Heisenberg Ferromagnet Research Articles

Critical Spontaneous Breaking of U(1) Symmetry at Zero Temperature in One Dimension.

The Hohenberg-Mermin-Wagner theorem states that there is no spontaneous breaking of continuous internal symmetries in spatial dimensions d≤2 at finite temperature. At zero temperature, the quantum-to-classical mapping further implies the absence of such symmetry breaking in one dimension, which is also known as Coleman's theorem in the context of relativistic quantum field theories. One route to violate this "folklore" is requiring an order parameter to commute with a Hamiltonian, as in the classic example of the Heisenberg ferromagnet and its variations. However, a systematic understanding has been lacking. In this Letter, we propose a family of one-dimensional models that display spontaneous breaking of a U(1) symmetry at zero temperature, where the order parameter does not commute with the Hamiltonian. While our models can be deformed continuously within the same phase, there exist symmetry-preserving perturbations that render the observed symmetry breaking fragile. We argue that a more general condition for this behavior is that the Hamiltonian is frustration-free.

Deciphering competing interactions of Kitaev–Heisenberg-Γ system in clusters: I. Static properties

Recently, the Kitaev-Heisenberg-Γ system has been used to explore various aspects of Kitaev spin liquid physics. Here, we consider a few small clusters of up to twelve sites and study them in detail to unravel many interesting findings due to the competition between all possible signs and various magnitudes of these interactions under the influence of an external magnetic field. When Heisenberg interaction is taken anti-ferromagnetic, one obtains plateaus in correlation functions where, surprisingly, the exact groundstate reduces to the eigenstate of Heisenberg interaction as well. On the other hand, for ferromagnetic Heisenberg interaction, its competition with Kitaev interaction results in non-monotonicity in the correlation functions. We discuss, in detail, the competing effects on low energy spectrum, flux operator, magnetization, susceptibility, and specific heat. Finally, we discuss how our findings could be helpful to explain some of the recent experimental and theoretical findings in materials with Kitaev interactions.

Breather–breather interactions, rational and semi-rational solutions of an integrable spin system in optical fibers

This paper investigates an integrable spin system (or Heisenberg ferromagnet-type equation or Nurshuak-IIA equation) which is an integrable generalization of the Heisenberg ferromagnet equation that raises from magneto-optical devices. The N-fold Darboux transformation of the Nurshuak-IIA equation will be constructed. With the selection of several parameters, a variety of the solutions will be produced, including breather–breather interactions, higher-order rogue waves and semi-rational solutions.

Emergence of vortex state in the S=1 Kitaev-Heisenberg model with single-ion anisotropy

The search for Kitaev spin liquid states has recently broadened to include a number of honeycomb materials with integer spin moments. The qualitative difference with their spin-1/2 counterparts is the presence of single-ion anisotropy (SIA). This motivates our investigation of the effects of SIA on the ground state of the spin-1 Kitaev-Heisenberg (KH) model using the density-matrix renormalization group which allows construction of detailed phase diagrams around the Kitaev points. We demonstrate that positive out-of-plane SIA induces an in-plane vortex state without the need for off-diagonal interactions. Conversely, negative SIA facilitates the emergence of a state in the presence of antiferromagnetic Heisenberg interactions, whereas a state can emerge for ferromagnetic Heisenberg coupling. These findings, pertinent even for weak SIA, not only enhance our theoretical understanding of the spin-1 KH model but also suggest experimental prospects for observing these novel magnetic states in material realizations. Published by the American Physical Society 2024

Ground state of the spin-1/2 Ising–Heisenberg distorted diamond chain with ferromagnetic Ising and antiferromagnetic Heisenberg interactions

Ground state of the spin-1/2 Ising–Heisenberg distorted diamond chain with ferromagnetic Ising and antiferromagnetic Heisenberg interactions

Spin-1/2 Ising-Heisenberg distorted diamond chain with antiferromagnetic Ising and ferromagnetic Heisenberg interactions

Spin-1/2 Ising-Heisenberg distorted diamond chain with antiferromagnetic Ising and ferromagnetic Heisenberg interactions

Polymeric dichlorido-bridged copper(II) offset stacked dimer coordination compounds and molecular spin stairs

A family of oxygen coordinated dichlorido-bridged n-X-2-pyridone (1, n = 3, X = Cl; 2, n = 3, X = Br; 3, n = 4, X = Cl; 4, n = 4, X = Br; 5, n = 5, X = Cl; 6, n = 5, X = Br) Cu(II) offset stacked dimer compounds, denoted molecular spin stairs, have been prepared and analyzed by single-crystal X-ray diffraction (1–4), powder X-ray diffraction (1–6), and variable temperature magnetic susceptibility measurements (1–6). Ferromagnetism is dominant in all compounds with a weaker antiferromagnetic exchange present in 1–4. The ferromagnetic exchange correlates to increased twist angles which are defined by the extent of the tetrahedral distortion of the copper coordination sphere and the identity of the halogen on the ring. The compounds were qualitatively fit to a ferromagnetic Heisenberg chain model with a Curie-Weiss interchain correction. The general Hamiltonian used is of the form H = − J ∑ S 1 S 2 . The ratio of exchange energies denoted α ( α = J FM | J AFM | ) ranges between 2.6 and 4.7. This family of compounds constitutes possible S = ½ Haldane-like materials.

