On the Induction of the First-Order Phase Magnetic Transitions by Acoustic Vibrations in MnSi

The main result of the paper contains the conclusion that the magnetic phase transition in MnSi always remains first order at any temperature and magnetic field. In these aims, a model of coupling of an order parameter with other degrees of freedom is used. The coupling of magnetic order parameters with long-wave acoustic phonons, in the presence of the nonsingular parts of the bulk and shear moduli, a first-order transition occurs, participle near the transition the heat capacity and the compressibility remain finite, if the heat capacity becomes infinite in the system disregarding the acoustic phonons. The role of the Frenkel heterophase fluctuations is discussed. The impurity effect shows that, for some phases, the heat capacity of the system remains continuous and finite at the transition point. It is supposed that the transition is progressively smoothed by these fluctuations at the application of the magnetic field.     

Theory for phase transitions in insulating V2O3.

We show that the recently proposed S = 2 bond model with orbital degrees of freedom for insulating V2O3 not only explains the anomalous magnetic ordering but also other mysteries of the magnetic phase transition. The model contains an additional orbital degree of freedom that exhibits a zero temperature quantum phase transition in the Ising universality class.     

Structural phase transition in pyrene - how does it occur?

The phase transition in pyrene crystal has been studied by a continuous differential optical transmittance measurement. The result suggests that the transition around 120 K is of the first order structural transition and occurs domain by domain which are formed temporarily in the crystal. In some crystals the phase transition does not occur around 120 K but only a variation of the lattice constants occurs.     

First order magnetic transition in doped CeFe2 alloys: phase coexistence and metastability

First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped CeFe2 alloys is studied with the micro-Hall probe technique. Clear visual evidence of magnetic phase coexistence on micrometer scales and the evolution of this phase coexistence as a function of temperature, magnetic field, and time across the first order FM-AFM transition is presented. Such phase coexistence and metastability arise as a natural consequence of an intrinsic disorder-influenced first order transition. The generality of these phenomena involving other classes of materials is discussed.     

Evidence for a magnetic collapse in the epsilon phase of solid oxygen

Solid oxygen is the only elementary molecular magnet. Under the very high pressure of 96 GPa oxygen transforms into a metal and a superconductor. Theory predicts a nonmagnetic state occurring before the transition into the superconducting xi phase. Nevertheless, until now there was no direct evidence of a magnetic collapse in high-pressure oxygen. For the first time direct information is provided on magnetic properties of the epsilon phase, which is sandwiched between the antiferromagnetic delta phase and the superconducting xi phase. We used magnetic neutron diffraction. The data show that the long-range magnetic order disappears at the delta-epsilon transition. The magnetic collapse occurs at P approximately equal to 8 GPa, far below the pressure of the insulator-metal (superconductor) transition. The collapse is preceded by a decrease in temperature of transition towards the long-range magnetically ordered state (T(LRO)) in the delta phase, at P = 7.6 GPa.     

Magnetic Phase Transitions in Nanoclusters and Nanostructures

New phenomena – the first order magnetic phase transitions were observed in nanoclusters and nanostructures. For isolated ferrihydrite nanoclusters (d ～ 1–2 nm) in porous materials, for α-,γ-Fe₂O₃ nanoclusters (d ～ 20–50 nm) and for composites of nanostructured metallic Eu with additives of α-, γ-Fe₂O₃ nanoclusters and adamantane the critical temperatures (T _C, T _N) and magnetic cluster critical sizes (R _cr) were determined by means of thermodynamic models and Mössbauer spectroscopy. The first order magnetic phase transitions (jump-like) proceed by such a way when magnetization and magnetic order disappear by jump without superparamagnetic relaxation. According to thermodynamic model predictions the cluster and interface defects were suggested to play the main role in magnetic behavior. Thus, for the defective α-, γ-Fe₂O₃ nanoclusters, at R ≤ R _cr, the presence of the first order (jump-like) magnetic phase transition was described in terms of magnetic critical size of cluster. The action of high pressure (up to 2 GPa) with shear (120–240°) was effective for defect generation and nanostructure formation. For nanosystems including iron oxide nanoclusters, adamantane and metallic europium and subjected to shear stress under high pressure loading the critical value of defect density was estimated by the study of the character of magnetic phase transition. First-to-second-order (nanostructured metallic Eu) and second-to-first-order (α-, γ-ferric oxide nanoclusters) changes of the character of magnetic phase transition were shown to accompany by the variation of critical temperatures compared to the corresponding bulk values.     

