Magnetooptical properties and FMR in granular nanocomposites (Co84Nb14Ta2) x (SiO2)100? x

For nanodimensional magnetically inhomogeneous amorphous granular films of the system (Co₈₄Nb₁₄Ta₂) _x (SiO₂)_100−x , 30≤x≤60 at.% the concentration dependences of the magnetooptical Kerr effect (MOKE) spectra and FMR have been investigated. The observed changes in the MOKE and FMR spectra are associated with transformations of microstructure and topology of the nanocomposites. For the compositions within the percolation region the transversal Kerr effect increases by an order of magnitude.     

Optical and magnetooptical properties of CoFeB/SiO<Subscript>2</Subscript> and CoFeZr/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> granular magnetic nanostructures

The optical and magnetooptical properties of the new granular nanocomposites (CoFeB)/(SiO₂) and (CoFeZr)/(Al₂O₃), which are grains of amorphous ferromagnetic alloys embedded in dielectric matrices, have been studied. The dependence of the optical, magnetooptical, and magnetic properties of the nanocomposites on their qualitative and quantitative composition, as well as on the conditions of their preparation, was investigated. Spectra of the dielectric functions ε = ε₁ ? iε₂ were obtained by the ellipsometric method in the range 0.6–5.4 eV. Above 4.2 eV, the absorption coefficient of the (CoFeB)/(SiO₂) composites was found to be close to zero for all magnetic-grain concentrations. The polar Kerr effect measured at a photon energy of 1.96 eV in dc magnetic fields of up to 15 kOe reaches values as high as 0.25°–0.3° for these nanocomposites and depends only weakly on the conditions of preparation. On the other hand, the (CoFeZr)/(Al₂O₃) nanostructures reveal a considerable difference in the concentration dependences of the Kerr effect between samples prepared in a dc magnetic field and in zero field.     

Magnetooptical properties of Co-SiO<Subscript>2</Subscript> granular films

The structural, optical, and magnetooptical properties of Co-SiO₂ granular films are studied. It is found that the magnetooptical response is considerably enhanced in samples with a metal component concentration close to the percolation threshold. The Co nanogranules formed in the SiO₂ matrix exhibit ferromagnetic properties and are ordered into the fcc structure, which is confirmed by magnetooptical investigations. The magnetooptical properties of nanocomposites are simulated in the effective-medium approximation. It is shown that the singularities found in the magnetooptical spectra are related to the excitation of plasmon resonances in granular films.     

Specific features of magnetooptical spectra of Co/SiO2 hybrid multilayers

Magnetooptical spectra of Co(x)/SiO₂(y) bilayers and [Co(x)/SiO₂(y)]_n hybrid multilayers (x and y are the layer thicknesses) are studied in the range 1.5–4.0 eV. In these layers, the ultrathin ferromagnetic layers are not continuous but consist of ellipsoidal Co nanoparticles with a concentration near the percolation threshold. It is found that (a) the sign, magnitude, and spectral shape of the magnetooptical signal depend strongly on the Co particle size; (b) the signal for the [Co(1.6 nm)/SiO₂(3.0 nm)]₆ multilayers at 3.0 eV has a record-high magnitude for Co-based structures, 3×10^?2, which exceeds that for bulk Co about fivefold and about 50-fold that for a uniform Co film of an equivalent thickness; (c) the magnetooptical signal of multilayers depends non-linearly on the number of periods and the thickness of the dielectric layer y; and (d) the dependence of the signal of bilayers and multilayers with x=1.3 and 1.6 nm on the wavelength is nonmonotonic and exhibits clearly pronounced extrema. The data obtained for bilayers are attributed to the strong influence of percolation on the optical and magnetooptical parameters of the structure and interpreted in the framework of the effective medium approximation and macroscopic Fresnel magnetooptics.     

