Magnetic phase diagram of spinel spin-glasses

The article by Villain [Z. Phys. B — Condensed Matter33, 31 (1979)] is discussed and a modified magnetic phase diagram is suggested for the spinel system (AB₂O₄) in which theA andB sites are partially (or completely) occupied by magnetic atoms. This diagram takes into account the antiferromagnetic exchange interactionsJ _AA,J _BB andJ _AB between nearest neighbor cations of various types. Regions of paramagnetic, antiferromagnetic, ferrimagnetic and possible spin glass behaviour are indicated on the diagram.Supported by the National Science Foundation under Grant ISP-80-11451     

Modulated Phase of a Potts Model with Competing Binary Interactions on a Cayley Tree

We study the phase diagram for Potts model on a Cayley tree with competing nearest-neighbor interactions J ₁, prolonged next-nearest-neighbor interactions J _p and one-level next-nearest-neighbor interactions J _o . Vannimenus proved that the phase diagram of Ising model with J _o =0 contains a modulated phase, as found for similar models on periodic lattices, but the multicritical Lifshitz point is at zero temperature. Later Mariz et al. generalized this result for Ising model with J _o ≠0 and recently Ganikhodjaev et al. proved similar result for the three-state Potts model with J _o =0. We consider Potts model with J _o ≠0 and show that for some values of J _o the multicritical Lifshitz point be at non-zero temperature. We also prove that as soon as the same-level interactionJ _o is nonzero, the paramagnetic phase found at high temperatures for J _o =0 disappears, while Ising model does not obtain such property. To perform this study, an iterative scheme similar to that appearing in real space renormalization group frameworks is established; it recovers, as particular case, previous work by Ganikhodjaev et al. for J _o =0. At vanishing temperature, the phase diagram is fully determined for all values and signs of J ₁,J _p and J _o . At finite temperatures several interesting features are exhibited for typical values of J _o /J ₁.     

Magnetic and non-magnetic ground states of the Kondo lattice

We use the variational method to investigate the ground state phase diagram of the Kondo lattice Hamiltonian for arbitraryJ/W, and conduction electron concentrationn _c (J is the Kondo coupling andW the bandwidth). We are particularly interested in the question under which circumstances the globally singlet (collective Kondo) Fermi liquid type ground state becomes unstable against magnetic ordering. For the collective Kondo singlet we use the lattice generalization of Yosida's wavefunction which implies the existence of a large Fermi volume, in accordance with Luttinger's theorem. Using the Gutzwiller approximation, we derive closed-form results for the ground state energy at arbitraryJ/W andn _c, and for the Kondo gap atn _c=1. We introduce simple trial states to describe ferromagnetic, antiferromagnetic, and spiral ordering in the small-J (RKKY) regime, and Nagaoka type ferromagnetism at largeJ/W. We study three particular cases: a band with a constant density of states, and the (tight binding) linear chain, and square lattice periodic Kondo models. We find that the lattice enhancement of the Kondo effect, which is described in our theory of the Fermi liquid state, pushes the RKKY-to-nonmagnetic phase boundary to much smaller values ofJ/W than it was previously thought. In our study of the square lattice case, we also find a region of itinerant, Nagaoka-type ferromagnetism at largeJ/W forn _c 1/3.     

Bethe-ansatz solution of a model for a mixed valent impurity with two magnetic configurations: II. Numerical solution of the thermodynamic equations

A mixed valence impurity with two magnetic configurations of total angular momentumJ ₂ andJ ₁=J ₂+1/2, respectively, coupled by conduction electrons with total angular momentum 1/2 via a hybridization matrix element is considered. The thermodynamic Bethe-ansatz equations derived previously are solved numerically for various values ofJ ₂. Thef-level occupation, the entropy, the magnetic susceptibility and the specific heat are obtained as a function of temperature for variousf-level positions. The magnetic field dependence is also discussed in the limit of integer valence (exchange model).Supported by the CONICET, Argentina     

Chaotic properties of multipoint correlation functions of an Ising model with long-range interactions on the Sierpiński—Gasket lattice

