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Dormann, J.L.; D’Orazio, F.; Lucari, F.; Tronc, E.; Prené, P.; Jolivet, Jean‐Pierre; Fiorani, D.; Cherkaoui, R.; Noguès, M.

doi:10.1103/physrevb.53.14291

Cited by 291 publications

(159 citation statements)

References 17 publications

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“…This is important in view of the very recent experimental results of Dormann et al 18 on ␥ Fe 2 O 3 with weak interparticle interactions which indicate that a may be as small as 0.05, hence the influence of a on 1 will be substantial.…”

Section: Introductionmentioning

confidence: 99%

Effect of an oblique magnetic field on the superparamagnetic relaxation time. II. Influence of the gyromagnetic term

Coffey

Crothers

Dormann³

et al. 1998

Phys. Rev. B

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The effect of a uniform magnetic field applied at an oblique angle to the easy axis of magnetization on the superparamagnetic ͑longitudinal or Néel͒ relaxation time is investigated by numerically solving the FokkerPlanck equation for the smallest nonvanishing eigenvalue. It is demonstrated that the reciprocal of the asymptotic formula for the Kramers escape rate in the intermediate to high damping limit for general nonaxially symmetric potentials when applied to the present problem, yields an acceptable asymptotic approximation to the Néel time for moderate to high values of the damping. Alternatively the corresponding Kramers low dissipation formula ͑energy controlled diffusion͒ provides an acceptable approximation for very small values of the damping. The effect of the gyromagnetic term which gives rise to coupling between the longitudinal and transverse modes of motion generally corresponds to an increase of the smallest nonvanishing eigenvalue and so to a decrease of the Néel relaxation time. The integral relaxation time or area under the slope of the curve of the decay of the magnetization is also evaluated. It is demonstrated that for sufficiently high values of the uniform field ͑much less, however, than that required to destroy the bistable nature of the potential͒ the reciprocal of the lowest nonvanishing eigenvalue ͑proportional to the Néel time, or the time of reversal of the magnetization͒ and the integral relaxation time may differ exponentially from one another signifying the contributions of modes other than that associated with the overbarrier ͑Néel͒ relaxation process to the overall relaxation process. The overall behavior is qualitatively similar ͑apart from the azimuthal dependence͒ to that of the axially symmetric case which arises due to the depletion of the shallower of the two potential wells by the uniform field, so that the fast processes in the deeper of the two wells may come to dominate the relaxation process at sufficiently high values of the uniform field. ͓S0163-1829͑98͒04229-5͔

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Section: Introductionmentioning

confidence: 99%

Effect of an oblique magnetic field on the superparamagnetic relaxation time. II. Influence of the gyromagnetic term

Coffey

Crothers

Dormann³

et al. 1998

Phys. Rev. B

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“…Although the values of bulk anisotropy constants K C can be obtained indirectly by magnetic measurements, the surface contribution K S to the anisotropy energy is more difficult to evaluate. However, for maghemite K C ≃ 4.7 × 10 4 erg/cm 3 [14,15] and K S has been estimated as K S ≃ 0.06 erg/cm 2 from Mössbauer experiments [16,17]. Therefore, in the simulations for the maghemite particle, we will vary the core anisotropy values in the range k C = 0.01 − 1 K and those of the surface anisotropy in the range k S = 1 − 100 K.…”

Section: Modelmentioning

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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“…For instance, it is known from the literature that, in general, the stronger the interaction the higher the effective energy barrier, i.e., σ e f f = σ + σ di p , with σ di p > 0. A possible theoretical approach to estimate σ di p is the DBF model, 41 which yields for the regime of strong interactions…”

Section: Interparticle Interactionsmentioning

confidence: 99%

Field dependent transition to the non-linear regime in magnetic hyperthermia experiments: Comparison between maghemite, copper, zinc, nickel and cobalt ferrite nanoparticles of similar sizes

et al. 2012

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Further advances in magnetic hyperthermia might be limited by biological constraints, such as using sufficiently low frequencies and low field amplitudes to inhibit harmful eddy currents inside the patient's body. These incite the need to optimize the heating efficiency of the nanoparticles, referred to as the specific absorption rate (SAR). Among the several properties currently under research, one of particular importance is the transition from the linear to the non-linear regime that takes place as the field amplitude is increased, an aspect where the magnetic anisotropy is expected to play a fundamental role. In this paper we investigate the heating properties of cobalt ferrite and maghemite nanoparticles under the influence of a 500 kHz sinusoidal magnetic field with varying amplitude, up to 134 Oe. The particles were characterized by TEM, XRD, FMR and VSM, from which most relevant morphological, structural and magnetic properties were inferred. Both materials have similar size distributions and saturation magnetization, but strikingly different magnetic anisotropies. From magnetic hyperthermia experiments we found that, while at low fields maghemite is the best nanomaterial for hyperthermia applications, above a critical field, close to the transition from the linear to the non-linear regime, cobalt ferrite becomes more efficient. The results were also analyzed with respect to the energy conversion efficiency and compared with dynamic hysteresis simulations. Additional analysis with nickel, zinc and copper-ferrite nanoparticles of similar sizes confirmed the importance of the magnetic anisotropy and the damping factor. Further, the analysis of the characterization parameters suggested core-shell nanostructures, probably due to a surface passivation process during the nanoparticle synthesis. Finally, we discussed the effect of particle-particle interactions and its consequences, in particular regarding discrepancies between estimated parameters and expected theoretical predictions.

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Thermal variation of the relaxation time of the magnetic moment ofγ-Fe2

Cited by 291 publications

References 17 publications

Effect of an oblique magnetic field on the superparamagnetic relaxation time. II. Influence of the gyromagnetic term

Effect of an oblique magnetic field on the superparamagnetic relaxation time. II. Influence of the gyromagnetic term

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

Field dependent transition to the non-linear regime in magnetic hyperthermia experiments: Comparison between maghemite, copper, zinc, nickel and cobalt ferrite nanoparticles of similar sizes

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