<Emphasis Type="Italic">I–V</Emphasis> behavior of transition metal oxides′ nanoparticles confined in ion tracks

Behavior of transition metal oxide (TMO) nanoparticles (Fe₃O₄, NiO, Co₃O₄, and Mn₃O₄) inserted in ion tracks has been studied in the presence of magnetic field. The special structure of ion tracks in dielectric layer on semiconductors is known as TEMPOS—‘Tunable Electronic Materials with Pores in Oxide on Silicon’. TEMPOS structure offers a high surface to volume ratio resulting in fast response time and high sensitivity of sensors fabricated by inserting suitable materials in the ion tracks. We have already reported the behavior of ferrofluids (aqueous and non-aqueous) inserted in the TEMPOS structures and its feasibility as earth’s magnetic field sensor. In continuation to this study, a comparative study between different transition metal oxides inserted in the ion tracks is being presented here with an aim to understand their response in confined geometry. This study shows that Fe₃O₄ (ferrofluid) is the best choice for ion track-based magnetic field sensor as compared to NiO, Co₃O₄, and Mn₃O₄. Its response to magnetic field can be tailored by the dilution of the ferrofluid and annealing.     

Growth induced anisotropy of cobalt in cobalt/organic semiconductor films

We present the study of Co/organic semiconductor (OS) stacks both from the morphological and magnetic point of view. Co has been successfully used up to now as top contact of hybrid vertical devices. While the properties of Co grown on amorphous layers are well established, its deposition on soft materials presents critical aspects such as interfacial damage that affects its electrical and magnetic properties. In this work we focus on the influence of the morphology of the organic underlayer in the magnetic behavior of a Co thin film: tris(8-hydroxyquinoline) aluminum (Alq₃) grown in different conditions by molecular beam evaporation have been considered. A further considered aspect is the effect of the presence of a thin oxide barrier (Al₂O₃) on the Co magnetic properties.     

Synthesis and magnetic properties of Co-Cr2O3 nanocomposites

Co:Cr₂O₃ nanocomposites were prepared with phase separated metallic Co clusters in a wide range of concentrations namely 10, 20, 30, 40 and 50 wt% Co. Samples were annealed at different temperatures to study the effects of crystallization of Cr₂O₃ and the growth of Co metal clusters on the magnetic behavior of nanocomposites. Enhanced crystallinity of antiferromagnetic (Cr₂O₃) matrix and growth of Co clusters with higher annealing temperatures strongly affects the coercivity, saturation and magnetic viscosity in these hybrid materials. Amorphous Cr₂O₃ acts as a paramagnetic matrix for Co particles. Exchange anisotropy stabilizes magnetic moments of Co embedded in Cr₂O₃ only if Cr₂O₃ is crystalline. This exchange anisotropy leads to the enhancement of coercivity. Relaxation measurements confirm that exchange anisotropy is higher for samples with lower Co content.     

Magnetization reversal mechanism of magnetic tunnel junctions

Using the ion-beam-sputtering technique,we have fabricated Fe/Al2O3/Fe magnetic tunnelling junctions (MTJs).We have observed double-peaked shapes of curves,which have a level summit and a symmetrical feature,showing the magnetoresistance of the junction as a function of applied field.We have measured the tunnel conductance of MTJs which have insulating layers of different thicknesses.We have studied the dependence of the magnetoresistance of MTJs on tunnel conductance.The microstructures of hard-and soft-magnetic layers and interfaces of ferromagnets and insulators were probed.Analysing the influence of MJT microstructures,including those having clusters or/and granules in magnetic and non-magnetic films,a magnetization reversal mechanism(MRM) is proposed,which suggests that the MRM of tunnelling junctions may be explained by using a group-by-group reversal model of magnetic moments of the mesoscopical particles.We discuss the influence of MTJ microstructures,including those with clusters or/and granules in the ferromagnetic and non-magnetic films,on the MRM.     

Domain wall dynamics and interlayer interactions in magnetic trilayer systems studied by XMCD-PEEM

We have used time-resolved x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM) to investigate the layer-resolved microscopic magnetization reversal in FeNi/X/Co (with X=Cu, Al₂O₃) trilayer systems. These measurements were performed in pump-probe mode, synchronizing magnetic pulses with synchrotron x-ray pulses. The good magnetic contrast observed for most samples reveals that in many cases the magnetization reversal is reproducible. We have used the measurements to obtain domain wall propagation speeds as a function of applied magnetic field, and to investigate the influence of domain wall interactions on the magnetic switching.     

