Competing interactions in the ammonium halides

The indirect interaction of two ammonium tetrahedra via the polarizable halide ions in between is investigated. It tends to order the tetrahedra in an antiferromagnetic way, whereas the direct interaction favors the ferromagnetic state. A mechanism is suggested that reduces the antiferromagnetic interaction and thus can account for the ferromagnetic order in NH₄Cl. Strong fluctuations are expected along the [1,1,0]-directions in reciprocal space.     

A study of the effective magnetic anisotropy and the corresponding spin configurations in two dimensional ferromagnetic/antiferromagnetic system

The spin configurations of two dimensional ferromagnetic/antiferromagnetic system were investigated using model calculations and Monte-Carlo simulation methods. The lowest energy state was obtained under various coupling conditions to investigate the role of interfacial interaction on anisotropy. We found that the total ferromagnetic layer anisotropy is contributed not only from its own crystalline anisotropy but also from the antiferromagnetic layer spin flop effect. The overall ferromagnetic layer effective anisotropy is calculated as a function of the exchange energy of antiferromagnetic layer and the interfacial interaction energy. If the effective anisotropy from the spin flop effect is comparable with the crystalline anisotropy, the asymmetric spin configuration is generated. In this configuration, the magnetization direction of the ferromagnetic layer is neither perpendicular nor parallel to the antiferromagnetic spin direction. Temperature effect on the perpendicular-to-collinear coupling transition was also investigated using Monte-Carlo simulation, and the relationship between the effective anisotropy and the temperature was obtained.     

Phase Transitions in the Antiferromagnetic Heisenberg Model on a Body-Centered Cubic Lattice with Allowance for the Next-Nearest-Neighbor Interactions

Based on the replica exchange Monte Carlo algorithm and histogram analysis of data, the phase transitions in the three-dimensional antiferromagnetic Heisenberg model on a body-centered cubic lattice with allowance for the next-nearest-neighbor interaction are studied. The study is performed for the nextnearest- neighbor exchange interaction ratio of r = 1. It is established that, for this model, the transition from the antiferromagnetic to paramagnetic phase is a first-order phase transition.     

Effect of magnetoelectrical interactions on the multiferroic domain walls

The antiferromagnetic domain structure of a multiferroic has been investigated in the presence of a ferroelectric domain structure. It has been demonstrated that an inhomogeneous magnetoelectric (flexomagnetoelectric) interaction leads to pinning of antiferromagnetic domain walls at the walls of the ferroelectric domains and to a change in the structure of antiferromagnetic domain walls.     

A two-leg spin ladder model for fumarate-bridged chain-like polymer of Cu(II)

In this paper, the transfer matrix renormalization group method is used to study the thermodynamic and the magnetic properties of a novel one-dimensional fumarate-bridged Cu(II) chain by a two kinds of models. One simplified model is that of the dominant ferromagnetic and antiferromagnetic alternating chain, which is only in rough agreement with the experimental results. Further research shows a possible mechanism of alternating strong ferromagnetic and weak antiferromagnetic interaction among one chain. Considering the weak antiferromagnetic interaction existing in the actual materials, we propose a two-leg spin ladder model with intrachain ferromagnetic and weak interchain antiferromagnetic interaction. The obtained theoretical results are in quite good agreement with experimental curves, which indicates the two-leg spin ladder model is appropriate to describe the fumarate-bridged chain-like polymer of Cu(II).     

具有Dzyaloshinskii-Moriya相互作用的三量子比特海森伯模型中的对纠缠

研究了一维三量子比特海森伯模型中的对纠缠的提高和控制问题,在该系统中引入了Dzyaloshinskii-Moriya（DM)相互作用,通过求解共生来计算两量子比特之间的热纠缠,结果表明：对于XXX模型,引入DM相互作用D,可以诱导铁磁和反铁磁自旋链产生热纠缠,尽管它们产生热纠缠所需的D值大小不同.对于XXZ模型,引入DM相互作用后,可以使原本不存在热纠缠的反铁磁自旋链产生纠缠,而且对于铁磁和反铁磁这两种XXZ自旋链,DM相互作用和各向异 关键词： 纠缠 XXX模型')" href="#">XXX模型 XXZ模型')" href="#">XXZ模型 Dzyaloshinskii-Moriya相互作用     

Antiferromagnetism and the node structure of the superconducting order parameter of UPd Al

