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Ultra-low-density BiFeO3 nanoparticles have been prepared by pulsed laser deposition and their structure and magnetic properties have been studied. Annealing increases crystallinity and the size of the particles leading to an alteration of magnetic properties, observed from magnetic studies and evaluated using high-resolution transmission electron microscopy , selected area electron diffraction and x-ray diffraction patterns analysis. Transmission electron microscopy results show that the BiFeO3 as-deposited nanoparticles annealed up to 400 °C exhibit a orthorhombic distorted perovskite structure without secondary phase and with diameters varying from 9 nm (as-deposited) to 17 nm (annealed at 400 °C). Magnetic data exhibit exchange bias and magnetic blocking effects at low temperatures and typical superparamagnetic behavior at high temperatures. Meanwhile, the BiFeO3 nanoparticles annealed at 500 °C exhibit a rhombohedrally distorted perovskite structure with typical antiferromagnetic properties and diameter of about 56 nm. The analysis of magnetic relaxation time using the Arrhenius equation suggests a superparamagnetic blocking process of ferromagnetic clusters on the surface of the nanoparticles at low temperature. The magnetic properties are discussed considering the interactions between nanoparticles and the co-existence of different magnetic phases within the nanoparticles: an ordered antiferromagnetic core and ferromagnetic clusters on the surface. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The role played by dipolar interactions in the coercive field and remanence of nanoparticle systems is studied using different concentrations of nearly monodisperse nanoparticles of γ-Fe2O3. The 9.0 nm particles are isolated by organic encapsulation and diluted in paraffin. Magnetic properties were investigated by means of hysteresis obtained at different temperatures, zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves, and the so-called ΔM technique. The analysis of the experimental data points to the existence of a demagnetizing role played by the dipolar interaction in the coercive field. [ABSTRACT FROM AUTHOR]

A detailed study of the magnetic properties of colloidal Ni nanoparticles (∼5.2 nm) that do not present long range order is presented. Two magnetic contributions were observed at high temperatures (>25 K), a dominating paramagnetic one and a ferromagnetic contribution, detected by ferromagnetic resonance and low field magnetic measurements. Interestingly, the ferromagnetic behavior does not follow a classical superparamagnetic description. The effective anisotropy increases at low temperatures due to interactions among the ferromagnetic clusters, which leads the system to a frustrated state. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]