Semiclassical approach to spin dynamics of a ferromagnetic S = 1 chain

Abstract Motivated by recent experimental progress on the quasi-one-dimensional quantum magnet NiNb2O6, we study the spin dynamics of an S = 1 ferromagnetic Heisenberg chain with single-ion anisotropy by using a semiclassical molecular dynamics approach. This system undergoes a quantum phase transition from a ferromagnetic to a paramagnetic state under a transverse magnetic field, and the magnetic response reflecting this transition is well described by our semiclassical method. We show that at low temperature the transverse component of the dynamical structure factor depicts clearly the magnon dispersion, and the longitudinal component exhibits two continua associated with single- and two-magnon excitations, respectively. These spin excitation spectra show interesting temperature dependence as effects of magnon interactions. Our findings shed light on the experimental detection of spin excitations in a large class of quasi-one-dimensional magnets.

Magnetization reversal phenomenon of higher-order lump and mixed interaction structures on periodic background in the (2+1)-dimensional Heisenberg ferromagnet spin equation

This paper mainly researches a useful model that can describe the rapid magnetization reversal process, namely the (2+1)-dimensional Heisenberg ferromagnet spin equation. Based on its Lax pair, we construct the iterative generalized (r,N−r)-fold Darboux transformation (DT), and obtain new position-controllable higher-order lump, periodic wave and rich mixed lump–breather interaction structures on periodic background. In particular, with the increase of N in the iterative generalized (r,N−r)-fold DT, we find that the structure of the lump and periodic wave can flip over with respect to the background. Moreover, we also find a bright lump structure with two peaks and two valleys, which does not flip with the increase of N. We study the magnetization trajectory and magnetization on the Bloch sphere, from which we discover that the magnetization is distributed in an incomplete unit sphere for the flipped case, while the magnetization is distributed in a complete unit sphere for the non-flipped case. These magnetization reversal phenomena are expected to have potential applications in understanding the rapid magnetization reversal process.

Rogue wave excitations and hybrid wave structures of the Heisenberg ferromagnet equation with time-dependent inhomogeneous bilinear interaction and spin-transfer torque.

In this paper, we focus on the localized rational waves of the variable-coefficient Heisenberg spin chain equation, which models the local magnetization in ferromagnet with time-dependent inhomogeneous bilinear interaction and spin-transfer torque. First, we establish the iterative generalized (m,N-m)-fold Darboux transformation of the Heisenberg spin chain equation. Then, the novel localized rational solutions (LRSs), rogue waves (RWs), periodic waves, and hybrid wave structures on the periodic, zero, and nonzero constant backgrounds with the time-dependent coefficients α(t) and β(t) are obtained explicitly. Additionally, we provide the trajectory curves of magnetization and the variation of the magnetization direction for the obtained nonlinear waves at different times. These phenomena imply that the LRSs and RWs play the crucial roles in changing the circular motion of the magnetization. Finally, we also numerically simulate the wave propagations of some localized semi-rational solutions and RWs.

Effect of interlayer exchange coupling interaction on topological phase of a bilayer honeycomb Heisenberg ferromagnet

Layered magnetic topological materials are material systems that exhibit both magnetic ordering and topological properties in their smallest two-dimensional units. Studying these systems may lead to the observation of new physical properties and phenomena, which has attracted considerable attention from researchers. The effect of interlayer exchange coupling interactions on bilayer honeycomb Heisenberg ferromagnets with interlayer coupled topological phase is investigated by using linear spin wave theory. The influence of introducing two additional types of interactions, i.e. interlayer exchange coupling interaction and interlayer easy-axis anisotropy interaction, on the topological phase transition are also explored in this work. By calculating the magnon dispersion relations at various interlayer exchange coupling interaction intensities, it is found that the band gaps of high energy band and low energy band both close and reopen at the Dirac points when the system reaches the critical value of interlayer exchange coupling interaction. In magnon systems, such physical phenomena typically relate to topological phase transitions. When calculating the Berry curvature and Chern numbers for the bands in the aforementioned process, it is found that the sign of the Berry curvature reverses and the Chern numbers change when the critical value of interlayer exchange coupling interaction strength is reached, confirming that a topological phase transition occurs indeed. Introducing two other types of interlayer exchange coupling interactions in this process can lead various novel topological phases to occur in the system. The enhancement of interlayer easy-axis anisotropy interactions is likely to impede the topological phase transitions occurring in the system. We find that a major distinction between bilayer honeycomb ferromagnets and their single-layer counterparts lies in the fact that during a topological phase transition, the sign of the magnon thermal Hall coefficient does not change; on the contrary, abrupt shift in the thermal Hall coefficient curve occurs which can be seen as an indicator of topological phase transition of bilayer honeycomb ferromagnets, and is also reflected in the change in magnon Nernst coefficient. The research results of this work can provide theoretical support for developing novel spintronic devices with enhanced information transmission capabilities by using bilayer honeycomb ferromagnetic materials, and can also provide theoretical reference for studing other bilayer ferromagnetic systems.

