194 research outputs found
Unified model of hyperthermia via hysteresis heating in systems of interacting magnetic nanoparticles
We present a general study of frequency and magnetic field dependence of the
specific heat power produced during field-driven hysteresis cycles in magnetic
nanoparticles with relevance to hyperthermia applications in biomedicine.
Employing a kinetic Monte-Carlo method with natural time scales allows us to go
beyond the assumptions of small driving field amplitudes and negligible
inter-particle interactions, which are fundamental to applicability of the
standard approach based on linear response theory. The method captures the
superparamagnetic and fully hysteretic regimes and the transition between them.
Our results reveal unexpected dipolar interaction-induced enhancement or
suppression of the specific heat power, dependent on the intrinsic statistical
properties of particles, which cannot be accounted for by the standard theory.
Although the actual heating power is difficult to predict because of the
effects of interactions, optimum heating is in the transition region between
the superparamagnetic and fully hysteretic regimes
Role of geometrical symmetry in thermally activated processes in clusters of interacting dipolar moments
Thermally activated magnetization decay is studied in ensembles of clusters
of interacting dipolar moments by applying the master-equation formalism, as a
model of thermal relaxation in systems of interacting single-domain
ferromagnetic particles. Solving the associated master-equation reveals a
breakdown of the energy barrier picture depending on the geometrical symmetry
of structures. Deviations are most pronounced for reduced symmetry and result
in a strong interaction dependence of relaxation rates on the memory of system
initialization. A simple two-state system description of an ensemble of
clusters is developed which accounts for the observed anomalies. These results
follow from a semi-analytical treatment, and are fully supported by kinetic
Monte-Carlo simulations.Comment: 9 pages, 6 figure
Non-converging hysteretic cycles in random spin networks
Behavior of hysteretic trajectories for cyclical input is investigated as a
function of the internal structure of a system modeled by the classical random
network of binary spins. Different regimes of hysteretic behavior are
discovered for different network connectivity and topology. Surprisingly,
hysteretic trajectories which do not converge at all are observed. They are
shown to be associated with the presence of specific topological elements in
the network structure, particularly with the fully interconnected spin groups
of size equal or greater than 4.Comment: 4 pages, 3 figure
Bloch points in nanostrips
Complex magnetic materials hosting topologically non-trivial particle-like objects such as skyrmions are under intensive research and could fundamentally change the way we store and process data. One important class of materials are helimagnetic materials with Dzyaloshinskii-Moriya interaction. Recently, it was demonstrated that nanodisks consisting of two layers with opposite chirality can host a single stable Bloch point of two different types at the interface between the layers. Using micromagnetic simulations we show that FeGe nanostrips consisting of two layers with opposite chirality can host multiple coexisting Bloch points in an arbitrary combination of the two different types. We show that the number of Bloch points that can simultaneously coexist depends on the strip geometry and the type of the individual Bloch points. Our simulation results allow us to predict strip geometries suitable for an arbitrary number of Bloch points. We show an example of an 80-Bloch-point configuration verifying the prediction
An in vivo coil setup for AC magnetic field-mediated magnetic nanoparticle heating experiments
In vitro and in vivo evaluation of magnetic nanoparticles in relation to magnetic fluid hyperthermia (MFH) treatment is an on-going quest. This current paper demonstrates the design, fabrication, and evaluation of an in vivo coil setup for real-time, whole body thermal imaging. Numerical calculations estimating the flux densities, and in silico analysis suggest that the proposed in vivo coil setup could be used for real-time thermal imaging during MFH experiments (within the limitations due to issues of penetration depth). Suchin silicoevaluations provide insightsinto the designofsuitable AMF applicators for AC magnetic field-mediated in vivoMNP heating as demonstrated in this study
Consistent energy barrier distributions in magnetic particle chains
We investigate long-time thermal activation behaviour in magnetic particle chains of variable length. Chains are modelled as Stoner–Wohlfarth particles coupled by dipolar interactions. Thermal activation is described as a hopping process over a multidimensional energy landscape using the discrete orientation model limit of the Landau–Lifshitz–Gilbert dynamics. The underlying master equation is solved by diagonalising the associated transition matrix, which allows the evaluation of distributions of time scales of intrinsic thermal activation modes and their energy representation. It is shown that as a result of the interaction dependence of these distributions, increasing the particle chain length can lead to acceleration or deceleration of the overall relaxation process depending on the initialisation procedure
Giant and reversible extrinsic magnetocaloric effects in La0.7Ca0.3MnO3 films due to strain
Large thermal changes driven by a magnetic field have been proposed for
environmentally friendly energy efficient refrigeration, but only a few
materials which suffer hysteresis show these giant magnetocaloric effects. Here
we create giant and reversible extrinsic magnetocaloric effects in epitaxial
films of the ferromagnetic manganite La0.7Ca0.3MnO3 using strain mediated
feedback from BaTiO3 substrates near a first-order structural phase transition.
