Magnetic relaxation in a classical spin chain (original) (raw)
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Magnetic relaxation in a classical spin chain as model for nanowires
1999
With decreasing particle size, different mechanisms dominate the thermally activated magnetization reversal in ferromagnetic particles. We investigate some of these mechanisms for the case of elongated, single-domain nanoparticles which we describe by a classical Heisenberg spin chain driven by an external magnetic field. For sufficiently small system size the magnetic moments rotate coherently. With increasing size a crossover to a reversal due to soliton-antisoliton nucleation sets in. For even larger systems many of these soliton-antisoliton pairs nucleate at the same time. These effects give rise to a complex size dependence of the energy barriers and characteristic time scales of the relaxation. We study these quantities using Monte Carlo simulations as well as a direct integration of the Landau-Lifshitz-Gilbert equation of motion with Langevin dynamics and we compare our results with asymptotic solutions for the escape rate following from the Fokker-Planck equation. Also, we investigate the crossover from coherent rotation to soliton-antisoliton nucleation and multi-droplet nucleation, especially its dependence on the system size, the external field and the anisotropy of the system.
Dynamic properties of a classical anisotropic Heisenberg chain under external magnetic field
Physica B: Condensed Matter, 2005
We investigate the dynamic properties of a classical anisotropic Heisenberg chain interacting with an external magnetic field at different temperatures. Properties such as time-dependent energy autocorrelation and space-time spin-spin correlation are obtained by solving the dynamic equation _ S i ¼ S i  r Si H: While the static spin-spin correlation length decreases as the system is heated up, it increases when an external magnetic field is present. The timedependent spin-spin correlation decreases when the system is heated up, resulting in a decrease of the spin-diffusion speed. The presence of the magnetic field contributes to the order, and therefore produces an increase of the spindiffusion speed. In contrast, the single-ion type anisotropy behaves dynamically as a local field, inducing disorder in the chain.
Ising-like dynamics and frozen states in systems of ultrafine magnetic particles
Physical Review B, 2007
We use Monte-Carlo simulations to study aging phenomena and the occurence of spinglass phases in systems of single-domain ferromagnetic nanoparticles under the combined influence of dipolar interaction and anisotropy energy, for different combinations of positional and orientational disorder. We find that the magnetic moments oriente themselves preferably parallel to their anisotropy axes and changes of the total magnetization are solely achieved by 180 degree flips of the magnetic moments, as in Ising systems. Since the dipolar interaction favorizes the formation of antiparallel chain-like structures, antiparallel chain-like patterns are frozen in at low temperatures, leading to aging phenomena characteristic for spin-glasses. Contrary to the intuition, these aging effects are more pronounced in ordered than in disordered structures.
Relaxation to equilibrium in models of classical spins with long-range interactions
Journal of Statistical Mechanics: Theory and Experiment, 2019
For a model long-range interacting system of classical Heisenberg spins, we study how fluctuations, such as those arising from having a finite system size or through interaction with the environment, affect the dynamical process of relaxation to Boltzmann-Gibbs equilibrium. Under deterministic spin precessional dynamics, we unveil the full range of quasistationary behavior observed during relaxation to equilibrium, whereby the system is trapped in nonequilibrium states for times that diverge with the system size. The corresponding stochastic dynamics, modeling interaction with the environment and constructed in the spirit of the stochastic Landau-Lifshitz-Gilbert equation, however shows a fast relaxation to equilibrium on a sizeindependent timescale and no signature of quasistationarity, provided the noise is strong enough. Similar fast relaxation is also seen in Glauber Monte Carlo dynamics of the model, thus establishing the ubiquity of what has been reported earlier in particle dynamics (hence distinct from the spin dynamics considered here) of longrange interacting systems, that quasistationarity observed in deterministic dynamics is washed away by fluctuations induced through contact with the environment.
Relaxation and Magnetization Dynamics of Frustrated Spin-Chain Systems
Journal of Low Temperature Physics, 2010
A 3D model of the magnetization dynamics in frustrated triangular spinchain systems Ca 3 Co 2 O 6 is proposed. This model is a generalization of the 2D model developed earlier. The spins are assumed to interact with the nearest neighbors and external agency (heat reservoir and external magnetic field) that causes them to change their states randomly with time. A probability of a single spin-flip process is assumed in a Glauber-like form. The 3D model allows describing the step-like behavior of a magnetization curves experimentally observed in a strong magnetic field.