A class of multispin correlation functions of an Ising model with ferromagnetic nearest neighbor interactionsK and constant (distance-independent) long-range interactionsQ ₁=Q,l=1,2,..., on the Sierpiski-gasket lattice is considered. Using an exact method for calculating thermodynamic functions of hierarchically constructed Ising systems, it is shown that, for a set of values ofQ and for almost all values ofK, someM _k-spin correlation functions, whereM _k=3 ^k +3 withk=1,2,...,n andn=1,2,... being the order of lattice construction, change chaotically asn, k, and therebyM _k increase to infinity. Accordingly, in the thermodynamic limit, these correlation functions prove to be nonanalytic for appropriate values ofQ andK. SinceM _k-point correlation functions withk being finite, i.e., correlation functions involving finite numbers of spins, remain analytic asn tends to infinity, there is a smooth crossover between analytic properties of correlation functions of the two types.     

Critical and Compensation Temperatures of the Ising Bilayer System Consisting of Spin-1/2 and Spin-1 Atoms

The critical and compensation temperatures of the bilayer Bethe lattices with one of the layers having only spin-1/2 atoms and the other having only spin-1 atoms placed symmetrically are studied by using exact recursion relations in a pairwise approach. The Hamiltonian of the model consist of the bilinear intralayer coupling constants of the two layers J ₁ and J ₂ for the interactions of the atoms in layers with spin-1/2 and spin-1, respectively, and the bilinear interlayer coupling constant J ₃ between the adjacent atoms with spin-1/2 and spin-1 of the layers. After obtaining the ground state phase diagram with J ₁ > 0, the variations of the order-parameters and the free energy are investigated to obtain the phase diagram of the model by considering only the ferromagnetic ordering of the layers, i.e. J ₁ > 0 and J ₂ > 0, and ferromagnetic or antiferromagnetic ordering of the adjacent spins of the layers, J ₃ > 0 or J ₃ < 0, respectively. It was found that the system presents both second- and first-order phase transitions and, tricritical points. The compensation temperatures was also observed for the appropriate values of the system parameters. PACS: 05.50.+q 05.70.Fh 64.60.Cn 75.10.Hk     

Phase diagrams of competing quadruple and binary interactions on Cayley tree-like lattice: Triangular Chandelier

We study the phase diagrams for the Ising model on a Cayley tree-like lattice, called Triangular Chandelier, with competing nearest-neighbour interactions J₁, prolonged next-nearest-neighbour interactions J_p and one-level next-nearest-neighbour quadruple interactions J_l1. The phase diagrams display the multicritical points (the Lifshitz points) that are at nonzero temperature and many modulated phases. To perform this study, an iterative scheme similar to that appearing in real space renormalization group frameworks is established; it recovers, as particular case, previous work of Vannimenus extension result given by Ganikhodjaev and U?uz for k=3. At vanishing temperature, the phase diagram is fully determined for all values and signs of J₁,J_p and J_l1. At finite temperatures several interesting features are exhibited for typical values of J_l1/J₁ and −J_p/J₁.     

A mean-field study of the temperature dependence of the layer magnetizations in a semi-infinite ising model

Summary We present a mean-field study of the layer magnetization of a semi-infinite Ising model with intra-surfaceJ _B and surface-bulkJ ₁ interaction modified with respect to the bulk exchange interactionJ. This allows to discuss the extent of the spatial inhomogeneity perpendicular to a free surface. By following the magnetization of each layer, we are able to see how the critical exponents evolve from the bulk to the surface values. Furthermore, we introduce a new characteristic temperature,T _em, where the surface magnetization departs from the bulk behaviour, as a possible candidate for the experimental determination ofJ ₈ andJ ₁. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Accurate and Precise Measurement of Heteronuclear Long-Range Couplings by a Gradient-Enhanced Two-Dimensional Multiple-Bond Correlation Experiment

We propose a phase-sensitive gradient-enhanced two-dimensional heteronuclear multiple-bond correlation (psge-2D HMBC) experiment for speedy, accurate, and precise measurement of²J_CHand³J_CH. The experiment does not suppress one-bond correlations. Rather, the value of a desired long-rangeJ_CHis obtained from the pertinent cross-peak pattern in the HMBC spectrum, using the corresponding¹J_CHcorrelation pattern as reference. The application of the proposed experiment is illustrated for the trisaccharide raffinose.     