高择优取向铌酸锶钡薄膜的射频磁控溅射制备

采用溶胶-凝胶法在(100)Si单晶上预先制备出掺钾(K)的铌酸锶钡(SBN)缓冲层,利用射频磁控溅射法在缓冲层KSBN上沉积出高择优取向的铌酸锶钡薄膜,获得了磁控溅射法制备择优取向铌酸锶钡薄膜的相关工艺参数,研究发现,KSBN缓冲层能够很有效地克服衬底与SBN薄膜之间较大的晶格失配,在氧气氩气的比例为1∶2,工作气压为10 Pa,溅射功率300 W,衬底温度300℃,退火温度为800℃的工艺条件下,能够获得c轴高度择优取向的铌酸锶钡铁电薄膜.利用X射线衍射仪,原子力显微镜等仪器分析了薄膜 关键词： 磁控溅射 高择优取向 p-n结效应     

Analysis of glass tesserae from the mosaics of the ‘Villa del Casale’ near Piazza Armerina (Enna, Italy). Chemical composition, state of preservation and production technology

This paper represents a contribution to the safeguard project of The ‘Villa del Casale’ near Piazza Armerina (3rd–early 4th century AD), among the most important examples of Sicilian mosaic art, glass materials used for the mosaics execution have been investigated by several techniques. Optical Microscopy and Scanning Electron Microscopy (SEM) were used to give information about the porosity (microstructure) of glass pastes and the presence of crystalline areas or alteration layers at the surface inside the glass paste itself. SEM-EDX, X-ray Photoelectron Spectroscopy (XPS), X-ray Fluorescence (XRF) and UV-VIS-NIR reflectance analysis have provided information to obtain the chemical composition of the tesserae and on the chromophore groups. X-ray diffraction (XRD) was employed to identify opacifiers. The analytical results indicate that all tesserae are silica-soda-lime and silica-soda-lime-lead glasses. Chromophore ions consist of Cu(II) (green and pale blue tesserae), Cu(0) (red sample), Fe(II) (pale blue and the green glasses), Co(II) (deep blue samples) and Mn as a decolourant, while opacifiers found out are Pb _x Sb_2−z (O,OH,H₂O)₆ (bindheimite) in the yellow and green tesserae, CaSb₂O₆ (calcium antimony oxide) in the pale blue samples, and Ca₂Sb₂O₇ (calcium antimony oxide) in the blue tesserae. The glasses that show the greatest signs of degradation processes are those of a pale blue hue.     

X-ray spectral and diffraction studies of germanides of iron-group metals in the amorphous and crystalline states

This article examines results of x-ray emission spectroscopy and x-ray diffractometry of amorphous and crystalline layers of certain germanides of iron-group transition metals. Combined analysis of the K- and L-bands of Me and Ge in Me₂ (Fe, Co, Ni)Ge and data from model interpretation of radial distribution functions of atoms in amorphous layers are used to make conclusions on the character of the chemical bond and the structure of the short-range order in the materials studied. It was established that there is no significant change in the character of the chemical bond in germanides Me₂(Fe, Co, Ni)Ge in the transition from the crystalline to the amorphous state. It was also found that short-range order in the investigated amorphous layers can be described by a complex poly-structural atom distribution, i.e. it is characterized by a complex, microscopically nonuniform model of the type Me₂Ge+Me.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 39–43, January, 1986.     

Crystalline instability of Bi-2212 superconducting whiskers near room temperature

We report new evidences for the thermodynamic instability of whisker crystals in the Bi–Sr–Ca–Cu–O (BSCCO) system. Annealing treatments at 90°C have been performed on two sets of samples, which were monitored by means of X-Rays Diffraction (XRD) and Atomic Force Microscopy (AFM) measurements, respectively. Two main crystalline domains of Bi₂Sr₂CuCa₂O_8+x (Bi-2212) were identified in the samples by the XRD data, which underwent an evident crystalline segregation after about 60 hours. Very fast dynamics of the surface modifications was also described by the AFM monitoring. Two typologies of surface structures formed after about 3 annealing hours: continuous arrays of dome shaped bodies were observed along the edges of the whiskers, while in the central regions a dense texture of flat bodies was found. These modifications are described in terms of the formation of simple oxide clusters involving a degradation of the internal layers.     

Effects of oxygen vacancy location on the electronic structure and spin density of Co-doped rutile TiO2 dilute magnetic semiconductors

According to density functional theory （DFT） using the plane wave base and pseudo-potential, we investigate the effects of the specific location of oxygen vacancy （Vo） in a （Ti,Co）06 distorted octahedron on the spin density and magnetic properties of Co-doped rutile Ti02 dilute magnetic semiconductors. Our calculations suggest that the Vo location has a significant influence on the magnetic moment of individual Co cations. In the case where two Co atoms are separated far away from each other, when the Vo is located at the equatorial site of a Co-contained octahedron, the ground state of the two Co cations is d6（t3g↑, t23g ↓） without any magnetic moment. However, if the Vo is located at the apical site, these two Co sites have different ground states and magnetic moments. The spin densities are also observed to be modified by the exchange coupling between the Co cations and the location of Vo. Some positive spin polarization is induced around the adjacent O ions.     

