer growth mode has been found for Fe growth at room temperature. For iron coverages below 1 ML, Fe²⁺ species are formed at the Fe/Al₂O₃ interface, followed by the formation of a metallic iron overlayer. The bonding of Fe at very low coverages occurs by charge transfer from Fe to surface oxygen atoms, and neither hybridisation of Fe and Al states nor reduction of the Al₂O₃ substrate are observed. The thermal stability of the interface has been also studied in the range 673–873 K. Annealing produces Fe agglomeration in such a way that some areas of the Al₂O₃ substrate become fully Fe-depleted. In these Fe-depleted areas, Fe²⁺ completely disappears and Al⁰ reduced species are formed. This behaviour would explain the decrease in the magnetoresistance performance of magnetic tunnel junctions after annealing above 573 K.     

Structural and magnetic properties of La/Fe multilayers

The structural and magnetic properties of La/Fe multilayers were investigated by X-ray diffraction, RHEED, magnetometry and⁵⁷Fe Mössbauer spectroscopy. Comparison is made with previous results obtained for Ce/Fe multilayers. Remarkably sharp interfaces are found, with roughness between 2 and 2.5 Å. The magnetic interface in the Fe sublayers resulting from the distribution of magnetic hyperfine fields distinctly exceeds the extension of the structural interface and points to a magnetic proximity effect. This is discussed in relation to a strong 3d-5d hybridization recently found in measurements of magnetic circular X-ray dichroism. Both the structural and magnetic La/Fe interface is less extended than the interface in Ce/Fe multilayers. Below a thickness of about 25 Å, the individual Fe layers grow in an amorphous structure on the La layers. In this case, Curie temperatures are below 200 K and the Fe-layer saturation magnetization is reduced up to 50%, and there is evidence of a non-collinear spin structure. It is argued that this mainly reflects the properties of pure amorphous Fe.     

Polarized neutron reflectometry of the influence of Fe/Si,Fe/FeSi2 and Au/Fe interfaces on the magnetic properties of epitaxial Fe films

The magnetic and structural properties of epitaxial Fe films grown on Si(1 1 1) are investigated by polarized neutron reflectometry (PNR) at room temperature. The influence of different types of interfaces, Fe/Si, Fe/FeSi₂ and Au/Fe on the magnetic properties of Fe films deposited by molecular beam epitaxy onto Si(1 1 1) are characterized. We observe a drastic reduction of the magnetic moment in the entire Fe film deposited directly on the silicon substrate essentially due to strong Si interdiffusion throughout the whole Fe layer thickness. The use of a silicide FeSi₂ template layer stops the interdiffusion and the value of the magnetic moment of the deposited Fe layer is close to its bulk value. We also evidence the asymmetric nature of the interfaces, Si/Fe and Fe/Si interfaces are magnetically very different. Finally, we show that the use of Au leads to an enhancement of the magnetization at the interface.     

Molecular dynamics simulation comparison of atomic scale intermixing at the amorphous Al2O3/semiconductor interface for a-Al2O3/Ge, a-Al2O3/InGaAs, and a-Al2O3/InAlAs/InGaAs

The structural properties of a-Al₂O₃/Ge, a-Al₂O₃/In_0.5Ga_0.5As and a-Al₂O₃/In_0.5Al_0.5As/InGaAs interfaces were investigated by density-functional theory (DFT) molecular dynamics (MD) simulations. Realistic a-Al₂O₃ samples were generated using a hybrid classical-DFT MD “melt and quench” approach. The interfaces were formed by annealing at 700 K/800 K and 1100 K with subsequent cooling and relaxation. The a-Al₂O₃/Ge interface demonstrates pronounced interface intermixing and interface bonding exclusively through Al–O–Ge bonds generating high interface polarity. In contrast, the a-Al₂O₃/InGaAs interface has no intermixing, Al–As and O–In/Ga bonding, low interface polarity due to nearly compensating interface dipoles, and low substrate deformation. The a-Al₂O₃/InAlAs interface demonstrated mild intermixing with some substrate Al atoms being adsorbed into the oxide, mixed Al–As/O and O–Al/In bonding, medium interface polarity, and medium substrate deformation. The simulated results demonstrate strong correlation to experimental measurements and illustrate the role of weak bonding in generating an unpinned interface for metal oxide/semiconductor interfaces.     

