J.L. Dormann

On the models for interparticle interactions in nanoparticle assemblies: comparison with experimental results

Abstract The models proposed to account for the variation of the blocking temperature of a magnetic particle in an assembly of interacting particles are re-discussed. Experimental results on the thermal variation of the relaxation time are reported in support of the discussion. Except few cases related to a collective state, all data are well explained by the Dormann–Bessais–Fiorani model accounting for the interparticle interactions, in the framework of the superparamagnetic Neel–Brown model. The energy barrier unambiguously increases with increasing interactions, even when they are very weak, in disagreement with the Morup–Tronc model.

Magnetic behaviour of γ-Fe2O3 nanoparticles by mössbauer spectroscopy and magnetic measurements

Abstractγ-Fe2O3 nanoparticles with varying state of dispersion in a polymer have been investigated by Mössbauer spectroscopy, static magnetic measurements at low applied field, and alternative susceptibility measurements over a large range of frequencies (2×10−2–104 Hz). The dynamical behaviour was characterized through the variation of the blocking temperature with the characteristic time of the measurement. The Mössbauer blocking temperature was determined according to a procedure described. For quasi-isolated particles an Arrhenius law is demonstrated. Effects of interparticle interactions in concentrated and aggregated systems are satisfactorily explained by the previous model. Dependence of the superparamagnetic susceptibility on the experimental conditions interpreted using the Lorentz or Onsager fields is mentioned.

Collective magnetic state in nanoparticles systems

Studies of static and dynamic properties of γ-Fe2O3 nanoparticles (4.7 nm) with interparticle interactions of increasing strength reveal a progressive change of regime from pure superparamagnetic, for very weak interactions, to collective for strong interactions. The collective regime presents a glass low-temperature state similar to the spin-glass state, but the glass transition shows distinct features.

Magnetic properties of ultrafine α-Fe2O3 antiferromagnetic particles

Abstract Granular films consisting of finely dispersed α-Fe 2 O 3 grains (o= 30 A) in an Al 2 O 3 matrix have been studied by means of magnetization, susceptibility and ESR measurements. The results show a superparamagnetic behaviour above T b = 145 K (blocking temperature from dc measurements, applying H = 25 Oe) and give evidence of the antiferromagnetic nature of the particles, bearing a net uncompensated moment. The ESR spectra show a broad Lorentzian line, which broadens and shifts towards lower fields as the temperature decreases. The shift of the resonance fields are proportional to the second power of the linewidth, in agreement with a phenomenological model implying partially oriented particles.

The initial susceptibility in the FC and ZFC magnetisation processes

Abstract A new ‘blocking temperature’ model for the field-cooled and zero-field-cooled magnetisation processes for fine ferromagnetic particle systems is presented. On the basis of Stoner-Wohlfarth model and the discrete two-level model for the thermal relaxation, both blocked and superparamagnetic contributions to the sample magnetic moment are calculated. The effects of volume and easy-axis orientation distributions and also of the temperature rate are taken into account; a Lorentz mean field term is also considered. The simulation results are discussed.

Adsorption phenomena and magnetic properties of γ-Fe2O3 nanoparticles

Abstract The synthesis of powders of γ-Fe2O3 nanoparticles with different surface chemical states (hydration, sulphato and phosphato complexes), their characterization and zero-field-cooled magnetisation measurements are reported.

Magnetic Properties of Fe2O3 Particles Prepared by Sol-Gel Method

Fe2O3 fine particles dispersed in a silica gel have been prepared by a new sol-gel method and characterized. Various samples have been studied by means of Mössbauer spectroscopy for temperatures ranging between 4.2 K and 300 K. The results show that the average particle size and the kinds of Fe2O3 phase obtained (α, γ or ε) are strongly dependent of the temperature on the final heat treatment.

Magnetic dynamics of γ-Fe2O3 nanoparticles

Abstract The dynamical properties of γ-Fe2O3/polymer composites with particle size, aggregation state and concentration controlled independently have been studied by Mossbauer spectroscopy and various magnetic measurements. The main results are surveyed.

Collective glass state in a magnetic nanoparticle system

Studies of static and dynamic properties of γ-Fe2O3 particle (5 nm) assemblies reveal the existence of three magnetic regimes with increasing strength of the interparticle interactions: pure superparamagnetic for very weak interactions, modified superparamagnetic for weak-medium interactions, and collective for strong interactions. The last presents a low-temperature state similar to the spin-glass state, but the glass transition shows distinct features.

Nanophase magnetic materials: size and interaction effects on static and dynamical properties of fine particles

Some magnetic properties of nanophase particles are reviewed and discussed. Theoretical models are proposed for the static and dynamical properties accounting for volume distribution, random position of anisotropy axis and interparticle interactions. Statistics allows the determination of the additional energy barrier coming from the interparticle interactions. The behavior of the superparamagnetic susceptibility is interpreted using the Onsager field and thermoremanent magnetization treatment is discussed. The models predictions are in agreement with results obtained on Fe and γ-Fe2O3 particles.