Easy-axis Exchange Anisotropy Research Articles

Investigation of quantum and thermal fluctuations ferromagnetic spin chains with easy-plane anisotropy

One-dimensional ferromagnetic S = 1/2 and S = 1 spin chains with easy-plane and easy-axis exchange anisotropy are studied by the double-time Green’s function method. The XYZ model can be used to describe several chain systems as the ferromagnetic spin chain of (CHAB). Alternatively, ferromagnetic spin-spirals with Dzyaloshinsky–Moriya interaction. To study quantum and thermal fluctuations the magnetization and susceptibility are calculated and analyzed as a function of the external field, the strength of the anisotropy and the chain length.

Higgs Mode of Planar Coupled Spin Ladders and its Observation in C9H18N2CuBr4

Polarized inelastic neutron scattering experiments recently identified the amplitude (Higgs) mode in C_{9}H_{18}N_{2}CuBr_{4}, a two-dimensional near-quantum-critical spin-1/2 two-leg ladder compound, which exhibits a weak easy-axis exchange anisotropy. Here, we theoretically examine the dynamic spin structure factor of such planar coupled spin-ladder systems using large-scale quantum MonteCarlo simulations. This allows us to provide a quantitative account of the experimental neutron scattering data within a consistent quantum spin model. Moreover, we trace the details of the continuous evolution of the amplitude mode from a two-particle bound state of coupled ladders in the classical Ising limit all the way to the quantum spin-1/2 Heisenberg limit with fully restored SU(2) symmetry, where it gets overdamped by the two-magnon continuum in neutron scattering.

Continual approach at T=0 in the mean field theory of incommensurate magnetic states in the frustrated Heisenberg ferromagnet with an easy axis anisotropy

The algorithm of approximate solution was developed for the differential equation describing the anharmonical change of the spin orientation angle in the model of ferromagnet with the exchange competition between nearest and next nearest magnetic neighbors and the easy axis exchange anisotropy. The equation was obtained from the collinearity constraint on the discrete lattice. In the low anharmonicity approximation the equation is resulted to an autonomous form and is integrated in quadratures. The obvious dependence of the angle velocity and second derivative of angle from angle and initial condition was derived by expanding the first integral of the equation in the Taylor series in vicinity of initial condition. The ground state of the soliton solutions was calculated by a numerical minimization of the energy integral. The evaluation of the used approximation was made for a triple point of the phase diagram.

Refining the Spin Hamiltonian in the Spin-12Kagome Lattice AntiferromagnetZnCu3(OH)6Cl2Using Single Crystals

We report thermodynamic measurements of the S=1/2 kagome lattice antiferromagnet ZnCu3(OH)6Cl2, a promising candidate system with a spin-liquid ground state. Using single crystal samples, the magnetic susceptibility both perpendicular and parallel to the kagome plane has been measured. A small, temperature-dependent anisotropy has been observed, where χ(z)/χ(p)>1 at high temperatures and χ(z)/χ(p)<1 at low temperatures. Fits of the high-temperature data to a Curie-Weiss model also reveal an anisotropy. By comparing with theoretical calculations, the presence of a small easy-axis exchange anisotropy can be deduced as the primary perturbation to the dominant Heisenberg nearest neighbor interaction. These results have great bearing on the interpretation of theoretical calculations based on the kagome Heisenberg antiferromagnet model to the experiments on ZnCu3(OH)6Cl2.

Magnetic properties and exchange integrals of the frustrated chain cuprate linarite PbCuSO4(OH)2

We present a detailed study in the paramagnetic regime of the frustrated $s$ = 1/2 spin-compound linarite, PbCuSO$_4$(OH)$_2$, with competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions. Our data reveal highly anisotropic values for the saturation field along the crystallographic main directions, with $\sim$ 7.6, $\sim$ 10.5 and $\sim$ 8.5\,T for the $a$, $b$, and $c$ axes, respectively. In the paramagnetic regime, this behavior is explained mainly by the anisotropy of the \textit{g}-factor but leaving room for an easy-axis exchange anisotropy. Within the isotropic $J_1$-$J_2$ spin model our experimental data are described by various theoretical approaches yielding values for the exchange interactions $J_1$ $\sim$ -100\,K and $J_2$ $\sim$ 36\,K. These main intrachain exchange integrals are significantly larger as compared to the values derived in two previous studies in the literature and shift the frustration ratio $\alpha = J_2/|J_1|$ $\approx$ 0.36 of linarite closer to the 1D critical point at 0.25. Electron spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements further prove that the static susceptibility is dominated by the intrinsic spin susceptibility. The Knight shift as well as the broadening of the linewidth in ESR and NMR at elevated temperatures indicate a highly frustrated system with the onset of magnetic correlations far above the magnetic ordering temperature $T_\mathrm{N}$ = 2.75(5)\,K, in agreement with the calculated exchange constants.

