Invar and covar behavior of simple ferromagnets: Thermodynamic simulation

Thermodynamic model calculations are performed with the aim to show that the invar behavior of a ferromagnet (when its thermal expansion coefficient is close to zero in a climatic temperature range) results from a certain optimum relation between the thermodynamic parameters characterizing the ferromagnetic state. The same is true for the covar behavior of a ferromagnet, when its thermal expansion coefficient is nonzero and almost constant in a certain temperature range. As a first approximation, calculations for simple ferromagnets are performed in terms of the thermodynamic Landau model.     

Invar and covar behavior of ferromagnets with allowance for magnetophonon interaction: A thermodynamic simulation

Based on the Landau theory of second-order phase transitions with allowance for interaction between the magnetic subsystem and crystal lattice (magnetophonon interaction, MPI), a consistent thermodynamic model of the invar and covar behavior of ferromagnets with MPI is constructed. Model calculations carried out at real temperatures show that the invar (covar) behavior of ferromagnets results from an optimal relationship between the thermodynamic parameters of the ferromagnetic phase. It is demonstrated that MPI is a fundamentally important factor responsible for the invar and covar effects. With the thermodynamic parameters obeying a certain relationship, the invar behavior of ferromagnets is predicted in a much wider temperature range than in known invars.     

Critical behavior of Griffiths ferromagnets

From a heuristic calculation of the leading order essential singularity in the distribution of Yang-Lee zeroes, we obtain new scaling relations near the ferromagnetic-Griffiths transition, including the prediction of a discontinuity on the analogue of the critical isotherm. We show that experimental data for the magnetization and heat capacity of La(0.7)Ca(0.3)MnO(3) are consistent with these predictions, thus supporting its identification as a Griffiths ferromagnet.     

Thermodynamic properties of ferromagnets with uniaxial and biaxial anisotropy

The thermodynamic properties of spin-one Heisenberg ferromagnets with uniaxial and biaxial anisotropy are investigated in the molecular field approximation (MFA) and random phase approximation (RPA). In MFA a full phase diagram comprising three lines of bicritical points, is obtained.Using the standard-basis operator method, the collective excitation spectrum is studied in detail; moreover the softening of the excitations at the phase boundaries is discussed. A self-consistent version of RPA with emphasis on the role of kinematic restrictions with regard to the standard-basis operators is analyzed.Moreover, the phase transitions are considered in the Ising model, where a line of tricritical points occurs, and the planar model, where a line of bicritical points, two lines of tricritical points and a line of triple points, occur.     

Quantum critical behavior of clean itinerant ferromagnets

We consider the quantum ferromagnetic transition at zero temperature in clean itinerant electron systems. We find that the Landau-Ginzburg-Wilson order parameter field theory breaks down since the electron-electron interaction leads to singular coupling constants in the Landau- Ginzburg-Wilson functional. These couplings generate an effective long-range interaction between the spin or order parameter fluctuations of the form 1 <r ² d?1, with d the spatial dimension. This leads to unusual scaling behavior at the quantum critical point in 1 < d ≤ 3, which we determine exactly. We also discuss the quantum-to-classical crossover at small but finite temperatures, which is characterized by the appearance of multiple temperature scales. A comparison with recent results on disordered itinerant ferromagnets is given.     

Thermodynamic grounds for the invar and ellinvar effects in ferromagnets

In terms of the standard thermodynamic approach (the Debye-Grüneisen model of solid and the Landau theory of second-order phase transitions), approximate conditions providing the constancy of the volumetric thermal expansion coefficient (the Invar effect) and of the bulk modulus (the Ellinvar effect) of a ferromagnet are established. Conditions under which a ferromagnet may exhibit Invar and Ellinvar properties simultaneously are found. Interaction between the magnetic, phonon, and electron subsystems of a ferromagnet is shown to be a crucial factor in the occurrence of the Invar and Ellinvar effects.     

Thermodynamic functions of both simple (monomeric) and polymeric melts: MFA approach and Monte Carlo simulation

The influence of equilibrium polymerization on the thermodynamic properties of model systems consisting of building units with well-defined characteristics and interactions is investigated. The systems under study are thought to resemble more or less accurately undercooled melts, and the calculations performed give the configurational part of the thermodynamic functions of these melts. The whole investigation is performed by using two approaches. In the first one, the temperature courses of the entropies and the specific heat of the systems as well as the average flexibility and the mean chain length of the polymer molecules are obtained in the framework of a mean field approximation (MFA). In the second approach, the bulk characteristics and the configurational properties of the model systems are obtained by using Monte Carlo simulations (MCS). The correspondence and the differences between the thermodynamic properties for the same systems in the two approaches are analyzed and discussed. The model used in our MCS is a modified version of the so-called independent monomer state model proposed earliar by Jari? and Bennemann. The temperature course of the thermodynamic functions of the systems under investigation is analyzed and compared with existing experimental data. The existence of phase transition corresponding to melt-crystal or order-disorder transformation in the system is discussed based on the two approaches.     

