Low frequency acoustic and dielectric measurements on glasses

The acoustic and dielectric properties of different glasses at audio frequencies and temperatures below 1 K have been investigated with the vibrating reed and a capacitance bridge technique. We found the temperature dependence of the absorption of vitreous silica (Suprasil W) to agree with the predictions of the tunneling model which is commonly used to explain the low temperature behaviour of amorphous materials. The variation of the sound velocity and of the dielectric constant, however, shows significant deviations from the expected behaviour which cannot be accounted for by a simple modification of the model. Instead, it seems to be necessary to introduce a temperature dependence of some relevant model parameters. Moreover, at very low temperatures (T < 0.1 K) the sound velocity strongly depends on the excitation levels. The absence of this effect at higher temperatures proves that it can be ascribed to a nonlinear response of tunneling systems. Similar results were found in sound velocity measurements on a cover glass and on a superconducting metallic glass (Pd₃₀Zr₇₀, T_c = 2.6 K), which indicates that these features are a general aspect of the dynamics of tunneling states in glasses. In contrast to the insulating glasses we found that in Pd₃₀Zr₇₀ also the internal friction is strain dependent.     

Non-adiabatic small-polaron hopping conduction in VN–PbO–TeO2 glasses

The dc conductivity of VN–PbO–TeO₂ glasses with different mole percentages of VN, PbO and TeO₂ has been measured in the temperature range 125–450?K. The conductivity of the glasses increases with increasing VN content for a fixed mole percentage of PbO. Neither Mott's variable-range hopping (VRH) model at low temperatures (T<Θ_D/4, where Θ_D is the Debye temperature) nor Greaves’ VRH model at intermediate temperatures (Θ_D/?4<T<Θ_D/2) describe the dc conductivity data for these glasses. Multiphonon tunnelling transport of strongly coupled electrons is also unable to account for the carrier transport. However, at high temperatures (T?>?Θ_D/2), conduction is shown to be due to small-polaron hopping in the non-adiabatic regime. Alteration of the VN content causes a change in the model parameters achieved from best-fitting curves for the glasses. Modulated differential scanning calorimetry analysis shows that the glass transition temperatures T _g in this system vary from 269 to 302°C.     

“Glassy” properties of crystalline Li3N at low temperatures

We have studied the acoustic and dielectric behaviour of the superionic conductor Li₃N at temperatures between 100 K and 20 mK. At low temperatures its properties are determined by low-energy excitations similar to those observed in glasses. In Li₃N these excitations are ascribed to the tunneling of hydrogen impurities, whereas thermally activated hopping of lithium ions accounts for the behaviour at higher temperatures.     

Spin glasses with short range interaction

The theory is presented of spin glasses for which the condition n _m ^1/3 l « dl is valid, l being the range of interaction forces between spins and n _m the volume concentration of spins. Various examples where such an interaction appears are considered. Thermodynamic and kinetic characteristics of such systems are calculated at high and low temperatures. Particular attention is paid to the transition region; for its analysis percolation theory methods are applied. The theory of one-dimensional spin glasses of several types is constructed. A ‘kinetic’ equation is derived for the cluster distribution function in the transition region and the behaviour of this function is studied.     

Semiconductor-metal-semiconductor like transitions in amorphous Ba1-xKxBiO3-δ oxides

In this paper we report the dc conductivities of the Ba_1-xK_xBiO_3-δ (x = 0,0.2,0.4,0.625 and 0.75) glasses at different temperatures (80 to 450 K). Distinct anomalies in the temperature dependence of the dc conductivities, indicating semiconductor-metal-semiconductor (SMS) like transitions, are observed for all the glasses at temperatures ranging between 297 K and 325 K (depending on x). A similar anomaly is also observed from the dielectric constant measurements around the same temperature region. Such a SMS transition is very uncommon in oxide glasses. The orientational motion of the BiO₃ or BiO₆ structural units present in the glasses (as observed from the infrared spectra) is considered to be responsible for such anomalous behavior. A model Hamiltonian considering strong electron-phonon interaction giving rise to a lattice instability is suggested to describe such a SMS type transition observed in the present glassy systems.     