Isotropic positive magnetoresistance in Co-Al<Subscript>2</Subscript>O<Subscript>n</Subscript> nanocomposites

The magnetotransport properties of Co_x(Al₂O_n)_{100 ? x} nanocomposites were studied in a wide concentration range (34 ≤ x ≤ 74 at %). Negative tunnel magnetoresistance reaching 6.5% in a field of 10 kOe was established. In addition to the negative magnetoresistance, the Co_x(Al₂O_n)_{100 ? x} composites were found to exhibit positive magnetoresistance reaching 1.5% in fields of 10 kOe over the concentration range corresponding to the percolation threshold (54 ≤ x ≤ 67 at %). The positive magnetoresistance is assumed to be due to the simultaneous existence in the composite structure of clusters and individual nanoparticles characterized by different values of the magnetic anisotropy and due to the dipole-dipole interaction between the clusters and nearest neighbor particles.     

Magnetoreflection and magnetooptical Kerr effect in La<Subscript>2/3</Subscript>Ba<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript> films at room temperature

Results of studies of magnetooptical Kerr effect and magnetoreflection of natural light in La_2/3Ba_1/3MnO₃/SrTiO₃ films of different thickness are presented. The Kerr effect was shown to be the most prominent in visible and near IR range; magnetoreflection was found to achieve its maximum of about 10% in the mid-IR range near the room temperature. Physical mechanisms defining the value and sign of the effects and the influence of the thin-film state on the magnetooptical properties are discussed. Magnetoreflection is estimated in the framework of the magnetorefractive effect theory.     

Thermopower of composites containing metallic cobalt nanoparticles embedded in the Al<Subscript>2</Subscript>O<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> dielectric matrix

The concentration and temperature dependences of the thermopower of composites containing Co nanoparticles embedded in the Al₂O _n dielectric matrix are investigated. Below the percolation threshold, i.e., in the tunneling conduction region, the absolute values of the thermopower of the composites under investigation are less than those above the percolation threshold. It is revealed that, in the tunneling conduction region, the slope of the temperature dependences of the thermopower changes at a temperature of ～205 K. This can indicate that the thermopower is sensitive to a change in the mechanism of conduction from the Mott law ln(σ) ∝ (1/T)^1/4 to a power relation that corresponds to the model of inelastic resonant tunneling through a chain of localized states in the dielectric matrix. The introduction of oxygen in the course of sputtering brings about a decrease in the absolute values of the thermopower; however, the character of variation in the concentration and temperature dependences of the thermopower remains unchanged.     

Generation of second optical harmonic and magnetooptical Kerr effect in ferromagnet-semiconductor heterostructures CaF2/MnAs/Si(111)

The second optical harmonic generation and magnetooptical Kerr effect are investigated for the light (λ=800 nm) reflected by ferromagnet-semiconductor heterostructures CaF₂/MnAs/Si(111). The observed change in the second-harmonic intensity is odd in magnetization. A phenomenological analysis of possible contributions to the second harmonic is carried out, and the sources of optically nonlinear signals are determined from the experimental azimuthal dependences of the light intensity at double frequency. The difference in the field dependences of the second harmonic and the magnetooptical Kerr effect is observed.     

Inverse magnetoresistance in (FeCoB)-(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) magnetic granular composites

The magnetoresistance, magnetization, and microstructure of granular composites with the general formula (Fe₄₀Co₄₀B₂₀)_x(Al₂O₃)_100?x were studied for contents of the amorphous metallic component both above and below the percolation threshold (x≈43). The low-temperature transverse magnetoresistance of the composites is negative at x=41 and practically zero for x=49. For metal contents below the percolation threshold (x=31), a noticeable (7–8%) positive magnetoresistance, reached in magnetic fields of about 17 kOe, was observed. Possible mechanisms of the generation of inverse (positive) magnetoresistance are discussed.     

Anomalous temperature dependence of photoluminescence in GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films and GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/SiO<Subscript>2</Subscript> nano-heterostructures

The optical properties of GeO _x film and GeO _x /SiO₂ multilayer heterostructures (with thickness of GeO _x layers down to 1 nm) were studied with the use of Raman scattering and infrared spectroscopy, ellipsometry and photoluminescence spectroscopy including temperature dependence of photoluminescence. The observed photoluminescence is related to defect (dangling bonds) in GeO _x and interface defects for the case of GeO _x /SiO₂ multilayer heterostructures. From analysis of temperature dependence of photoluminescence intensity, it was found that rate of nonradiative transitions in GeO _x film has Berthelot type, but anomalous deviations from Berthelot type temperature dependence were observed in temperature dependences of photoluminescence intensities for GeO _x /SiO₂ multilayer heterostructures.     

