Possibility of a new universality class: The anisotropic Heisenberg model with dipolar interactions

Theory of Two-Step Magnetic Ordering Phenomena in a Geometrically Frustrated Heisenberg Pyrochlore Antiferromagnet with Long Range Dipolar Interactions

A model for a Heisenberg antiferromagnet on a pyrochlore lattice with exchange and dipole-dipole interactions is studied via mean-field theory. In treating the dipoles by use of the Ewald method, a soft (critical) mode with a unique ordering wave vector is selected for all strengths of the dipole-dipole coupling. For weak dipoles a partially ordered, three sublattice spin structure (P state), with q_{ord}= (1/2 1/2 1/2), is selected. A fully ordered, four sublattice spin structure (F state), with q = (000), competes with the P state and becomes the stable structure as the temperature is reduce…

Universality in the three-dimensional random bond quantum Heisenberg antiferromagnet

The three-dimensional quenched random bond diluted $(J_1-J_2)$ quantum Heisenberg antiferromagnet is studied on a simple-cubic lattice. Using extensive stochastic series expansion quantum Monte Carlo simulations, we perform very long runs for $L \times L \times L$ lattice up to $L=48$. By employing standard finite-size scaling method, the numerical values of the Néel temperature are determined with high precision as a function of the coupling ratio $r=J_2/J_1$. Based on the estimated critical exponents, we find that the critical behavior of the considered model belongs to the pure classical $3…

Universality of the Berezinskii-Kosterlitz-Thouless type of phase transition in the dipolar XY-model

We investigate the nature of the phase transition occurring in a planar XY-model spin system with dipole-dipole interactions. It is demonstrated that a Berezinskii-Kosterlitz-Thouless (BKT) type of phase transition always takes place at a finite temperature separating the ordered (ferro) and the disordered (para) phases. The low-temperature phase corresponds to an ordered state with thermal fluctuations, composed of a "gas" of bound vortex-antivortex pairs, which would, when considered isolated, be characterized by a constant vortex-antivortex attraction force which is due to the dipolar inter…

阿基米德晶格与拉夫斯晶格上的经典偶极海森堡模型

Classical dipolar Heisenberg models on the Archimedean and Laves lattices

本研究系统计算了15种阿基米德与拉夫斯平面晶格上经典偶极海森堡模型的基态与自旋波谱，揭示了倾斜行为、量子修正与晶格几何的关联，为相关材料与人工磁体实验提供预测。

Far-from-equilibrium universality in the two-dimensional Heisenberg model

We characterize the universal far-from-equilibrium dynamics of the two-dimensional quantum Heisenberg magnet isolated from its environment. For a broad range of initial conditions, we find a long-lived universal prethermal regime characterized by self-similar behavior of spin-spin correlations. We analytically derive the spatial-temporal scaling exponents and find excellent agreement with numerics using phase space methods. The scaling exponents are insensitive to the choice of initial conditions, which include coherent and incoherent spin states with values of total magnetization and energy i…

Evidence of Unconventional Universality Class in a Two-Dimensional Dimerized Quantum Heisenberg Model

The two-dimensional $J$-$J^\prime$ dimerized quantum Heisenberg model is studied on the square lattice by means of (stochastic series expansion) quantum Monte Carlo simulations as a function of the coupling ratio \hbox{$α=J^\prime/J$}. The critical point of the order-disorder quantum phase transition in the $J$-$J^\prime$ model is determined as \hbox{$α_\mathrm{c}=2.5196(2)$} by finite-size scaling for up to approximately $10 000$ quantum spins. By comparing six dimerized models we show, contrary to the current belief, that the critical exponents of the $J$-$J^\prime$ model are not in agreemen…