Sensitivity to temperature perturbations of the ageing states in a re-entrant ferromagnet

Dynamic magnetic properties and ageing phenomena of the re-entrant ferromagnet (Fe_0.20Ni_0.80)₇₅P₁₆B₆Al₃ are investigated by time dependent zero field cooled magnetic relaxation, m(t), measurements. The influence of a temperature cycling (perturbation), ,(prior the field application) on the relaxation rate is investigated both in the low temperature re-entrant spin glass “phase” and in the ferromagnetic phase. In the ferromagnetic phase the influence of a positive and a negative temperature cycle (of equal magnitude) on the response is almost the same (symmetric response). The result at lower temperatures, in the RSG “phase” is asymmetric, with a strongly affected response for positive, and hardly no influence on the response for negative temperature cycles. The behaviour at low temperatures is similar to what is observed in ordinary spin glasses. Received 20 August 1998     

High pressure electrical resistivity studies on Ga-Te glasses

Abstract

Bulk, semiconducting Ga_xTe_100-x (17 ≤ x ≤ 25) glasses are prepared by the meltingh quenching method and electrical resistivity measurements are carried out at high pressures up to 8 GPa and low temperatures down to 77 K in a Bridgman anvil system. It is found that all the Ga_xTe_100-x, glasses exhibit metallization under pressure, with a continuous decrease in electrical resistivity and activation energy for conduction. Further, the high pressure metallic phases of Ga_xTe_100-x samples are found to be crystalline.     

Absorption phenomena in multicomponent phosphate glasses with Mn

The absorption and ESR spectra of multicomponent alumino-phosphate glasses doped with manganese (0·2–20 mol %) has been measured and compared with the absorption spectra of manganous and manganic ions in model solutions. The unirradiated glasses show in ultraviolet region increasing absorption below 200 nm with the shoulder at 235 nm and the absorption bands due to manganous oxygen complexes in octahedral symmetry. They are similar to the absorption bands of Mn²⁺ in concentrated H₃PO₄ acid. In irradiated glasses the bands at 200, 235, 275 (only if Mn is present) and a broad band at 540 nm appear. After the annealing at 450 C all radiation-induced bands disappear and the bands at 235 nm is more pronounced. The Mn³⁺ in H₃PO₄ solution exhibit the absorption spectrum with a weak band at 530 nm and a very intense band at 270 nm. It is therefore proposed that both the 540 and 275 nm bands in irradiated glasses can be assigned to octahedral oxygen complexes of Mn³⁺, i.e. to hole centres. The band at 200 nm which is practically independent of the modifiers (Be, Mg, Ca, Sr and Ba) is, therefore, associated with electron centres (electrons trapped in non-bridging oxygen vacancies). It is suggested that the band at 235 nm in irradiated and annealed glasses is associated with irreversible structural changes.The authors wish to express their appreciation to H.Dvoáková for preparing the solutions and E.Linhartová for careful measurements.     

Low temperature electrical conductivity and magnetoconductivity of Fe39Ni39Mo4Si6B12 and Co58Ni10Fe5Si11B16 metallic glass alloys

A detailed study on the weak localization phenomenon vis-a-vis electron-electron interaction effects in magnetic metallic glasses has been carried out. We measured the electrical conductivity and magnetoconductivity within the temperature range 1.8≤T≤300K. A maximum on the conductivity versus temperature curve exists atT=T _m. The conductivity was observed to follow aT ^1/2 law forT<T _m andT ² law forT>T _m. Magnetoconductivity data of these alloys indicate the prominence of electron-electron interaction at low temperatures. The authors have determined the inelastic scattering field and spin-orbit scattering field from the magnetoconductivity data. The inelastic scattering field obeys aT ^p law (p=2) at low temperatures.     

