Magnetic relaxation of a one-dimensional model for small particle systems with dipolar interaction: Monte Carlo simulation

Ribas, R.; Labarta, A.

doi:10.1063/1.363502

Cited by 19 publications

(12 citation statements)

References 37 publications

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“…37 Moreover, it seems that scaling is accomplished over a wider range of T the stronger the interaction is, whereas in the weak interaction regime, scaling is fulfilled over a narrower range of times and T. As we will explain latter, this fact is due to the different variation of the effective energy barriers contributing to the relaxation in the two regimes. This power-law behavior has also been found by Ribas et al 40 in a 1D model of Ising spins and by Sampaio et al 15,41 and Toloza et al 42 in Monte Carlo simulations of the time dependence of the magnetic relaxation of 2D array of Ising spins under a reversed magnetic field. It has also been observed experimentally in arrays of micromagnetic dots tracked by focused ion beam irradiation on a Co layer with perpendicular anisotropy, 13,14 and also in discontinuous multilayers.…”

Section: T Ln"t / 0 … Scaling In the Presence Of Interactionsupporting

confidence: 77%

Magnetic relaxation in terms of microscopic energy barriers in a model of dipolar interacting nanoparticles

Iglesias

Labarta

2004

Phys. Rev. B

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The magnetic relaxation and hysteresis of a system of single domain particles with dipolar interactions are studied by Monte Carlo simulations. We model the system by a chain of Heisenberg classical spins with randomly oriented easy-axis and log-normal distribution of anisotropy constants interacting through dipoledipole interactions. Extending the so-called T ln͑t / 0 ͒ method to interacting systems, we show how to relate the simulated relaxation curves to the effective energy barrier distributions responsible for the long-time relaxation. We find that the relaxation law changes from quasilogarithmic to power-law when increasing the interaction strength. This fact is shown to be due to the appearance of an increasing number of small energy barriers caused by the reduction of the anisotropy energy barriers as the local dipolar fields increase.

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Section: T Ln"t / 0 … Scaling In the Presence Of Interactionsupporting

confidence: 77%

Magnetic relaxation in terms of microscopic energy barriers in a model of dipolar interacting nanoparticles

Iglesias

Labarta

2004

Phys. Rev. B

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“…This power-law behaviour has also been found by Ribas et al [45] in a 1D model of Ising spins and by Sampaio et al [48; 47] in a Monte Carlo simulation of the time dependence of the magnetic relaxation of 2D array of Ising spins under a reversed magnetic field. It has also been observed experimentally in arrays of micromagnetic dots tracked by focused ion beam irradiation on Co layer with perpendicular anisotropy [20; 2].…”

Section: T Ln(t/τ 0 ) Scaling In Presence Of Interactionsupporting

confidence: 80%

- Controlling the Structure and Properties of Nanostructured Magnetic Materials Produced by Depositing Gas-Phase Nanoparticles

Trohidou¹

2014

Magnetic Nanoparticle Assemblies

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Magnetic nanoparticle systems are difficult systems to model due to the interplay between intrinsic and collective effects. The first ones are associated to the magnetic properties of an individual particle and require considering the atomic spins of the magnetic ions, their mutual exchange interactions and magnetocrytalline anisotropy. Due to its finite size, an individual particle has different magnetic properties from the bulk counterpart material. Morover, the high proportion of surface spins with reduced coordination influences the equilibrium magnetic configuration of individual particle, with spin noncollinearities being a consequence of the distinct surface anisotropy. The collective effects have to do with interactions among the nanoparticles in an ensemble such as long range dipole-dipole interactions and can be tackled more easily by considering each nanoparticle as one effective spin having the magnitude of the total magnization of the particle. Therefore, at this level of description, nanoparticle ensembles can be modeled by a collection of macrospins having their own anisotropy axes and interacting through dipolar interactions, neglecting their internal structure, which inturn is equivalent to assuming that the interactomic exchange coupling is strong enough to keep atomic magnetic ions aligned along the global magnetization direction. Whereas in atomic magnetic materials the exchange interaction usually dominates over dipolar interactions, the opposite happens in many nanoscale particle or clustered magnetic systems, for which the interparticle interactions are mainly of dipolar origin. Therefore, one spin models (OSP) should, in principle, provide a correct description of non-interacting systems and, to a first approximation be valid also to account for the main features observed in more concentrated samples where interactions cannot be neglected. It has been shown also recently [17; 54] that spin non-collinearities due to surface anisotropy can even be incorporated within the OSP approach if an effective cubic anisotropy term is added to the original uniaxial anisotropy energy. However, incorporation of dipolar interactions along these lines does not seem feasable within the present theoretical frameworks.While we have a valid theoretical framework to compute equilibrium magnetic properties (such as thermal dependence M(T), isothermal field dependence M(H), low temperature configurations,...) analytically or numerically within the scope of OSP models [19] for non-interacting systems, models including dipolar interactions can only compute these quantities using perturbative thermodynamic theory and, even so, anaytical expressions can only be obtained under certain limits and approximations. In contrast, dynamic properties (such as hysteresis loops, FC-ZFC processes, susceptibility or magnetic relaxation) are non-equilibrium phenomena for which a unique theoretical framework covering the wide range of time June 15, 2018 22:41 PSP Book -9.75in x 6.5in Book_PSP_Nobibtex_Condmat 2 Time dependent pheno...