Quantum spin fluctuations in the dipolar Heisenberg-like rare earth pyrochlores

The magnetic pyrochlore oxide materials of general chemical formula R2Ti2O7 and R2Sn2O7 (R = rare earth) display a host of interesting physical behaviours depending on the flavour of rare earth ion. These properties depend on the value of the total magnetic moment, the crystal field interactions at each rare earth site and the complex interplay between magnetic exchange and long-range dipole-dipole interactions. This work focuses on the low temperature physics of the dipolar isotropic frustrated antiferromagnetic pyrochlore materials. Candidate magnetic ground states are numerically determined…

Anisotropy-temperature phase diagram for the two-dimensional dipolar Heisenberg model with and without magnetic field

We investigate phase transitions in the two-dimensional dipolar Heisenberg model with uniaxial anisotropy with a specific ratio between the exchange and dipolar constants, $δ=1$. We obtain the $η$--$T$ (anisotropy vs. temperature) phase diagrams for typical values of magnetic field by a Monte Carlo method with an $O(N)$ algorithm. We find that at lower fields, the $η$--$T$ phase diagram consists of the planar ferromagnetic (F), (perpendicular) stripe-ordered (SO), and paramagnetic (P) phases, and is characterized by the triple point. In the SO phase realized at larger $η$ and smaller $T$, the …

纯偶极三角晶格临界行为的平衡与非平衡蒙特卡洛模拟分析

Critical Behavior Analysis of Pure Dipolar Triangular Lattice via Equilibrium and Non-Equilibrium Monte Carlo Simulations

本研究利用优化的Tomita蒙特卡洛算法，通过平衡与非平衡两种模拟方法，发现XY偶极三角晶格的临界行为可能属于接近二维伊辛普适类的新普适类。

各向异性Heisenberg模型的相图与Ashkin-Teller universality

Phase diagram and Ashkin-Teller universality in the classical square-lattice Heisenberg-compass model

研究揭示了Heisenberg交换与compass各向异性如何共同作用，形成不同的临界行为，包括Ashkin-Teller和Ising临界性。

Exotic vs. conventional scaling and universality in a disordered bilayer quantum Heisenberg antiferromagnet

We present large-scale Monte-Carlo simulations of a two-dimensional (2d) bilayer quantum Heisenberg antiferromagnet with random dimer dilution. In contrast to the exotic scaling scenarios found in many other random quantum systems, the quantum phase transition in this system is characterized by a finite-disorder fixed point with power-law scaling. After accounting for strong corrections to scaling, characterized by a leading irrelevant exponent of ω= 0.48, we find universal, i.e., disorder-independent, critical exponents z=1.310(6) and ν=1.16(3). We discuss the consequences of these findings a…

Quantum Critical Universality and Singular Corner Entanglement Entropy of Bilayer Heisenberg-Ising model

We consider a bilayer quantum spin model with anisotropic intra-layer exchange couplings. By varying the anisotropy, the quantum critical phenomena changes from XY to Heisenberg to Ising universality class, with two, three and one modes respectively becoming gapless simultaneously. We use series expansion methods to calculate the second and third Renyi entanglement entropies when the system is bipartitioned into two parts. Leading area-law terms and subleading entropies associated with corners are separately calculated. We find clear evidence that the logarithmic singularity associated with th…

Critical exponents and equation of state of the three-dimensional Heisenberg universality class

We improve the theoretical estimates of the critical exponents for the three-dimensional Heisenberg universality class. We find gamma=1.3960(9), nu=0.7112(5), eta=0.0375(5), alpha=-0.1336(15), beta=0.3689(3), and delta=4.783(3). We consider an improved lattice phi^4 Hamiltonian with suppressed leading scaling corrections. Our results are obtained by combining Monte Carlo simulations based on finite-size scaling methods and high-temperature expansions. The critical exponents are computed from high-temperature expansions specialized to the phi^4 improved model. By the same technique we determine…

偶极铁磁体中不具备共形不变性的标度性

Scale without Conformal Invariance in Dipolar Ferromagnets

本文重新研究强偶极相互作用各向同性铁磁体的临界现象，阐明偶极不动点无共形不变性的标度性机制，分析不同材料中偶极与短程作用对应的临界行为差异。

三维海森堡普适类的边界临界行为

Boundary critical behavior of the three-dimensional Heisenberg universality class

