58 research outputs found
Towards high-performance electrochemical thermal energy harvester based on ferrofluids
The ionic liquid-based thermo-electrochemical cells receive increasing attention as an inexpensive alternative to solid-state thermo-electrics for waste heat harvesting applications. Recently, it has been demonstrated that magnetic nanoparticles (MNPs) in liquid-based thermoelectric materials result in enhancement of the Seebeck effect opening new perspectives to the design of a thermoelectric device with relatively high efficiency and cost effectiveness. Here, the role of an interacting assembly of MNPs in the thermoelectric signal is studied for the first time. Based on a thermodynamic approach, an analytic expression has been derived for the Seebeck coefficient that includes the inter-particle magnetic interactions in the assembly and the nanoparticle's magnetic characteristics (saturation magnetization, magnetic anisotropy). Mesoscopic scale modelling with the implementation of the Monte Carlo Metropolis algorithm is performed to calculate their contribution to the Seebeck coefficient, for diluted assemblies of \u3b3-Fe2O3 and CoFe2O4 nanoparticles, materials commonly used in ferrofluids. The results demonstrate the increase of the size and temperature range of the Seebeck coefficient with the increase of nanoparticles\u2019 magnetic anisotropy paving the way for the detailed study of the magneto-thermal effects in high-performance thermoelectric materials based on ferrofluids
Dynamical and thermal effects in nanoparticle systems driven by a rotating magnetic field
We study dynamical and thermal effects that are induced in nanoparticle
systems by a rotating magnetic field. Using the deterministic Landau-Lifshitz
equation and appropriate rotating coordinate systems, we derive the equations
that characterize the steady-state precession of the nanoparticle magnetic
moments and study a stability criterion for this type of motion. On this basis,
we describe (i) the influence of the rotating field on the stability of the
small-angle precession, (ii) the dynamical magnetization of nanoparticle
systems, and (iii) the switching of the magnetic moments under the action of
the rotating field. Using the backward Fokker-Planck equation, which
corresponds to the stochastic Landau-Lifshitz equation, we develop a method for
calculating the mean residence times that the driven magnetic moments dwell in
the up and down states. Within this framework, the features of the induced
magnetization and magnetic relaxation are elucidated.Comment: 18 pages, 5 figure
Assembly-mediated Interplay of Dipolar Interactions and Surface Spin Disorder in Colloidal Maghemite Nanoclusters
Controlled assembly of single-crystal, colloidal maghemite nanoparticles is
facilitated via a high-temperature polyol-based pathway. Structural
characterization shows that size-tunable nanoclusters of 50 and 86 nm diameters
(D), with high dispersibility in aqueous media, are composed of 13 nm
(d) crystallographically oriented nanoparticles. The interaction effects are
examined against the increasing volume fraction, , of the inorganic
magnetic phase that goes from individual colloidal nanoparticles (= 0.47)
to clusters (= 0.72). The frozen-liquid dispersions of the latter exhibit
weak ferrimagnetic behavior at 300 K. Comparative Mossbauer spectroscopic
studies imply that intra-cluster interactions come into play. A new insight
emerges from the clusters temperature-dependent ac susceptibility that displays
two maxima in ''(T), with strong frequency dispersion. Scaling-law
analysis, together with the observed memory effects suggest that a superspin
glass state settles-in at T 160-200 K, while at
lower-temperatures, surface spin-glass freezing is established at T
40- 70 K. In such nanoparticle-assembled systems, with increased ,
Monte Carlo simulations corroborate the role of the inter-particle dipolar
interactions and that of the constituent nanoparticles surface spin disorder in
the emerging spin-glass dynamics
Monte Carlo simulations of ferromagnetism in p-CdMnTe quantum wells
Monte Carlo simulations, in which the Schrodinger equation is solved at each
Monte Carlo sweep, are employed to assess the influence of magnetization
fluctuations,short-range antiferromagnetic interactions, disorder, magnetic
polaron formation, and spin-Peierls instability on the carrier-mediated Ising
ferromagnetism in two-dimensional electronic systems. The determined critical
temperature and hysteresis are affected in a nontrivial way by the
antiferromagnetic interactions. The findings explain striking experimental
results for modulation-doped p-CdMnTe quantum wells.Comment: 4 pages, 4 figures, to be published in Physical Review Letters;
replaced figure 4; revised tex
Correlation between tunneling magnetoresistance and magnetization in dipolar coupled nanoparticle arrays
The tunneling magnetoresistance (TMR) of a hexagonal array of dipolar coupled
anisotropic magnetic nanoparticles is studied using a resistor network model
and a realistic micromagnetic configuration obtained by Monte Carlo
simulations. Analysis of the field-dependent TMR and the corresponding
magnetization curve shows that dipolar interactions suppress the maximum TMR
effect, increase or decrease the field-sensitivity depending on the direction
of applied field and introduce strong dependence of the TMR on the direction of
the applied magnetic field. For off-plane magnetic fields, maximum values in
the TMR signal are associated with the critical field for irreversible rotation
of the magnetization. This behavior is more pronounced in strongly interacting
systems (magnetically soft), while for weakly interacting systems (magnetically
hard) the maximum of TMR (Hmax) occurs below the coercive field (Hc), in
contrast to the situation for non-interacting nanoparticles or in-plane fields
(Hmax=Hc). The relation of our simulations to recent TMR measurements in
self-assembled Co nanoparticle arrays is discussed.Comment: 21 pages, 8 figures, submitted to Physical Review
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