Critical behavior of diluted Heisenberg ferromagnets with competing interactions

The pure and the site-diluted classical Heisenberg model on the face centered cubic (fcc) lattice with ferromagnetic exchange J_nn between nearest neighbors and antiferromagnetic exchange J_nn = −J_nn/2 between next nearest neighbors is studied by Monte Carlo simulation. Data are generated by the heat bath algorithm for lattice sizes L = 4, 8, 12, 16, 20 and 24, using histogram reweighting techniques and sampling up to several hundred configurations of the random site disorder. From a finite size scaling analysis both the critical temperature and the critical exponents are estimated. For the pure system, the data are in very good agreement with the critical exponent estimates 1/v ≈ 1.42, β/v ≈ 0.51 obtained from other methods (as a check of the accuracy of our approach, we also study the nearest neighbor model — where J_nn ≡ 0− and again obtain very good agreement with the known behavior). However, for the diluted systems evidence for a new universality class is found. While for concentration c = 0.875 of occupied sites strong crossover phenomena preclude us from giving exponent estimates, for c = 0.75 we find 1/v ≈ 1.2 and β/v ≈ 0.45. Possible reasons why the Harris criterion may not apply for this system are discussed. The application of this study to experiments on Eu_xSr_1−xS is briefly mentioned.     

Thermodynamic properties of noble metal clusters: molecular dynamics simulation

The thermodynamics properties of noble metal clusters Au_N, Ag_N, Cu_N, and Pt_N (N = 80, 106, 140, 180, 216, 256, 312, 360, 408, 500, 628, 736, and 864) are simulated by micro-canonical molecular dynamics simulation technique. The potential energy and heat capacities change with temperature are obtained. The results reveal that the phase transition temperature of big noble metal clusters (N ? 312 for Au, 180 for Ag and Cu, and 360 for Pt) increases linearly with the atom number slowly and approaches gently to bulk crystals. This phenomenon indicates that clusters are intermediate between single atoms and molecules and bulk crystals. But for the small noble clusters, the phase transition temperature changes irregularly with the atom number due to surface effect. All noble metal clusters have negative heat capacity around the solid-liquid phase transition temperature, and hysteresis in the melting/freezing circle is derived in noble metal clusters.     

Thermodynamic properties of noble metal clusters:molecular dynamics simulation

The thermodynamics properties of noble metal clusters Au_N, Ag_N, Cu_N, and Pt_N (N = 80, 106, 140, 180, 216, 256, 312, 360, 408, 500, 628, 736, and 864) are simulated by micro-canonical molecular dynamics simulation technique. The potential energy and heat capacities change with temperature are obtained. The results reveal that the phase transition temperature of big noble metal clusters (N ⩾ 312 for Au, 180 for Ag and Cu, and 360 for Pt) increases linearly with the atom number slowly and approaches gently to bulk crystals. This phenomenon indicates that clusters are intermediate between single atoms and molecules and bulk crystals. But for the small noble clusters, the phase transition temperature changes irregularly with the atom number due to surface effect. All noble metal clusters have negative heat capacity around the solid-liquid phase transition temperature, and hysteresis in the melting/freezing circle is derived in noble metal clusters.     

Thermodynamic properties and self-diffusion coefficients of simple liquids

Pair distribution functions and thermodynamic properties for a system of 108 particles interacting with a hard-sphere modified Lennard-Jones (12, 6) potential have been calculated by the Monte Carlo technique for a range of reduced liquid densities and two reduced temperatures. It is shown that the introduction of a hard core significantly modifies the Lennard-Jones parameters for the noble gases. The friction coefficients proposed in the transport theories of Rice and Allnatt and of Palyvos and Davis have been calculated using the exact pair distribution functions. In the normal liquid range, contributions to the frictional force on a particle arising from hard sphere collisions and quasi-Brownian motion are shown always to be comparable. This result is contrary to the conditions under which the principal assumption of Brownian theory is applicable.     