本文通过高精度蒙特卡洛模拟改进晶格模型，研究三维海森堡普适类的边界临界行为，证明存在具有异常指数的特殊相变和对数衰减关联的异常相，解释了量子自旋模型边界临界行为的疑难结果。

Two-dimensional quantum spin-1/2 Heisenberg model with competing interactions

We study the quantum spin-1/2 Heisenberg model in two dimensions, interacting through a nearest-neighbor antiferromagnetic exchange ($J$) and a ferromagnetic dipolar-like interaction ($J_d$), using double-time Green's function, decoupled within the random phase approximation (RPA). We obtain the dependence of $k_B T_c/J_d$ as a function of frustration parameter $δ$, where $T_c$ is the ferromagnetic (F) transition temperature and $δ$ is the ratio between the strengths of the exchange and dipolar interaction (i.e., $δ= J/J_d$). The transition temperature between the F and paramagnetic phases dec…

二维海森堡反铁磁体中的远离平衡态普适性

Far-from-equilibrium universality in two-dimensional Heisenberg antiferromagnets

研究孤立二维海森堡反铁磁体的远离平衡动力学，发现长寿命预热区中交错磁化涨落呈现自相似标度行为，且无需能量精细调节，其时空标度指数与铁磁体不同，揭示了不同的普适类。

旋子Heisenberg链中电流算符的随机矩阵理论普遍性

Random matrix theory universality of current operators in spin-$S$ Heisenberg chains

本文通过数值研究证明，在自旋S=1/2,1,3/2的Heisenberg链中，自旋电流算符的统计特性符合随机矩阵理论的普遍性预测。

Phase diagram and magnons in quasi-one-dimensional dipolar antiferromagnets

We investigate antiferromagnetic spin chains, which are coupled by a weak antiferromagnetic exchange interaction on a hexagonal lattice. We particulary study the role of the dipole-dipole interaction within the framework of a Heisenberg model with nearest-neighbor exchange and additional dipolar interaction. We find several commensurate and incommensurate phases depending on the ratio of dipolar energy to interchain-exchange energy due to their competing qualtity. The ground-state analysis is supplemented by a stability analysis by means of a linear spin-wave theory. In comparison with experim…

Persistence of the flat band in a kagome magnet with dipolar interactions

The weathervane modes of the classical Heisenberg antiferromagnet on the kagome lattice constitute possibly the earliest and certainly the most celebrated example of a flat band of zero-energy excitations. Such modes arise from the underconstraint that has since become a defining criterion of strong geometrical frustration. We investigate the fate of this flat band when dipolar interactions are added. These change the nearest-neighbour model fundamentally as they remove the Heisenberg spin-rotational symmetry while also introducing a long- range component to the interaction. We explain how the…

Peculiar from-Edge-to-Interior Spin Freezing in a Magnetic Dipolar Cube

By molecular dynamics simulation, we have investigated classical Heisenberg spins, which are arrayed on a finite simple cubic lattice and interact with each other only by the dipole-dipole interaction, and have found its peculiar it from-Edge-to-interior freezing process. As the temperature is decreased, spins on each edge predominantly start to freeze in a ferromagnetic alignment parallel to the edge around the corresponding bulk transition temperature, then from each edges grow domains with short-range orders similar to the corresponding bulk orders, and the system ends up with a unique mult…

Nature of Long-Range Order in Stripe-Forming Systems with Long-Range Repulsive Interactions

We study two dimensional stripe forming systems with competing repulsive interactions decaying as $r^{-α}$. We derive an effective Hamiltonian with a short range part and a generalized dipolar interaction which depends on the exponent $α$. An approximate map of this model to a known XY model with dipolar interactions allows us to conclude that, for $α<2$ long range orientational order of stripes can exist in two dimensions, and establish the universality class of the models. When $α\geq 2$ no long-range order is possible, but a phase transition in the KT universality class is still present. Th…

