E. De Biasi

Size and anisotropy determination by ferromagnetic resonance in dispersed magnetic nanoparticle systems

Abstract We present results for the FMR line shape modelling of non-interacting magnetic nanoparticle systems. We compare the results of the Smit and Beljers formalism and the usual linear-model, where the effective anisotropy field, HAeff, in the superparamagnetic regime is considered as a perturbation to the Zeeman interaction and added to the applied field, H. While the difference between these approaches is negligible for small HAeff (high temperature regime), it becomes more pronounced when HAeff≈H. We show how these results influence the determination of the parameters characterizing an array of random particles.

Competing interparticle interactions and surface anisotropy in NiO nanoparticles

We report unconventional magnetic properties on NiO nanoparticles of an average diameter ∼5.8(7) nm obtained by coprecipitation method. To investigate the effect of the intra and interparticles interactions in the magnetic properties nanoparticles were dispersed in a polyvinyl-pyrrodone matrix at two different concentrations. X-ray, ac, and dc magnetization and ferromagnetic resonance experiments were carried out on powder and dispersed NiO systems. Our results show that dispersed and concentrated samples exhibit following two different magnetic behaviors: (i) a high temperature peak related to the blocking of the particle core and (ii) a low temperature maximum likely related to the freezing of the frustrated spins on surface particle. Besides, we have observed that the low temperature maximum is not field-dependent and depend strongly on the distance among particles. This result can be understood taking account the decreasing of the dipolar interaction to more dispersed samples.

The nature and enhancement of magnetic surface contribution in model NiO nanoparticles

We report an alternative synthesis method and novel magnetic properties of Ni-oxide nanoparticles (NPs). The NPs were prepared by thermal decomposition of nickel phosphine complexes in a high-boiling-point organic solvent. These particles exhibit an interesting morphology constituted by a crystalline core and a broad disordered superficial shell. Our results suggest that the magnetic behavior is mainly dominated by strong surface effects at low temperature, which become evident through the observation of shifted hysteresis loops (approximately 2.2 kOe), coercivity enhancement (approximately 10.2 kOe) and high field irreversibility (>or=50 kOe). Both an exchange bias and a vertical shift in magnetization can be observed in this system below 35 K after field cooling. Additionally, the exchange bias field shows a linear dependence on the magnetization shift values, which elucidate the role of pinned spins on the exchange fields. The experimental data are analyzed in terms of the interplay between the interface exchange coupling and the antiferromagnetically ordered structure of the core.

Magnetic behavior of Ni nanoparticles with high disordered atomic structure

This report concerns the magnetic properties of colloidal Ni nanoparticles (NPs) obtained by chemical reduction of Ni(II) salt in an organic solvent. The NPs present a complex and disordered atomic structure, where small clusters of a few Ni atoms appear to coexist within each NP. These NPs exhibit interesting magnetic properties, with a low temperature ferromagnetic order followed by a transition from ferromagnetic to a “spin-glass-like” state as the temperature decreases. The results are discussed considering the role of the atomic ordering of the NPs on the corresponding magnetic behavior.

Comment on ``generalized Stoner-Wohlfarth Model and the Non-Langevin Magnetism of Single-Domain Particles'' by M.A. Chuev

The model recently proposed by M.A. Chuev [JETP Lett.85, 611 (2007)] has been analyzed. It has been shown that the anisotropy corrections near the blocking temperature always lead to a magnetization lower than the equilibrium value, contrary to the findings in Chuev’s work. In addition, the asymptotic high-temperature limit of Chuev’s model is inconsistent with the expected thermodynamic limit. Moreover, even at a low temperature, only a careful implementation of this theory can guarantee arriving at the correct result, avoiding some wrong conclusions in Chuev’s work.