Dynamic compensation temperature in the kinetic spin-1 Ising model in an oscillating external magnetic field on alternate layers of a hexagonal lattice

The dynamic behavior of a two-sublattice spin-1 Ising model with a crystal-field interaction (D) in the presence of a time-varying magnetic field on a hexagonal lattice is studied by using the Glauber-type stochastic dynamics. The lattice is formed by alternate layers of spins σ=1 and S=1. For this spin arrangement, any spin at one lattice site has two nearest-neighbor spins on the same sublattice, and four on the other sublattice. The intersublattice interaction is antiferromagnetic. We employ the Glauber transition rates to construct the mean-field dynamical equations. Firstly, we study time variations of the average magnetizations in order to find the phases in the system, and the temperature dependence of the average magnetizations in a period, which is also called the dynamic magnetizations, to obtain the dynamic phase transition (DPT) points as well as to characterize the nature (continuous and discontinuous) of transitions. Then, the behavior of the total dynamic magnetization as a function of the temperature is investigated to find the types of the compensation behavior. Dynamic phase diagrams are calculated for both DPT points and dynamic compensation effect. Phase diagrams contain the paramagnetic (p) and antiferromagnetic (af) phases, the p+af and nm+p mixed phases, nm is the non-magnetic phase, and the compensation temperature or the L-type behavior that strongly depend on the interaction parameters. For D<2.835 and H₀>3.8275, H₀ is the magnetic field amplitude, the compensation effect does not appear in the system.     

Nonequilibrium phase transition in the kinetic Ising model on a two-layer square lattice under the presence of an oscillating field

The nonequilibrium or dynamic phase transitions are studied, within a mean-field approach, in the kinetic Ising model on a two-layer square lattice consisting of spin- 1/2 ions in the presence of a time varying (sinusoidal) magnetic field has been studied by using Glauber-type stochastic dynamics. The dynamic equations of motion are obtained in terms of the intralayer coupling constants J₁ and J₂ for the first and second layer, respectively, and interlayer coupling constant J₃ between these two layers. The nature (first- or second-order) of the transitions is characterized by investigating the behavior of the thermal variations of the dynamic order parameters. The dynamic phase transitions are obtained and the dynamic phase diagrams are constructed in the plane of the reduced temperature versus the amplitude of the magnetic field and found fourteen fundamental types of phase diagrams. Phase diagrams exhibit one, two or three dynamic tricritical points for various values of J₂/|J₁| and J₃/|J₁|. Besides the paramagnetic (p), ferromagnetic (f) and compensated (c) phases, there were the f+c,f+sf,c+sf,af+p,m+p,f+m and c+af, where the af, sf and m are the antiferromagnetic, surface ferromagnetic and mixed phases respectively. Coexistence phase regions also exist in the system.     

Dynamical mean-field theory of a simplified double-exchange model

Simplified double-exchange model including transfer of the itinerant electrons with spin parallel to the localized spin in the same site and the indirect interaction J of kinetic type between localized spins is comprihensively investigated. The model is exactly solved in infinite dimensions. The exact equations describing the main ordered phases (ferromagnetic and antiferromagnetic) are obtained for the Bethe lattice with (z is the coordination number) in analytical form. The exact expression for the generalized paramagnetic susceptibility of the localized-spin subsystem is also obtained in analytical form. It is shown that temperature dependence of the uniform and the staggered susceptibilities has deviation from Curie-Weiss law. Dependence of Curie and Néel temperatures on itinerant-electron concentration is discussed to study instability conditions of the paramagnetic phase. Anomalous temperature behaviour of the chemical potential, the thermopower and the specific heat is investigated near the Curie point. It is found for J=0 that the system is unstable towards temperature phase separation between ferromagnetic and paramagnetic states. A phase separation connected with antiferromagnetic and the paramagnetic phases can occur only at . Zero-temperature phase diagram including the phase separation between ferromagnetic and antiferromagnetic states is given. Received 28 May 1999 and Received in final form 14 July 1999     

Phase diagrams of a nonequilibrium mixed spin-3/2 and spin-2 Ising system in an oscillating magnetic field