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“…Therefore, the overall dipolar interactions are demagnetizing and their effect can be considered through a mean magnetic field which reduces the height of the energy barrier associated to the intrinsic anisotropy of the particles. As a consequence, a slight shift to lower energies is observed in the energy barrier distribution 20 ͑see inset of Fig. 4͒.…”

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confidence: 89%

Erasing the glassy state in magnetic fine particles

1999

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magnetic fine particles exhibit most of the features attributed to glassy behavior, e.g., irreversibility in the hysteresis loops and in the zero-field-cooling and field-cooling curves extends up to very high fields, and aging and magnetic training phenomena occur. However, the multivalley energy structure of the glassy state can be strongly modified by a field-cooling process at a moderate field. Slow relaxation experiments demonstrate that the intrinsic energy barriers of the individual particles dominate the behavior of the system at high cooling fields, while the energy states corresponding to collective glassy behavior play the dominant role at low cooling fields. ͓S0163-1829͑99͒05421-1͔Fine magnetic particle systems show most of the features of glassy systems due to the random distribution of anisotropy axis, interparticle interactions, and surface effects. These main features 1 include the flattening of the field cooling susceptibility, 2 an increase in the magnetic viscosity, 3 the occurrence of aging effects, 4 the critical slowing down observed by ac susceptibility, 5 and the increase in the nonlinear susceptibility as the blocking temperature is approached from above.3 These features do not seem to be associated with a true spin-glass transition. Nevertheless, some authors claim that they reveal the existence of some kind of collective state. 3,6 Although this state is mostly attributed to the frustration induced by magnetic interactions between randomly distributed particles, 6 some studies suggest the dominant role of surface spin disorder.7 One of the facts that makes the behavior of these systems complex is the coexistence of the freezing associated with frustration and the intrinsic blocking of the particles. Consequently, depending on the time window of the experimental technique, one or both phenomena are observed. For example, blocking effects usually determine the results of Mössbauer spectroscopy, since the measured blocking temperature decreases with increasing interactions, 8 while freezing phenomena determine the thermal dependence of the cusp of the real part of the ac susceptibility for concentrated samples, which moves to higher temperatures with increasing interactions. In this paper, we show that the glassy state of strong interacting particles can be destroyed by a field-cooling process at a moderate magnetic field, which precludes a true phase transition. We also demonstrate that the dynamics of these systems is strongly affected by the initial magnetic moment configuration, in such a way that the glassy state determines the dynamic behavior only in low-cooling-field experiments, while at high cooling fields the dynamics is mostly dominated by the intrinsic energy barriers of the individual particles. These conclusions result from comparing the effective distribution of energy barriers obtained from the T ln (t/ 0 ) analysis of the magnetic relaxation 10 measured after field cooling the sample at different fields. The results of some aging experiments also reinforce these conclusio...

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Magnetic relaxation of a one-dimensional model for small particle systems with dipolar interaction: Monte Carlo simulation

Cited by 19 publications

References 37 publications

Magnetic relaxation in terms of microscopic energy barriers in a model of dipolar interacting nanoparticles

Magnetic relaxation in terms of microscopic energy barriers in a model of dipolar interacting nanoparticles

- Controlling the Structure and Properties of Nanostructured Magnetic Materials Produced by Depositing Gas-Phase Nanoparticles

Erasing the glassy state in magnetic fine particles

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