Phase transition in the two-dimensional dipolar Planar Rotator model

In this work we have used extensive Monte Carlo simulations and finite size scaling theory to study the phase transition in the dipolar Planar Rotator model (dPRM), also known as dipolar XY model. The true long-range character of the dipolar interactions were taken into account by using the Ewald summation technique. Our results for the critical exponents does not fit those from known universality classes. We observed that the specific heat is apparently non-divergent and the critical exponents are $ν=1.277(2)$, $β=0.2065(4)$ and $γ=2.218(5)$. The critical temperature was found to be $T_c=1.20…

Phase transition in two-dimensional magnetic systems with dipolar interactions

In this work we have used extensive Monte Carlo calculations to study the planar to paramagnetic phase transition in the two-dimensional anisotropic Heisenberg model with dipolar interactions (AHd) considering the true long-range character of the dipolar interactions by means of the Ewald summation. Our results are consistent with an order-disorder phase transition with unusual critical exponents in agreement with our previous results for the Planar Rotator model with dipolar interactions. Nevertheless, our results disagrees with the Renormalization Group results of Maier and Schwabl [PRB, 70,…

Kardar-Parisi-Zhang physics in the quantum Heisenberg magnet

Equilibrium spatio-temporal correlation functions are central to understanding weak nonequilibrium physics. In certain local one-dimensional classical systems with three conservation laws they show universal features. Namely, fluctuations around ballistically propagating sound modes can be described by the celebrated Kardar-Parisi-Zhang (KPZ) universality class. Can such universality class be found also in quantum systems? By unambiguously demonstrating that the KPZ scaling function describes magnetization dynamics in the SU(2) symmetric Heisenberg spin chain we show, for the first time, that …

Critical dynamics of a uniaxial and dipolar ferromagnet

We study the critical dynamics of three-dimensional ferromagnets with uniaxial anisotropy by taking into account exchange and dipole-dipole interaction. The dynamic spin correlation functions and the transport coefficients are calculated within a mode coupling theory. It is found that the crossover scenario is determined by the subtle interplay between three length scales: the correlation length, the dipolar and uniaxial wave vector. We compare our theoretical findings with hyperfine interaction experiments on Gd and find quantitative agreement. This analysis allows us to identify the universa…

Vortex Structure in Magnetic Nanodots: Dipolar Interaction, Mobile Spin Model, Phase Transition and Melting

We study in this article properties of a nanodot embedded in a support by Monte Carlo simulation. The nanodot is a piece of simple cubic lattice where each site is occupied by a mobile Heisenberg spin which can move from one lattice site to another under the effect of the temperature and its interaction with neighbors. We take into account a short-range exchange interaction between spins and a long-range dipolar interaction. We show that the ground-state configuration is a vortex around the dot central axis: the spins on the dot boundary lie in the $xy$ plane but go out of plane with a net per…

Nonequivalence of ensembles in the Curie-Weiss anisotropic quantum Heisenberg model

The microcanonical entropy s(e,m) as a function of the energy e and the magnetization m is computed analytically for the anisotropic quantum Heisenberg model with Curie-Weiss-type interactions. The result shows a number of interesting properties which are peculiar to long-range interacting systems, including nonequivalence of ensembles and partial equivalence. Furthermore, from the shape of the entropy it follows that the Curie-Weiss Heisenberg model is indistinguishable from the Curie-Weiss Ising model in canonical thermodynamics, although their microcanonical thermodynamics in general differ…

Monte Carlo study of the critical properties of noncollinear Heisenberg magnets: $O(3)\times O(2)$ universality class

The critical properties of the antiferromagnetic Heisenberg model on the three-dimensional stacked-triangular lattice are studied by means of a large-scale Monte Carlo simulation in order to get insight into the controversial issue of the criticality of the noncollinear magnets with the $O(3)\times O(2)$ symmetry. The maximum size studied is $384^3$, considerably larger than the sizes studied by the previous numerical works on the model. Availability of such large-size data enables us to examine the detailed critical properties including the effect of corrections to the leading scaling. Strong…

Universality Class of Ferromagnetic Transition in Three-Dimensional Double-Exchange System - O(N) Monte Carlo Study -

Curie temperature and exponents are studied for the three-dimensional double-exchange model. Applying the O(N) Monte Carlo algorithm, we perform systematic finite-size scaling analyses on the data up to $20^3$ sites. The obtained values of the critical exponents are consistent with those of the Heisenberg universality class, and clearly distinct from the mean-field values.

