Multiferroic behavior in silicate glass nanocomposite having a core–shell microstructure

Nanosized iron core and barium titanate shell microstructure was generated within a silicate glass of composition 23.1 Na₂O, 23.1 BaO, 23.0 TiO₂, 7.6 B₂O₃, 5.8 Fe₂O₃, 17.4 SiO₂ by first reducing it at 893 K for ½ h and then subjecting it to heat treatment at 759 K for 4 h. Transmission electron microscopy showed the composite particles to have a mean diameter of 3.9 nm. The nanocomposite exhibited both ferroelectric and ferromagnetic behavior. The dielectric constant peak was not prominent because of a small thickness of the barium titanate phase. The magnetic hysteresis loop showed an asymmetric behavior giving rise to a small exchange bias field. This is believed to arise due to exchange interaction between the ferromagnetic iron core and the thin layer of Fe₃O₄ on the core surface with a spin glass-like behavior. The magnetization under zero-field cooled (ZFC) and field cooled (FC) conditions indicated superparamagnetic behavior at temperatures higher than 300 K. The optical absorption spectra exhibited a peak at around 325 nm. This was analyzed satisfactorily on the basis of a metal core–oxide shell nanoconfiguration. The extracted values of metal core conductivity showed a metal insulator transition for iron core diameters less than 2.4 nm. The present synthesis approach will lead to newer multiferroic nanocomposites and glasses with multifunctionalities.     

Magnetic properties of the micro-silica/cement matrix with carbon-coated cobalt nanoparticles and free radical DPPH

Samples of micro-silica/cement containing iron oxide, Fe₂O_3, and doped with carbon-coated cobalt nanoparticles and free radical DPPH were prepared and studied by the magnetic resonance method. The concrete’s main components (silica and cement) produced very complicated FMR/EPR (ferromagnetic and electron paramagnetic resonance) spectra. The temperature dependence of the FMR/EPR spectra was recorded in the 90–300 K temperature range. The cement/micro-silica matrix produced a very broad FMR line originating from iron oxide particles and two EPR lines originating from iron(III) ions in the crystal field of low-symmetry (centered at g_eff ～ 4.3) and from manganese(II) ions (g_eff ～ 2) of hyperfine structure. Additionally, a very narrow line and a very broad EPR/FMR line were registered and, respectively, attributed to DPPH and cobalt nanoparticles. The isolated paramagnetic iron(III) and manganese(II) centers displayed increasing intensity of the EPR spectra with decreasing temperature, while no influence of the magnetic nanoparticles was observed. The intensity of the FMR spectrum of iron oxide decreased strongly and the resonance field was effectively shifted toward low magnetic fields with decreasing temperature. The observed FMR behavior is similar to what was registered for iron oxide magnetic nanoparticles. The introduction of magnetic nanoparticles influenced the EPR spectrum of the free radical DPPH significantly: its intensity decreased above 260 K and increased slightly below this temperature, while the resonance field changed with decreasing temperature. This behavior may be associated with the porous state of cement and/or the reaction of the multi-component magnetic system. The FMR/EPR method could be very useful for the characterization of matrices containing small amounts of magnetic nanoparticles.     

Exchange-bias and superparamagnetic behaviour of Fe nanoparticles embedded in a porous carbon matrix

Combining different experimental techniques, the physical properties of Fe nanoparticles (NPs) randomly dispersed in a porous carbon have been investigated. From the analysis of TEM images and the broad maximum observed in ZFC–FC magnetization measurements under an applied magnetic field of 2.5 mT, a broad-size distribution NPs (5–50 nm) is estimated. The nature of these NPs (α-Fe and γ-Fe) was determined from X-ray diffraction. Furthermore, magnetization measurements suggest that these Fe-NPs remain completely blocked below 60 K due to a magnetic exchange-bias coupling induced on the surface of these NPs. However, the true nature of the core–shell responsible for this exchange bias within the NPs, is not totally understood yet. In addition, the system begins to be magnetically unblocked above 60 K, becoming almost superparamagnetic above 200 K. Room temperature Mössbauer spectroscopy confirms the existence of both α- and γ-Fe phases; an additional quadrupole component is required to properly fit the spectrum, which could be associated with the NP’s shell or to an oxide phase probably located on the surface of the NPs.     

