Low-frequency Magnetic Response Research Articles

Low-frequency magnetic response of gold nanoparticles

Gold nanoparticles (AuNPs) exposed to low frequency magnetic fields have shown promise in enhancing biological processes, such as cellular reprogramming. Despite the experimental evidence, a comprehensive understanding of the underlying physical principles and the corresponding theory remains elusive. The most common hypothesis is that functionalized nanoparticles transiently amplify magnetic fields, leading to improved cellular reprogramming efficiency. However, a detailed investigation on this topic is lacking. This paper bridges this knowledge gap by conducting a comprehensive investigation on the magnetic response of surface-modified AuNPs exposed to magnetic fields with frequencies up to hundreds of MHz. Starting with the inherent properties of bulk gold material, we explore a wide range of magnetic susceptibilities that might result from the redistribution of charge carriers due to bond molecules on the particle surfaces. Through analytical models and numerical electromagnetic simulations, we examine various geometric factors that can enhance the magnetic response, including the number of particles, spatial distribution, size, and shape. Our broad investigation provides researchers with analytical and numerical estimates of the magnetic response of nanoparticles, and the associated limits that can be expected. We found that a magnetic field enhancement comparable to the incident field requires very high magnetic susceptibilities, well beyond the values measured in functionalized gold nanoparticles thus far.

Numerical evaluation of active source magnetics as a method for imaging high-resolution near-surface magnetic heterogeneity

We have evaluated a geophysical method that uses a low-frequency magnetic source to image subsurface magnetic heterogeneity. This active source approach can be used to image magnetic features at higher resolutions than the conventional passive geomagnetic method. Importantly, this frequency-domain active source approach is independent of the effects of remanent magnetization, which complicates the interpretation of geomagnetic data. We carried out forward modeling of frequency-domain electromagnetic (EM) data and we found that, at frequencies of a few hertz, the magnetostatic response due to the induced magnetization dominates the EM induction response. The result suggests that it is possible to make magnetic interpretation of low-frequency EM data without having to consider the conductivity structure and the corresponding EM induction effect. We compare the anomalous magnetic responses with magnetic noise components and find that the proposed active source magnetic (ASM) method has a depth of investigation of approximately 300 m. Free-space field and inductive noise are considered as the most important issues affecting the depth of investigation. We also determine the potential for linear interpretation of magnetic heterogeneity under 0.1 SI by showing that the low-frequency magnetic response can be approximated by a linear magnetic response. In our synthetic experiments, inversion of the ASM data shows a marked enhancement in resolution, with no effect of the remanent magnetization, in contrast to geomagnetic inversion. These results show that the ASM method is a useful geophysical tool, especially when high-resolution imaging of magnetic susceptibility is required or where strong remanent magnetization complicates the magnetic interpretation.

Low-frequency magnetic response characteristics of magnetoelectric laminate composites considering non-isometric strain

This paper presents a low-frequency magnetoelectric response model of magnetoelectric laminate composites considering non-isometric strain. The stress and strain of the magnetostrictive and piezoelectric layers are analyzed by the stress function based on the shear stress transferring strain. Under the open circuit condition, low-frequency magnetoelectric response characteristics of the magnetoelectric laminate composites working on L-T mode are obtained by using equal strain at the adhesive surface of composites. This model can be used to analyze the impact of multiple parameters on the magnetoelectric voltage coefficient, such as the compliance coefficient, piezoelectric coefficient, thickness ratio of the magnetostrictive material and the magnetomechanical coupling coefficient, piezoelectric coefficient of the piezoelectric material. The theoretical results of this paper were compared with that of the equivalent circuit method, two equivalent circuit method consideration of demagnetization and the experiment. The experimental results of the Terfenol-D/PZT/Terfenol-D laminates show that the theoretical and experimental results error is 5.42% and the theoretical results of this paper are better agree with the experimental results than that of other methods.

