Spontaneous magnetization related to internal energy in an assembly with long-range interaction: Predicted universal features fingerprints in experimental data from Fe and Ni

In recent work we have dealt with antiferromagnetism in materials with long-range interactions. Here, we extend low-temperature theoretical study of Grout and March on metallic ferromagnets. Using experimental data on Fe and Ni on spontaneous magnetization M(T) and specific heat c(T), we relate M(T) and internal energy E(T) for 0<T<TC with TC the Curie temperature. An appropriate plot shows that the experimental data on Fe and Ni then collapse on to a common curve. To gain theoretical insight the Blume-Emery-Griffiths infinite-range model is used to connect M and E. A related “universality” is exposed.     

New insights from variable-temperature and variable-pressure studies into coupling and decoupling processes for ion transport in polymer electrolytes and glasses

A fresh analysis of literature data shows how the influences of temperature and pressure on ion transport and structural relaxation in glass-forming systems may be combined within the framework of ‘master plots’ based on the equation E_A = M · V_A, to reveal new insights into coupling and decoupling effects in a wide range of systems. E_A,σ and V_A,σ are, respectively, instantaneous activation energies and volumes for ionic conductivity and the parameter, M_σ, is a corresponding ‘process modulus’. For structural relaxations occurring at the glass transition, the appropriate modulus is given by M_s = T_g · dP/dT_g. We can now identify typical behaviour patterns for fragile liquids on the one hand, and typical inorganic glasses on the other. Thus, the parameters, M_σ and M_s, for fragile systems such as molten Ca(NO₃)₂:KNO₃ (CKN) or a typical polymer electrolyte such as a complex of LiCF₃SO₃ in PPG, are found to remain constant over a wide range of temperatures down to T_g, despite changes in the temperature (and pressure) dependences of the ionic conductivities, as indicated for example by a return to Arrhenius behaviour in the case of CKN, or by so-called Stickel plots and changes in the VTF parameters for the polymer electrolytes. If E* and V* are activation energies and volumes assigned to elementary steps, when again E* = M · V*, we can go further and identify the microscopic processes driving forward structural relaxation. In the case of inorganic glasses, where usually we find the decoupling index R_τ ≈ 10¹², we identify two distinct decoupling paradigms represented by strong and fragile systems respectively, where in both cases the activation volumes for ion transport are very similar to the corresponding ionic volumes. In the former case (typified by the strongly cross-linked silicate and aluminosilicate systems), the negative activation volumes for structural relaxation (negative values of dT_g/dP) are clearly indicative of a ‘water-like’ behaviour attributable to the collapse of the network under pressure. On the other hand, for the more fragile fast-ion conducting silver iodomolybdate glass, the experimental results show that M_s (at T_g) ≈ M_σ (in glass), implying some recoupling of structural relaxation to ion transport. Arguments based on the dynamic structure model lead us to predict that a similar close link should exist between M_s (at T_g) and M_σ in the relatively fragile lithium and sodium borate glasses, thus highlighting the need for more information concerning the effects of pressure on the glass transition temperatures of common inorganic glasses.     

Structure and stability of two-dimensional systems of C<Subscript>20</Subscript> fullerenes

Two-dimensional systems of C₂₀ fullerenes connected to each other by strong covalent bonds have been investigated. Several isomers differing in the type of intercluster bonds have been revealed. The lifetimes τ of the (C₂₀) _{M × M} complexes with M = 2 and 3 at T = 1800–3300 K have been directly calculated using the molecular dynamics method. It has been shown that these complexes lose their periodic cluster structure due usually to the coalescence of two or several neighboring C₂₀ fullerenes. The activation energy of this process determined by analyzing the τ(T) dependence appears to be E _a ≈ 2.5 eV in agreement with the calculations of the heights of the potential barriers preventing the coalescence. At high temperatures T > 2400 K, the decay of C₂₀ fullerenes entering into the complex is possible.     

Components of the low-temperature heat capacity of rare-earth hexaborides

The temperature dependence of heat capacity C _p(T) was studied for nine rare-earth hexaborides MB₆(M=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) at temperatures of 5–300 K. Using the correspondence principle for lattice heat capacities of isostructural compounds, the lattice contribution C ₁(T) and the excess contribution ΔC(T) to the heat capacity of the hexaborides were determined. The lattice heat capacity C ₁(T) is represented as the sum of the Debye contributions of the metal and boron sublattices: C ₁(T)=C _M (T)+6C _B(T). The Debye temperatures π_M and π_B of the metal and boron sublattices were determined. The anomalies in the excess heat capacity ΔC(T)=C _p (T)?C ₁(T) are related to the magnetic ordering effects, the Schottky contribution, and the Jahn-Teller effect.     