Spin disorder in Fe-doped manganites

Ln_0.7M_0.3MnO₃ compounds are well-known ferromagnets mediated by a double exchange mechanism. As Mn atoms are substituted by Fe in the ratio Mn_1−xFe_x the magnetic structure dramatically changes, because the ferromagnetic double exchange chain is broken. At low Fe concentrations all compounds are magnetically ordered. For intermediate values ferro (FM), antiferro (AF) and paramagnetic (PM) phases co-exist in a large temperature range, and at x ≈ 0.2 spin or cluster-glass behavior is found. Magnetization, Mössbauer, polarized and low angle neutron scattering as well as muon spin relaxation experiments have been performed on 0 ⩽ x ⩽ 0.30 compounds showing the transit from long range ferromagnetism to spin glass. Co-existence of FM and AF clusters of different size has been found for all doped compounds.     

Influence of maghemite concentration on magnetic interactions in maghemite/PTT-block-PTMO nanocomposite

Temperature dependence of dc magnetization and ferromagnetic resonance (FMR) of two samples containing γ-Fe₂O₃ (maghemite) magnetic nanoparticles dispersed at low concentration (0.1 and 0.3 wt%) in a nanocomposite based on a poly(ether–ester) multiblock copolymer (PTT-block-PTMO) matrix was investigated. The polymer filler was in a powder form consisting of small-sized magnetic nanoparticles arranged in agglomerates 2–3 μm long and 100 nm thick. The studied samples were characterized by SEM spectroscopy. The SEM showed that the concentration of magnetic nanoparticles was homogenous in both samples The temperature dependence of the dc magnetization revealed that the blocking was about 100 K and the ZFC (zero-field cooling) mode at low magnetic fields uncovered the presence of magnetic interactions between magnetic nanoparticles depending on the properties of the matrix. FMR measurements were carried out in the temperature range 4.2–300 K. An intense resonance absorption line attributed to γ-Fe₂O₃ nanoparticles was recorded with a slightly asymmetric lineshape. At room temperature the resonance line was centered at H_r = 3241(2) and 3253(2) G, with linewidths of ΔH = 1069(1) and 1070(1) G for samples with concentrations of 0.1 and 0.3 wt%, respectively. All FMR parameters showed an anomalous behavior at matrix critical temperatures. It was shown that the difference in concentration of magnetic nanoparticles could be responsible for the observed differences in the thermal behavior of the FMR spectra.     

Magnetic properties of nanocrystalline BaFe12O19 prepared by hydrothermal method

Barium ferrite nanoparticles were synthesized by hydrothermal method. The grain size of BaFe₁₂O₁₉ produced chemically at temperatures between 150 and 200 °C with a reaction time of 12 h varies from 9 to 15 nm. At 4.2 K a grain size independent coercive field of about 1.2 kOe was found. However, at 300 K, the coercivity changes from 230 Oe up to 590 Oe. The magnetization at 4.2 and 300 K increases linearly with grain size. In TEM images it can be seen, that the materials are composed of plate-like and stick-like particles. The amount of the stick-like particles increases with synthesis temperature. The low values of the coercivity at 300 K are discussed in terms of demagnetising field and the nearness of the blocking temperature. The increase of the magnetization with reaction temperature may be attributed to the increase of grain size.     

Nonlinear optical studies of lead lanthanum borate glass doped with Au nanoparticles

Synthesis, characterization and optical nonlinearity of lead lanthanum borate glass embedded with gold nanoparticles have been investigated. DSC thermogram shows characteristics glass transition temperature at T_g = 775 K. Glasses doped with Au were subjected to heat treatment at 823 K with different annealing time and then, slowly cooled to room temperature show striking ruby color. SAED and TEM analyses have confirmed that f.c.c. Au nanoparticles of ~ 40 nm size are present in these glasses. An absorption peak centered on 563 nm has been observed in heat treated samples, which is attributed to surface plasmon resonance of gold nanoparticles. Nonlinear optical studies with open aperture Z-Scan technique show saturable absorption for heat treated samples at low intensity and reverse saturable absorption in samples without heat treatment at high intensity.     

Electronic conductivity in zinc iron phosphate glasses

The electrical properties of (40−x)ZnO–xFe₂O₃–60P₂O₅ (x = 10, 20, 30 mol%) glasses were measured by impedance spectroscopy in the frequency from 0.01 Hz to 4 MHz and the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe₂O₃ content and Fe(II)/Fe_tot ratio. The increase in dc conductivity for these glasses is attributed to the increase in Fe₂O₃ content from 10 to 30 mol%. With increasing Fe(II) ion content from 6% to 17% the dc conductivity increases. This indicated that the conductivity arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electron conduction in these glasses. By applying scaling on conductivity data measured at different temperatures, single master curve was obtained for each glass. On the other hand, deviation from the master curve at high frequencies was observed for glasses with different compositions. This deviation originates from a various mobility of charge carriers in different glass structures. Raman spectra showed the change of structure, from metaphosphate to pyrophosphate, with increasing Fe₂O₃ content from 10 to 30 mol%.     

