Topological edge states are predicted to be responsible for the high efficient thermoelectric response of topological insulators, currently the best thermoelectric materials. However, to explain ...their figure of merit the coexistence of topological electrons, entropy and phonons can not be considered independently. In a background that puts together electrodynamics and topology, through an expression for the topological intrinsic field, we treat relativistic phonons within the topological surface showing their ability to modulate the Berry curvature of the bands and then playing a fundamental role in the thermoelectric effect. Finally, we show how the topological insulators under such relativistic thermal excitations keep time reversal symmetry allowing the observation of high figures of merit at high temperatures. The emergence of this new intrinsic topological field and other constraints are suitable to have experimental consequences opening new possibilities of improving the efficiency of this topological effect for their based technology.
Within a relativistic quantum formalism we examine the role of second-order corrections caused by the application of magnetic fields in two-dimensional topological and Chern insulators. This allows ...to reach analytical expressions for the change of the Berry curvature, orbital magnetic moment, density of states and energy determining their canonical grand potential and transport properties. The present corrections, which become relevant at relatively low fields due to the small gap characterizing these systems, determine the zero-field diamagnetic susceptibility of non-zero Berry curvature systems and unveil additional contributions from the magnetic field.
The oriented attachment of magnetic nanoparticles is recognized as an important pathway in the magnetic-hyperthermia cancer treatment roadmap, thus, understanding the physical origin of their ...enhanced heating properties is a crucial task for the development of optimized application schemes. Here, we present a detailed theoretical analysis of the hysteresis losses in dipolar-coupled magnetic nanoparticle assemblies as a function of both the geometry and length of the array, and of the orientation of the particles’ magnetic anisotropy. Our results suggest that the chain-like arrangement biomimicking magnetotactic bacteria has the superior heating performance, increasing more than 5 times in comparison with the randomly distributed system when aligned with the magnetic field. The size of the chains and the anisotropy of the particles can be correlated with the applied magnetic field in order to have optimum conditions for heat dissipation. Our experimental calorimetrical measurements performed in aqueous and agar gel suspensions of 44 nm magnetite nanoparticles at different densities, and oriented in a magnetic field, unambiguously demonstrate the important role of chain alignment on the heating efficiency. In low agar viscosity, similar to those of common biological media, the initial orientation of the chains plays a minor role in the enhanced heating capacity while at high agar viscosity, chains aligned along the applied magnetic field show the maximum heating. This knowledge opens new perspectives for improved handling of magnetic hyperthermia agents, an alternative to conventional cancer therapies.
Progress in the design of nanoscale magnets for localized hyperthermia cancer therapy has been largely driven by trial-and-error approaches, for instance, by changing of the stoichiometry ...composition, size, and shape of the magnetic entities. So far, widely different and often conflicting heat dissipation results have been reported, particularly as a function of the nanoparticle concentration. Thus, achieving hyperthermia-efficient magnetic ferrofluids remains an outstanding challenge. Here we demonstrate that diverging heat-dissipation patterns found in the literature can be actually described by a single picture accounting for both the intrinsic magnetic features of the particles (anisotropy, magnetization) and experimental conditions (concentration, magnetic field). Importantly, this general magnetic-hyperthermia scenario also predicts a novel non-monotonic concentration dependence with optimum heating features, which we experimentally confirmed in iron oxide nanoparticle ferrofluids by fine-tuning the particle size. Overall, our approach implies a magnetic hyperthermia trilemma that may constitute a simple strategy for development of magnetic nanomaterials for optimal hyperthermia efficiency.
The likelihood of magnetic nanoparticles to agglomerate is usually estimated through the ratio between magnetic dipole-dipole and thermal energies, thus neglecting the fact that, depending on the ...magnitude of the magnetic anisotropy constant (K), the particle moment may fluctuate internally and thus undermine the agglomeration process. Based on the comparison between the involved timescales, we study in this work how the threshold size for magnetic agglomeration (daggl) varies depending on the K value. Our results suggest that small variations in K-due to, e.g., shape contribution, might shift daggl by a few nm. A comparison with the usual superparamagnetism estimation is provided, as well as with the energy competition approach. In addition, based on the key role of the anisotropy in the hyperthermia performance, we also analyse the associated heating capability, as non-agglomerated particles would be of high interest for the application.
The performance of magnetic nanoparticles is intimately entwined with their structure, mean size and magnetic anisotropy. Besides, ensembles offer a unique way of engineering the magnetic response by ...modifying the strength of the dipolar interactions between particles. Here we report on an experimental and theoretical analysis of magnetic hyperthermia, a rapidly developing technique in medical research and oncology. Experimentally, we demonstrate that single-domain cubic iron oxide particles resembling bacterial magnetosomes have superior magnetic heating efficiency compared to spherical particles of similar sizes. Monte Carlo simulations at the atomic level corroborate the larger anisotropy of the cubic particles in comparison with the spherical ones, thus evidencing the beneficial role of surface anisotropy in the improved heating power. Moreover we establish a quantitative link between the particle assembling, the interactions and the heating properties. This knowledge opens new perspectives for improved hyperthermia, an alternative to conventional cancer therapies.
