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Magnetic dipolar interaction between nanoparticles affects the relaxation time and consequently the superparamagnetic behavior of the assembly. Understanding how the relaxation mechanisms depend on the interaction is important due to the wide use of these materials in the different fields of nanotechnology. In this work we have investigated the effect of dipolar interaction on superparamagnetic behavior of magnetic particle assemblies. The interaction was considered through an interaction magnetic field and was calculated using the mean field model. The relaxation time of an interacting system can be written in two equivalent ways, the first as a perturbation in the energy barrier of the isolated particle and the second as a perturbation in the thermal energy. With this equivalence, it was possible to determine that the shift in the blocking temperature is associated with the type of interaction that predominates in the assembly. Blocking temperature increases (decreases) if ferromagnetic (antiferromagnetic) interaction is the dominant interaction. [ABSTRACT FROM AUTHOR]

In the present paper the temperature effect on the interplay of Brownian and Néel relaxation processes, in a water-based magnetic fluid with mixed magnetite and tetragonal maghemite particles was analyzed. In consequence, the frequency (f=ω/2π) and temperature (T) dependencies of the complex magnetic permeability, μ(f,T)=μ′(f,T)−iμʺ(f,T), over the frequency range 3kHz–2MHz and at various values of temperatures within the range (25–90)°C were measured. The imaginary component of the complex magnetic permeability μʺ(f,T) shows two maxima: the first maximum is a large one, in the frequency range (10–40) kHz, being present only at temperatures below 60°C and the second maximum, around the frequency of 1MHz, is present at all temperatures at which measurements were made. The first maximum is assigned to the Brownian relaxation process of particle agglomerations within the sample, which have the hydrodynamic diameter in order of (30–33) nm. The second maximum of μʺ(f,T) component is assigned to the Néel relaxation process of tetragonal maghemite particles. The results are intended to clarify some fundamental issues concerning the magnetic properties of magnetic fluids in low-frequency field and can be used to study particle agglomeration processes in magnetic fluids, but also for biomedical applications such as cancer treatment by magnetic hyperthermia of tissues. [ABSTRACT FROM AUTHOR]

We investigate the local (particle level) and averaged magnetic relaxation characteristics in linear chain-like agglomerates of k magnetic nanoparticles (MNPs) or k -mers using computer simulations. The local relaxation behaviour is dictated by the corresponding dipolar field acting on the nanoparticle, irrespective of k -mer size. There is a wide variation in local relaxation characteristics as a function of nanoparticle position in a small nanocluster. On the other hand, there is more uniformity in the local relaxation response with a larger k -mer, except for MNPs at the boundary. Interestingly, there is a smooth decay of magnetization with small h d , independent of cluster size. In contrast, the magnetization ceases to relax in the presence of substantial dipolar interaction strength. Remarkably, the local Néel relaxation time τ N i is directly proportional to the corresponding dipolar field. Likewise, the averaged Néel relaxation time τ N also depends strongly on the k -mer size and h d . • State of the art kinetic Monte Carlo algorithm is implemented to probe the local and averaged relaxation characteristics in the small cluster of nanoparticles. • The dipolar interaction is found to dictate the local and averaged relaxation mechanism. • The form of the local magnetization-decay curve is precisely the same as that of the corresponding time evolution of the dipolar field. • There is a wide variation in the local Néel relaxation time in the tiny agglomerates of nanoparticles. • Such studies are performed for the first time to the best of our knowledge. [ABSTRACT FROM AUTHOR]

Superparamagnetic ZnxFe3−xO4 magnetic nanoparticles (0 ≤ x < 0.5) with spherical shapes of 16 nm average diameter and different zinc doping level have been successfully synthesized by co-precipitation method. The homogeneous zinc substitution of iron cations into the magnetite crystalline structure has led to an increase in the saturation magnetization of nanoparticles up to 120 Am2/kg for x ~ 0.3. The specific absorption rate (SAR) values increased considerably when x is varied between 0 and 0.3 and then decreased for x ~ 0.5. The SAR values are reduced upon the immobilization of the nanoparticles in a solid matrix being significantly increased by a pre-alignment step in a uniform static magnetic field before immobilization. The SAR values displayed a quadratic dependence on the alternating magnetic field amplitude (H) up to 35 kA/m. Above this value, a clear saturation effect of SAR was observed that was successfully described qualitatively and quantitatively by considering the non-linear field's effects and the magnetic field dependence of both Brown and Neel relaxation times. The Neel relaxation time depends more steeply on H as compared with the Brown relaxation time, and the magnetization relaxation might be dominated by the Neel mechanism, even for nanoparticles with large diameter. [ABSTRACT FROM AUTHOR]