Exploitable magnetic anisotropy and half-metallicity controls in multiferroic van der Waals heterostructure

Two-dimensional (2D) XY ferromagnets have drawn pronounced interest in recent years, but the characteristic of easy-plane magnetization restricts their application in spintronics to some extent. Here, we propose a general strategy for constructing multiferroic van der Waals heterostructures, aiming to achieve electrical control over the magnetic anisotropy in 2D XY ferromagnets. The validity of this strategy is verified by the heterostructure composed of ferromagnetic VBi2Te4 and ferroelectric In2Se3 monolayers. By manipulating the polarized states of In2Se3, the VBi2Te4 can be reversibly transformed between 2D XY and Heisenberg ferromagnets, characterized by the switching of easy magnetization axis between in-plane and out-of-plane directions. More interestingly, accompanied by the changes in magnetic anisotropy, the VBi2Te4 also demonstrates a phase transition from a semiconductor to a half-metal state, which can be ascribed to the band alignment and interfacial charge transfer. The switchable magnetic and electronic properties enable the heterostructure to be utilized in nonvolatile memory and logic devices. Additionally, the half-metallicity and magnetocrystalline anisotropy energy of the heterostructure can be effectively tuned by biaxial strain. These findings not only pave the way for electrically nonvolatile control of 2D XY ferromagnet, but also facilitate the development of interfacial magnetoelectric physics and applications.

Magnetic properties of a quasi-one-dimensional antiferromagnet Eu2BiS4

We have synthesized c-axis oriented polycrystals of a Eu-based orthorhombic compound Eu2BiS4 and measured transport, magnetic, and thermal properties down to 1.8 K. The crystal structure of Eu2BiS4 was confirmed by powder x-ray diffraction to be orthorhombic CaFe2O4-type with one-dimensional chains of Eu2+ ions along the c-axis. The electrical resistivity ρ increases on cooling from 300 K, which shows that this compound is a semiconductor. Both magnetic susceptibility χ and specific heat C exhibit an obvious peak at TN = 2.5 K, indicating an antiferromagnetic order. Anisotropic behaviors of isothermal magnetization M and χ below TN suggest that the magnetic moments align along the c-axis. Moreover, a tail appears in C(T) in the range of TN < T < 8 K, where χ(T) is reproduced by that calculated with a one-dimensional ferromagnetic Heisenberg model. These behaviors in C(T) and χ(T) are attributed to developed short-range ferromagnetic correlations in the Eu2+ chains.

Critical behavior at ferromagnetic to paramagnetic phase transition in single crystalline MnNiSi ferromagnet

Ferromagnetic single crystalline MnNiSi samples were first fabricated through a Sn-flux growth technique, followed by measurements of their structural characteristics and intrinsic magnetic properties. Additionally, the critical behavior for second-order ferromagnetic to paramagnetic phase transition was investigated through utilization of techniques such as the modified Arrott plot, the Kouvel–Fisher method, and the magnetocaloric effect scaling law method. Through different methods of analysis, reliable critical exponents were obtained. Renormalization of interactions around the Curie temperature indicates the reliability of the obtained exponents. The obtained critical exponents are close to those theoretically predicted for a three-dimensional isotropic short-range Heisenberg ferromagnet but shift toward the long-range mean-field estimates. This may arise from the coupling of short- and long-range interactions as well as the competition between localized Mn–Mn magnetic interactions and the hybridization between p- and d-type orbitals.

Dynamical analysis of multi-soliton and breather solutions on constant and periodic backgrounds for the (2+1)-dimensional Heisenberg ferromagnet equation

Dynamical analysis of multi-soliton and breather solutions on constant and periodic backgrounds for the (2+1)-dimensional Heisenberg ferromagnet equation

Geometric phases for pseudo-hyperbolic magnetic particles with ferromagnetic media

In this study, we analyze the electromagnetic parameters and associated geometric phases of a magnetic particle moving in pseudo-hyperbolic space associated with Minkowski 3-space. To do this, we make use of curves representing the trajectories of electromagnetic waves, the electric and the magnetic field equations, and in particular the Heisenberg equations (ferromagnetic spin chain) that serve spin wave theory. First of all, we obtain the Lorentz forces and magnetic fields of these magnetic particles, which we call pseudo-hyperbolic (p-hyperbolic) magnetic particles. Next, we calculate electric fields and power flows with the help of the equation relating Lorentz and Newtonian forces. We then characterize some geometric phases of these power flows, which give data on energy density and magnitude. Finally, we characterize the geometric phases of a power flow in a ferromagnetic media with the help of the ferromagnetic Heisenberg model. We also give simulations of Heisenberg ferromagnetic power flows, including observations of the results obtained at the end of this section.