Magnetic and structural characterization of silver-iron oxide nanoparticles obtained by the microemulsion technique

In the present paper we report the magnetic characterization of silver-iron oxide nanocomposite obtained by the chemical microemulsion method. TEM images and X-ray diffractograms show that the nanocomposite consists of Ag nanoparticles of ～ 7 nm surrounded by a quasiamorphous matrix. The ZFC–FC curves and Mössbauer spectra obtained at different temperatures show a typical evolution of a system composed of weakly interacting nanoparticles with a blocking temperature (T_b) of ～50 K. The analysis of the magnetic data reveals that the matrix is formed by γ-Fe₂O₃ phase with a structural range order of ～2 nm.     

Regenerator performance below 4 K in Tm-based bulk metallic glasses

The main obstacle for reaching low temperatures below 4 K using cryocoolers was the inefficiency of the regenerator materials. We report that large volumetric specific heat below 4 K in a wide temperature range has been obtained in Tm-based bulk metallic glasses, and the peak temperature of specific heat peak is tunable. We show that the amorphization of rare-earth based crystalline alloys and the lower magnetic transition temperature of spin glass behavior in the glasses result in the specific heat anomaly below 4 K. The large and tunable specific heat anomaly below 4 K indicates the potential regenerator performance of the glassy materials for sub-4 K cryocoolers.     

Recombination luminescence of oxygen-deficient centers in silica

The luminescence of silica glass, prepared by plasma chemical vapor deposition (PCVD) and quartz glass of type IV (trade mark KS-4V) methods, were studied while irradiated with pulses of ArF laser (193 nm) light in the range of sample temperatures between 10 and 300 K. The samples contain less than 0.1 ppm metallic and hydroxyl impurities. The samples synthesized by PCVD were of two kinds. The first one (amorphous) was as-deposited from plasma at a substrate tube temperature of ～1200 °C. The second one (fused) was prepared from the first by the tube collapsing with an external burner. In this process, a section of the substrate tube with the deposited glass was installed in a lathe and processed at a temperature of ～2100 °C during ～20 min until the tube was transformed to a rod. After such processing, the rod was cooled down to room temperature in air at an average rate of about 400 °C per min. The only observed luminescence possesses two broad bands, with not well defined position, one at 2.6–2.9 eV (a blue band) and another in the range of 4.4 eV (an UV band). There is a correspondence in luminescence properties between KS-4V silica and fused PCVD silica. Those bands have been attributed to oxygen deficient centers (ODC). No luminescence is observed in amorphous PCVD silica under irradiation with 193 nm laser light. So, formation of the sample by melting at least stimulates formation of ODCs at 193 nm. The blue band decays obeys to power law ～t^?1 and is detected in the range of time 10 ns to 300 μs. The UV band possesses a fast, practically repeating excitation pulse, and a slow component (～30 μs). The obtained new kinetics data are compared with known in literature for lone twofold-coordinated silicon having exponential decay for the blue band equal to 10 ms and 4.5 ns for the UV band. That shows the blue band of new studied samples under ArF laser possesses decay component faster and the UV band slower than that of the twofold-coordinated silicon center. This corresponds to the recombination process of luminescence excitation by laser. We propose a model of the processes as charge separation under excitation with creation of a nearest self-trapped hole and electron trapped on the twofold-coordinated silicon, modified by its surrounding atoms or ions. This pair is recombining then with luminescence.     

Octahedral fluoride glasses: Raman spectra and structure of niobium pentafluoride

Raman spectroscopy is used to characterize the NbF₅ phases in the temperature range 80–500 K. A new clear glass is formed by quenching the melt to liquid nitrogen temperatures having a glass transition at ～206 K and crystallization at ～233 K. For all phases including the melt, the glass, the supercooled liquid, the crystalline solid and the gas, the Raman spectra show a rather common high frequency band at ～760 cm^?1 which is attributed to the Nb–F terminal frequency of partially bridged ‘NbF₆’ octahedra. Based on the systematics of the Raman spectra for all phases and the literature physicochemical data a model is proposed for the glass and the liquid phases where ‘NbF₆’ octahedral bridged in cis and/or trans configurations form a variety of cyclic and/or chain structures which intermix building up the overall structure. At exceptionally low energies (<11 cm^?1) a rather weak in intensity Boson peak is observed in the glass which shifts to even lower energies with increasing temperature. Librational and/or tortional motions of the bridged octahedra participating in the glass structure are possible candidates for the origin of this peak.     