Low-Frequency Magnetic Response in the Pseudogap Phase of Cuprate Perovskites

We calculate the magnetic susceptibility in the pseudogap phase of cuprates using the two-dimensional t-J model and the Mori projection operator technique. In this phase, the Fermi arcs lead to a quasi-elastic incommensurate magnetic response for low temperatures. The theory accounts for the reorientation of the susceptibility maxima from the axial to the diagonal direction occurring at small hole concentrations in La2−xSrxCuO4. A small cusp of the hole dispersion near the Fermi level, which is connected with the spin-polaron band, affords the growth of the maxima with decreasing frequency. As in the superconducting phase, the susceptibility maxima have an hourglass dispersion. The assumption of the phase separation into regions of superconducting and pseudogap phases in underdoped cuprates explains the lack of the superconducting gap in their susceptibility. The observed rapid change of widths of the susceptibility maxima with frequency is related to the contribution of the pseudogap phase. This contribution may also explain the strengthening of the quasi-elastic response by modest magnetic fields and by small impurity concentrations.

Casimir interactions between graphene sheets and metamaterials

The Casimir force between graphene sheets and metamaterials is studied. Theoretical results based on the Lifshitz theory for layered, planar, two-dimensional systems in media are presented. We consider graphene-graphene, graphene-metamaterial, and metal-graphene-metamaterial configurations. We find that quantum effects of the temperature-dependent force are not apparent until the submicron range. In contrast to results with bulk dielectric and bulk metallic materials, no Casimir repulsion is found when graphene is placed on top of a magnetically active metamaterial substrate, regardless of the strength of the low-frequency magnetic response. In the case of the metal-graphene-metamaterial setting, repulsion between the metamaterial and the metal-graphene system is possible only when the dielectric response from the metal contributes significantly.

Impurity effects on the spin excitation spectrum in anisotropic superconductors.

Effects of nonmagnetic and magnetic impurities on [ital d]-wave and extended-[ital s]-wave superconductors in two dimensions are studied. After the effects on the density of states are investigated, the effects on the dynamical spin susceptibility are studied. In the clean limit, the magnetic response has significant wave-vector dependence if the gap has a large anisotropy. When impurities are introduced this dependence is smeared. In the extended-[ital s]-wave superconductor with a fully open gap, the response at low frequencies is strongly suppressed at low temperatures in the clean limit. Magnetic impurities can give rise to low-lying excitations in the gap and enhance the low-frequency magnetic response at low temperatures. It is found, however, that the intensity is still quite weak with a moderate concentration of the impurities. It is discussed how these results can help in the determination of the symmetry of the order parameter of the high-[ital T][sub [ital c]] superconductors.

Fractal vortices in disordered superconductors

We measure the low-frequency magnetic response χ = χ′ + iχ″ of halogenated ceramics YBa 2Cu 3O 7−δF x as functions of x, AC field amplitude H 0 and temperature T. The field of first flux penetration displays scaling behaviour: H 1( x, T) = H 1( x, 0) (1 − T/ T c ) ζ , with T c the coherence temperature and ζ = 2.7 ± 0.2; in all cases H 1 ⪢ H c1 expected for the ideal material. A model where deviations from stoichiometry are assumed to result in local critical temperatures T c( r ), is described. Superconductivity is due to regions where T< T c( r ) with concentration p(T). The coherence temperature is defined by p( T c) = p c, the percolation threshold. Critical exponents appear to be related to percolation exponents, in particular ν = 2 ν p/3 β p≅ 1.33, in agreement with the experimental value found in granular superconductors. Expressions are found for the magnetic fields observed in Euclidean space while being due to currents restricted to fractal percolation clusters. The free energy of the resulting fractal vortices accounts for the experimental value of the exponent in the temperature dependence of H 1( T).

Low-frequency magnetic response in valence fluctuating and heavy fermion systems

By starting from an Anderson impurity Hamiltonian, a simple approximation for the spectral densities of the occupied and the empty f state is suggested. With it such thermodynamic quantities like the partial f occupancies go over, in the zero temperature limit, into the results of variational calculations. The approximation is applied in the presence of a crystalline electric field in order to calculate χ″( w,T). A comparison with previous, more elaborate calculations is provided for the case of vanishing CEF splittings.