The node structure of the superconducting order parameter of the heavy-fermion system is analyzed within the weak-coupling theory. A pairing interaction induced by the exchange of antiferromagnetic spin excitations is assumed as suggested by recent inelastic neutron scattering experiments and tunneling spectroscopy. The multi-sheeted Fermi surface is taken into account. Based on a model susceptibility for the simple antiferromagnetic structure of , line nodes result at the rim of the magnetic Brillouin zone. Received 29 July 1999     

Entanglement and quantum phase transition in alternating XY spin chain with next-nearest neighbouring interactions

By using the method of density-matrix renormalization-group to solve the different spin spin correlation functions, the nearest-neighbouring entanglement （NNE） and the next-nearest-neighbouring entanglement （NNNE） of one-dimensional alternating Heisenberg XY spin chain are investigated in the presence of alternating the-nearestneighbouring interaction of exchange couplings, external magnetic fields and the next-nearest neighbouring interaction. For a dimerised ferromagnetic spin chain, the NNNE appears only above a critical dimerized interaction, meanwhile, the dimerized interaction a effects a quantum phase transition point and improves the NNNE to a large extent. We also study the effect of ferromagnetic or antiferromagnetic next-nearest neighbouring （NNN） interaction on the dynamics of NNE and NNNE. The ferromagnetic NNN interaction increases and shrinks the NNE below and above a critical frustrated interaction respectively, while the antiferromagnetic NNN interaction always reduces the NNE. The antiferromagnetic NNN interaction results in a large value of NNNE compared with the case where the NNN interaction is ferromagnetic.     

Effect of Lattice Distortion on Charge Order in Manganites at Doping x = 0.5

In the present paper, we continue our investigation on the antiferromagnetic origin of the charge order observed in the half-doped manganese. By introducing a Su-Schrieffer-Heeger (SSH) type of perturbation interaction to the double-exchange Hamiltonian, we calculate again its ground-state phase diagram at filling x=0.5 by the unrestricted real-space Hartree-Fock approximation method. We find that, as the SSH electron-phonon interaction increases, the charge order parameter decreases to zero rapidly but the CE-type antiferromagnetic order becomes more stable. In other words, the charge order is much more fragile than the CE-type or the Neel-type antiferromagnetic orders under the electron-phonon perturbation. These results support the proposed theory in the recent publications that the charge order in these systems is induced by the antiferromagnetic correlations.     

Cyclic period-3 window in antiferromagnetic potts and Ising models on recursive lattices

The magnetic properties of the antiferromagnetic Potts model with two-site interaction and the antiferromagnetic Ising model with three-site interaction on recursive lattices have been studied. A cyclic period-3 window has been revealed by the recurrence relation method in the antiferromagnetic Q-state Potts model on the Bethe lattice (at Q < 2) and in the antiferromagnetic Ising model with three-site interaction on the Husimi cactus. The Lyapunov exponents have been calculated, modulated phases and a chaotic regime in the cyclic period-3 window have been found for one-dimensional rational mappings determined the properties of these systems.     

Slave-Boson Mean-Field Theory of Spin- and Orbital- Ordered States in the Degenerate Hubbard Model

The mean-field theory with the use of the slave-boson functional method has been generalized to take account of the spin- and/or orbital-ordered state in the doubly degenerate Hubbard model. Numerical calculations are presented of the antiferromagnetic orbital-ordered state in the half-filled simple-cubic model. The orbital order in the present theory is much reduced compared with that in the Hartree–Fock approximation because of the large orbital fluctuations. From a comparison of the ground-state energy, the antiferromagnetic orbital state is shown to be unstable against the antiferromagnetic spin state, although the situation becomes reversed when the exchange interaction is negative.     

On the antiferromagnetic resonance in superconductors: Influence of indirect exchange interaction

The special features of the antiferromagnetic resonance in metals in presence of a superconducting ordering are analyzed on the basis of the surface impedance. The indirect exchange interaction determines the shift of the resonace frequency and the spectrum of a new undamped mode near ω_r. A parallel is drawn bettween the antiferromagnetic resonances in normal and superconducting states.     

Spin-phonon coupling induced frustration in the exactly solved spin-1/2 Ising model on a decorated planar lattice

Spin-1/2 Ising model with a spin-phonon coupling on decorated planar lattices partially amenable to lattice vibrations is examined using the decoration-iteration transformation and harmonic approximation. It is shown that the magnetoelastic coupling gives rise to an effective antiferromagnetic next-nearest-neighbour interaction, which competes with the nearest-neighbour interaction and is responsible for a frustration of decorating spins. A strong enough spin-phonon coupling consequently leads to an appearance of striking partially ordered and partially disordered phase, where a perfect antiferromagnetic alignment of nodal spins is accompanied with a complete disorder of decorating spins. Thermal dependences of the specific heat are explored in detail.     

