Interaction effects and energy barrier distribution on the magnetic relaxation of nanocrystalline hexagonal ferrites

Batlle, X.; Muro, M. Garcı́a del; Labarta, A.

doi:10.1103/physrevb.55.6440

Cited by 69 publications

(64 citation statements)

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“…This change of behavior in the effective energy barrier distributions has been observed experimentally in ensembles of Ba ferrite fine particles, 33,45 in which evidence of T ln͑t / 0 ͒ scaling of the relaxation curves was demonstrated and the relevance of demagnetizing interactions in this sample was established by means of Henkel plots at different T. In this experiment, the authors also studied relaxation processes after different cooling fields and found that when increasing the cooling field, the effective distributions changed from a function with a maximum that extends to high energies to a narrower distribution with a peak at much lower energy scales for high cooling fields. The effective distribution at high H FC , which was there argued to be given by the intrinsic anisotropy barriers of the particles, appears shifted towards lower energy values with respect to the anisotropy distribution as derived from TEM due to the demagnetizing dipolar fields generated by the almost aligned spin configuration induced by the H FC .…”

Section: Effective Energy Barrier Distributions From T Ln"t / 0 …supporting

confidence: 64%

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: Effective Energy Barrier Distributions From T Ln"t / 0 …supporting

confidence: 64%

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 The low-field susceptibility ͑200 Oe͒ displayed a wide maximum in the ZFC curve at ca. 205 K and a flattened FC curve below this temperature, 15 being T 0 ϭϪ170Ϯ30 K, the value of the Curie temperature extrapolated from the reciprocal susceptibility, 15 which indicates strong interparticle interactions. Taking into account the aggregation state of the powder samples, direct exchange through the surface of neighboring particles and dipolar interactions are present.…”

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confidence: 99%

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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“…In particular, they display high closure fields with high values of the differential susceptibility and shifts in the hysteresis loops after field cooling [3,4,5,6] whose origin is still a matter of controversy. While some of these features have been attributed the existence of dipolar interactions among the particles [7,8,9,10], there are experimental evidences that finite-size and surface effects are crucial in order to understand this phenomenology. In this study, we present the results of Monte Carlo (MC) simulations of the magnetic properties of individual nanoparticles which aim at clarifying what is the specific role played by increased radial anisotropy at the surface on the magnetization processes.…”

Section: Introductionmentioning

confidence: 99%

Role of surface disorder on the magnetic properties and hysteresis of nanoparticles

Iglesias

Labarta

2004

Physica B: Condensed Matter

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We present the results of Monte Carlo simulations of a model of a single maghemite ferrimagnetic nanoparticle including radial surface anisotropy distinct from that in the core with the aim to clarify what is its role on the magnetization processes at low temperatures. The low temperature equilibrium states are analized and compared to those of a ferromagnetic particle with the same lattice structure. We have found that the formation of hedgehog-like structures due to increased surface anisotropy is responsible for a change in the reversal mechanism of the particles.

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Interaction effects and energy barrier distribution on the magnetic relaxation of nanocrystalline hexagonal ferrites

Cited by 69 publications

References 50 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

Erasing the glassy state in magnetic fine particles

Role of surface disorder on the magnetic properties and hysteresis of nanoparticles

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