Low-density expansions for the homogeneous dipolar Bose gas at zero temperature

The low-density expansions for the energy, chemical potential, and condensate depletion of the homogeneous dilute dipolar Bose gas are obtained by regularizing the dipole-dipole interaction at long distances. It is shown that the leading term, proportional to the density, allows a simple physical interpretation and consistently describes the thermodynamic stability of the system. The long-range asymptotics are obtained analytically for the normal and anomalous one-particle correlation functions and the pair distribution function. We discuss the properties of the two-body scattering with zero r…

Surface scaling behavior of isotropic Heisenberg systems: Critical exponents, structure factor, and profiles

The surface scaling behavior of classical isotropic Heisenberg magnets is investigated by Monte - Carlo methods in d=3 dimensions for various values of the surface - to - bulk coupling ratio J_1/J. For J_1/J <= 1.0 critical behavior according to the ordinary surface universality class is found. New estimates for magnetic surface exponents are presented and compared to older estimates and their theoretical counterparts. For J_1/J >= 2.0 scaling is still valid with effective exponents which depend on J_1/J. The surface structure factor S_1(p,L) is investigated at bulk criticality as function of …

具有偶极相互作用的平面转子链中的量子临界性

Quantum criticality in chains of planar rotors with dipolar interactions

本文利用密度矩阵重正化群方法研究了偶极相互作用的平面转子链，发现随转子平面取向变化会发生从无序到偶极有序的量子相变，并揭示了涵盖Ising至XY模型的不同普适类。

Phase transition in ultrathin magnetic films with long-range interactions: Monte Carlo simulation of the anisotropic Heisenberg model

Ultrathin magnetic films can be modeled as an anisotropic Heisenberg model with long-range dipolar interactions. It is believed that the phase diagram presents three phases: An ordered ferromagnetic phase I, a phase characterized by a change from out-of-plane to in-plane in the magnetization II, and a high-temperature paramagnetic phase III. It is claimed that the border lines from phase I to III and II to III are of second order and from I to II is first order. In the present work we have performed a very careful Monte Carlo simulation of the model. Our results strongly support that the line …

Universality class of S=1/2 quantum spin ladder system with the four spin exchange

We study s=1/2 Heisenberg spin ladder with the four spin exchange. Combining numerical results with the conformal field theory(CFT), we find a phase transition with central charge c=3/2. Since this system has an SU(2) symmetry, we can conclude that this critical theory is described by k=2 SU(2) Wess-Zumino-Witten model with Z$_2$ symmetry breaking.

Spin-1/2 Ising-Heisenberg model with the pair XYZ Heisenberg interaction and quartic Ising interactions as the exactly soluble zero-field eight-vertex model

The spin-1/2 Ising-Heisenberg model with the pair XYZ Heisenberg interaction and quartic Ising interactions is exactly solved by establishing a precise mapping relationship with the corresponding zero-field (symmetric) eight-vertex model. It is shown that the Ising-Heisenberg model with the ferromagnetic Heisenberg interaction exhibits a striking critical behavior, which manifests itself through re-entrant phase transitions as well as continuously varying critical exponents. The changes of critical exponents are in accordance with the weak universality hypothesis in spite of a peculiar singula…