The phase diagrams of the nonequilibrium mixed spin-3/2 and spin-2 Ising ferrimagnetic system on square lattice under a time-dependent external magnetic field are presented by using the Glauber-type stochastic dynamics. The model system consists of two interpenetrating sublattices of spins σ=3/2 and S=2, and we take only nearest-neighbor interactions between pairs of spins. The system is in contact with a heat bath at absolute temperature T_abs and the exchange of energy with the heat bath occurs via one-spin flip of the Glauber dynamics. First, we investigate the time variations of average order parameters to find the phases in the system and then the thermal behavior of the dynamic order parameters to obtain the dynamic phase transition (DPT) points as well as to characterize the nature (first- or second-order) phase transitions. The dynamic phase diagrams are presented in two different planes. Phase diagrams contain paramagnetic (p), ferrimagnetic (i₁, i₂, i₃) phases, and three coexistence or mixed phase regions, namely i₁+p, i₂+p and i₃+p mixed phases that strongly depend on interaction parameters.     

Percolation on patchwise heterogeneous surfaces under equilibrium conditions

The site percolation problem on square lattices whose sites are grouped in two types of energetically different patches is studied. Several lattices formed by collections of either randomly or orderly localized and no overlapped patches of different sizes are generated. The system is characterized by two parameters, namely, the size of each patch, l, and the energy difference between the two kind of sites, ΔE. Particles are adsorbed at equilibrium on the lattice. The critical coverage is determined by means of Monte Carlo simulations and finite-size scaling analysis. The percolative behavior of the system as a function of the parameters characterizing the heterogeneity of the energetic surface topography is presented and discussed.     

On the magnetic phase diagram of (Mn,Fe)WO4 wolframite

The magnetic phase diagram of mixed crystals of type (Mn, Fe)WO₄ is discusses within molecular field approximation. The two species of magnetic ions tend to align in different orderings. Depending on the interaction strengths the two ordered phases are separated either by a phase in which both orderings are superimposed or by a first order transition.     

New types of short-range orientational ordering in ortho-para hydrogen monolayers

Orientational ordering in monolayers of solid hydrogen is discussed in view of recent experimental findings in NMR studies of (ortho)_c-(para)_1−c-hydrogen mixtures on boron nitride substrate reported for ortho-H₂ concentrations 0.35≤c≤0.92 and temperatures 0.14≤T≤1.80 K. Analysis of the temperature-concentration behavior for the observed NMR frequency splitting is given on the basis of a two-dimension (J=1)_c-(J=0)_1−c-rotor model for which the fundamental parameters, quadrupolar coupling constant Γ₀=0.50±0.03 K and the crystal-field amplitude V₀=0.70±0.10 K, are derived from experiment. The two distinct para-rotational (PR) short-range ordered structures driven by positive and negative crystalline fields are described in terms of the local alignment and orientation of the polar principal axis. It is shown that they are rather different from the ferromagnetic-type ordering suggested earlier by Harris and Berlinsky as a unique PR structure. The common quadrupolar glass and a new hindered rotor phases found below the 2D site percolation threshold c_p=0.72 are also discussed.     

Disorder-order phase transition induced by size effect in bulk La2/3Sr1/3MnO3

Bulk La_2/3Sr_1/3MnO₃ ceramic samples prepared by thermal decomposition are investigated using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). An abnormal phenomenon, where three kinds of La_2/3Sr_1/3MnO₃ phases with different structures and the same composition coexist in the same grain, has been observed. Besides the stable rhombohedral majority phase, the two other phases are a simple cubic structure with a=0.389 nm and a new hexagonal structure with a=0.544 nm, c=0.668 nm. The simple cubic phase is a residual phase of high-temperature due to the size effect and bondage of twin boundary. Image simulations have suggested that the new hexagonal phase is the La-Sr ordered structure with space group , which is converted from the disordered simple cubic phase. The formation mechanism of the ordered phase is explained from volume energy and interface energy considerations.     

Effect of Y-doping on the magnetic and charge orderings in Nd2/3Ca1/3MnO3

The shifts of the magnetic and charge ordering transition temperatures caused by Nd substitution for Y in Nd_2/3Ca_1/3MnO₃ CMR narrow-band perovskite manganite have been studied. At low temperatures, three different long-range magnetic orderings consistent with a phase separation scenario have been observed in the doped compound (Nd_0.9Y_0.1)_2/3Ca_1/3MnO₃ by neutron-diffraction study: the antiferromagnetic orderings of PCE and DE types existing below ∼110 and ∼60 K, respectively, and the ferromagnetic one of B type existing below ∼42. Magnetic phase transformations temperatures as well as those of charge ordering have been found to be structural-dependent. Y-doping leads to the decrease of the anisotropy of the orthorhombic Pnma crystal lattice b/√2c, which causes a decrease of the indirect exchange parameters in the system and thus a decrease in the magnetic transformation temperatures for 20-30 K in the doped compound. Doping leads as well to the higher level of the coherent Jahn-Teller distortions of the MnO₆ octahedra in the 200-300 K temperature region, which results in the increase of the charge ordering temperature for ∼80 K.     

