Spin coupling and magnetic field effects on the finite-size free energy and its non-extensivity for 1-D Ising model with nearest and next-nearest neighbor interactions in nanosystem

The effects of second-neighbor spin coupling interactions and a magnetic field are investigated on the free energies of a finite-size 1-D Ising model. For both ferromagnetic of nearest neighbor (NN) and next-nearest neighbor (NNN) spin coupling interactions, the finite-size free energy first increases and then approaches a constant value for any size of the spin chain. In contrast, when NNN and NN spin coupling interactions are antiferromagnetic and ferromagnetic, respectively, the finite-size free energy gradually decreases by increasing the competition factor and eventually vanishes for large values of it. When a magnetic field is applied, the finite-size free energy decreases with respect to the case of zero magnetic fields for both ferromagnetic and antiferromagnetic spin coupling interactions. Deviation of free energy per size for finite-size systems relative to the infinite system increases when the spin coupling interactions as well as the f parameter (the ratio of the magnetic field to NN spin coupling interaction) increase.     

Quantum XY Spin-Glass Model with Ferromagnetic Couplings and Plane Dzyaloshinski-Moriya interactions

In the generalized quantum replica symmetric approximation, a quantum XY spin-glass model with ferromagnetic coupling and planar Dzyaloshinski-Mariya (DM) interactions is investigated theoretically. Entropy and specific heat are calculated numerically as well as spin self-interactions and spin-glass order parameters for spin S = 1. It is found that the doublecusp of the specific heat, which corresponds to the collinear ferromagnetic and the paramagnetic ferromagnetic transitions respectively, .coincides completely due to DM-anisotropy interactions. Additionally, the local susceptibility and the spontaneous magnetization are evaluated numerically.     

Quantum HeisenbergS=1/2 spin glass with ferromagnetic coupling and random Dzyaloshinskii-Moriya interactions

By means of the generalized static replica symmetric spin glass theory, a quantum HeisenbergS=1/2 spin glass model with the infinite-ranged random Dzyaloshinskii-Moriya (DM) interaction and ferromagnetic coupling is investigated. The dependence of entropy, specific heat, susceptibility and the corresponding order parameters on temperature is studied numerically for different ferromagnetic interactions and fixed anisotropy. Two spin glass phases has been found including transverse and mixed spin glass phases. It has been shown that the local susceptibility exhibits double-cusp features for different ferromagnetic coupling (J ₀). Phase transition poins are found in the specific heat-temperature plane at various ferromagnetic coupling values. Additionally, the dependence of the spontaneous moment on temperature is calculated.     

Exact ground state of a spin ladder with a quantum phase transition

We introduce a spin ladder with Ising interactions along the legs and intrinsically frustrated Heisenberg-like ferromagnetic interactions on the rungs. The model is solved exactly in the subspaces relevant for the ground state by mapping to the quantum Ising model, and we show that a first order quantum phase transition separates the classical from quantum regime, with the spin correlations on the rungs being either ferromagnetic or antiferromagnetic, and different spin excitations in both regimes. The present case resembles the quantum phase transition found in the compass model in one and two dimensions.     

Electromagnetic waves reflected from the plate of a magnetic with a ferromagnetic spiral

The spectrum of coupled spin and electromagnetic waves is obtained for a magnetic with a ferromagnetic spiral structure determined by nonuniform exchange and relativistic interactions. It is shown that resonant interaction between spin and electromagnetic waves is possible. The electromagnetic wave reflectance from the plate of a magnetic with a ferromagnetic spiral is calculated for different spiral angles.     

具有三自旋相互作用XX模型纠缠特性研究

利用Concurrence判据,研究了具有三自旋相互作用的XX模型的纠缠特性;分别在铁磁和反铁磁模型中研究了三自旋相互作用J_2和温度T对两自旋纠缠度的影响.结果表明,三自旋相互作用J_2提高系统的两体纠缠度,但是提高程度会因最近邻自旋间发生铁磁、反铁磁相互作用而有所差异;并且J_2影响两自旋系统纠缠消失的临界温度T_C,T_C会随J_2的增大而减小.系统温度T影响两体纠缠度,随着温度的降低,纠缠度会得到提高.此外,分别在系统本征态和基态中研究了两自旋的纠缠度,求出了系统发生量子相变的量子临界点.     