具有磁偶极相互作用的Fcc Kagome反铁磁体中的有序态

Ordered States in Fcc Kagome Antiferromagnets with Magnetic Dipolar Interactions

本文通过数值求解Landau-Lifshitz方程，研究了fcc晶格ABC堆叠kagome平面上同时含最近邻交换和磁偶极相互作用的经典Heisenberg模型，发现随两种相互作用相对强度变化可出现涡旋自旋结构、玻璃态及多种非均匀层状120度自旋结构等有序态。

Determination of the universality class of Gadolinium

We resolve a longstanding puzzle for the static and dynamic critical behavior of Gadolinium by a combined theoretical and experimental investigation. It is shown that the spin dynamics of a three dimensional ferromagnet with hcp lattice structure and a spin-spin interaction given by both exchange and dipole-dipole interaction belongs to a new dynamic universality class, model J$^*$. Comparing results from mode coupling theory with results from three different hyperfine interaction probes we find quantitative agreement. The crossover scenario for the wavevector dependence of the hyperfine relax…

Weak universality of spin-glass transitions in three-dimensional $\pm J$ models

We find a possibility of a weak universality of spin-glass phase transitions in three-dimensional $\pm J$ models. The Ising, the XY and the Heisenberg models seem to undergo finite-temperature phase transitions with a ratio of the critical exponents $γ/ν\sim 2.4$. Evaluated critical exponents may explain corresponding experimental results. The analyses are based upon nonequilibrium relaxation from a paramagnetic state and finite-time scaling.

Using the Energy probability distribution zeros to obtain the critical properties of the two-dimensional anisotropic Heisenberg model

In this paper we present a Monte Carlo study of the critical behavior of the easy axis anisotropic Heisenberg spin model in two dimensions. Based on the partial knowledge of the zeros of the energy probability distribution we determine with good precision the phase diagram of the model obtaining the critical temperature and exponents for several values of the anisotropy. Our results indicate that the model is in the Ising universality class for any anisotropy.

Driven Heisenberg Magnets: Nonequilibrium Criticality, Spatiotemporal Chaos and Control

We drive a $d$-dimensional Heisenberg magnet using an anisotropic current. The continuum Langevin equation is analysed using a dynamical renormalization group and numerical simulations. We discover a rich steady-state phase diagram, including a critical point in a new nonequilibrium universality class, and a spatiotemporally chaotic phase. The latter may be `controlled' in a robust manner to target spatially periodic steady states with helical order.

Ill-Behaved Convergence of a Model of the Gd3Ga5O12 Garnet Antiferromagnet with Truncated Magnetic Dipole-Dipole Interactions

Previous studies have found that calculations which consider long-range magnetic dipolar interactions truncated at a finite cut-off distance Rc predict spurious (unphysical) long-range ordered phases for Ising and Heisenberg systems on the pyrochlore lattice. In this paper we show that, similar to these two cases, calculations that use truncated dipolar interactions to model the Gd3Ga5O12 garnet antiferromagnet also predict unphysical phases with incommensurate ordering wave vector q_ord that is very sensitive to the dipolar cut-off distance Rc.

Classical dipoles on the kagome lattice

Motivated by recent developments in magnetic materials, frustrated nanoarrays and cold atomic systems, we investigate the behaviour of dipolar spins on the frustrated two-dimensional kagome lattice. By combining the Luttinger-Tisza approach, numerical energy minimization, spin-wave analysis and parallel tempering Monte-Carlo, we study long-range ordering and finite-temperature phase transitions for a Hamiltonian containing both dipolar and nearest-neighbor interactions. For both weak and moderate dipolar interactions, the system enters a three-sublattice long-range ordered state, with each tri…

Quantum Phase Transition in a Heisenberg Antiferromagnet on a Square Lattice with Strong Plaquette Interactions

We present numerical results for an $S=1/2$ Heisenberg antiferromagnet on a inhomogeneous square lattice with tunable interaction between spins belonging to different plaquettes. Employing Quantum Monte Carlo, we significantly improve on previous results for the the critical point separating singlet-disordered and Néel-ordered phases, and obtain an estimate for the critical exponent $ν$ consistent with the three-dimensional classical Heisenberg universality class. Additionally, we show that a fairly accurate result for the critical point can be obtained from a Contractor Renormalization (CORE)…