Structural properties and electrical characteristics of high-k Dy2O3 gate dielectrics

This paper describes the structural properties and electrical characteristics of thin Dy₂O₃ dielectrics deposited on silicon substrates by means of reactive sputtering. The structural and morphological features of these films after postdeposition annealing were studied by X-ray diffraction and X-ray photoelectron spectroscopy. It is found that Dy₂O₃ dielectrics annealed at 700 °C exhibit a thinner capacitance equivalent thickness and better electrical properties, including the interface trap density and the hysteresis in the capacitance-voltage curves. Under constant current stress, the Weibull slope of the charge-to-breakdown of the 700 °C-annealed films is about 1.6. These results are attributed to the formation of well-crystallized Dy₂O₃ structure and the reduction of the interfacial SiO₂ layer.     

Solid state reaction synthesis of NiFe2O4 nanoparticles

Ni-ferrite (NiFe₂O₄) nanoparticles have been synthesized via a solid state reaction process. Ni and Fe bi-metallic nanoparticles in the form of Ni₃₃Fe₆₇ alloy nanopowder are first synthesized by simultaneous evaporation of the required amounts of pure Ni and Fe metals followed by rapid condensation of the evaporated metal flux into solid state by means of an inert gas, helium, using the process of inert gas condensation (IGC). In order to form the NiFe₂O₄ structure, as-synthesized samples (Ni₃₃Fe₆₇) are annealed for 12 h in ambient conditions at different annealing temperatures. Structural analyses show that NiFe₂O₄ starts to form at around 450 °C and gets progressively well defined with increasing annealing temperatures yielding particle with size ranging between 15 and 50 nm. Besides successfully forming NiFe₂O₄, NiO/Fe₃O₄ core/shell nanoparticles have also been synthesized by adjusting the annealing conditions. Three different structures, Ni₃₃Fe₆₇, NiO/Fe₃O₄, and NiFe₂O₄, obtained in this study are compared with respect to their structural and magnetic properties.     

Sol-gel preparation and characterization of CoFe2O4-SiO2 nanocomposites

Magnetic nanocomposites formed by cobalt ferrite particles dispersed in a silica matrix were prepared by a sol-gel process. The effects of the thermal treatment temperature and the salt concentration on the structural and magnetic properties of the composites were investigated. By controlling these parameters, CoFe₂O₄/SiO₂ nanocomposites with different crystallite size and magnetic properties were obtained. By increasing the annealing temperature and salt concentration, composites with a progressive increase in the coercive field and of the density of magnetization were produced. In particular, a nanocomposite, with a Fe/Si molar concentration of 21%, obtained by drying the gel at 150 °C and further annealing at 800 °C, has a coercivity of 2000 Oe, which is more than twice higher than the coercivity of bulk cobalt ferrite.     

Aqueous synthesis and enhanced photocatalytic activity of ZnO/Fe2O3 heterostructures

We report a facile synthesis of ZnO/Fe₂O₃ heterostructures based on the hydrolysis of FeCl₃ in the presence of ZnO nanoparticles. The material structure, composition, and its optical properties have been examined by means of transmission electron microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and diffuse reflectance UV–visible spectroscopy. Results obtained show that 2.9 nm-sized Fe₂O₃ nanoparticles produced assemble with ZnO to form ZnO/Fe₂O₃ heterostructures. We have evaluated the photodegradation performances of ZnO/Fe₂O₃ materials using salicylic acid under UV-light. ZnO/Fe₂O₃ heterostructures exhibited enhanced photocatalytic capabilities than commercial ZnO due to the effective electron/hole separation at the interfaces of ZnO/Fe₂O₃ allowing the enhanced hydroxyl and superoxide radicals production from the heterostructure.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Structural relation and epitaxial properties of hexagonal InN and oxidized cubic In2O3