Evolution of magnetooptical and transport properties of La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> films depending on silver concentration

Evolution of optical, magnetooptical, and transport properties of La_1?x Ag _x MnO₃/SrTiO₃ epitaxial films depending on the silver concentration (x = 0.05, 0.10, 0.15, and 0.25) is studied. The highest values of the Curie temperature (T _C ≈ 317 K), magnetoresistance (～16%), magnetotransmission (～8%), and transverse Kerr effect (δ ～ 20 × 10^?3) are attained for a concentration x = 0.10 of Ag⁺ ions. Comparison of the temperature dependences of the transmission of IR radiation, resistivity, magnetotransmission, magnetoresistance, and Kerr effect indicates electronic and magnetic inhomogeneity of the films in spite of saturation of films with silver. This feature of the film state is explained using the concepts of epitaxial stresses and metastable point defects.     

Fractal magnetic microstructure in the (Co<Subscript>41</Subscript>Fe<Subscript>39</Subscript>B<Subscript>20</Subscript>)<Subscript>x</Subscript>(SiO<Subscript>2</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> nanocomposite films

Magnetostructural methods are applied to determine the exchange bond percolation limit in (Co₄₁Fe₃₉B₂₀)_x(SiO₂)_1?x nanocomposites (x _c = 0.30 ± 0.02), which separates the phase plane along the metal concentration axis into a superparamagnetic region and a ferromagnetic region. It is shown that, with respect to the singularities of the magnetization up to the magnetization saturation curves, the ferromagnetic region is further subdivided into three regions differing in the character of the spatial propagation of the magnetization ripples or in the magnetic correlation function characteristics. The fractal dimension of the nanocomposite magnetic microstructure near the percolation threshold is determined.     

Effect of annealing on the magnetic and magnetooptical properties of Ni films

The magnetic and magnetooptical properties of 50-to 200-nm-thick Ni films, both as-deposited and annealed at T_ann = 300, 400, or 500°C, were studied. Volume and near-surface hysteresis loops were measured with a vibrating-sample magnetometer (VSM) and with the use of the transverse Kerr effect (TKE). The annealing temperature was found to exert a strong effect on the magnetic characteristics of the samples under study. It was established, in particular, that the coercivity H _C of Ni films increases and the remanent magnetization decreases with increasing annealing temperature. The observed dependences of the magnetic properties of the films on film thickness and annealing temperature are explained as being due to microstructural characteristics of the samples. It was found that, while TKE spectra obtained in the incident-photon energy region from 1.5 to 6 eV have the same shape for all the Ni films studied, the magnitude of the TKE decreases with increasing T_ann. This experimental observation is accounted for by the decreased saturation magnetization of the annealed films.     

Effect of an interface boundary on the magnetooptical and magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> heterostructures

The optical, magnetooptical (Kerr effect and magnetotransmission), and magnetotransport properties of La_2/3Ca_1/3MnO₃/La_2/3Sr_1/3MnO₃ and La_2/3Ca_1/3MnO₃/SrTiO₃/La_2/3Sr_1/3MnO₃ heterostructures on SrTiO₃ substrates are studied. The contribution of the interface boundary to the magnetotransmission is typical of a material with a transitional composition. It is found that a 2-nm-thick SrTiO₃ spacer does not influence the shape and position of the magnetotransmission peak in a field normal to the surface of the heterostructure but increases the contribution of the upper layer to the magnetotransmission in the Voigt geometry and also enhances the magnetoresistance that is due to the tunneling of spin-polarized carriers through the spacer. The Kerr spectra taken of the heterostructures are typical of single-layer single-crystal films.     