Ground and excited states of spinor fermi gases in tight waveguides and the Lieb-Liniger-Heisenberg model

The ground and excited states of a one-dimensional (1D) spin-1/2 Fermi gas (SFG) with both attractive zero-range odd-wave interactions and repulsive zero-range even-wave interactions are mapped exactly to a 1D Lieb-Liniger-Heisenberg (LLH) model with delta-function repulsions depending on isotropic Heisenberg spin-spin interactions, such that the complete SFG and LLH energy spectra are identical. The ground state in the ferromagnetic phase is given exactly by the Lieb-Liniger (LL) Bethe ansatz, and that in the antiferromagnetic phase by a variational method combining Bethe ansatz solutions of the LL and 1D Heisenberg models. There are excitation branches corresponding to LL type I and II phonons and spin waves, the latter behaving quadratically for small wave numbers in the ferromagnetic phase and linearly in the antiferromagnetic phase.     

Spin glass models with ferromagnetically biased couplings on the Bethe lattice: analytic solutions and numerical simulations

We derive the zero-temperature phase diagram of spin glass models with a generic fraction of ferromagnetic interactions on the Bethe lattice. We use the cavity method at the level of one-step replica symmetry breaking (1RSB) and we find three phases: a replica-symmetric (RS) ferromagnetic one, a magnetized spin glass one (the so-called mixed phase), and an unmagnetized spin glass one. We are able to give analytic expressions for the critical point where the RS phase becomes unstable with respect to 1RSB solutions: we also clarify the mechanism inducing such a phase transition. Finally we compare our analytical results with the outcomes of a numerical algorithm especially designed for finding ground states in an efficient way, stressing weak points in the use of such numerical tools for discovering RSB effects. Some of the analytical results are given for generic connectivity.     

The Phase Diagram of a Spin Glass Model

A spin glass problem on a Cayley tree with ferromagnetic interactions is solved rigorously. Using a level-I large deviation argument together with the martingale approach used by Buffet, Patrick and Pulé,⁽¹⁾ explicit expressions for the free energy are derived in different regions of the phase diagram. It is found that there are four phases: a paramagnetic phase, a spin-glass phase, a ferromagnetic phase and a mixed phase. The nature of the phase diagram depends on the power with which the ferromagnetic term occurs in the Hamiltonian.     

A family of equilibrium states relevant to low temperature behavior of spin 1/2 classical ferromagnets. Breaking of translation symmetry

An analysis of a family of equilibrium states is performed which, combined with our previous work, allows to describe all translation invariant equilibrium states of spin 1/2 classical ferromagnetic systems with finite range interactions at low temperatures. A model is described with continuously many equilibrium states for low temperatures.     

Energy Level of Coupled Harmonic Oscillator Model Born of Heisenberg Ferromagetic Spin Chain Derived by Invariant Eigen-operator Method

Based on the coupled harmonic oscillator model, which is born of Heisenberg ferromagnetic spin chain with nearest-neighbor interactions, we adopt the invariant eigen-operator method to derive its energy level, which provides an alternate approach to dealing with the dynamics of spin chain.     

低温下铁磁和反铁磁体系的运动学相互作用

我们发展了处理低温下铁磁和反铁磁运动学相互作用的一个新方法。所有运动学相互作用都能用各级关联函数来表示。应用热力学微扰理论,导得低温下s=1/2的铁磁体系的确没有T³项。对反铁磁体系,我们发现运动学相互作用即使在低温下也十分大,它甚至超过动力学相互作用的修正,我们的理论适用于s≥1/2的一切体系。     

Magnetization dynamics of an integrable model of 3D ferromagnetic spin system

We formulate the dynamical equation of a 3 dimensional Heisenberg ferromagnetic (FM) spin system with bilinear and anisotropic interactions in the semiclassical limit. In the continuum limit the dynamics is found to be governed by a (3+1) dimensional nonlinear Schrödinger equation. We check the integrability of the dynamics by constructing Lax pair of operators. To express the nonlinear spin excitations in terms of magnetic soliton, we use Darboux transformation(DT) and Hirota bilinearization procedure .     

