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Heisenberg Model (heisenberg + model)

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Physics and Astronomy	100%

Selected Abstracts

Reentrant phenomenon in the exactly solvable mixed spin-1/2 and spin-1 Ising,Heisenberg model on diamond-like decorated planar lattices

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2010
Lucia, anová
Abstract Ground-state and finite-temperature behaviour of the mixed spin-1/2 and spin-1 Ising,Heisenberg model on decorated planar lattices consisting of inter-connected diamonds is investigated by means of the generalised decoration,iteration mapping transformation. The obtained exact results clearly point out that this model has a rather complex ground state composed of two unusual quantum phases, which is valid regardless of the lattice topology as well as the spatial dimensionality of the investigated system. It is shown that the diamond-like decorated planar lattices with a sufficiently high coordination number may exhibit a striking critical behaviour including reentrant phase transitions with two or three consecutive critical points. [source]

Cooling-field dependence of exchange bias and asymmetric reversal modes in a nanoparticles system with ferromagnetic core and antiferromagnetic matrix morphology

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2009
Yong Hu
Abstract Interest in exchange bias (EB) in magnetic nanoparticles has increased in the past few years by virtue of its potential for application in fields such as ultrahigh-density magnetic recording. A modified Monte Carlo Metropolis method is performed to simulate the effect of cooling field on EB and asymmetric reversal modes of a granular system of ferromagnetic (FM) nanoparticles embedded in an antiferromagnetic (AFM) matrix, based on three-dimensional classical Heisenberg model. The results show that the EB first decreases slightly due to the energy barriers in the antiferromagnet, while the coercivity and vertical magnetization shift increase with the increase of cooling field, finally, they all level off as the cooling field is strong enough. Whereas the cooling- and measuring-field angular dependence of asymmetric reversal modes reveal asymmetric reversal mechanism and interesting rotation process of FM spins, confirming the existence of the net magnetization on the surface of AFM matrix. The reason may be due to the energy competition and geometric frustration of system. However, the strong interfacial coupling may change the intrinsic atomic configuration of antiferromagnet to influence the EB and reversal modes. [source]

Thermodynamic properties of the Ising and Heisenberg S = 1 ferromagnet with biquadratic exchange and uniaxial anisotropy

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2006
M. Manojlovi
Abstract We study the thermodynamic properties of S = 1 Ising and Heisenberg ferromagnets with both bilinear and biquadratic exchange and/or uniaxial anisotropy of both easy axis/plane character. Using the mean-field (MF) approximation we evaluated the free energy enabling us to study in detail the behavior of the order parameters and the dependence of the critical point on the anisotropy. We show that in the presence of biquadratic interaction, there is a difference in the behavior between the Ising and the Heisenberg model even in the MF approximation, which is not the case for the bilinear interaction. Combining the equations of motion for Green's functions with identities particular to S = 1, we managed to perform the random phase approximation without the decoupling of the operators at the same site, avoiding the peculiarities of the Callen,Anderson decoupling. This allowed us to improve the phase diagram for the Heisenberg model. The important result is the demonstration of the effect of anisotropy to nonvanishing of the quadrupolar order parameter at the Curie point. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Quantum and thermal effects in the double exchange ferromagnet

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003
N. Shannon
Abstract The physics of the ferromagnetic phase of the "double exchange" model has been widely discussed in the context of the CMR manganites. Usually, the double exchange ferromagnet is treated is classically, by mapping it onto an effective Heisenberg model. However this mapping does not permit a correct treatment of quantum or thermal fluctuation effects, and the results obtained lack many of the interesting features seen in experiments on the manganites. Here we outline a new analytic approach to systematically evaluating quantum and thermal corrections to the magnetic and electronic properties of the double exchange ferromagnet. [source]

Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro- and antiferromagnets

ANNALEN DER PHYSIK, Issue 10-11 2009
B. Vogt
Abstract The physical properties of quantum systems, which are described by the anisotropic Heisenberg model, are influenced by thermal as well as by quantum fluctuations. Such a quantum Heisenberg system can be profoundly changed towards a classical system by tuning two parameters, namely the total spin and the anisotropy field: Large easy-axis anisotropy fields, which drive the system towards the classical Ising model, as well as large spin quantum numbers suppress the quantum fluctuations and lead to a classical limit. We elucidate the incipience of this reduction of quantum fluctuations. In order to illustrate the resulting effects we determine the critical temperatures for ferro- and antiferromagnets and the ground state sublattice magnetization for antiferromagnets. The outcome depends on the dimension, the spin quantum number and the anisotropy field and is studied for a widespread range of these parameters. We compare the results obtained by: Classical Mean Field, Quantum Mean Field, Linear Spin Wave and Random Phase Approximation. Our findings are confirmed and quantitatively improved by numerical Quantum Monte Carlo simulations. The differences between the ferromagnet and antiferromagnet are investigated. We finally find a comprehensive picture of the classical trends and elucidate the suppression of quantum fluctuations in anisotropic spin systems. In particular, we find that the quantum fluctuations are extraordinarily sensitive to the presence of small anisotropy fields. This sensitivity can be quantified by introducing an "anisotropy susceptibility". [source]

Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro- and antiferromagnets

Rigorous solution of a Hubbard model extended by nearest-neighbour Coulomb and isotropic exchange interaction on a triangle and tetrahedron

ANNALEN DER PHYSIK, Issue 4 2009
R. Schumann
In connection with the study of the extended Hubbard model on an isosceles triangular cluster, we discovered a factor two error in the coding of the Ising part of the Heisenberg term in the Hamiltonian. In consequence the rigorous results remain valid, but not for the isotropic Heisenberg model, but for an anisotropic Heisenberg model. [source]

Effect of pure phase decoherence on entanglement teleportation

ANNALEN DER PHYSIK, Issue 5 2008
M. Ge
Abstract We study entanglement teleportation in the two-qubit XX Heisenberg model with pure phase decoherence taken into account. For some initial entangled states, pure phase decoherence has no effect on the teleported entanglement, while for others pure phase decoherence has a pronounced effect. In addition, entanglement sudden death happens in the latter case. [source]