Magnetostriction in the vicinity of spin-reorientation phase transitions in singlecrystal DyFe11Ti

The field, temperature, and angular dependences of longitudinal λ_‖ and transverse λ_⊥ magnetostriction in single-crystal DyFe₁₁Ti are investigated. Tensometric measurements were made in the temperature range from 78 to 300 K in magnetic fields up to 12 kOe. Measurements of the magnetostriction of single-crystal DyFe₁₁Ti, YFe₁₁Ti, and LuFe₁₁Ti imply that the sublattice of 3d transition metals makes only a small contribution to the magnetostriction in compounds RFe₁₁Ti, and that the primary contribution to the magnetostriction of these compounds comes from the rare-earth metal sublattice. The primary microscopic mechanism for magnetostriction is single-ion magnetostriction caused by the interaction of the anisotropic orbital electron cloud around the Dy³⁺ magnetic ion with the crystal field of the lattice. Fiz. Tverd. Tela (St. Petersburg) 41, 1647–1649 (September 1999)     

Magnetostriction studies of magnetic phase transitions in the copper metaborate CuB2O4

The field dependences of the longitudinal and transversal magnetostriction of the copper metaborate CuB₂O₄ were measured at various temperatures below the Néel point in magnetic fields directed along the tetragonal axis or in the basal plane. Magnetostriction was found to exhibit jumps at magnetic-field-induced phase transitions to a commensurate weak ferromagnetic state, as well as to grow smoothly in fields above and below the critical level. The magnetostriction observed in a magnetic field directed along the tetragonal axis is shown to be primarily caused by volume dilatation of the crystal. The experimental data obtained were used to construct the magnetic phase diagram of copper metaborate magnetized along the tetragonal axis.     

Absence of second-order phase transitions in the Dobrushin uniqueness region

The Dobrushin uniqueness theorem assures that in a very large class of high-temperature classical statistical mechanical lattice models with short or long range, many-body interactions, and arbitrary compact spin space there are no first-order phase transitions. It will be shown that for the same class of interactions there are also no second-order phase transitions.Research partially supported by the National Science Foundation under Grant MCS 78-00688.     

Thermoelectric power, magnetoresistance of lead chalcogenides in the region of phase transitions under pressure

The longitudinal and transverse thermomagnetic Nernst-Ettingshausen (N-E) effects were measured at ultrahigh pressure up to 20 GPa under closure of semiconductor gap at NaCl- and GeS-type phases of n-PbTe, p-PbSe and p-PbS. Near ∼3 GPa, the maxima of N-E effects and magnetoresistance (and hence of mobility of charge carriers) attributed to gapless state for PbTe and PbSe were established. The reversible sign inversion of transverse N-E effect indicating the change in scattering mechanism of charge carries have been revealed at high pressure phase of PbSe. The lowering of thermomagnetic effects with pressure gave the evidence of indirect semiconductor gap at high pressure GeS-type phases in contrary to NaCl-phases.     

Magnetostriction of rare earth-Fe2 laves phase compounds

Single crystal magnetostriction measurements were made as a function of temperature on TbFe₂ and DyFe₂. From these, the intrinsic magnetoelastic coupling coefficients were determined for the rare earth-Fe₂ compounds. Employing X-ray techniques, certain multicomponent rare earth-Fe₂ compounds were identified to maximize the magnetostriction to anisotropy ratio.     

Thermodynamic theory of phase transitions in a ferroelectric binary system with morphotropic region

A thermodynamic theory of phase transitions is developed for binary ferroelectric systems with a morphotropic phase region. Ah expansion of the thermodynamic potential of a perovskite-structure ferroelectric with coefficients that depend on the temperature and concentration yields equations for the isotherms for the tetragonal and rhombohedral phases, whose stability boundaries and phase transition points are determined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 26–29, February, 1981.     

Cosmic phase transitions

The sequence of phase transitions during the hot history of the universe is followed within a phenomenological framework. Particular emphasis is put on the QCD confinement transition, which is at reach under earth laboratory conditions. A tepid inflationary scenario on the GUT scale with bubble growth at moderate supercooling is discussed.     

On the order of itinerant antiferromagnetic phase transitions and superconducting phase transitions in an exchange field

The coefficients of the second and fourth order terms in a Landau expansion of the free energy are evaluated for an itinerant antiferromagnetic transition. The choice of spatial dependence is found to play an important role. A first order transition is obtained for a range of values of the band structure parameters. However if values appropriate to Cr are used it is found that the contribution from the shift in the Fermi level is sufficiently large to give second order transitions even when magnetostrictive effects are included. In the mathematically similar problem of a superconductor in an exchange field it is found that the transition from normal to superconducting states is first order near the triple point with an upper and possibly a lower critical point where the transition changes to second order.     

量子相变

量子相变是一种发生在绝对零度,由量子涨落而非热涨落导致的相变现象,满足著名的海森堡不确定关系。通过零温量子临界点的研究,可获知物质系统更广泛范围的行为,包括稀土磁性绝缘体,高温超导体和二维电子气体等。     

Two-dimensional electron crystal in magnetic field. Topological phase transitions and stability region.