Phase diagram of a simple model of Eu p Sr1−p S

A random site Ising model on the checkerboard square lattice with first neighbor interactionsJ in all first neighbor bonds and second neighbor interactionsJ in red squares is considered as a simple model of the dilute spin glass of Eu _p Sr_1–p S. The phase boundary between the paramagnetic and ferromagnetic phases, and that between the paramagnetic and spin glass phases are calculated. The obtained phase diagram is qualitatively similar to the experimental result by Maletta and Convert.     

Spin state and exchange in the quasi-one-dimensional antiferromagnet KFeS2

We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS₂. Measurements have been carried out to ascertain the spin state of Fe³⁺ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using aS=1/2 spin ground state for the Fe³⁺ ion. The features in the optical spectra have been assigned to transitions within thed-electron manifold of the Fe³⁺ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state atT _N=250 K. The intra and interchain exchange constants,J andJ′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, andχ _max,T _max, andT _N for a spin 1/2 one-dimensional chain. The values areJ=−440.71 K, andJ′=53.94 K. Using these values ofJ andJ′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used belowT _N. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infiniteS=1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS₂. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2.75×10⁻⁴ emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2.7×10⁻⁴ emu/mol) obtained from the experimental data.     

Magnetization and susceptibility of disordered alloy with competitive interactions

A model calculation of magnetization and susceptibility of disordered alloy (A _pB_{1 −p} ) where bothA andB represent the magnetic atoms is presented. It is based on the cluster-variational method where interactions within the clusters of all possible configurations are treated exactly and the rest of the interaction is replaced by an effective variational field. The frustration effect is introduced taking the exchange interactionsJ _AB orJ _BB or both to be antiferromagnetic whereas the exchangeJ _AA is ferromagnetic. The results are qualitatively similar to the observed behaviour of moment and susceptibility in some metallic glasses. The critical concentration for ferromagnetic state is determined in the presence of competitive interactions.     

Monte Carlo study of the antiferromagnetical Ising model on a centred honeycomb lattice

We use the Monte Carlo method to study an antiferromagnetical Ising spin system on a centred honeycomb lattice, which is composed of two kinds of 1/2 spin particles A and B. There exist two different bond energies J_A-A and J_A-B in this lattice. Our study is focused on how the ratio of J_A-B to J_A-A influences the critical behaviour of this system by analysing the physical quantities, such as the energy, the order parameter, the specific heat, susceptibility, {etc} each as a function of temperature for a given ratio of J_A-B to J_A-A. Using these results together with the finite-size scaling method, we obtain a phase diagram for the ratio J_A-B / J_A-A. This work is helpful for studying the phase transition problem of crystals composed of compounds.     

Measurement of Magnitude and Sign of Heteronuclear Coupling Constants in Transition Metal Complexes

Sets of specifically tailored E.COSY-type correlation experiments and double-quantum/zero-quantum (DQ/ZQ) experiments are presented which enable the determination of sign and size of small heteronuclear coupling constants across the metal center of transition metal complexes. For the octahedrally coordinated complexes, [Ru(TPM)(H)(CO)(PPh₃)]⁺[BF₄]⁻(1) and [Ir(TPM)(H)(CO)(CO₂CH₃)]⁺[BF₄]⁻(2), 14 of 15 and 15 of 15 possible two-bond scalar coupling constants across the metal center were measured, respectively, using¹⁵N and¹⁵N/¹³C enriched samples (TPM = tris(1-pyrazolyl)methane)). The reduced coupling constants²K_X-M-Y= 4π²²J/(hγ_Xγ_Y) were found to be positive when the coupled nuclei X and Y weretranswith respect to the metal center, and negative when the coupled nuclei were incisposition. The validity of this sign rule was verified forJ_CC,J_NN,J_PN,J_PC,J_CN,J_HP,J_HC, andJ_HNcouplings. Idiosyncracies associated with 2D NMR spectra for the sign determination of coupling constants with¹⁵N which lead to corrections for the signs ofJ_HN,J_PN, andJ_CNcouplings reported previously are discussed.     

