Insulating SiO2 Matrix Research Articles

Superconducting Properties of VN-SiO2Sol-Gel Derived Thin Films

in this work studies of structure and superconducting properties of vnsio2 films are reported. the fi lms were obtained through thermal nitridation (ammonolysis) of solgel derived v2o3sio2 coatings (in a proper v2o3/sio2 ratio) at 1200 ◦c. this process leads to the formation of disordered structure with vn metallic grains dispersed in the insulating sio2 matrix. the structural transformations occurring in the lms as a result of ammonolysis were studied using x-ray photoelectron spectroscopy (xps). the critical superconducting parameters are obtained. the magnetoresistance at high magnetic fields has been investigated.

Structural properties of nanocomposite SiO2 (Si) films obtained by ion-plasma sputtering and thermal annealing

The nanocomposite SiO2(Si) films containing Si nanoclusters inside insulating SiO2 matrix are promising for many nanoelectronics applications. The ion-plasma sputtering of Si in O2 containing gas mixture and following thermal annealing have been used to form nanocomposite SiO2(Si) films. The structural properties of the obtained films have been studied using several methods. Among them, there were ellipsometry, IR spectroscopy, Raman spectroscopy, and AFM. Transition of SiOx matrix into insulating SiO2 matrix has been revealed by IR spectroscopy. The shift of the transmittance spectra toward high frequency region and the increase in their intensity have been observed. The existence of amorphous and nanocrystalline phases into SiO2(Si) films have been confirmed using Raman spectroscopy. Two material phases on the film surface, namely SiO2 and Si, and surface density of silicon nanoclusters have been determined using AFM. It was shown that the size of silicon nanoclusters and their surface density depend on the level of enrichment with silicon of the initial SiOx film after ion-plasma sputtering and the temperature of subsequent annealing.

Exchange bias in CoFe–SiO2 granular nanostructure

Exchange-biased (EB) granular nanostructures are fabricated by sputtering. The EB granular system consists of Co90Fe10 ferromagnetic (FM) nanograins embedded into the insulating SiO2 matrix, which is sandwiched between antiferromagnetic (AFM) Ir20Mn80 layers. The average size of the CoFe nanograins is about 5 nm separated by SiO2 and the average distance between CoFe nanograins is 1.6 nm for the volume fraction of 11.5% SiO2. Significant asymmetry in the M–H loop of this EB granular nanostructure reveals that different magnetization reversal mechanisms are involved in the increasing- and decreasing-field branches of the hysteresis loops. The values of the anisotropy field, Hk = 96 Oe, and the exchange field, Hex = 60 Oe, for the IrMn/CoFe–SiO2/IrMn granular nanostructure are obtained which are two times larger than those of the continuous IrMn/CoFe/IrMn film (Hk = 50 Oe and Hex = 30 Oe). The enhancement of coercivity by 1.5 times is also found. Such a large enhancement is attributed to the breakup of large FM domains into small ones due to the isolation of FM nanograins by the nonmagnetic phase. This EB granular nanostructure is valuable for understanding theoretical phenomena in EB nanostructures as well as for practical applications.

Superconducting properties of an NbN–SiO2 disordered system obtained by ammonolysis of NbN–SiO2 sol–gel derived coatings

Studies in superconducting properties of NbN–SiO2 films are reported. The films were obtained through nitridation of sol–gel derived Nb2O5–SiO2 coatings at 1200°C, a process leading to the formation of disordered structures with NbN metallic grains dispersed in the insulating SiO2 matrix. Electrical resistivity was measured with the conventional four-terminal method in the temperature range from 5 to 280K. The samples’ superconducting properties, examined with magnetically modulated microwave absorption (MMMA), depend on the NbN/SiO2 molar ratio and the film’s thickness.