The phase diagram of a 2-dim. electron system in the magnetic field $\text{H}$ is analyzed. We obtain ab initio the free energy F{U_iκ} of a deformed electron crystal when the long-range Coulomb forces and $\text{H}$ are taken into account. Using F{u_iκ} we find H-dependent interaction between the dislocations and the curve of the topological melting due to the dissociation of dislocation pairs. The topological melting curve for H → ∞ asimptotically approaches to the melting curve of a classical crystal. This result is obtained using Feynman variational technique as well. We examine also the electron liquid crystal phase and the stability region of a 2-dim. crystal at $\text{H}$ ≠ 0. The first-order phase transitions from the crystal and liquid crystal to the isotropic liquid are predicted to take place in a definite region of the phase diagram. We find the increase of the crystal and liquid crystal region on the T-n phase diagram with H increase. The various phenomenological stability criteria are ånaylzed.     

Structural phase transitions in ZnS

X-ray diffraction techniques have been used to investigate structural phase transitions in ZnS between 20 and 1200°C. These measurements imply that the transition from the cubic 3C structure to the hexagonal 2H structure is a first-order phase transition while transitions between the 2H, 4H, and the 6H(33) hexagonal structures were found to obey the symmetry rules of second-order phase transitions. Direct transitions from the cubic 3C structure to the 4 or 6H hexagonal structures are not observed.     

Pressure-induced phase transitions in gypsum

Abstract

We report high-pressure Raman scattering spectroscopy and energy dispersive X-ray diffraction investigations on gypsum, CaSO₄ · 2H₂O, at room temperature in a diamond cell. With increasing pressure, measurements indicate that CaSO₄ · 2H₂O undergoes two stages of crystalline-state phase transitions at 5 and 9 GPa, and then converts to a disordered phase above 11 GPa. The structures of the three high-pressure phases of gypsum have not been determined yet. These phases are tentatively named as “post-gypsum-I” (PG-I), “post-gypsum-II” (PG-II) and “disordered” according to the sequence of their appearance with pressure.

Gypsum shows anisotropic compressibility along three crystallographic axes with b > c > a below 5 GPa. The difference in the behavior of the two OH stretching modes in gypsum is attributed to the different reduction rate in the hydrogen bonding distances by the anisotropic axial compressibility.     

Crystal-glass phase transitions in the adatom layers

Phase transitions in the two-dimensional crystalline films adsorbed onto an imperfect substrate (with impurities and defects) are analyzed. It is shown that the phase transition from the commensurate long-range-order (LRO) crystal to the glass state occurs with increase of impurity concentration. The orientational correlation function is shown to tend algebraically to zero at large distances in the glass state. The influence of defects on the phase transition from the LRO-commensurate crystal to the disordered state at the definite temperature T_m is considered.     

Coarsening in fluid phase transitions

We review the understanding of the kinetics of fluid phase separation in various space dimensions. Morphological differences, percolating or disconnected domains, based on overall composition in a binary liquid or on density in a vapor–liquid system, are discussed. Depending upon the morphology, various possible mechanisms for domain growth are pointed out and discussions of corresponding theoretical predictions are provided. On the computational front, useful models and simulation methodologies are presented. Theoretically predicted growth laws have been tested via molecular dynamics simulations of vapor–liquid transitions. In the case of a disconnected structure, the mechanism has been confirmed directly.     

Singularities in first-order phase transitions

Several theories of phase transitions and their inter-relations have been criticized, focusing on the problem of whether z _c, the value of the fugacity corresponding to the point of condensation, is given by z _s, the smallest real positive singularity of the analytic function defined by the power series using volume-independent cluster integrals, or not. The present situation has been analysed and it is made clear that none of the existing theories can give the answer to this problem. Plausibility arguments for an affirmative or negative answer are discussed.     

Elastic phase transitions in plutonium

The rich phase diagram of plutonium with a large number of different transitions in a narrow temperature interval has been puzzling scientists for decades. We offer a theoretical proof that most of the structural transformations in plutonium at temperatures exceeding the Debye temperature are the elastic phase transitions. The proof is given in the framework of the Landau theory of phase transitions and space group theory taking into account the anomalously small value of the elastic shear constants related to tetragonal and orthorhombic lattice deformations.     

Isostructural phase transitions in solids

A model of electrons interacting with lattice vibrations is shown to exhibit an isostructural phase transition as a function of applied force by relating the Hamiltonian to that of an Ising model in magnetic field.     

Magnetic phase transitions in garnets

The phenomenological theory broadly applicable to magnetic transitions in ferrimagnetic garnets is discussed briefly. The experimental techniques, particularly nuclear magnetic resonance and Mössbauer effect spectroscopy, are then reviewed. Finally, there is a review of the results on specific garnets which undergo such transitions. Some remaining problems are pointed out.