Influence of temperature on raman spectra of the FeS2 single crystal with pyrite structure

Raman scattering in a natural FeS₂ single crystal with the pyrite structure was investigated in the temperature range of 80–300 K. All five Raman-active modes E _g , T _g (1), T _g (2), A _g , and T _g (3) were observed under normal conditions (T = 23°C). The influence of temperature on the Raman spectra was studied in the HH configurations (polarizations of the incident and scattered radiations are parallel), which made it possible to detect the strongest spectral lines A _g and E _g . Widths of modes E _g and A _g were substantially smaller than those given in previous publications. It was found that the temperature dependences of frequencies and widths (FWHM) of modes A _g and E _g are approximated well by the Klemens model, which describes the three-phonon scattering mechanism.     

Temperature dependence of the fine structure of the C and E absorption bands in RbMnF3 below the Néel temperature

The variation in the parameters (width, position, intensity) of the fine structure lines in the C[⁶ A _1g → ⁴ A _1g , ⁴ E _g(⁴ G)] and E[⁶ A _1g → ⁴ E _g(⁴ D)] bands in RbMnF₃ with temperature is studied in the temperature range 10–70 K. In the C band, two narrow (<6 cm^?1) lines are are distinguished at distances of 77 and 80 cm^?1 from the exciton line at T = 10 K. The other lines in the C band and all lines in the E band are more than 20 cm^?1 wide. It is demonstrated that the narrow lines become allowed because of the spin-exchange interaction within a long-range magnetic order model and originate from the excitation of exciton-magnon bound states and that the other lines are made allowed by the exchange-vibronic mechanism within a short-range magnetic order model and originate from the excitation of bound states composed of an exciton, magnon, and oddparity phonon. The vibrational replicas of the main exciton-magnon-phonon lines are due to the quadratic vibronic interaction with odd-parity vibrations. Variations of the intensities and widths of the absorption lines with temperature indicate that these parameters are affected by relaxation and delocalization of the bound states.     

Nuclear spin relaxation in liquids due to interaction with paramagnetic ions having anisotropic g tensors

Analytical expressions are derived for spin-lattice and spin-spin relaxation times (T_1M and T_2M) of a nucleus arising from magnetic interactions (dipolar and contact) with a paramagnetic ion which has an anisotropic g tensor. The relaxation rates depend on the orientation of the ion-nucleus vector in the principal-axis system of the g tensor. The deviations caused by this angular dependence with reference to the corresponding relaxation rates for an isotropic g tensor are numerically illustrated by considering a case typical for Co (II) complexes.     

Exchange bias in bulk Ni-Mn-In-based Heusler alloys

The crystal structure and exchange bias of the bulk Heusler alloys Ni₅₀Mn_50−xIn_x with 14.5?x?15.2 have been investigated using X-ray diffraction and magnetization measurements, respectively. Magnetic measurements were performed with SQUID magnetometry after samples were zero-field cooled and field cooled (FC) in positive magnetic fields up to H=50 kOe, from a temperature T=380 K. Three temperatures of the phase transitions, T₁<T_M<T_C, and a shift of the FC (50 kOe) magnetic hysteresis loops up to 120 Oe at 5 K have been detected for all samples. The exchange bias field (H_E) was almost constant for intermediate In concentrations 14.8<x<15.2, and sharply decreased to about 20 Oe on the borders of this concentration interval (x?14.5; 15.2?x). The changes of H_E have been related to changes in the ratio of T₁ to T_M: the overlapping of transitions at T₁ and T_M (for x=14.8, and 15.2) results in a decrease in H_E.     

Effects of component ratio and easy axes distribution on the exchange bias in ferromagnetic-antiferromagnetic random alloys

For a ferromagnetic (FM)-antiferromagnetic (AFM) system with composition x(FM)+(AFM)_1−x, a modified Monte Carlo Metropolis method is performed to study the effects of x and easy axes distribution at the FM/AFM nearest neighbors on exchange bias field H_E, coercivity H_C, and vertical magnetization shift M_E after cooling under different magnetic fields h_CF. When the orientations of easy axes are uniform, the x dependence of H_E and M_E undergo a non-monotonous to monotonous process with the increase of h_CF, whereas H_C shows a more complex behavior. On the other hand, for the case of the random orientation, H_C has a peak around x=0.5, while M_E decreases with the increase of x. H_E exhibits negative extrema at small x and disappears for larger x. However, abnormal positive H_E observed depends on the frustration and the distinct trends of two coercive fields with x in such a special model.     