Numerical simulation of the generation of magnetic inhomogeneities in ferromagnets with inhomogeneous parameters

The generation and evolution of magnetic inhomogeneities of the stationary breather type, which appear in a flat layer with the magnetic anisotropy and exchange interaction parameters that are different from those in the bulk of an infinite ferromagnet after transmission through a 180° domain wall, have been investigated theoretically. The dependences of the amplitude and frequency of vibrations on the parameters of the defect have been constructed for the revealed magnetic inhomogeneities, and the ranges of the parameters determining the possibility of their existence have been found.     

Thermodynamic behavior of the system benzene-ethylene dichloride

The Maron theory of nonelectrolyte solutions has been used to analyze the thermodynamic behavior of the system benzene-ethylene dichloride. Excellent agreement with experiment has been obtained for all the thermodynamic properties involved. The analysis also shows that the interaction parameter for this system is small and positive, but it varies sharply with both concentration and temperature. As a result the system is nonideal and complex in behavior, despite the fact that vapor pressure data would tend to suggest near ideality.     

Thermodynamic perturbation theory for simple polar fluids,I

The application of thermodynamic perturbation theory to the computation of the properties of simple polar fluids is considered. The Helmholtz free energy of a fluid of molecules interacting via a Stockmayer potential v ^S is computed through fourth order in μ, where μ is the dipole moment. Numerical results are obtained on the basis of both the ‘exact’ Monte-Carlo computations for a Lennard-Jones system and the Verlet-Weis perturbation-theoretic computations for that system. The results obtained on the latter basis are then compared with results for a fluid of molecules interacting via a central-force potential v ^ES that is conformal with the Lennard-Jones 6–12 potential and equivalent to the Stockmayer potential through order μ⁴. To facilitate the comparison the v ^ES results are computed according to the Verlet-Weis method. The v ^ES results and the fourth-order (in μ) v ^S results constitute two different approximations to the thermodynamics of the Stockmayer potential; the compressibility factors as well as the free energies of the two approximations are compared.

It is concluded that the thermodynamic contribution of the dipole term of v ^S is significant over the whole liquid region when μ²=εσ³, where ε and σ are the usual Lennard-Jones parameters. For this μ, the two approximations we consider give results in close agreement.     

Thermodynamic perturbation theory for simple polar fluids. II

The study of polar fluids begun in a previous paper is continued. Calculations for the Stockmayer potential are extended to include the term of order μ⁶, where μ is the dipole moment. The effects of higher-order terms are then approximated by means of a simple Padé extrapolation procedure, and the liquid-gas coexistence curve is located in this approximation. An orientation-independent but temperature-dependent potential that is thermodynamically equivalent to an arbitrary orientation-dependent potential is introduced and used to assess the lowest-order thermodynamic effects that result from the presence of quadrupole and octupole terms in the pair-potential. Several values of quadrupole and octupole moments representative of a dipolar molecule (HCl) as well as linear molecules (N₂, O₂ and CO₂, for which μ=0) are considered.     

NMR analysis of ferromagnets: Fe oxides

A short historical review is given on internal field NMR of ferromagnets, illustrated with recent pulsed NMR spectra of the elemental ferromagnets Fe, Co and Ni and the Fe-oxides magnetite, maghemite and hematite, which, with the exception of maghemite, have resonance frequencies first reported over 45 years ago. Since the magnetic hyperfine field at the nucleus is not known a priori, the original search frequency motivations are discussed along with the mechanisms for the initially much larger than expected (～10³) NMR signals that were observed. The ⁵⁷Fe spectra of the three principal Fe-oxide ferromagnets, magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃), obtained here under uniform spectroscopic conditions, are then discussed in more detail, with a focus on the influence of particle size and vacancy content on the hyperfine fields     

Crossover behavior of the anomalous Hall effect and anomalous nernst effect in itinerant ferromagnets

The anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) are experimentally investigated in a variety of ferromagnetic metals including pure transition metals, oxides, and chalcogenides, whose resistivities range over 5 orders of magnitude. For these ferromagnets, the transverse conductivity sigma{xy} versus the longitudinal conductivity sigma{xx} shows a crossover behavior with three distinct regimes in accordance qualitatively with a recent unified theory of the intrinsic and extrinsic AHE. We also found that the transverse Peltier coefficient alpha{xy} for the ANE obeys the Mott rule. These results offer a coherent and semiquantitative understanding of the AHE and ANE to an issue of controversy for many decades.