The epitaxial properties and structural relation between hexagonal InN and cubic In₂O₃ phases were studied by synchrotron X-ray scattering and X-ray photoelectron spectroscopy. The cubic bixbyite In₂O₃ phase on the sapphire(0001) substrate was formed after an annealing time of 10 min at 10⁻⁵ Torr after the hexagonal InN film was grown at 550 °C, above the dissociation temperature of InN, by RF-magnetron sputtering. The crystal orientation was cubic In₂O₃(222), parallel to Al₂O₃(0001) and parallel to hexagonal InN(0002) before the oxidation process. The cubic In₂O₃ phase was believed to be formed layer by layer by the oxidation of the hexagonal InN phase.     

Inverse TMR in a nominally symmetric CoFe/AlOx/CoFe junction induced by interfacial Fe3O4 investigated by STEM-EELS

We have found inverse tunneling magnetoresistance (TMR) with a non-symmetric bias voltage dependence in a nominally symmetric Si (001)/Ag/CoFe/AlO_x/CoFe/IrMn/Ag magnetic tunnel junction after field cooling. The O K edge fine structure extracted from electron energy loss spectroscopy spectrum images taken at the interfaces of junctions with inverse TMR shows a thin, discontinuous Fe₃O₄ layer at the CoFe/AlO_x interfaces. The Fe L_2,3 edge core level shifts are also consistent with those of Fe₃O₄. We find no Fe₃O₄ layer in junctions with normal TMR. We believe this Fe₃O₄ layer is responsible for the inverse TMR.     

Tunneling magnetoresistance in Fe-oxide film

We fabricate Fe/Fe oxide granular film by DC sputtering and study the magnetic and transport properties in the insulator region. X-ray photoelectron spectroscopy and transmission electron microscopy confirm the coexistence of iron and Fe₂O₃. Accompanied with the nonlinear I-V curve and magnetic measurement, we investigate mechanism of sizable magnetoresistance in detail and found the spin in the interface has crucial contribution to the spin tunneling process.     

Thermomagnetic history effects across the first order magneto-structural transition in the giant magnetocaloric Fe-Rh alloy

We investigate the electronic structure of Sr₂FeMoO₆/SrTiO₃ (SFMO/STO) multilayers using the ab initio Full Potential Linearized Augmented Plane Wave method in order to study their properties within the GGA and GGA+U methods. We examin more especially the role of the interface on the magnetic and transport properties of these multilayers taking into account a possible Fe deficiency at the interface and we show that bulk behaviour is rapidly recovered due to the strong localization of the interfacial perturbation. For perfect interfaces, the whole structure is found half-metallic within the GGA+U method; the situation being ambiguous within the GGA method where SFMO is at the limit of being half-metallic depending on the structural deformation induced by the STO layer. This leads us to the conclusion that such a system could be used as injection electrode and tunnel barrier in magnetic tunnel junctions with a fully spin polarized injected current. For Fe deficient interfaces, we show that the interfacial densities of states are nearly unpolarized showing that this kind of imperfection has potentially a strong impact on the properties of the multilayers.     

Fe/Al2O3/Fe隧道结特性分析

用离子束溅射方法制备磁性隧道结(MTJ). 研究MTJ样品的隧道结磁电阻(TMR)效应.用X射线 光电子能谱分析了MTJ的软、硬磁层和非磁层及其界面的化学组成与微结构.研究了MTJ的微 结构对氧化铝势垒高度与有效宽度和TMR效应的影响. 关键词： 磁性隧道结 X射线光电子能谱 隧道结磁电阻     

Effect of thermal annealing on the ac impedance of Co/Al2O3/Co/Al2O3/Co double-barrier MTJs