Voltammetric approach to redox behavior of various elements in cathode ray tube glass melts

The redox behavior of various elements (Fe, Sb, Ce, Ti, Zn) was investigated in alkali–alkaline earth-silica CRT (Cathode ray tube: TV panel) model glass melts using the square-wave voltammetry (SWV). The current–potential curve, so called voltammogram, was produced at temperature range of 1000–1400 °C under the scanned potential between 0 and –800 mV at 100 Hz. The Fe, Ti and Zn doped melts exhibited one peak due to reduction reaction. In the case of the Sb doped melts, two traces doubted as peak were found in the voltammogram at low temperature while only one peak existed at high temperature. The peak potential was shifted to the negative direction with decrease of temperature, however, its temperature dependence showed linearity. On the other hand, no peak was found in voltammogram of the melts doped with Ce. Based on the temperature dependence of the peak potential, standard enthalpy (ΔH⁰) and standard entropy (ΔS⁰) for the reduction of Fe³⁺ to Fe²⁺, Sb³⁺ to Sb⁰ and Zn²⁺ to Zn⁰ in CRT model glass melts were calculated.     

Magnetic properties of the Mg2FeV3O11−x site-disordered vanadate

The magnetic properties of the Mg₂FeV₃O_11−x ternary vanadate, characterized by disorder between diamagnetic Mg²⁺ and high-spin Fe³⁺ ions, are studied using dc magnetization and electron paramagnetic resonance (EPR). The dc susceptibility shows antiferromagnetic interactions between Fe³⁺ spins with a Curie–Weiss temperature of Θ = −50(1) K, followed by spin-glass-like freezing at T_f ≈ 2.8 K, suggesting significant spin frustration. Temperature-dependent EPR measurements confirm the antiferromagnetic coupling of Fe³⁺ spins at high temperatures, while a distinct divergence is observed at T ≈ 50 K. This behavior is associated with the formation of spin clusters providing two different energy scales for the magnetic interactions. The magnetic response of Mg₂FeV₃O_11−x is similar to that of the Zn-analogue compound, though the observed differences of the implicated energy scales indicate that magnetic inhomogeneity depends on the extent of cation disorder.     

Tracer diffusion of 22Na and 45Ca,ionic conduction and viscosity of two standard soda-lime glasses and their undercooled melts

The tracer diffusivities of ⁴⁵Ca in two different high purity standard soda-lime silica glasses have been measured by the radiotracer method below and above their calorimetric glass transition temperatures. Calorimetric glass transition temperatures (T_g) of 845 K and 867 K have been obtained for standard glasses I and II, respectively, using differential scanning calorimetry (DSC) at a heating rate of 20 K/min. In this paper, we focus on the results of ⁴⁵Ca diffusion and conductivity of the two standard soda-lime glasses and compare them with ²²Na diffusivities also obtained in our laboratory [E.M. Tanguep Njiokep, H. Mehrer, Solid State Ionics 177 (2006) 2839, E.M. Tanguep Njiokep, H. Mehrer, Defect Diffus Forum 237–240 (2005) 282]. The ⁴⁵Ca diffusion coefficients obtained are found to follow the Arrhenius law, both below (Tanguep Njiokep and Mehrer, 2006, 2005) and above T_g. In the Arrhenius diagram a change of slope of the ⁴⁵Ca diffusivities appears at 835 K for standard glass I and at 790 K for standard glass II. At the same time, the ionic conductivities display a change in slope at 790 K and 778 K for standard glasses I and II, respectively. These temperatures are somewhat smaller than the calorimetric glass transition temperatures obtained at a heating rate of 20 K/min. Rather, they appear to be close to values of T_g obtained by extrapolation to a vanishing heating rate (Tanguep Njiokep and Mehrer, 2006). The viscosity diffusion of standard glass I is considerably smaller than the conductivity diffusion coefficient and both tracer diffusivities. In both glasses the ionic conductivity is essentially due to the motion of Na ions. The contribution of Ca ions to the conductivity is negligible.     