Second-order phase transition in the Heisenberg model on a triangular lattice with competing interactions

We discover an example where the dissociation of the Z2 vortices occurs at the second-order phase transition point. We investigate the nature of phase transition in a classical Heisenberg model on a distorted triangular lattice with competing interactions. The order parameter space of the model is SO(3)xZ2. The dissociation of the Z2 vortices which comes from SO(3) and a second-order phase transition with Z2 symmetry breaking occur at the same temperature. We also find that the second-order phase transition belongs to the universality class of the two-dimensional Ising model.

二维量子海森堡反铁磁体表面的特殊相变与非常规相

Special Transition and Extraordinary Phase on the Surface of a Two-Dimensional Quantum Heisenberg Antiferromagnet

本文通过调节二维量子临界海森堡模型的表面耦合强度，发现从普通相到非常规相的直接特殊相变，并确定了一组新的临界指数，揭示了三维O(3) Wilson-Fisher理论的一个新表面普适类。

Two-Dimensional Heisenberg Model with Nonlinear Interactions: 1/N Corrections

We investigate a two-dimensional classical $-vector model with a generic nearest-neighbor interaction $W(\bsigma_i\cdot \bsigma_j)$ in the large-N limit, focusing on the finite-temperature transition point at which energy-energy correlations become critical. We show that this transition belongs to the Ising universality class. However, the width of the region in which Ising behavior is observed scales as $1/N^{3/2}$ along the magnetic direction and as 1/N in the thermal direction; outside a crossover to mean-field behavior occurs. This explains why only mean-field behavior is observed for $N=\…

Reals-space renormalization group study of the anisotropic antiferromagnetic Heisenberg model on the square lattice

In this work we apply two different real-space renormalization-group (RSRG) approaches to the anisotropic antiferromagnetic spin-1/2 Heisenberg model on the square lattice. Our calculations allow for an approximate evaluation of the $T$ vs. $Δ$ phase-diagram: the results suggest the existence of a critical value of $Δ>0$, at which the critical temperature goes to zero, and the presence of reentrant behavior on the critical line between the ordered and disordered phases. This whole critical line is found to belong to the same universality class as the Ising model. Our results are in accordance …

Magnetic history dependence of metastable states in systems with dipolar interactions

We present the results of a Monte Carlo simulation of the ground state and magnetic relaxation of a model of a thin film consisting on a two-dimensional square lattice of Heisenberg spins with perpendicular anisotropy K, exchange J and long-range dipolar interactions g. We have studied the ground state configurations of this system for a wide range of the interaction parameters J/g, K/g by means of the simulated annealing procedure, showing that the model is able to reproduce the different magnetic configurations found in real samples. We have found the existence of a certain range of K/g, J/g…

Chiral universality class behavior of a non-chiral antiferroquadrupole system

The planar Heisenberg system with antiferroquadrupolar exchange on 3D stacked triangular lattice is shown to belong to the new "chiral" universality class, predicted for chiral antiferromagnets. The present system, however, displays no such chirality, which, according to the currently widely accepted concept, is supposed to be a key ingredient for the new critical behavior. Our claim that the new universality class should not be limited only to the chiral antiferromagnets is based on a simple mapping between the chiral antiferromagnetic and antiferroquadrupolar systems and supported by actual …

Effect of anisotropy on small magnetic clusters

The effect of dipolar interaction and local uniaxial anisotropy on the magnetic response of small spin clusters where spins are located on the vertices of icosahedron, cuboctahedron, tetrahedron and square geometry have been investigated. We consider the ferromagnetic and antiferromagnetic spin-1/2 and spin-1 Heisenberg model with uniaxial anisotropy and dipolar interaction and apply numerical exact diagonalization technique in order to study the influence of frustration and anisotropy on the ground state properties of the spin-clusters. The ground state magnetization, spin-spin correlation an…