Theory of spin-resolved photoemission from Fe at finite temperature: The disordered local moment picture

We present first principles calculations of the spin and angle-resolved photoemission spectra for Fe(100) at T = 0 and T = T_c, based on the KKRCPA treatment of the disordered local moment model. At T = 0 the calculations are in generally good agreement with experiment, and are shown to be quite sensitive to the surface potential and magnetisation. The high T calculations are also consistent with existing data, and a strong photon polarisation dependence is predicted as a consequence of the different exchange splitting of the T_2g and E_g states.     

Critical behavior of the optical absorption edge in EuO

Measurements of the temperature dependence of the optical absorption edge, E_g(T), in EuO, at the close vicinity of its critical temperature, T_c, are reported. In agreement with theoretical expectations the analysis of the results, with the constraints of continuous E_g and dE_g/dT, yielded the same critical exponents (α = α' ≈ -0.044) and amplitude ratio (|A/Á| ≈ 1.22) as the specific heat data analysis.     

Influence of superalkali oxides on structural,electrical and optical properties of C24 fullerene nanocluster: A theoretical study

The influence of superalkali oxides on structural, electrical and the optical properties of 6-5 isomer of C₂₄ fullerene nanocluster was investigate using DFT calculations. It was shown that the structure of fullerene was drastically affected by adsorption of M₃O on its surface and as a result of all bonding lengths, was subsequently changed. DOS results showed that the E_g of C₂₄ in presence of M₃O (M=Li, Na, K) was fairly changed so that the electronic properties and electrical conductance of C₂₄ were not affected by M₃O adsorption. Based on the results of NBO calculations, specifically on MEP surfaces and HOMO-LUMO distribution, the charge was transferred from M₃O to C₂₄ fullerene; therefore, the M₃O is the positive head of complexes and in an electric field it is oriented toward the negative pole. The calculated polarisability(α) and the first hyperpolarisability(β₀) values indicated that the adsorption of M₃O on exterior surface of C₂₄ fullerene has had a drastic effect on the optical response of C₂₄ fullerene and caused a large increase in α and β₀ values. TD-DFT calculations were also carried out in order to confirm the β₀ values. This work theoretically devises novel C₂₄-based nanostructures via interaction with Superalkali Oxides in order to promote, their potential applications in electronic devices and high-performance NLO materials.     

The exchange bias and coercivity of FM/AFM films: Monte Carlo simulation

The exchange bias (H_E) and coercivity (H_C) of the ferromagnet/antiferromagnet (FM/AFM) films have been simulated with Monte Carlo method. The simulated results indicate that, the value of H_E decreases with increasing temperature, and the values of H_E and the blocking temperature T_b at which H_E=0 reduce evidently with decreasing absolute value of interlayer exchange coupling JI. It also is found that for the large absolute values of JI, the maximum in H_C occurs very close to T_b. At the same time, it is observed that the diluted ratio of FM at FM/AFM interface influences clearly the value of H_E. The simulated results are consistent with the experimental facts. The maximum behaviour in the H_C−T curves has been explained by the interplay of the softening of some fraction of the spins in the AFM layer near T_N′ and the disorder of the spins in FM layer near Curie temperature T_C.     

Electron paramagnetic resonance of Cr<Superscript>3+</Superscript>-Li<Superscript>+</Superscript> centers in (Cr,Li): Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> synthetic forsterite

Synthetic single crystals of chromium-and lithium-doped forsterite, namely, (Cr,Li): Mg₂SiO₄, are studied using electron paramagnetic resonance spectroscopy. It is revealed that, apart from the known centers Cr³⁺(M1) and Cr³⁺(M2) (with local symmetries C_i and C_s, respectively), these crystals involve two new types of centers with C₁ symmetry, namely, Cr³⁺(M1)′ and Cr³⁺(M2)′ centers. The standard parameters D and E in a zero magnetic field [zero-field splitting (ZFS) parameters expressed in GHz] and principal components of the g tensor are determined as follows: D=31.35, E=8.28, and g=(1.9797, 1.9801, 1.9759) for Cr³⁺(M1)′ centers and D=15.171, E=2.283, and g=(1.9747, 1.9769, 1.9710) for Cr³⁺(M2)′ centers. It is found that the lowsymmetric effect of misalignment of the principal axes of the ZFS and g tensors most clearly manifests itself (i.e., its magnitude reaches 19°) in the case of Cr³⁺(M2)′ centers. The structural models Cr³⁺(M1)-Li⁺(M2) and Cr³⁺(M2)-Li⁺(M1) are proposed for the Cr³⁺(M1)′ and Cr³⁺(M2)′ centers, respectively. The concentrations of both centers are determined. It is demonstrated that, upon the formation of Cr³⁺-Li⁺ ion pairs, the M1 position for chromium appears to be two times more preferable than the M2 position. Reasoning from the results obtained, the R₁ line (the ²E → ⁴A₂ transition) observed in the luminescence spectra of (Cr,Li): Mg₂SiO₄ crystals in the vicinity of 699.6 nm is assigned to the Cr³⁺(M1)′ center.     