Effect of germanium on the microstructure and the magnetic properties of Fe–B–Si amorphous alloys

In this work, we present a systematic study on the crystallization kinetics and the magnetic properties of melt-spun Fe₈₀B₁₀Si_10 ? xGe_x (x = 0.0 ? 10.0) amorphous alloys. The activation energy for crystallization, determined by differential scanning calorimetry, displayed a strong dependence on the Ge content, reflecting a deleterious effect on the alloys' thermal stability and their glass forming ability with increasing Ge concentration. On the other hand, the alloys exhibited excellent soft magnetic properties, i.e., high saturation magnetization values (around 1.60 T), alongside Curie temperatures of up to 600 K. Complementary, for increasing Ge substitution, the ferromagnetic resonance spectra showed a microstructural evolution comprising at least two different magnetic phases corresponding to a majority amorphous matrix and to Fe(Si, Ge) nanocrystallites for x ≥ 7.5.     

FT-IR,ESI and EPR studies of a Dy(III) Schiff base podand complex

IR spectroscopic, electrospray ionization mass spectrometry, structure and magnetic properties of a dysprosium(III) macroacyclic tripodal Schiff base tris((2-(5-chlorosalicylideno)amino)ethyl)amine (tren-5Clsal) complex are reported. The positions of ν(CN) stretching bonds indicate that the azomethine group nitrogen is coordinated to the rare earth ion. Two main spectral components are observed in the low magnetic field region (<300 mT) in the EPR spectra, independently of temperature. These components are attributed to the Dy³⁺ ion with an effective spin of S = 1/2. The magnetic anisotropy between 3 and 23 K is practically constant as evidenced only by a marginal change of the spread of g-factors. The g_y-factor increases strongly with an increase in temperature above 23 K, giving rise to a very high magnetic anisotropy of the complex. Such behavior of the magnetic anisotropy at a higher temperature could be an effect of the existence of an excited doublet. A very small value of the Curie–Weiss constant and the lack of any meaningful thermal changes of linewidths indicates the absence of any significant interactions between Dy(III) complexes below 20 K. The temperature dependence of the total integrated intensity above 20 K indicates the existence of an excited doublet, occurring about 33(1) K above the ground state.     

Dominance of magnetic scattering in the electrical-transport of Al70.5Pd22Mn7.5 icosahedral quasicrystal

Conductivity vs. temperature (σ–T) at zero and 8-T field, magneto-resistance (Δρ/ρ), magnetization vs. temperature (M–T) and magnetization vs. field (M–H) of Al_70.5Pd₂₂Mn_7.5 quasicrystal have been studied in the temperature range of 1.4 K to 300 K. The σ–T variations in both the field conditions show a σ–T minima. In addition to this the σ–T variation at 8 T shows a maxima also at ∼6 K. Comparative analysis shows that the observed σ–T minima arises due to competing inelastic scattering events in the presence of weak-localization effect. These events are e–ph scattering in the dirty-metallic limit (τ_i ∝ T⁻²), and the Kondo-type spin-flip scattering (τ_sf). The maxima observed for the σ–T variation at 8 T, has been attributed to suppression of the spin-flip scattering in the presence of field. The magneto-resistance is found to be large and positive. It was properly accounted only when the Stoner-enhancement, found in the case of spin fluctuating systems, was taken into consideration.     

Structural and magnetic study of mechanically alloyed Fe30Cr70 by neutron thermo-diffractometry and magnetization measurements

The temperature dependence of magnetization together with neutron powder thermo-diffraction show that nominal ball milled Fe₃₀Cr₇₀ for 110 h is formed of two mixed phases (Fe₂₀Cr₈₀ + Fe), both of them with body centered cubic crystal structure and very close values for the lattice parameter (～2.87 Å). On heating above 900 K, the system exhibits an irreversible structural transformation, giving rise to the homogenization of the material, and then recovering the well-defined starting composition Fe₃₀Cr₇₀. Subsequent heating-cooling neutron thermo-diffraction cycles do not show any additional transformation, thus explaining the overlapping M(T) curves after the first heating.     

Linear and nonlinear optical properties of glasses doped with Bi nanoparticles

Transparent glass samples doped with bismuth nanoparticles are prepared by heat treatment of as-made glass samples. According to the results of X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectra, Bi nanoparticles are well distributed inside glasses after heat treatment. The average size of Bi nanoparticles increases with the increasing of heat treatment temperature. Because of the size effect and multiple scattering of nanoparticles, the fundamental absorption edge shows a red-shift behavior with the increasing of heat treatment temperature. Nonlinear optical properties of Bi nanoparticles doped glasses are investigated by using Z-scan technique. The maximum value of χ⁽³⁾ of the glasses is estimated to be 2.49 × 10^? 7 esu at 800 nm. These results indicate that Bi nanoparticles doped glasses may be promising as material for optical switching.