Double-barrier magnetic tunnel junctions (DBMTJs) were prepared from Co(75 nm)/Al₂O₃(2.3 nm)/Co(5 nm)/Al₂O₃(2.3 nm)/Co(50 nm) sputtering pentalayer films. The ac electrical properties of as-deposited DBMTJs and those annealed in a vacuum at 100–350 °C for 30 min were then investigated using a complex impedance spectroscopic technique. The ac impedance responses as a function of annealing temperature were further analyzed based on Maxwell's layered dielectric barrier and Maxwell–Wagner capacitor models after considering the DBMTJs as having double-capacitor-type structures. The effect of thermal annealing on the ac transport behavior of the DBMTJs was interpreted by examining the equivalent electric circuits fitted to Nyquist plots of each different sample. The effects were found to be due to changes in the structural characteristics in both bulk and interface morphologies of Co and Al₂O₃ layers. The structural morphology determined the different ac transport modes that occurred in the DBMTJs.     

Barium borate glass and transparent glass ceramics doped with Pb4Lu2YbF17

Barium borate glasses doped with complex Pb₄Lu₂YbF₁₇ fluoride have been synthesized and investigated. Heat treatment produced glass ceramics containing the crystalline BaF₂:Yb³⁺ phase. The changes in the structural and optical properties of the glass ceramics were revealed by X-ray diffraction, Raman spectroscopy, and luminescence spectroscopy of Yb³⁺ ions in polycrystalline Pb₄Lu₂YbF₁₇, initial glass, and glass ceramics.     

Magnetic,electronic and structural properties of ZnxFe3−xO4

A series of samples Zn_xFe_3−xO₄ have been prepared by the chemical coprecipitation technique and characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and X-ray photoelectron spectroscopy (XPS). XRD demonstrates all the samples of Zn_xFe_3−xO₄ have a spinel structure same as Fe₃O₄. The magnetic hysteresis loops of Zn_xFe_3−xO₄ obtained from VSM indicate that the saturation magnetization has a maximum when x is ∼1/3. The chemical states of Fe atoms and Zn atoms in zinc ferrites have been measured using XPS and Auger electron spectroscopy (AES). The Fe 2p core-level XPS spectra and Zn L₃M₄₅M₄₅ Auger peaks have been analyzed and the results have been discussed in correlation with the samples’ magnetic properties. These results suggest most of Zn atoms occupy the tetrahedral sites and a small amount of them occupy the octahedral sites.     

Influence of Lu2O3 on electrical and microstructural properties of CaCu3Ti4O12 ceramics

In this work, the influence of Lu₂O₃ doped on the dielectric and electrical properties of CaCu₃Ti₄O₁₂ was reported. Lu₂O₃-doped CCTO was prepared by a conventional solid state technique using CuO, TiO₂, and CaCO₃ as starting materials. The samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM); dielectric measurements were measured in the 10² Hz–10⁷ Hz frequency range at room temperature; and the nonlinear behavior of all samples was measured. The doping of Lu₂O₃ resulted in an increase in the dielectric constant of CCTO, but decreased the stability of the frequency dependence. Increasing concentrations of Lu₂O₃ resulted in decreasing nonlinear coefficients.     

Influence of annealing temperature on structural, optical and magnetic properties of Zn0.95Cu0.02Cr0.03O powders

Zn_0.95Cu_0.02Cr_0.03O powders have been synthesized by the sol-gel method and sintered in argon atmosphere under different temperatures. The structural, optical and magnetic properties of the powders were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and vibrating sample magnetometer (VSM). The XRD results demonstrated that Cr and Cu ions are incorporated into ZnO successfully when annealing temperatures were 600 and 700 °C. But when the samples were annealed at 500 °C, the crystallinity of the samples was not very good. However, when the annealing temperature was increased to 800 °C, the secondary phase of Cu and ZnCr₂O₄ appeared in the samples. The PL spectra revealed that the position of the ultraviolet (UV) emission peak of the samples showed a blue shift and the green emission peak enhanced significantly with the annealing temperature increasing from 600 to 700 °C. Magnetic measurements indicated that the room temperature ferromagnetism of Zn_0.95Cu_0.02Cr_0.03O was intrinsic in nature. In addition, the saturation magnetization (Ms) increased from 0.0078 to 0.0088 emu/g with the annealing temperature increased from 600 to 700 °C.