Our findings should inspire the discovery of giant magnetocaloric effects in a
wide range of magnetic materials, and the parallel development of
nanostructured bulk samples for practical applications.Comment: 32 pages, 1 Table, 5 figures, supplementary informatio
Effects of interactions on the relaxation processes in magnetic nanostructures
Controlling the relaxation of magnetization in magnetic nanostructures is key to optimizing magnetic storage device performance. This relaxation is governed by both intrinsic and extrinsic relaxation mechanisms and with the latter strongly dependent on the interactions between the nanostructures. In the present work we investigate laser induced magnetization dynamics in a broadband optical resonance type experiment revealing the role of interactions between nanostructures on the relaxation processes of granular magnetic structures. The results are corroborated by constructing a temperature dependent numerical micromagnetic model of magnetization dynamics based on the Landau-Lifshitz-Bloch equation. The model predicts a strong dependence of damping on the key material properties of coupled granular nanostructures in good agreement with the experimental data. We show that the intergranular, magnetostatic and exchange interactions provide a large extrinsic contribution to the damping. Finally we show that the mechanism can be attributed to an increase in spin-wave degeneracy with the ferromagnetic resonance mode as revealed by semianalytical spin-wave calculations
History-dependent domain and skyrmion formation in 2D van der Waals magnet Fe3GeTe2
The discovery of two-dimensional magnets has initiated a new field of research, exploring both fundamental low-dimensional magnetism, and prospective spintronic applications. Recently, observations of magnetic skyrmions in the 2D ferromagnet Fe3GeTe2 (FGT) have been reported, introducing further application possibilities. However, controlling the exhibited magnetic state requires systematic knowledge of the history-dependence of the spin textures, which remains largely unexplored in 2D magnets. In this work, we utilise real-space imaging, and complementary simulations, to determine and explain the thickness-dependent magnetic phase diagrams of an exfoliated FGT flake, revealing a complex, history-dependent emergence of the uniformly magnetised, stripe domain and skyrmion states. The results show that the interplay of the dominant dipolar interaction and strongly temperature dependent out-of-plane anisotropy energy terms enables the selective stabilisation of all three states at zero field, and at a single temperature, while the Dzyaloshinksii-Moriya interaction must be present to realise the observed NĂ©el-type domain walls. The findings open perspectives for 2D devices incorporating topological spin textures
Thermodynamics of interacting magnetic nanoparticles
We apply the concepts of stochastic thermodynamics combined with transition-state theory to develop a framework for evaluating local heat distributions across the assemblies of interacting magnetic nanoparticles (MPs) subject to time-varying external magnetic fields. We show that additivity of entropy production in the particle state-space allows separating the entropy contributions and evaluating the heat produced by the individual MPs despite interactions. Using MP chains as a model system for convenience, without losing generality, we show that the presence of dipolar interactions leads to significant heat distributions across the chains. Our study also suggests that the typically used hysteresis loops cannot be used as a measure of energy dissipation at the local particle level within MP clusters, aggregates, or assemblies, and explicit evaluation of entropy production based on appropriate theory, such as developed here, becomes necessary
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