Finite-temperature critical behaviors in 2D long-range quantum Heisenberg model

The Mermin-Wagner theorem states that spontaneous continuous symmetry breaking is prohibited in systems with short-range interactions at spatial dimension D ≤ 2. For long-range interactions with a power-law form (1/rα), the theorem further forbids ferromagnetic or antiferromagnetic order at finite temperature when α ≥ 2D. However, the situation for α ∈ (2, 4) at D = 2 is not covered by the theorem. To address this, we conduct large-scale quantum Monte Carlo simulations and field theoretical analysis. Our findings show spontaneous breaking of SU(2) symmetry in the ferromagnetic Heisenberg model with 1/rα-form long-range interactions at D = 2. We determine critical exponents through finite-size analysis for α < 3 (above the upper critical dimension with Gaussian fixed point) and 3 ≤ α < 4 (below the upper critical dimension with non-Gaussian fixed point). These results reveal new critical behaviors in 2D long-range Heisenberg models, encouraging further experimental studies of quantum materials with long-range interactions beyond the Mermin-Wagner theorem’s scope.

Effect of random antiferromagnetic exchange on the spin waves in a three-dimensional Heisenberg ferromagnet

Effect of random antiferromagnetic exchange on the spin waves in a three-dimensional Heisenberg ferromagnet

Enhanced magnetocaloric effect in melt-spun Mn[formula omitted]Fe[formula omitted]Ge3 (x = 0 and 1) ribbons with anisotropy induced spin-dimensionality crossover

We report the structural, magnetic, magnetocaloric and ferromagnetic resonance properties of melt-spun Mn5−xFexGe3 (x = 0 and 1) ribbons. The melt-spun Mn5Ge3 ribbons exhibit hexagonal structure with preferential growth along c-axis while Fe doping retains the structure but changes preferential growth in in-plane direction. Fe doping enhances the ferromagnetic to paramagnetic phase transition temperature of Mn5Ge3 ribbons. Elaborate isothermal magnetization data analyses in the critical region yield critical exponents β= 0.344(1), γ= 1.284(1) and δ= 4.70(1) with critical temperature TC= 303.64(1) K for x = 0 while β= 0.389(1), γ= 1.334(1) and δ= 4.441(1) with TC= 330.84(1) K for x = 1. Further, the magnetization data in the vicinity of TC satisfies the magnetic scaling equations of state characteristic of a second-order phase transition. The magnetocaloric performance is reflected through maximum entropy change −ΔSmmax(T) ∼ 5.0(2.9) J kg−1 K−1; refrigerant capacity RC ∼ 505(302) J kg−1 and temperature-averaged entropy TEC(10 K) ∼ 4.92(2.28) J kg−1 K−1 at an applied field of 5 T for x = 0(1) ribbons, respectively. −ΔSmmax(T) embracing the critical region is found to be well-described by the critical exponents that are consistent with those determined by the modified Arrott plots and Kouvel–Fisher analyses. Renormalization group calculations further confirm that Mn5Ge3 and Mn4FeGe3 systems behave as 3D Ising and 3D Heisenberg ferromagnets in the critical region wherein short-range extended magnetic interactions decay as J(r) ≈ r−4.995 and r−4.895 with intermoment distance, respectively. Ferromagnetic resonance study reveals that with the introduction of Fe, the magnetocrystaline anisotropy (Ku∼1.4×106 erg/cm3) of Mn5Ge3 gets suppressed so much so that the system exhibits a crossover from 3D Ising to 3D Heisenberg universality class in the critical region.

Multiple nonlinear wave solutions of a generalized Heisenberg ferromagnet model and their interactions

Under investigation in this paper is a generalized Heisenberg ferromagnet (HF) equation which is named the Zhanbota-IIA equation. It is one of the integrable generalizations of the HF equation that plays an important role in nonlinear magnetization dynamics. Through the establishment of the N-fold Darboux transformation, a series of solutions will be obtained, including multi-solitons, one- and two-breathers, first- and higher-order rogue waves. Dynamic behaviors of those solutions will be analyzed, including several structures of rogue waves such as fundamental structure, triangular structure, ring structure and ring-fundamental structure, the coexistence of rogue waves and breathers, i.e. semi-rational solution and the interaction of two breathers.