Search

Magnetic effects caused by dipolar interactions in single-domain magnetic ensembles at finite temperatures are described. A modified superparamagnetic approach based on the mean field theory and random anisotropy model has been developed to describe the magnetization curves of nanoparticle assemblies. The model was used to fit experimental zero-field-cooled and field-cooled magnetization curves in Fe 3 O 4 nanoparticles embedded in paraffin. The fitting parameters were based on structural properties of the materials and the strength of the interactions between nanoparticles. The model provides a quantitative description of the effects of the nanoparticle interaction with good agreement with the experiment. In addition, the model was adapted to describe magnetic properties of a NiFe thin film patterned into a nanodot array, showing potential to be used as a framework to predict magnetic interaction effects in high-density 2D arrays such as bit patterned media. [ABSTRACT FROM AUTHOR]

In this work, we investigated the effects of postdeposition heat treatment on structural and magnetic properties of CoFe2O4 nanoparticles produced by pulsed laser deposition. Structural analysis by X-ray diffraction, transmission electron microscopy (TEM) and Mössbauer spectroscopy indicate the formation of a single phase cobalt ferrite nanoparticles with the size ranging from 4.3 to 33.3 nm depending on the annealing temperature. The magnetic properties of the samples were investigated in a wide temperature range (50–400 K). Noticeable effects of the cubic magnetocrystalline anisotropy on the magnetization process of nanoparticles were observed for samples annealed at 450 °C and 600 °C, while for samples as-deposited and annealed at 300 °C the magnetization properties were dominated by a uniaxial effective anisotropy. ΔM technique was used to investigate the magnetic interaction among the nanoparticles. Only demagnetizing interactions were observed for the sample annealed up to 300 °C, while for the samples treated at 450 °C and 600 °C, both magnetizing and demagnetizing interactions were observed. The results are discussed considering the evolution of the nanoparticles’ nanostructure with anneals and its effects on the magnetic properties. [ABSTRACT FROM AUTHOR]

We have investigated the magnetic properties of Ni nanoaggregates produced by ion implantation in ZnO single crystals. Several deviations from classical models usually adopted to describe the magnetic properties of nanoparticle systems were found. The strain between host and Ni nanoaggregates induces a magnetic anisotropy with a preferred direction. We show that these anisotropy effects can be misinterpreted as a ferromagnetic or antiferromagnetic coupling among the nanoaggregates similar to that of an oriented, interacting nanocrystal ensemble. [ABSTRACT FROM AUTHOR]

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]

We report on the investigation of the exchange bias effect in Fe layers on EuTe(111), an antiferromagnetic semiconductor. For this ferromagnet (FM)/semiconducting antiferromagnet (AFM) exchange bias system, we have found positive and negative exchange bias effect (EB). Fresh samples exhibit positive EB, independently of the applied cooling field, indicating antiferromagnetic coupling between the FM and the AFM layers at the Fe/EuTe(111) interface. The change in EB with time, from positive EB for fresh samples to negative EB after short time,is attributed to aging effects at the Fe/EuTe interface. [ABSTRACT FROM AUTHOR]

The effects of the dipolar interaction on the coercive filed and remanence were investigated using nanosized Gamma-[Fe.sub.2][O.sub.3] nanoparticles isolated by organic encapsulation and diluted in paraffin. The experimental data, on analysis revealed the existence of a demagnetizing role played by the dipolar interaction in the coercive field.

Explains the thermal behavior of the coercivity and remanence of melt-spun copper-cobalt ribbons using Monte Carlo simulations and a model of balance between the magnetization of superparamagnetic and blocked granules of cobalt. Monte Carlo simulations; Balance model of coercivity; Conclusions.

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]

Low temperature measurements of specific heat and magnetic properties were performed on granular Cu90Co10 ribbons prepared by melt spinning technique. The thermal and magnetic behavior of an as-quenched sample is compared to that of an annealed sample at 500 °C for 1 h. It was found that the electronic specific heat γ decreases about 50% and the magnetic properties change from spin-glass to superparamagnetic with the annealing process. These results are interpreted and discussed considering the role of isolated Co atoms in the RKKY interaction among small nanoparticles. [ABSTRACT FROM AUTHOR]

The magnetic properties of sub-micrometric magnetic structures of Ni and permalloy (Ni80Fe20) patterned stripes are studied as functions of temperature. The samples are produced by combining an interferometric lithographic technique with sputtering. At room temperature, ferromagnetic resonance and hysteresis data indicate the existence of an in-plane uniaxial anisotropy in both samples. The NiFe sample has an easy axis along the stripes direction in the entire studied temperature range (2 < T < 300 K). On the other hand, an interesting change in the easy magnetization direction is observed in Ni samples, from parallel to perpendicular to the axis of the stripes, when the temperature is decreased. The results are interpreted in terms of a competition between shape and magnetoelastic anisotropies. [ABSTRACT FROM AUTHOR]