The processes for generation, amplification, and protraction of oscillating signals—often for information transfer—are inherently associated with dispersive decoherence and nonlinear phenomena. One such example is the case of wave patterns of non-negligible amplitudes that can emerge due to dispersion of a signal propagating through matter; for a continuously applied drive these patterns precede the main signal. Here, we investigate how spin wave generation inherently results in dispersive decoherence in the form of precursors. By quantifying three different frequency regimes, we investigate how decoherence is affected, or predetermined by the shape of the spin wave dispersion relation and, what is perhaps most interesting, it does not require non-linearity. Understanding the relationship between spin wave dispersion and decoherence can enable engineering magnonic devices supporting well-resolved signals for magnonic computing and signal processing.

Standard magnetization measurements on samples of small magnetic particles may generate conflicting results. We compare the mass magnetization of MgZn ferrite particles in a compressed bulk material and in dry powder and find that at low fields the values can differ by as much as 50%. We show here that embedding the particles in a silicone matrix and measuring the NMR linewidth in combination with simulations establishes a new method to evaluate the magnetization of the particles at different fields and temperatures. The NMR results agree with the direct magnetization measurements of the powder samples and the magnetization measurements of the particles embedded in silicone. This work is motivated, in part, by studies on using small magnetic particles as MRI temperature indicators, and we compare the effectiveness of these particles for low-field and high-field MRI thermometry.

We demonstrate the nonlinear generation of spin-wave edge modes with half the frequency of the applied oscillating field in a Co25Fe75 ferromagnetic stripe through micromagnetic simulations and experiments. The generation of half-frequency modes depends on the simultaneous presence of resonances near both the driving frequency and the half-frequency in different regions of the material. The half-frequency generation occurs in a system that is thin enough that typical three-magnon decay would not be allowed in a ferromagnetic resonance experiment in an extended film. We find that a limited range of driving frequencies will produce a half-frequency for a given set of system parameters. This range can be tuned by the strength of the oscillating field and the strength of the static external field. Our experimental results agree well with the findings from the simulations.

In this article, the Curie-Weiss type behavior and the appearance of an "interaction" or "ordering" temperature for a collection of magnetic nanoparticles is explored theoretically. We show that some systems where an interaction temperature is reported are too dilute for dipolar interactions to play a role unless at least some of the particles are clumped together. We then show using the most simple type of clumps (particle pairs) that positive and negative interaction temperatures are possible due to dipolar interactions. The clump orientation dramatically changes this result. Finally, we show that an apparent interaction temperature can be measured in magnetic nanoparticle systems that have no interactions between particles, due to some alignment of anisotropy easy axes. These results show that nanoscale physical structures affect the measured magnetic response of nanoparticles.

Magnetic particles are widely employed in applications ranging from medical to industrial. Their magnetic state, a single-domain versus a multi-domain configuration, depends on their size as well as the strength and frequency of an applied magnetic field. The critical size of a single-domain ferromagnetic/ferrimagnetic particle can be determined by minimizing the total free energy, which should include the Zeeman energy term when an external field is applied. To study the effect of an applied field on the critical size of single-domain particles as a function of their size, we performed numerical simulations using MuMax3 software for Fe, Co, Fe3O4, MgZn ferrites, and BaFe12O19. These materials exhibit a wide range of values for magnetic properties such as saturation magnetization and anisotropy. We determined the critical field at which a given sized particle becomes a single-domain magnetic particle. Our MuMax3 calculations for an iron spherical particle were compared with Kondorskii's analytical theory.

We study the behavior of nonlinear spin motion in one-dimensional magnetic chain models. We explore how the various parameters affect the phase transitions between linear and nonlinear behaviors both with and without the Dzyaloshinskii-Moriya interaction (DMI). A Fourier analysis method is employed to investigate the modes in the structure with DMI. In all cases, we find that uniaxial anisotropy is necessary for the Fermi-Pasta-Ulam effect to appear. We investigate the DMI influence through the use of phase diagrams that examine the transitions between the three fundamental states: linear, Fermi-Pasta-Ulam, and ergodic. The DMI does make significant changes in these phase diagrams because it allows for additional routes for the distribution of energy.

Recently there has been an explosion of research related to the Dzyaloshinskii-Moriya interaction (DMI) in magnetic and multiferroic materials. This article reviews the key themes in this research and provides insight into the consequences of the DMI through simple theoretical models. The topics covered include new magnetic structures such as skyrmions and changes in domain wall structures along with their motion under a variety of driving fields. In addition, the influence of DMI on linear and nonlinear spin wave behavior is discussed. Multiferroic materials and new two-dimensional materials with DMI are briefly discussed. Finally, we also present an overview of different DMI materials and their characteristic parameters and potential applications.

The processes for generation, amplification, and protraction of oscillating signals - often for information transfer - are inherently associated with dispersive decoherence and nonlinear phenomena. One such example are shock waves, which can be generated alongside, or instead of, the desired signal while dispersion and broadening can limit the propagation range of spin waves. Here, we develop a numerical model to investigate a generation process which results in dispersive shockwaves (DSW) being observed. In contrast to previous work, our system is strictly linear and does not require surface spin waves to observe DSW. We demonstrate how decoherence associated to DSWs is an inherent property of signals propagating through spin wave systems. We then discuss use cases for this behaviour.

We develop a computational method, based on Gauss's law, for calculating the effective permeability tensor of materials with magnetic inclusions. Electromagnetic composites find application in a variety of modern technologies since they have properties that are different from their constituent materials. However, many effective medium calculations for the permittivity or permeability tensor detailed over the past 120 years have issues with a limited range of applicability or with ignoring the coupling between different components of the tensor. Here, a computational approach is developed for calculating the effective tensor permeability of a composite material, which eliminates these issues. We show its validity by comparing to analytic results in the relevant limits, namely composites comprised of thin films and regular inclusions of low filling fraction. Examples of irregularly shaped magnetic inclusions and materials with high magnetic filling fractions are then presented for which analytic effective medium methods cannot make predictions.