The magnetic order of two-dimensional anisotropic antiferromagnets

We study the two-dimensional quantum Heisenberg antiferromagnet on the square lattice with easy-axis exchange anisotropy by means of Green’s function approach within random phase and Callen’s approximations. The Néel temperature TN, energy gap w0 and staggered magnetization m are calculated. The theoretical predictions of TN and w0 for K2NiF4, Rb2MnF4, K2MnF4, Rb2MnCl4 and (CH3NH3)2MnCl4 fit well to the measured values. The power law behavior of w(T)w(0)=β[m(T)m(0)]ν is also investigated. The exponents β and ν for K2NiF4 are in excellent agreement with the experimental results.

Comparative study between a two-dimensional anisotropic Heisenberg antiferromagnet with easy-axis single-ion anisotropy and one with easy-axis exchange anisotropy

Generally, in literature, easy-axis single ion anisotropy and easy-axis exchange anisotropy was treated in indistinct way. In this work we propose to perform a comparative study of the effects of these two easy-axis anisotropies on the behavior of the magnetization and the critical temperature ( T c ) in the 2D classical Heisenberg antiferromagnetic model. In order to study the low-temperature thermodynamics of this model, we should consider the contribution of anisotropic spin waves, using a self-consistent harmonic approximation (SCHA) theory. We compare the predictions of SCHA with numerical simulations on L × L square lattices using Monte Carlo (MC) simulations, which include effects due to all thermodynamically allowed excitations. Our SCHA results are in good agreement with our MC simulations results and have shown that the strong K limit gives two different Ising-like behavior. In the exchange anisotropic case, the dependence of T c on anisotropic parameter K becomes linear and in the single-ion anisotropic case, T c becomes independent of K . Also, using MC simulations and finite size scaling, we show that the critical exponents in the two anisotropic case are compatible with the 2D Ising values α = 0.125 and γ = 1.75 .

Monte Carlo study of the anisotropic three-dimensional Heisenberg model in a crystal field

We study the phase diagram of the three-dimensional classical ferromagnetic Heisenberg model with an easyplane crystalline anisotropy and an easy-axis exchange anisotropy through Monte Carlo simulations. We employ the Metropolis algorithm together with single-histogram techniques in order to characterize the transitions in each region of the phase diagram. Our results reveal, besides the disordered phase, the existence of Ising-like and XY-like ordered phases which are separated by a first-order transition line.

Pyrochlore photons: TheU(1)spin liquid in aS=12three-dimensional frustrated magnet

We study the S=1/2 Heisenberg antiferromagnet on the pyrochlore lattice in the limit of strong easy-axis exchange anisotropy. We find, using only standard techniques of degenerate perturbation theory, that the model has a U(1) gauge symmetry generated by certain local rotations about the z-axis in spin space. Upon addition of an extra local interaction in this and a related model with spins on a three-dimensional network of corner-sharing octahedra, we can write down the exact ground state wavefunction with no further approximations. Using the properties of the soluble point we show that these models enter the U(1) spin liquid phase, a novel fractionalized spin liquid with an emergent U(1) gauge structure. This phase supports gapped S^z = 1/2 spinons carrying the U(1) ``electric'' gauge charge, a gapped topological point defect or ``magnetic'' monopole, and a gapless ``photon,'' which in spin language is a gapless, linearly dispersing S^z = 0 collective mode. There are power-law spin correlations with a nontrivial angular dependence, as well as novel U(1) topological order. This state is stable to ALL zero-temperature perturbations and exists over a finite extent of the phase diagram. Using a convenient lattice version of electric-magnetic duality, we develop the effective description of the U(1) spin liquid and the adjacent soluble point in terms of Gaussian quantum electrodynamics and calculate a few of the universal properties. The resulting picture is confirmed by our numerical analysis of the soluble point wavefunction. Finally, we briefly discuss the prospects for understanding this physics in a wider range of models and for making contact with experiments.