Faraday effect in Co/TiO2 films

It is shown that in a layered medium consisting of a magnetic metal (Co) and a nonmagnetic insulator (TiO₂), the layer thickness being less than the wavelength of the light, a fundamental change occurs in the character of the Faraday-effect spectra if the number of layers is sufficiently large. When the number of pairs of layers changes from 5 to 10, a maximum appears near ~ 1.7 eV in the Faraday rotation spectrum and the ellipticity passes through zero. The results obtained are compared with the existing theory of the Faraday effect in granular structures consisting of a magnetic metal and nonmagnetic insulator. The surface plasmon energy in thin Co layers is estimated. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 531–534 (10 April 1997)     

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.     

Exchange bias in thin-film (Co/Pt)3/Cr2O3 multilayers

We have fabricated exchange-biased Co/Pt layers ((0.3 nm/1.5 nm)×3) on (0 0 1)-oriented Cr₂O₃ thin films. The multilayered films showed extremely smooth surfaces and interfaces with root mean square roughness of ≈0.3 nm for 10 μm×10 μm area. The Cr₂O₃ films display sufficient insulation with a relative low leakage current (1.17×10⁻² A/cm² at 380 MV/m) at room temperature which allowed us to apply electric field as high as 77 MV/m. We find that the sign of the exchange bias and the shape of the hysteresis loops of the out-of-plane magnetized Co/Pt layers can be delicately controlled by adjusting the magnetic field cooling process through the Néel temperature of Cr₂O₃. No clear evidence of the effect of electric field and the electric field cooling was detected on the exchange bias for fields as high as 77 MV/m. We place the upper bound of the shift in exchange bias field due to electric field cooling to be 5 Oe at 250 K.     

Growth of magnetic materials and structures on Si(0 0 1) substrates using Co2Si as a template layer

We report in this paper the use of Co₂Si silicide as a template layer for the integration of magnetic materials and structures on silicon substrate. By undertaking Co deposition on silicon at a temperature of about 300 °C, we show that it is possible to obtain a smooth and epitaxial Co₂Si layer, which can act as a template layer preventing the reaction between Co and other transition metals with silicon. Two examples of over-growth of magnetic materials and structures on this template layer will be presented: growth of ferromagnetic Co layers and of magnetic tunnel junctions (Co(Fe)/AlO_x/NiFe).     

Order-disorder transitions: structural and magnetic ordering in double perovskites

Using an effective lattice gas-like model for B-B′ site ordering we study order-disorder transitions in double perovskite materials A₂BB′O₆. We motivate this effective model from a microscopic hamiltonian. Using this model, we are able to address several experimentally observed issues including nonmonotonic dependence of the degree of order on annealing temperature, and the rapid decrease of order upon overdoping with either B or B′ species. We also study ordering in the ‘ternary’ compounds A₂BB′_1−yB″_yO₆, using a variant of the Blume-Emery-Griffiths model. Several issues related to structural and magnetic ordering are discussed.     

Synthesis,structural and physical properties of <Emphasis Type="Italic">δ</Emphasis>’-FeSe<Subscript>1</Subscript><Subscript><Emphasis Type="Italic">-x</Emphasis></Subscript>

We report on synthesis, structural characterization, resistivity, magnetic and thermal expansion measurements on the as yet unexplored δ’-phase of FeSe_{1 -x} , here synthesized under ambient- (AP) and high-pressure (HP) conditions. We show that in contrast to β-FeSe_{1 -x} , monophasic superconducting δ’-FeSe_{1 -x} can be obtained in off-stoichiometric samples with excess Fe atoms preferentially residing in the van der Waals gap between the FeSe layers. The AP δ’-FeSe_{1 -x} sample studied here (T _c @\simeq 8.5 K) possesses an unprecedented residual resistivity ratio RRR @\simeq 16. Thermal expansion data reveal a small feature around ~90 K, which resembles the anomaly observed at the structural and magnetic transitions for other Fe-based superconductors, suggesting that some kind of “magnetic state” is formed also in FeSe. For HP samples (RRR @\simeq 3), the disorder within the FeSe layers is enhanced through the introduction of vacancies, the saturated magnetic moment of Fe is reduced and only spurious superconductivity is observed.     