The ground states of the classical heisenberg and planar models on the triangular and plane hexagonal lattices

The energies and the spin configurations of the ground states of the classical Heisenberg and classical planar (XY) models with first- and second-neighbor interactions on the triangular and plane hexagonal lattices are obtained. The phase diagrams in theJ ₁–J ₂ plane are determined, whereJ ₁ andJ ₂ are the coefficients of the first- and second-neighbor interactions, respectively. It is noted for the system on the plane hexagonal lattice, that an infinite degeneracy of the ground states occurs in some region of theJ ₁–J ₂ plane and then the study is made under an introduction of an infinitesimal third-neighbor interaction, removing the degeneracy.     

A simulation of the mixed spin 3–spin 3/2 ferrimagnetic Ising model

The mixed spin 3–spin 3/2 ferrimagnetic Ising model was simulated using cooling algorithm on cellular automaton (CA). The simulations were carried out in the intervals ?4 ≤ D_A/J ≤ 8 and ?4 ≤ D_B/J ≤ 8 for the square lattices with periodic boundary conditions. The ground-state phase diagram of the model has different types of ferrimagnetic phases. Although only the antiferromagnetic nearest-neighbor interaction was contained in the Hamiltonian, the compensation points emerged through D_A/J = 2 at kT/J = 0. The values of the critical exponents (ν, α , β and γ) were estimated within the framework of the finite-size scaling theory and power-law relations for the selected D_A/J values (?2, 0, 1, 2, and 4). The estimated critical exponent values were in good agreement with the universal values of the two-dimensional Ising model (ν = 1, α = α′ = 0, β = 0.125, β′ = 0.875 and γ = γ′ = 1.75).     

New theoretical analysis ofv 2 0→1Q-bands of ammonia

Summary The analysis ofQ-band infra-red transitionsv ₂0→1 of ammonia has been performed by using existing data and a simple polynomial expansion up to the tenth order ofJ andK. The lines withK=3 require an additionalJ-dependent term because of theK-splitting effect. In this manner, the spectrum has been reproduced within the accuracy of the experimental data. New experimental lines withJ≦20 have been reported or assigned. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

On equivalent-neighbour,random site models of disordered systems

Models of random systems whose Hamiltonian reads , where and _i ,=1,...,n are independent, identically distributed random variables are discussed.J _ij are assumed to be symmetric, with respect toJ ₀, random variables and also symmetric functions of components of . A question of dependence of a phase diagram on a probability distribution of is addressed. A class of distributions and interactionsJ _ij , which give rise to phase diagrams called typical is selected. Then a problem of obtaining typical phase diagrams, containing a certain region with an infinite number of pure phases, is studied.     

Continuum of compensation points in the mixed spin Ising ferrimagnet with four-spin interaction and next-nearest neighbor coupling

We investigate the magnetic properties of the mixed spin-1/2 and spin-1 Ising ferrimagnetic system with four-spin interaction J₄ and next-nearest neighbor (NNN) coupling J′. We perform exact ground-state calculations and use the finite cluster approximation, based on a single cluster theory, to derive the state equations for the two-dimensional square lattice. The main attention has been paid to the study of the phase diagram for both the transition and compensation temperatures. We find a number of characteristic behaviors. The model with only NNNs induces one compensation point while the four-spin interaction does not. The investigation of the model with both interactions shows a number of characteristic behaviors. In particular, the presence of the four-spin interaction, according to J₄ and J′, may lead to one, two or possibly a continuum of compensation points. This phenomenon may have important applications in technology such as thermomagnetic writing and erasing at the compensation point.     

Phase diagram of the Ising square lattice with competing interactions

We restudy the phase diagram of the 2D-Ising model with competing interactions J₁ on nearest neighbour and J₂ on next-nearest neighbour bonds via Monte-Carlo simulations. We present the finite temperature phase diagram and introduce computational methods which allow us to calculate transition temperatures close to the criticalpoint at J₂ = J₁/2. Further on we investigate the character of the different phase boundariesand find that the transition is weakly first order formoderate J₂ > J₁/2.