Structural and magnetic ordering in the two-dimensional coordination polymer Co(ox)(bpy-d8), (ox=C2O4, bpy-d8=4,4′-bipyridine-d8)

Neutron diffraction and magnetic susceptibility studies of the two-dimensional coordination polymer Co(ox)(bpy-d8) are presented, where ox=C₂O₄²⁻ and bpy-d8=4,4′-bipyridine-d8 (fully deuterated). The neutron powder diffraction data reveal a second-order crystallographic phase transition at 290 K. Above 290 K, a disordered structure, space group Immm, is observed that is closely related to the ordered structure previously proposed on the basis of single crystal X-ray diffraction. At low temperatures, the structure is an ordered variant of the high-temperature phase with space group I222. In both phases, the Co ions are linked by the oxalate forming infinite chains that are crosslinked by the bpy ligands.The magnetic susceptibility follows qualitatively a quasi one-dimensional chain behavior. It exhibits a broad maximum around 35 K, corresponding to a strong antiferromagnetic coupling through the oxalate bridges. A kink at 9 K marks the onset of long-range antiferromagnetic ordering due to much weaker interchain magnetic interactions.The magnetically ordered structure determined from the low-temperature neutron diffraction data can be described with the propagation vector (1/2, 1/2, 1/2), i.e. a doubling of the unit cell in each principal direction. It is concluded that a significant antiferromagnetic interaction is mediated through the bpy ligands, although the Co-Co distance along these bridges is 11.4 Å.     

Spontaneous-search method and short-time dynamics: applications to the Domany-Kinzel cellular automaton

The one-dimensional Domany-Kinzel cellular automaton is investigated by two numerical approaches: (i) the spontaneous-search method, which is a method appropriated for a search of criticality; (ii) short-time dynamics. Both critical frontiers of the system are investigated, namely, the one separating the frozen and active phases, as well as the critical line determined by damage spreading between two cellular automata, that splits the active phase into the nonchaotic and chaotic phases. The efficiency of the spontaneous-search method is established herein through a precise estimate of both critical frontiers, and in addition to that, it is shown that this method may also be used in the determination of the critical exponent ν_⊥. Using the critical frontiers obtained, other exponents are estimated through short-time dynamics. It is verified that the critical exponents of both critical frontiers fall in the universality class of directed percolation.     

Quantum-mechanically induced asymmetry in the phase diagrams of spin-glass systems

The spin-1/2 quantum Heisenberg spin-glass system is studied in all spatial dimensions d by renormalization-group theory. Strongly asymmetric phase diagrams in temperature and antiferromagnetic bond probability p are obtained in dimensions d>or=3. The asymmetry at high temperatures approaching the pure ferromagnetic and antiferromagnetic systems disappears as d is increased. However, the asymmetry at low but finite temperatures remains in all dimensions, with the antiferromagnetic phase receding from the ferromagnetic phase. A finite-temperature second-order phase boundary directly between the ferromagnetic and antiferromagnetic phases occurs in d>or=6, resulting in a new multicritical point. In d=3, 4, 5, a paramagnetic phase reaching zero temperature intervenes asymmetrically between the ferromagnetic and reentrant antiferromagnetic phases. There is no spin-glass phase in any dimension.     

Symmetry properties of the phase of coexistence of superconductivity and antiferromagnetism in 2D systems with strong electron correlations

Using the diagram technique for Hubbard operators, the effect of quasi-two-dimensionality and hybridization of the 4f electrons of cerium ions and p electrons of indium ions on the properties of the antiferromagnetic, superconducting, and mixed phases in heavy-fermion intermetallic compounds of cerium is studied. It is shown that taking into account quasi-two-dimensionality, low-energy hybridization processes renormalize the antiferromagnetic and superconducting order parameters in the broken time-reversal symmetry phase. Estimates of the critical temperatures of antiferromagnetic ordering and Cooper instability, obtained by the developed approach, are in good agreement with experimental data for cerium-based intermetallic compounds.     

Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2

Low-temperature specific-heat measurements on YbRh₂Si₂ at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements.     