Kramers-Kronig analysis of the reflectance spectra of Pb-doped Bi-4334 type glass and the corresponding glass-ceramic superconductor

The room temperature reflectance spectra in UV-VIS-NIR region (energy range of 0.6 to 6.2 eV) for glassy, partially crystalline and its fully crystalline superconducting ceramic phases of Bi_3.9Pb_0.1Sr₃Ca₃Cu₄O _x have been studied by Kramers-Kronig (KK) analysis. A comparative study of the energy loss function [− Im (1/ε)] and the absorption coefficient [α(E)] has been done. Excitions in the superconducting phase hitherto evidenced by the authors are located in the polarizable layers of the superconducting cuprate and their implications for superconductivity have been pointed out. An estimate of the optical band gap energy (E _g) has also been made from the linear fit ofα ² vs.E curve for the superconducting phase. Jezierski’s method ofR-extrapolations in the higher energy has been used to show that both methods yield results that agree quantitatively and can be relied upon.     

NMR studies of interfaces,strain and anisotropy in Co/Cu multilayers

⁵⁹Co NMR studies of multilayers are able to give three direct pieces of information: (i) the crystal phase of Co, fcc (217.4 MHz), hcp (220–228 MHz) and in exotic cases bcc (198 MHz) for films measured at T= 4.2 K, (ii) the nature of the interfaces from low frequency satellite lines, and (iii) the strain state deduced from small changes in the line positions. Extensive studies of Co/Cu multilayer interfacial structures as a function of deposition technique, layer thickness, substrate/buffer layer structure and annealing temperature have been undertaken. This work has shed new light on the relationship between interfacial structure and magnetoresistance and in particular has demonstrated that flat, atomic scale, interfaces lead to greater magnetoresistance. The difference between the Co and Cu lattice constant results in an extensive, tensile in-plane strain developing in Co layers provided that some epitaxial registry is present. Information on strain effects can be obtained from the position and width of the NMR lines. The magnetic anisotropy field can be determined by measuring the field dependence of the enhancement effect due to electronic magnetisation. This provides unique insight into the distribution of magnetic anisotropy within the Co layers, as the enhancement can be investigated independently for each NMR line and, hence, provides environment specific information on magnetic anisotropy at the interfaces and in the interior of the layers. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of crystallinity of Co layer on perpendicular exchange bias in Au-capped ultrathin Co film on Cr2O3(0 0 0 1) thin film

The effect of the crystalline quality of ultrathin Co films on perpendicular exchange bias (PEB) has been investigated using a Au/Co/Au/α-Cr₂O₃ thin film grown on a Ag-buffered Si(1 1 1) substrate. Our investigation is based on the effect of the Au spacer layer on the crystalline quality of the Co layer and the resultant changes in PEB. An α-Cr₂O₃(0 0 0 1)layer is fabricated by the thermal oxidization of a Cr(1 1 0) thin film. The structural properties of the α-Cr₂O₃(0 0 0 1) layer including the cross-sectional structure, lattice parameters, and valence state have been investigated. The fabricated α-Cr₂O₃(0 0 0 1) layer contains twin domains and has slightly smaller lattice parametersthan those of bulk-Cr₂O₃. The valence state of the Cr₂O₃(0 0 0 1) layer is similar to that of bulk Cr₂O₃. The ultrathin Co film directly grown on the α-Cr₂O₃(0 0 0 1) deposited by an e-beam evaporator is polycrystalline. The insertion of a Au spacer layer with a thickness below 0.5 nm improves the crystalline quality of Co, probably resulting in hcp-Co(0 0 0 1). Perpendicular magnetic anisotropy (PMA) appears below the Néel temperature of Cr₂O₃ for all the investigated films. Although the PMA appears independently of the crystallinequality of Co, PEB is affected by the crystalline quality of Co. For the polycrystalline Co film, PEB is low, however, a high PEB is observed for the Co films whose in-plane atom arrangement is identical to that of Cr³⁺ in Cr₂O₃(0 0 0 1). The results are qualitatively discussed on the basis of the direct exchange coupling between Cr and Co at the interface as the dominant coupling mechanism.     

Magnetic relaxation in nanocrystalline iron-oxides

Nanocrystalline Fe oxides are compacted solids of fine particles. By compacting interfaces are formed between adjacent particles. Since collective magnetic relaxation and superparamagnetism depend on the anisotropic energy, it is expected that the existence of interfaces and related interface anisotropy will change the relaxation behavior of nanocrystalline Fe oxides. Measurements on nanocrystalline α-Fe₂O₃ and γ-Fe₂O₃ have shown that the exchange interaction in the interfaces is more effective in suppressing superparamagnetic relaxation upon compacting than the dipole interaction.