Low-temperature static and dynamic behavior of the easy-axis Heisenberg antiferromagnet on the Kagome lattice

The antiferromagnetic Heisenberg model with easy-axis exchange anisotropy on the Kagome lattice is studied by means of Monte Carlo simulations. From equilibrium properties, we find that the values of the critical exponents associated with the magnetization at the critical temperature ${T}_{c}$ vary with the magnitude of the anisotropy. On the other hand, the spin-spin autocorrelation functions have a stretched exponential behavior with a power-law divergence of the relaxation time at a glasslike temperature ${T}_{g}\ensuremath{\sim}{T}_{c}.$ From nonequilibrium dynamics at a fixed temperature below ${T}_{g},$ aging effects are found which obey the same scaling laws as in spin glasses and polymers.

Topological and dynamical excitations in a classical 2D easy-axis Heisenberg model

The properties of dynamical solitons (magnon droplets) in the classical, two-dimensional anisotropic Heisenberg model with easy-axis exchange anisotropy are studied. The solution of the Landau-Lifshitz equation in the continuum limit for the soliton with topological charge q = 1 is obtained numerically using a shooting method. We analized a wide range of the anisotropy parameter and our results are in good agreement with results obtained from spin dynamics simulations. The dependence of an internal precession frequency of the soliton on both the anisotropy parameter and the radius of the soliton is also investigated. Finally, the limits of applicability of the continuum approach are discussed.

The two-dimensional quantum Heisenberg antiferromagnet with Ising-like anisotropy

We study the two dimensional quantum Heisenberg antiferromagnet on the square lattice with easy-axis exchange anisotropy by the semiclassical method called pure-quantum self-consistent harmonic approximation. In particular, we focus on the problem of the existence of a nite-temperature transition in such a model, and study the corresponding critical temperature as the spin value and the anisotropy vary. We find that an Ising-like transition characterizes the model even when the anisotropy is of the order of 10-2J (J being the intra-layer exchange integral). The good agreement found between our theoretical results and the experimental data for the compounds Rb2MnF4, K2MnF4, and K2NiF4 shows that the insertion of the easy-axis exchange anisotropy, with quantum effects properly taken into account, provides a quantitative description and explanation of the real system's critical behaviour.

Finite-temperature ordering in two-dimensional magnets

We study the two-dimensional quantum Heisenberg antiferromagnet on the square lattice with easy-axis exchange anisotropy. By the semiclassical method called pure-quantum self-consistent harmonic approximation we analyze several thermodynamic quantities and investigate the existence of a finite temperature transition, possibly describing the low-temperature critical behavior experimentally observed in many layered real compounds. We find that an Ising-like transition characterizes the model even when the anisotropy is of the order of ${10}^{\ensuremath{-}2}J$ $(J$ being the intralayer exchange integral), as in most experimental situations. On the other hand, typical features of the isotropic Heisenberg model are observed for both values of anisotropy considered, one in the quasi-isotropic limit and the other in a more markedly easy-axis region. The good agreement found between our theoretical results and the experimental data relative to the real compound ${\mathrm{Rb}}_{2}{\mathrm{MnF}}_{4}$ shows that the insertion of the easy-axis exchange anisotropy, with quantum effects properly taken into account, provides a quantitative description and explanation of the experimental data, thus allowing us to recognize in such anisotropy the main agent for the observed onset of finite temperature long-range order.

Spontaneous dimerization in theS=12Heisenberg antiferromagnetic chain with competing interactions

Spontaneous dimerization is found in the $S=\frac{1}{2}$ isotropic Heisenberg antiferromagnetic chain with competing nearest- and next-nearest-neighbor exchange, $\frac{{J}_{2}}{{J}_{1}}\ensuremath{\gtrsim}\frac{1}{6}$, and results from the same umklapp processes that lead to the N\'eel state when easy-axis exchange anisotropy is present. Spontaneous and externally induced dimerizations are contrasted.