Magnetic bubble domain phase at the spin reorientation transition of ultrathin Fe/Ni/Cu(001) film

Magnetic domain phases of ultrathin Fe/Ni/Cu(001) are studied using photoemission electron microscopy at the spin reorientation transition (SRT). We observe a new magnetic phase of bubble domains within a narrow SRT region after applying a nearly in-plane magnetic field pulse to the sample. By applying the magnetic field pulse along different directions, we find that the bubble domain phase exists only if the magnetic field direction is less than approximately 10 degrees relative to the sample surface. A temperature dependent measurement shows that the bubble domain phase becomes unstable above 370 K.     

Lattice transformation in the superconductivity ZrV2 by neutron diffraction

The 120 K phase transition in ZrV₂ has been investigated for magnetic and crystallographic effects using neutron diffraction. A cubic-rhombohedral martensitic transformation was observed at 116.7 K. No local magnetic moments, either ordered or disordered, were found to exist between 5.4 K and 300 K.     

Microcanonical Analysis on a System with Long-Range Interactions

We study a long-range interacting spin chain placed in a staggered magnetic field using microcanonical approach and obtain the global phase diagram. We find that this model exhibits both first order phase transition and second order phase transition separated by a tricritical point, and temperature jump can be observed in the first order phase transition.     

Elastic anomalies near a surface-induced phase transition

An analysis is made of the influence of elastic degrees of freedom on a surface-induced phase transition. It is found that elastic interactions give rise to a nonlinear correction to the boundary conditions for the order parameter, with the result that the surface transition is of the second order despite the fact that a first order phase transition occurs in the bulk. The propagation of Rayleigh waves near such a transition is discussed.     

Monopoles and vortices in Yang-Mills plasma

We discuss the role of magnetic degrees of freedom in Yang-Mills plasma at temperatures above and of order of the critical temperature T _c . While at zero temperature the magnetic degrees of freedom are condensed and electric degrees of freedom are confined, at the point of the phase transition both magnetic and electric degrees of freedom are released into the thermal vacuum. This phenomenon might explain the observed unusual properties of the plasma.     

Observation of a second order magnetic phase transition in CsFeS2

Mössbauer experiments performed on CsFeS₂ at temperatures between 4.2 K and 300 K show that the orthorhombic high temperature phase undergoes a second order magnetic phase transition near 69 K, when the previously reported first order magnetic and structural transition to a triclinic modification near 75 K is suppressed by lattice defects or internal stresses. The saturation values of the hyperfine fields are 19.1 T for the triclinic and 15.5 and 14.1 T for the orthorhombic phase.     

On the possibility of direct measurement of the magnetic field-induced phase transition in hematite by means of magnetooptical method

It is shown that a rotation ? and a deformation κ of the optical indicatrix appear during the transverse magnetic field-induced phase transition in hematite. Analytic expressions for ? and κ are deduced from the magnetization-dependent electromagnetic energy in the crystal. It is shown that during the phase transition, induced by increasing the temperature, the electromagnetic energy in the crystal. It is shown that during the phase transition, induced by increasing the temperature, the antiferromagnetic vector L = M₁ - M₂ is rotating from the three-fold C₃ axis toward the basal plane, which implies that the main axis of the optical indicatrix is not aligned in a general case with the magnetic field or the crystallographic axis although the magnetic moment (M₁ + M₂) is always parallel to the field. The linear magnetic birefringence is very sensitive to the magnetic phase in hematite, as described in previous experimental work, but the present analysis shows that a direct determination of the transverse field-induced phase transition can be obtained in hematite, by means of a magnetooptical method, only when large and non-uniform rotation (up to ninety degrees) and variation of the shape of the indicatrix are taken into account.     

Gelation as a rod-glass transition

A sol-gel transition of rod-like macromolecules in solution is viewed as a rod-glass transition, similar to the spin-glass transition in dilute magnetic systems. A molecular field theory for the order parameters is presented. Varying the disorder parameter an ordered nematic-like phase is found for zero disorder, exhibiting first order transition, while a rod-glass second order transition is found for maximal disorder. A remanent nematic-like order always accompanies the glass phase, unlike the case of magnetic spin-glass.     