A new manganese-based single-molecule magnet with a record-high antiferromagnetic phase transition temperature

We perform both dc and ac magnetic measurements on the single crystal of Mn30（Et-sao）3（C104）（MeOH）3 single- molecule magnet （SMM） when the sample is preserved in air for different durations. We find that, during the oxidation process, the sample develops into another SMM with a smaller anisotropy energy barrier and a stronger antiferromagnetic intermolecular exchange interaction. The antiferromagnetic transition temperature observed at 6.65 K in the new SMM is record-high for the antiferromagnetic phase transition in all the known SMMs. Compared to the original SMM, the only apparent change for the new SMM is that each molecule has lost three methyl groups as revealed by four-circle x-ray diffraction （XRD）, which is thought to be the origin of the stronger antiferromagnetic intermolecular exchange interaction.     

Competition between ferromagnetic and antiferromagnetic spin ordering in nuclear matter

The possibility of ferromagnetic and antiferromagnetic phase transitions in symmetric nuclear matter is analyzed in Fermi liquid theory with the Skyrme effective interaction. The density dependence of the ferromagnetic and antiferromagnetic parameters of spin polarization at zero temperature is obtained for SkM* and SGII effective potentials. In the density region where both solutions of self-consistency equations exist, the ferromagnetic spin state is preferable over the antiferromagnetic spin state.     

Strings in charge-transfer Mott insulators: Effects of lattice vibrations and the Coulomb interaction

Applying the canonical transformation with the 1/λ perturbation expansion in the nonadiabatic and intermediate regime and the discrete generalization of Pekar’s continuous nonlinear equation in the extreme adiabatic regime, we show that there are no strings in narrow-band ionic insulators due to the Fröhlich electron-phonon interaction alone. The multipolaron system is a homogeneous state in a wide range of physically interesting parameters, no matter how strong the correlations are. At the same time, the Fröhlich interaction allows the antiferromagnetic interactions and/or short-range electron-phonon interactions to form short strings in doped antiferromagnetic insulators if the static dielectric constant is large enough.     

Magnetic susceptibility of the compound (CH2)10(NH3)2FeCl4

The magnetic susceptibility of the compound (CH₂)₁₀(NH₃)₂FeCl₄ is measured in the temperature range from liquid nitrogen temperature up to room temperature. The effect of thermal and magnetic history on the data obtained is also discussed. It is shown that the compound is antiferromagnetic with a Néel temperature of 93 K but it appears that the antiferromagnetic intra-layer exchange interaction co-exists with a weak ferromagnetic interlayer interaction.     

Antiferromagnetic edge states in carbon nanotubes with hydrogen line defect abundancy

We have investigated the antiferromagnetic edge states in carbon nanotubes with hydrogen line defects by using the density functional theory calculations. As the hydrogen line defects increase, the exchange energy gain stabilizing the antiferromagnetic edge states increases in each graphenic ribbon produced by the line defects, indicating that the antiferromagnetic edge states can be realized at high temperatures regardless of the nanotube size. The exchange energy gain in each ribbon is determined by the ribbon width of the flat ribbon and apparently by the curvature of the curved ribbon. The exchange interaction between the ribbons is seen to be negligibly small even in the presence of a nonmagnetic inter-ribbon interaction that is sensitive to the ribbon width.     

The determination of the ground-state energy of an antiferromagnetic lattice by means of a renormalization procedure

An approximate value for the ground-state energy of an antiferromagnetic lattice of spins one-half is determined by means of a repeated renormalization procedure in which the lattice is divided into cells with an effective interaction. This effective interaction is determined on the basis of the spin-hamiltonian formalism.     

Influence of two-particle excited states on the interatomic exchange interaction in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The effective spin Hamiltonian for undoped cuprates is constructed in the framework of the realistic multiband p-d model with the parameters calculated from first principles. The exchange interaction parameter is defined as the sum of the antiferromagnetic and ferromagnetic contributions, which are determined by the two-hole triplet terms. The ferromagnetic and antiferromagnetic contributions of the excited terms compensate each other to a large extent. It is shown that the antiferromagnetic contribution of the two-hole ground singlet ¹ A _1g to the exchange interaction is dominant. Original Russian Text. V.A. Gavrichkov, S.G. Ovchinnikov, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 6, pp. 1037–1041.