Electrical properties of Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-based composite systems

Composite electrolytes are well-known multiphase systems and exhibit maxima in the conductivity at certain second-phase concentration. An attempt has been made to investigate a number of sodium sulfate (Na₂SO₄)-based composite systems. The dispersoids that have been used are MgO, Al₂O₃, and SiO₂. The samples have been characterized using impedance spectroscopy, X-ray diffraction, and differential scanning calorimetry. The maximum conductivity has been observed for MgO dispersed system, and the percolation threshold has been observed at 30-mol% dispersoid, MgO concentration. Interestingly, two maxima have been observed in case of the Na₂SO₄–SiO₂ and Na₂SO₄–Al₂O₃ composite systems. In the Na₂SO₄–SiO₂ system, the first maximum occurs at lower concentration, i.e., in the range between 10 and 20 mol%, whereas the second occurs at the 40-mol% dispersoid concentration. For the Na₂SO₄–Al₂O₃ system, although slightly indistinguishable, two peaks in the conductivity vs composition plot have been observed around 12- and 30-mol% Al₂O₃ concentrations.     

Anisotropy of amorphous nanogranular composites CoNbTa-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> and CoFeB-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>

A comparative investigation of the magnetic properties of amorphous nanogranular composites (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} and (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} has been performed in the subpercolation region at temperatures in the range 4.2–300 K. The thermomagnetic dependences in the range 4.2–300 K and the processes of magnetization reversal and remanent magnetization relaxation at liquid-helium temperatures have been studied. It has been established that the average anisotropy constants of amorphous nanograins are equal to 3.6–7.0 kJ/m³ for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites and 5–8 kJ/m³ for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites. The fundamental differences in the concentration dependences of the anisotropy constant K _eff and the coercive force H _C have been revealed for the two systems under investigation. It has been demonstrated that, as the concentration of the metal phase increases, the quantities K _eff and H _C increase for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites and decrease for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites.     

Transport phenomena in metal nanocomposites

Ferromagnetic nanocomposites are the special case of metal composites; they are of practical interest for spintronics. Temperature dependences of resistivity ρ(T) and thermoelectric power α(T) of ferromagnetic nanocomposites with the composition Co _x (Al₂O₃)_{100 ? x} (36.6 ≤ x ≤ 52.5 at %) are investigated near the percolation threshold (x _p ≈ 43.3 at %) in a temperature range of 77–300 K. Sizes of Co nanoparticles are no larger than 25 nm. Specific features are observed in the dependences α(T) in the form of a kink at T _b ≈ 170 K. The analysis of the structural and electrical schematics as well as energy diagrams of percolation channels of electrons shows that only the diffusion thermoelectric power appears in Co nanoparticles, at which α(T) is the linear function. No mechanisms of the thermoelectric power caused by nanosizes of Co particles or by electron tunneling between them are found. The kink of the α(T) linear dependence is explained by the existence of the oxide shell of Co nanoparticles. It is assumed that the temperature dependences of energy barriers of oxide shells of metal nanoparticles (including ferromagnetic ones) in oxygen-containing dielectric matrices determine the features of α(T) and ρ(T) dependences of such nanocomposites.     

Impedance of Si/SiO<Subscript>2</Subscript> composites in the vicinity of the percolation threshold

The Si/SiO₂ composites, in which the concentration of the conducting silicon phase is close to the percolation threshold, have been prepared using the ceramic technology and studied at an alternating current. It has been found that an increase in the potential difference in a direct-current electric field leads to a decrease in the dispersion of time constants of dielectric spacers in the “Si grain-SiO₂ spacer-Si grain” structures forming a conducting cluster in the composite.     

Prussian blue modified FePt nanoparticles for the electrochemical reduction of H<Subscript>2</Subscript>O<Subscript>2</Subscript>

FePt nanoparticles were synthesized by polyol process with chloride salts, and the equiatomic composition was surface modified with prussian blue (PB). From the magnetic studies, the fraction of PB present in the surface-modified fcc-FePt was found to be 18 %. The FePt nanoparticles with an average particle size of 5 nm forms cluster like morphology, which were embedded in the PB matrix. The electrocatalytic reduction of hydrogen peroxide (H₂O₂) by the PB-modified FePt nanoparticles was studied. The reduction peak current showed linear response for H₂O₂ in the concentration range up to 3.5 mM. The FePt nanoparticles did not exhibit significant H₂O₂ reduction whereas the PB-modified FePt showed reduction of H₂O₂ with the addition of 0.35 mM of H₂O₂.