Relaxation in ordered systems of ultrafine magnetic particles: effect of the exchange interaction
Journal of Physics: Condensed Matter, 2011
We perform Monte Carlo simulations to study the relaxation of single-domain nanoparticles that are located on a simple cubic lattice with anisotropy axes pointing into the z-direction, under the combined influence of anisotropy energy, dipolar interaction and ferromagnetic interaction of strength J. We compare the results of classical Heisenberg systems with three-dimensional magnetic monments µi to the ones of Ising systems and find that Heisenberg systems show a much richer and more complex dynamical behavior. Contrary to Heisenberg systems, Ising systems need large activation energies to turn a spin and also possess a smaller configuration space for the orientation of the µi. Accordingly, Heisenberg systems possess a whole landscape of different states with very close-lying energies, while Ising systems tend to get frozen in one random state far away from the groundstate. For Heisenberg systems, we identify two phase transitions, (i) at intermediate J between domain and layered states and (ii) at larger J between layered and ferromagnetic states. Between these two transitions, the layered states change their apparence and develop a sub-structure, where the orientation of the µi in each layer depends on J, so that for each value of J, a new groundstate appears.
Demagnetization of spin systems at low temperature
Physical Review B, 1997
We report on analytical and numerical results for the time evolution of a ͑lattice͒ model of ferromagnetic particles after field inversion. Relaxation from the metastable phase occurs by superposition of two independent spin-flip processes: with probability 1Ϫp, the flip obeys the Metropolis rule at finite temperature, T, while the flip is performed at random, as for T→ϱ, with probability p. For small p, the latter process mimics quantum tunneling or any other sort of impurity that would induce both additional randomness and a nonequilibrium steady-state condition asymptotically. Critical avalanches and constant magnetic viscosity at low T ensue as two key features of the relaxation for any p 0. This has some implications for experiments on demagnetization and mesoscopic quantum coherence. ͓S0163-1829͑97͒08437-3͔ DEMAGNETIZATION OF SPIN SYSTEMS AT LOW . . .
Physical Review B, 2006
For studying the interplay of dipolar interaction and anisotropy energy in systems of ultrafine magnetic particles we consider simple cubic systems of magnetic dipoles with anisotropy axes pointing into the z-direction. Using Monte Carlo simulations we study the magnetic relaxation from several initial states. We show explicitely that, due to the combined influence of anisotropy energy and dipole interaction, magnetic chains are formed along the z-direction that organize themselves in frozen metastable domains of columnar antiferromagnetic order. We show that the domains depend explicitely on the history and relax only at extremely large time scales towards the ordered state. We consider this as an indication for the appearence of frozen metastable states also in real sytems, where the dipoles are located in a liquid-like fashion and the anisotropy axes point into random directions.
There is so far no clear-cut experimental analysis that can determine whether dipole-dipole interactions enhance or reduce the blocking temperature TB of nanoparticle assemblies. It seems that the samples play a central role in the problem and therefore, their geometry should most likely be the key factor in this issue. Yet, in a previous work, Jönsson and Garcia-Palacios did investigate theoretically this problem in a weak-interaction limit and without the presence of an external DC field. Based on symmetry arguments they reached the conclusion that the variation of the relaxation rate is monotonous. In the presence of an external magnetic field we show that these arguments may no longer hold depending on the experimental geometry. Therefore, the aim of this paper is to evaluate the variation of TB for a model system consisting of a chain of ferromagnetic nanoparticles coupled with long-range dipolar interaction with two different geometries. Rather than addressing a quantitative analysis, we focus on the qualitative variation of TB as a function of the interparticle distance a and of the external field h. The two following situations are investigated: a linear chain with a longitudinal axial anisotropy in a longitudinal DC field and a linear chain with a longitudinal axial anisotropy in a transverse field.
Multimagnon dynamics and thermalization in the S=1S=1S=1 easy-axis ferromagnetic chain
2021
Quasiparticles are physically motivated mathematical constructs for simplifying the seemingly complicated many-body description of solids. A complete understanding of their dynamics and the nature of the effective interactions between them provides rich information on real material properties at the microscopic level. In this work, we explore the dynamics and interactions of magnon quasiparticles in a ferromagnetic spin-1 Heisenberg chain with easy-axis onsite anisotropy, a model relevant for the explanation of recent terahertz optics experiments on NiNb2O6 [P. Chauhan et al., Phys. Rev. Lett. 124, 037203 (2020)], and nonequilibrium dynamics in ultra cold atomic settings [W.C. Chung et al., Phys. Rev. Lett. 126, 163203 (2021)]. We build a picture for the properties of clouds of few magnons with the help of exact diagonalization and density matrix renormalization group calculations supported by physically motivated Jastrow wavefunctions. We show how the binding energy of magnons effe...