Quantized spin excitations in a ferromagnetic microstrip from microwave photovoltage measurements

Quantized spin excitations in a single ferromagnetic microstrip have been measured using the microwave photovoltage technique. Several kinds of spin wave modes due to different contributions of the dipole-dipole and the exchange interactions are observed. Among them are a series of distinct dipole-exchange spin wave modes, which allow us to determine precisely the subtle spin boundary condition. A comprehensive picture for quantized spin excitations in a ferromagnet with finite size is thereby established. The dispersions of the quantized spin wave modes have two different branches separated by the saturation magnetization.     

Biaxial magnetic structures in rare-earth compounds: GdMg and HoP

The flopside spin structure, where the magnetic moments form two sublattices which at low temperatures are mutually perpendicular was first found in HoP and then in other rare-earth pnictides. There are large orbital contributions to the magnetic moments of these compounds and it had been thought that they play an important role in stabilizing the flopside spin structure. However, recently this spin structure has been found in GdMg. As Gd³⁺ is an S-state ion, there are negligible orbital effects. We have developed a model Hamiltonian which is able to explain both the occurence of initially a ferromagnetic phase and then at low temperature the flopside spin structure in two very dissimilar compounds GdMg and HoP. For GdMg we find that the competition between the near neighbor ferromagnetic and antiferromagnetic bilinear exchange interactions is such that while they produce a transition to a ferromagnetic phase at 110 K, an unusually small amount of biquadratic (quadrupolar) coupling is able to stabilize a flopside phase at low temperature which is able to resist collapse in a field as large as 150 kOe. For HoP we find that although anisotropic bilinear pair interactions - as for example pseudo-dipole - exist, they cannot be the primary origin of the flopside phase; quadrupole pair interactions are essential to explain the appearance of first the ferromagnetic and then the flopside phases found in HoP. On the basis of our model calculations we are able to explain the data extant on these compounds and we make some predictions which are open to experimental verification.     

Ferromagnetic resonance investigation of collective phenomena in two-dimensional periodic arrays of Co particles

The ferromagnetic resonance (FMR) method is used to study the collective phenomena in two-dimensional periodic arrays of disk-shaped Co particles. A study of geometrically similar structures with different periods reveals a broadening of the FMR resonance lines due to the excitation of additional size-dependent non-uniform spin waves. It is shown that these collective spin-wave modes are based on dipole–dipole interactions between the ferromagnetic particles in the array. Qualitative and quantitative data on magnetic interparticle interactions can thus be obtained from FMR spectra for two-dimensional periodic arrays of ferromagnetic particles. PACS 73.21.-b, 75.75.+a, 76.50.+g     

Ferromagnetically coupled Shastry-Sutherland quantum spin singlets in (CuCl)LaNb₂O₇

A thorough crystal structure determination at very low temperature of (CuCl)LaNb?O?, originally proposed as a spin-1/2 square-lattice antiferromagnet, is reported thanks to the use of single-crystal x-ray diffraction and powder neutron diffraction. State-of-the-art calculations (maximum entropy method) reveal that (CuCl)LaNb?O? is orthorhombic with Pbam symmetry. First-principles calculations demonstrate that the dominant magnetic interactions are antiferromagnetic between fourth nearest neighbors with a Cu-Cl-Cl-Cu exchange path, which lead to the formation of spin singlets. The two strongest interactions between the singlets are ferromagnetic, which makes (CuCl)LaNb?O? the first system of ferromagnetically coupled Shastry-Sutherland quantum spin singlets.     

Spin-1 chain doped with mobile S = 1/2 fermions

We investigate the doping of a two-orbital chain with mobile S = 1/2 fermions as a valid model for Y2-xCaxBaNiO5. The S = 1 spins are stabilized by strong, ferromagnetic Hund's rule couplings. We calculate correlation functions and thermodynamic quantities by density matrix renormalization group methods and find a new hierarchy of energy scales in the spin sector upon doping. Gapless spin excitations are generated at a lower energy scale by interactions among itinerant polarons created by each hole and coexist with the larger scale of the gapful spin-liquid background of the S = 1 chain accompanied by a finite string order parameter.