Microstructure and magnetism of FeCo–SiO2 nano-granular films for high frequency application

Excellent soft magnetic properties have been achieved in a wide metal volume fraction (x) range for as-deposited (Fe65Co35)x(SiO2)1−x granular films fabricated by magnetron sputtering. With the decrease in x from 0.7 to 0.5, the films exhibit small coercivity Hc not exceeding 4 Oe and high electrical resistivity ρ up to 5.48 × 103 µΩ cm. A minimum Hc value of 1.7 Oe was obtained for the sample of x = 0.57 with ρ = 2.86 × 103 µΩ cm. At a frequency lower than 1.3 GHz, the real part μ′ of complex permeability of this sample is about 170 and then decreases with frequency while the imaginary part μ″ is visible when f > 1 GHz, which may be caused by anisotropy dispersion. The FMR frequency is as high as 2.4 GHz, implying a high cut-off frequency for high frequency applications. A study of high resolution transmission electronic microscope indicates that the samples consist of Fe65Co35 particles uniformly embedded in an insulating SiO2 matrix. The excellent soft magnetic property is ascribed to exchange coupling among magnetic particles. The investigation of the Δm–H curve evidences the existence of this inter-granule interaction.

Electron field-emission properties of Ag–SiO2 nanocomposite layers

In this work, Ag–SiO2 nanocomposite layers were synthesized by introducing Ag nanoclusters into thermally oxidized SiO2 layers, using ion implantation. The field-emission (FE) properties of these layers were studied and correlated with the results from atomic force microscopy and transmission electron microscopy measurements. These nanocomposites exhibit good FE properties and give an emission current of 1nA at electric fields as low as 13V∕μm, for a dose of 5×1016Ag+∕cm2, compared with 204V∕μm for “bare” SiO2 layers. It is clearly demonstrated that the good FE properties of these nanocomposites are attributed to two types of local-field enhancement: one due to the surface morphology and the other due to electrical inhomogeneity. The isolated conductive Ag nanoclusters embedded in the electrically insulating SiO2 matrix provide a field enhancement due to the electrical inhomogeneity effect. Moreover, the implanted Ag ions diffuse to the surface, during the implantation process, and create dense surface-protrusion structure which provides a geometric local-field enhancement. The local-field-enhancement mechanisms in these samples are critically dependent on the implantation dose of Ag.

Study on mechanism of soft magnetic properties for high‐frequency application in Ni75Fe25–SiO2 granular films

AbstractExcellent soft magnetic properties have been achieved in a wide metal volume fraction (x) range for as‐deposited (Ni75Fe25)x–(SiO2)(1–x) granular films fabricated by magnetron sputtering. With x changes from 0.81 to 0.52, the films exhibit small coercivity (Hc) not exceeding 5 Oe and high electrical resistivity ρ. The minimum Hc value of 87.6 A m–1 was obtained for the sample of x = 0.81. At the frequency lower than 550 MHz, the real part μ′ of complex permeability keeps about 170 and the image part μ″ is less than 26. Especially, for the film with very low x (= 0.52), besides the low Hc (∼3 Oe), very high ρ over 1.18 × 104 μΩ cm has been obtained. A combined study of TEM and XRD indicates that the samples consist of Ni75Fe25 particles uniformly embedded in insulating SiO2 matrix. The excellent soft magnetic property is ascribed to the exchange coupling among magnetic particles due to the long exchange length of Ni75Fe25. The investigation of the Henkel plot evidences the existence of this intergranular interaction. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ion-Beam-Synthesised Ag-SiO2 Nanocomposite Layers for Electron Field Emission Devices

AbstractAg-SiO2 nanocomposite layers were synthesised by Ag+ implantation into thermally oxidised SiO2 layers and demonstrated to have excellent field emission (FE) properties. These nanocomposite layers can give an emission current of 1 nA at electric fields less than 20 V/μm, compared to several thousand volts per micrometre of pure metal surfaces. Their fabrication processes are fully compatible with existing integrated circuit technology. By correlating the FE results with other characterisation techniques including atomic force microscopy, Rutherford backscattering spectroscopy and transmission electron microscopy, it is clearly demonstrated that there are two types of field enhancement mechanisms responsible for the excellent FE properties of these cathodes. Firstly, the electrically conductive Ag nano-clusters embedded in the insulating SiO2 matrix give rise to a local electric field enhancement due to an electrical inhomogeneity effect and secondly, the dense surface protrusions provide a geometric local electric field enhancement. The FE properties of these layers are critically dependent on the size and distribution of the Ag clusters, which can be controlled by the Ag dose and modified by the post-implantation pulse annealing with a high power KrF Excimer laser operating at 248 nm.