Local structure of technologically modified g-GeS2: resonant Raman and absorption edge spectroscopy combined with ab initio calculations

We have used resonant Raman and absorption edge spectroscopy together with first-principle calculations in order to study the structure of GeS₂ glasses (g-GeS₂). The glasses were prepared under different melt temperatures and cooling rates, which are shown to significantly influence the g-GeS₂ structure at the nano-scale. The combined use of Raman spectroscopy and ab initio calculations reveals the origin of the molecular level electronic structure and its connection to the interesting technological features of the g-GeS₂. Local structure within the glasses is discussed in terms of atomic Ge _n S _m clusters. The band gaps computed for these clusters and their correlation to the experimental band gaps and the possible formation of band tail states are also discussed.     

Low activation energies in superionic glasses in the 4.2–77 K temperature region

The peaks in the ultrasonic attenuation observed in AgI-doped superionic glasses at temperatures higher than 77 K have always been explained in terms of classical, thermally activated, relaxation processes due to silver ions which are mobile in the glassy matrix. Here we complete an ultrasonic study with the loss characteristics and sound velocity in “binary” (Ag₂O·nB₂O₃) and “ternary” [(AgI)_x(Ag₂O·nB₂O₃) _{1 − x}] glasses in the 4.2–77 K temperature region. In such a way we obtain useful information about defects having low activation energy.     

Vitreous solid state electrolytes in the system of RNO3–Zn(NO3)2–KHSO4–P2O5

The structure and electric conductivity of glasses in the system RNO₃–Zn(NO₃)₂–KHSO₄–P₂O₅ (R = Na, K) obtained at different temperatures were investigated by IR and impedance spectroscopy. Glasses fused at 250–350 °C demonstrated an increase of their ionic conductivity in 10³ times in comparison with the same compositions fused at 550 °C. The influence of the chemical composition on the structure and properties of the obtained glasses was analyzed. It is proposed that the high ionic conductivity of glasses obtained at low temperatures is related to the incorporation of the nitrate ions between long (PO₃)_n chains, similar to the iodide ions; this resulted in a maximal coordination of the local conduction space for the cation associated with a disordered glass network.     

The crystal fields inα-alums—A high pressure magnetic resonance study

The origin of the crystal field and its variation with temperature in severalα-alums have been studied bynmr of²⁷Al andepr of Cr³⁺ at high hydrostatic pressures and low temperatures. The results lead to an explanation of the anomalous temperature variation of the axial crystal field at the trivalent ion site. The mechanism of the phase transition in ammonium alum is also explained. A correlation between the axial crystal field as determined bynmr (e ² qQ/h) and that determined byepr (D) has been obtained.     

Mixed polaronic-ionic conduction in lithium borate glasses and glass-ceramics containing copper oxide

The effect of electric field strength on conduction in lithium borate glasses doped with CuO with different concentration was studied and the value of the jump distance of charge carrier was calculated. The conductivity measurements indicate that the conduction is due to non-adiabatic hopping of polarons and the activation energies are found to be temperature and concentration dependent. Lithium borate glasses are subjected to carefully-programmed thermal treatments which cause the nucleation and growth of crystalline phases. X-ray diffraction analysis confirmed the amorphous nature for the investigated glass sample and the formation of crystalline phase for annealed samples at 650 °C. The main separated crystalline phase is Li₂B₈O₁₃. The scanning electron micrographs of some selected glasses showed a significant change in the morphology of the films investigated due to heat treatment of the glass samples. It was found that the dc-conductivity decreases with an increase of the HT temperature. The decrease of dc conductivity, with an increase of the HT temperature, can be related to the decrease in the number of free ions in the glass matrix. There is deviation from linearity at high temperature regions in the logσ-1/T plots for all investigated doped samples at a certain temperature at which the transition from polaronic to ionic conduction occurs. The hopping of small polarons is dominant at low temperatures, whereas the hopping of Li⁺ ions dominates at high temperatures. PACS 71.55.Jv; 72.60.+g; 72.80.Ng     

Glass-forming region in the Cd-Ge-As ternary system

In the Cd-Ge-As system, bulk glassy samples can be prepared by quenching of the melt for certain compositions. The glass-forming region is not very large. It includes the pure CdAs₂ compound, the strongest tendency to form a glass is at CdGe_0,3As₃. The characteristic phase transition temperatures for some glasses were determined by DTA. The studies also show that all these glasses recrystalize on heating. After melting and slow cooling they solidify as polycrystals.     