The Fe‐MgO core‐shell morphology is proposed within the single‐domain nanoparticle regime as an enhanced magnetically driven hyperthermia carrier. The combinatory use of metallic iron as a core ...material together with the increased particle size (37–65 nm) triggers the tuning of dipolar interactions between particles and allows for further enhancement of their collective heating efficiency via concentration control. A theoretical universal estimation of hysteresis losses reveals the role of dipolar interactions on heating efficiency and outlines the strong influence of coupling effects on hyperthermia opening a novel roadmap towards multifunctional heat‐triggered theranostics particles.
The correlations arising from dipole–dipole interactions and their influence on the hyperthermia efficacy are studied both theoretically and experimentally and found to be in quantitative agreement. The calculation represents an analytical model of hyperthermia in magnetic interacting particle systems to explain in a simple way the ubiquitous behavior observed in this class of materials.
The relative orientation between anisotropy easy axes of magnetization of magnetic nanoparticles (MNPs) and the applied magnetic field direction determines their heating properties and thus needs to ...be considered for accurate heating applications. In this work we systematically study the heating properties of a system of interacting MNPs with ferromagnetic-like behavior (i.e. in the blocked state), randomly distributed in space, as a function of the degree of collinearity of their easy anisotropy axes along the magnetic field direction. The easy-axes of the particles were distributed at random within cones of different aperture angles (0, 10, 22.5 and 45 degrees with respect to the field direction), under different conditions of magnetic field amplitude and interparticle interactions. Our results show that easy-axes collinearity marks a clear threshold for heat dissipation at low interacting conditions, but increasing interactions tends to attenuate this effect.
•Heating is strongly influenced by easy-axes collinearity and field direction.•Easy-axes collinearity marks a threshold for heat dissipation at low interaction.•Increasing interactions attenuates this influence.
Magnetic nanoparticles (MNPs) constitute promising nanomedicine tools based on the possibility of obtaining different actuations (for example, heating or mechanical response) triggered by safe remote ...stimuli. Particularly, the possibility of performing different tasks using the same entity constitutes a main research target towards optimizing the treatment. But such a goal represents, in general, a very difficult step because the requisites for achieving efficient responses for separate actuations are often disparate - if not completely incompatible. An example of this is the response of MNPs to external AC fields, which could in principle be exploited for either magneto-mechanical actuation (MMA) at low frequencies (tens of Hz); or heat release at high frequency (0.1-1 MHz range) for magnetic fluid hyperthermia (MFH). The problem is that efficient MMA involves large torque, the required material parameters for which are detrimental to high heating, thus hindering the possibility of effective alternation between both responses. To overcome such apparent incompatibility, we propose a simple approach based on the use of anisotropic MNPs. The key idea is that the AC-frequency change must be concurrent with a field-amplitude variation able to promote - or impede - the reversal over the shape-determined anisotropy energy barrier. This way it is possible to switch the particle response between an efficient (magnetically dissipationless) rotation regime at low-
f
, for MMA, and a "frozen" (non-rotatable) high-energy-dissipation regime at high-
f
, for MFH. Furthermore, we show that such an alternation can also be achieved within the same high-
f
MFH regime. We use combined Brownian dynamics and micromagnetic simulations, based on real experimental samples, to show how such a field threshold can be tuned to working conditions with hexagonal-disk shape anisotropy.
Hexagonal-shape magnetic nanoparticles for efficient alternation between magneto-mechanical actuation and heating.
Nitride coatings are increasingly demanded in the cutting- and machining-tool industry owing to their hardness, thermal stability and resistance to corrosion. These properties derive from strongly ...covalent bonds; understanding the bonding is a requirement for the design of superhard materials with improved capabilities. Here, we report a pressure-induced cubic-to-orthorhombic transition at approximately 1 GPa in CrN. High-pressure X-ray diffraction and ab initio calculations show an unexpected reduction of the bulk modulus, K0, of about 25% in the high-pressure (lower volume) phase. Our combined theoretical and experimental approach shows that this effect is the result of a large exchange striction due to the approach of the localized Cr:t3 electrons to becoming molecular-orbital electrons in Cr-Cr bonds. The softening of CrN under pressure is a manifestation of a strong competition between different types of chemical bond that are found at a crossover from a localized to a molecular-orbital electronic transition.
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