Characterization and analysis of a novel hybrid magnetoelectronic device for magnetic field sensing

Structures in which magnetic and electronic materials are combined offer a variety of possibilities for realization of devices with improvement functionality or performance compared to conventional devices. We have designed, characterized, and analyzed a novel hybrid magnetoelectronic device: a monolithic field-effect-transistor-amplified magnetic field sensor in which a granular tunnel magnetoresistive (TMR) thin film, consisting of Co nanoparticles embedded in an insulating SiO2 matrix, is incorporated into the gate of a p-channel Si metal-oxide-semiconductor field-effect transistor. In this structure, current flow through the TMR film leads to a buildup of electronic charge within the gate due to the Coulomb charging energy of the Co nanoparticles, and consequently to a transistor threshold voltage shift that varies with applied external magnetic field. In a prototype demonstration device, the relative current change induced by application of a 6 kOe external magnetic field at room temperature was amplified from 5% for the current through the TMR film to 21% for the transistor subthreshold current. The absolute current response in the saturation regime increased by a factor of about 500 compared to that of the TMR film alone. A detailed analysis of the device operation and of methods for optimization of performance are presented. It is anticipated that substantially better performance should be achievable with relatively straightforward improvements in device design and processing. This device concept is shown to compare particularly favorably with Hall bar sensors and thus may be very beneficial in sensor applications for medium field ranges up to about 1 T.

Spin-dependent tunneling through layered ferromagnetic nanoparticles

Spin-dependent tunneling has been investigated for nanostructured ferromagnetic tunnel junctions, in which an insulating nanostructured film made of layered hard ferromagnetic Co80Pt20 nanoparticles dispersed in an insulating SiO2 matrix is sandwiched between two soft ferromagnetic electrodes, an Fe/Co80Pt20 as a bottom electrode and a Co9Fe as a top electrode. The junctions investigated have three tunnel barriers and exhibited tunnel magnetoresistance of 9% at room temperature with a small resistance at a low field. The charging energy of the particles with an average diameter of 7 nm was estimated to be 45 meV from the temperature dependence of the resistance. Coulomb blockade was not observed for the junctions because of the low junction resistance.

Magnetic granular Fe-(SiO2) solids

We have synthesized a series of granular Fe-SiO2 solids consisting of Fe metal granules of nanometer sizes embedded in an insulating SiO2 matrix. The ultrafine microstructures, revealed by TEM, are achieved by using a high-rate magnetron sputtering system. The magnetic properties of such solids are determined by the granular size and the metal volume fraction. Below the percolation threshold, isolated single-domain particles having large magnetic coercivity in the blocked state exhibit superparamagnetic relaxation, which has been studied by using SQUID magnetometry and Mössbauer spectroscopy. Above the percolation threshold, magnetic coercivity drops dramatically and the magnetic properties approach that of pure Fe. The anisotropy energy of the magnetic granules, much larger than that due to magnetocrystalline anisotropy, has been determined.

Special committee on technical information formed

Excellent soft magnetic properties have been achieved in low metal volume fraction(x ∼ 0.52) of (Ni75Fe25)x(SiO2)1−x granular film fabricated by magnetron sputtering. The coercivity of 247 A m− 1 with very high resistivity ρ over 1.18 × 104 μΩ cm has been obtained. At the frequency lower than 1 GHz, the real part μ′ of the complex permeability keeps about 55 and the imaginary part μʺ is less than 2. A combined study of TEM and XRD indicates that the sample consists of Ni75Fe25 particles uniformly embedded in insulating SiO2 matrix. The excellent soft magnetic properties are ascribed to the magnetic dipole interaction between particles. The investigations of field cooled and zero field cooled curves as well as Henkle Plot prove the existence of this intergranular interaction.