Effect of pressure on electrical resistance of Pd82−xVxSi18 metallic glasses

Measurements have been made of the effect of hydrostatic pressure to 10 kbar, at room ambient temperatures, on the electrical resistance of a series of Pd_82?xV_xSi₁₈ metallic glasses with x varying from 0 to 6 at.%. These glasses show positive temperature coefficients of resistance at low vanadium concentrations, varying monotonically to negative coefficients at high concentrations. It is found that, independent of vanadium concentration, the pressure coefficient of resistance of all the glasses is identically zero to one part in 10⁴, indicating that the pressure derivative of the logarithm of the resistivity is exactly equal to the negative of the linear compressibility. The implication of this result with respect to theoretical models is discussed; only the Ziman liquid metal model provides a consistent explanation.     

Effective exponents for de almeida-thouless line

The experimental irreversibility line at the magnetic phase diagram H-T of spin glasses is often approximated using a power law Hα (T_G-T)^φ/2. The mean field theory of de Almeida and Thouless predicts φ=3 for temperatures T close to the spin-glass transition temperature T_G . For a range of reduced temperature involved in an experiment one should use, however, effective exponents (φ>3) obtained by fitting the power law to the exact expression for de Almeida-Thouless line. These compare favorably with experimental values of φ obtained for different spin glasses. The increase of effective exponent up to φ=7 on approaching the spin-glass-ferromagnetism multicritical point is also considered.     

Electricity and Magnetism,Volume 1: Third Edition,Electricity and Magnetism,Volume 2: Third Edition,by B.I. Bleaney and B. Bleaney

During the last decade the term ‘spin glass’ has become prominent in the literature on magnetism. It refers to magnetic alloys where the spins on the impurities become locked or frozen into random orientations below a characteristic temperature T ₀. In this article the properties of spin glasses are described with particular reference to the two archetypal examples AuFe and CuMn. Interest in spin glasses was mainly stimulated by some a.c. susceptibility measurements which showed sharp, cusp-like peaks, accurately defining T ₀ and suggesting that some type of phase transition was occurring. The Mossbauer effect and the anomalous Hall effect also showed clear features at T ₀ supporting this viewpoint. But measurements of the electrical resistivity and ‘specific heat’, here usually meaning the molar heat capacity, also the remanence, magnetic hysteresis and time-dependent effects observed in spin glasses were difficult to reconcile with a phase transition approach. This article discusses the results obtained from the very wide variety of experimental techniques which have been used to investigate spin glasses, and also deals with some of the important theoretical concepts which have arisen out of these studies. Then follows a short account of the many systems which have been found to exhibit spin glass behaviour and which suggest that it is a widespread magnetic state of matter. Lastly, an example is given which shows that some of the ideas of spin glasses are applicable to problems outside the sphere of magnetic alloys.     

Nuclear spin-electron coupling of copper at microkelvin temperatures

The nuclear spin-electron coupling of copper has been determined by establishing stationary temperature differences between nuclei and electrons at electronic temperatures of 15 K to 5 mK in magnetic fields of 2 mT to 440 mT using a well-defined heat flow from the electrons to the nuclear spin system. The measured Korringa constantk increases proportional toB/T _e ² with decreasing electronic temperatureT _e and with increasing magnetic fieldB. At the lowest temperaturesk is more than an order of magnitude larger than its high temperature value.     

Magnetic moments and the high field susceptibilities of MnFe2O4

An extended version of the noncollinear spin model bySawatzky et al. is developed to derive a dependence of the differential high field susceptibility on the degree of inversion of the ferrite MnFe₂O₄. Using the available data on the main molecular field coefficients it is shown that the present model imposes restrictions on values of. Determination of from the measurements of the field dependence of the saturated moments at low temperatures on samples with varying degrees of inversion reveals good quantitative agreement with the present model.