Phase Diagram and Structure of the Ground State of the Antiferromagnetic Ising Model on a Body-Centered Cubic Lattice

The Monte Carlo method has been used to study phase transitions and the structure of the ground state of the antiferromagnetic Ising model on a body-centered cubic lattice taking into account the interactions of nearest and next nearest neighbors. All possible magnetic structures of the ground state have been obtained for the first time as a function of the ratio of exchange interactions r. It is shown that six different orderings in the ground state are possible in the system as a function of the r value. The phase diagram of the dependence of the critical temperature on the interaction of the next nearest neighbors is constructed. For the first time, a narrow region (2/3 < r ≤ 0.75) is found in the diagram where the transition from the antiferromagnetic phase to the paramagnetic phase occurs as a first-order phase transition. It is shown that the competition between exchange interactions at the value r = 2/3 does not lead to the frustration and degeneracy of the ground state.     

Spin glass behavior upon diluting frustrated magnets and spin liquids: a Bethe-Peierls treatment

A Bethe-Peierls treatment to dilution in frustrated magnets and spin liquids is given. A spin glass phase is present at low temperatures and close to the percolation point as soon as frustration takes a finite value in the dilute magnet model; the spin glass phase is reentrant inside the ferromagnetic phase. An extension of the model is given, in which the spin glass/ferromagnet phase boundary is shown not to reenter inside the ferromagnetic phase asymptotically close to the tricritical point whereas it has a turning point at lower temperatures. We conjecture similar phase diagrams to exist in finite dimensional models not constraint by a Nishimori's line. We increase frustration to study the effect of dilution in a spin liquid state. This provides a “minimal” ordering by disorder from an Ising paramagnet to an Ising spin glass. Received 9 April 1999 and Received in final form 27 September 1999     

Investigations of the first order magnetic phase transition in dysprosium

The first order magnetic phase transition in polycrystalline sample of dysprosium (Dy) has been investigated in detail by dc-resistance vs. temperature (R-T) measurements. Distinct signature of coexistence of two magnetically ordered phases (i.e. ferromagnetic and antiferromagnetic), originating as a result of the metastable behavior associated with the first order phase transition (FOPT) is found near 90 K. By successfully recording the minor hysteresis loops within the hysteretic R-T regime of Dy, a varying degree of phase coexistence in the FOPT is demonstrated.     

Charge orderings in the atomic limit of the extended Hubbard model

The extended Hubbard model in the atomic limit (AL-EHM) on a square lattice with periodic boundary conditions is studied with use of the Monte Carlo (MC) method. Within the grand canonical ensemble the phase and order-order boundaries for charge orderings are obtained. The phase diagrams include three types of charge ordered phases and the nonordered phase. The system exhibits very rich structure and shows unusual multicritical behavior. In the limiting case of t_ij=0, the EHM is equivalent to the pseudospin model with single-ion anisotropy , exchange interaction W in an effective magnetic field . This classical spin model is analyzed using the MC method for the canonical ensemble. The phase diagram is compared with the known results for the Blume-Capel model.     

Phase diagram of lithium-doped copper oxide, Cu1−xLixO

Heat capacities of lithium-doped samples of CuO have been measured below room temperature by adiabatic calorimetry. The antiferromagnetic ordering transition to incommensurately modulated state was detected as a step in the heat capacity. Its concentration dependence was compatible with existing reports based on Li-NMR. The incommensurate-commensurate transition of lithium-doped copper oxide was clearly detected for the first time. The magnetic phase diagram of Cu_1−xLi_xO was thus constructed. The suppression of both transition temperatures by the Li doping is nearly twice as strong as that expected from mean-field and percolation theories.     

Charge and lattice dynamics of ordered state in La1/2Ca1/2MnO3: infrared reflection spectroscopy study

We report an infrared reflection spectroscopy study of La_1/2Ca_1/2MnO₃ over a broad frequency range and temperature interval which covers the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. The structural phase transition, accompanied by a ferromagnetic ordering at T_C=234 K, leads to enrichment of the phonon spectrum. A charge ordered antiferromagnetic insulating ground state develops below the Néel transition temperature T_N=163 K. This is evidenced by the formation of charge density waves and opening of a gap with the magnitude of 2Δ₀=(320±15) cm⁻¹ in the excitation spectrum. Several of the infrared active phonons are found to exhibit anomalous frequency softening. The experimental data suggest coexistence of ferromagnetic and antiferromangetic phases at low temperatures.