Relation between elastic modulus and glass softening temperature in the delocalized atom model

The ratio of softening temperature (glass transition temperature) to elastic modulus (T _g /E) is mainly determined by the limiting elastic deformation of an interatomic bond, which characterizes the transition of a structural microregion from an elastic into a viscous-flow state. In silicate glasses, this transition is caused by the limiting deformation of directed ionic-covalent Si-O-Si bonds. In the case of amorphous hydrocarbons, it is related to the relatively weak intermolecular bonds between regions in chain macromolecules, and the T _g /E ratio is significantly higher than in inorganic glasses. In glassy systems of one class, this ratio turns out to be constant (T _g /E ?? const), and a linear correlation is detected between softening temperature and elastic modulus, which can be explained in terms of the delocalized atom model. The values of T _g /E can be used to classify glasses similarly to the well-known Angell classification according to so-called fragility.     

Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

Radiative properties of α-ZnAl2S4:V spinel type single crystals

The absorption and luminescent properties of α-ZnAI₂S₄:V spinel type crystals in the temperature range 10-300 K are investigated. The spectra are assigned to the electronic transitions of trivalent vanadium ions located in octahedral sites. It is shown that at low temperatures the three main components of the revealed IR luminescence spectra are caused by the ¹A_1g(¹G)→¹E_g(¹D), ¹T_2g(¹D), ³T_2g(³F)→³T_1g(³F), and ¹E_g(¹D)→³T_1g(³F) transitions. The observed dependencies of the emission components intensities on temperature are explained assuming that there is a phonon assisted tunnelling between ³T_2g(³F) and ¹E_g(¹D) states. On temperature rise, the ³T_2g(³F)→³T_1g(³F) vibronic transitions suppress other emission channels, which leads to the enhancement of the integral luminescence intensity and to the broadening of the spectrum centred at λ=1.4 μm.     

Differential scanning calorimetric study of Ga5Se95 glass

Results of differential scanning calorimetry, at different heating rates, α, on Ga₅Se₉₅ glass are reported and discussed. From the heating rates dependence of values of T_g and T_p, the glass activation energy, E_g and the crystallization activation energy, E_c, are derived. The crystallization results are interpreted in terms of recent analyses developed for non-isothermal crystallization and also for the evaluation of E_c. The crystallization mechanism is then characterized. From the obtained results, the glassy Ga₅Se₉₅ has two-dimensional growth, the average value of the order of crystallization mechanism, n is 3. The average value of the glass activation energy, E_g and crystallization activation energy, E_c, for Ga₅Se₉₅ glass are 189±4 and 69±5 kJ/mol, respectively.     

(T1u+T1g)⊗(hg+τ1u)vibronic interaction and superconductivity in C 60 n− fullerides

The closeness of low-lying T_1u and T_1g levels of C ₆₀ ^? could enable their mixing under an odd parity vibration of (T_{1 u} + T_{1 g} ? (h_g + τ_{1 u})type. In addition, the two levels are susceptible to Jahn-Teller interaction due to five-fold degenerate h_g vibrations. This complex problem of (T_1u+T_1g)?(h_g+τ_1u) vibronic interaction is transformed to a form similar to T_2g ? (ε_g + τ_2g) vibronic problem of octahedral symmetry. The problem is analysed in an infinite coupling model and compared with the experimental spectroscopic results for the C ₆₀ ^? radical. The resulting parameters are used to calculate the pair-binding energy and superconducting transition temperature in C ₆₀ ^n? fullerides. Vibronic mixing with the T_1g level is found to be responsible for maximising the pair-binding energy at the doping level n=3. It is also found to be an important source of T_c enhancement.