Evidence of a canted magnetic state in self-doped LaMnO(3+δ) (δ?=?0.04): a magnetocaloric study

We report a detailed investigation of the magnetocaloric properties of self-doped polycrystalline LaMnO(3+δ) with δ?=?0.04. Due to the self-doping effect, the system exhibits a magnetic transition from a paramagnetic to ferromagnetic-like canted magnetic state (CMS) at ～120?K, which is associated with an appreciably large magnetocaloric effect (MCE). The CMS is an inhomogeneous magnetic phase developing due to a steady growth of antiferromagnetic correlation in its predominant ferromagnetic state below ～120?K. The stabilization of CMS in this material is concluded from a comprehensive analysis of magnetocaloric data using Landau theory, which is in excellent agreement with our neutron diffraction study. The magnetic entropy change versus temperature curves for different applied fields collapse into a single curve, revealing a universal behavior of MCE. Our studies suggest that investigation of MCE is an effective technique to acquire fundamental understanding about the basic magnetic structure of a system with complex competing interactions.     

Structural transformation induced by magnetic field and "colossal-like" magnetoresistance response above 313 K in MnAs

MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin metal hexagonal phase to a paramagnetic, lower-spin insulator orthorhombic phase at T(C)=313 K. Here, we report the results of neutron diffraction experiments showing that an external magnetic field, B, stabilizes the hexagonal phase above T(C). The phase transformation is reversible and constitutes the first demonstration of a bond-breaking transition induced by a magnetic field. The field-induced phase transition is accompanied by an enhanced magnetoresistance of about 17% at 310 K. The phenomenon appears to be similar to that of the colossal magnetoresistance response observed in the Mn [corrected] perovskite family.     

Effective scalar field theory for the electroweak phase transition

We investigate an effective model for the finite-temperature symmetry-restoration phase transition of the electroweak theory. It is obtained by dimensional reduction of the (3 + 1)-dimensional full theory and by subsequent integration over all static gauge degrees of freedom. The resulting theory corresponds to a 3-dimensional O(4) ferromagnet containing cubic and quartic terms of the field in its potential function. Possible nonperturbative effects of a magnetic screening mass are parametrically included in the potential. We analyse the theory using mean-field and numerical Monte Carlo (MC) simulation methods. At the value of the physical Higgs mass, m_H = 37 GeV, considered in the present investigation, we find a discontinuous symmetry-restoring phase transition. We determine the critical temperature, order parameter jump, interface tension and latent heat characteristics of the transition. The Monte Carlo results indicate a somewhat weaker first-order phase transition as compared to the mean-field treatment, demonstrating that non-perturbative fluctuations of the Higgs field are relevant. This effect is especially important for the interface tension. Any observation of hard first-order transition could result only from non-perturbative effects related to the gauge degrees of freedom.     

Singular renormalization group transformations and first order phase transitions (II). Monte Carlo renormalization group results

The first order phase transitions in the two-dimensional 10-state Potts model and in the two-dimensional Ising model with magnetic field are studied with Monte Carlo renormalization group methods. The deconfining phase transition of the four-dimensional U(1) lattice gauge theory is treated similarly. The results are not consistent with the standard discontinuity fixed point picture of first order phase transitions. In the U(1) case, where this possibility exists, they are not consistent with a second order phase transition either. The results show a discontinuous flow on the first order transition surface, which is a Monte Carlo renormalization group signal of singular RG transformations.     

Dimer to monomer phase transition in alkali-metal fullerides: magnetic susceptibility changes

Ab initio calculations have been employed to investigate the peculiar change in magnetic property (from diamagnetic to paramagnetic) of the dianionic C60-dimer phase in a rapidly cooled AC60 samples ( A: alkali metal). We first note that the triplet state of (C60)-22 which was never considered previously is nearly degenerate with the singlet state, and the transition barrier between the two states is reasonably small. This explains the susceptibility increase with an increase in temperature and the magnetic phase transition in the process of the dimer to monomer phase transition.     

Simulation of the (p,T) phase diagram of the temperature-driven metamagnet α-FeRh

We perform finite-pressure Monte Carlo simulations of an effective spin-analogous model with coupled magnetic and lattice degrees of freedom, which has previously been proposed in order to explain the anomalous temperature driven metamagnetic phase transition in α-FeRh. The results are in reasonable agreement with the experimental (p,?T) phase diagram. The critical behaviour of the system along the transition lines is analysed in detail.