We investigate the magnetic field induced spectral properties of metamaterials incorporating indium antimonide (InSb), using electromagnetic probes in the low terahertz frequency regime. An effective medium theory has been developed to explore the behavior of InSb interspersed with a simple dielectric in a gratinglike structure with various thicknesses and grating filling factors. The metamaterial grating structure impacts the light-matter interactions and substantially modifies reflectivity. Our numerical results demonstrate how nonreciprocal reflection can be enhanced and controlled in this spectral region through the composition of the magnetic grating. The current need for higher frequency communication technology drives the relevancy of this study for application to directional-dependent terahertz devices.

This study provides insight into the advantages and disadvantages of using ferrite particles embedded in agar gel phantoms as MRI temperature indicators for low-magnetic field scanners. We compare the temperature-dependent intensity of MR images at low-field (0.2 T) to those at high-field (3.0 T). Due to a shorter T1 relaxation time at low-fields, MRI scanners operating at 0.2 T can use shorter repetition times and achieve a significant T2⁎ weighting, resulting in strong temperature-dependent changes of MR image brightness in short acquisition times. Although the signal-to-noise ratio for MR images at 0.2 T MR is much lower than at 3.0 T, it is sufficient to achieve a temperature measurement uncertainty of about ±1.0 °C at 37 °C for a 90 μg/mL concentration of magnetic particles.

Electromagnetic interference shielding composite materials can be designed with properties that are tailored for specific applications. We have explored the magnetic properties of Ni-coated carbon fibers embedded in a polymer matrix through an extrusion process. This method can produce large, pliable flat sheets, which can be easily bent and shaped. Microscopy imaging shows that the extrusion process preferentially orders most of the fibers with their long axis along the extrusion direction. The measured samples reached 90% of the saturation magnetization value at a field of approximately 800 Oe along the extrusion direction, much lower than needed for other directions (in-plane perpendicular to the extrusion or perpendicular to the foil surface). A field on the order of 6 kOe must be applied to obtain the full saturation of the magnetization in all three directions. The absorption characteristics were measured, with zero external applied field to determine the shielding effectiveness of the composite materials for multiple orientations with respect to the TE10 mode of an S-band waveguide. The nickel-coated carbon fiber composite materials exhibit dramatic orientation-dependent shielding effectiveness. The most effective orientation has shielding effectiveness of up to 45 dB at 4.5 GHz, which is comparable to multi-walled carbon nanotube composites with similar volumetric filling fractions.

We explore the electromagnetic properties of a structure composed of two optically active materials—each layer contains a hyperbolic crystal with its anisotropy axis rotated with respect to the crystal surface. Through this, we can control the transmission spectra where at one frequency, light with a positive incidence angle is transmitted while it is absorbed for a negative incidence angle and the reverse occurs at a second frequency. Using a Gaussian beam analysis, we determine in which material layer the absorption occurs. In a radiating line current source study, we obtain tunable output collimated beams. From these discoveries, our structure can be applied as an efficient frequency or angle selector, demultiplexer, or filter.

Liquid crystal molecules tend to align with each other, often forming regions of opposite alignment that meet at a boundary-topological defects. These often offer information on configuration of the liquid crystal molecules with competing constraints on their order. Here, we experimentally demonstrate a mechanism to generate topological defects in the form of spatially oscillatory domain walls in nematic liquid crystals. We initially orient the molecules perpendicular to the substrate (i.e. homeotropic alignment) and when a horizontal electric field is applied, domain walls that change their shape with time emerge. These walls form at low frequencies of the applied electric field and remain stable as the frequency increases. If the initial biasing field is at larger frequencies (kHz regime), the domain walls still form, but are not oscillatory. We develop a general theory to predict the three-dimensional liquid crystal director evolution in any two-dimensional varying field. This theory gives the time dependence for the domain walls and confirms that both the oscillatory and straight walls are stable.

We have explored dynamic magnetic properties of micron-sized Ni-coated carbon fibers embedded in a polymer matrix for electromagnetic interference shielding applications. These hollow magnetic cylinders exhibit unusual dynamic magnetic properties, which were measured with a broad-band ferromagnetic resonance system (FMR). We observe three families of FMR modes, which are connected to different physical locations within the cylinder. We develop a simple analytic model to explain these results and corroborate resonant mode profiles with micromagnetic simulations. We find excellent agreement between experimental results and theoretical models. Our work indicates that global demagnetizing factors are not appropriate for understanding the spin motions in these hollow cylinders. The FMR absorption observed in these hallow cylinders is very different from those observed in nanowires or solid cylinders. The field-swept envelope of all the observed FMR resonances is very broad, approximately μ0H = 1 T, with a linewidth of individual modes around μ0ΔH = 250 mT. This can be important for electromagnetic shielding applications.

The consequences of non‐uniform exchange in magnetic systems are reported. The quantum mechanical exchange interaction between spins is responsible for the phenomenon of magnetic order, and is generally considered to be uniform across bulk magnetic systems. Partly inspired by the Dzyaloshinskii‐Moriya interaction—also known as antisymmetric exchange—a linearly varying exchange interaction is used along a magnetic strip as a route to spatial inversion symmetry‐breaking. It is found that, in addition to asymmetric modes and localization, spatially varying exchange can be used to design nonreciprocal magnetic signal excitation at frequencies that are tunable. Moreover, the authors’ work predicts nonreciprocity to occur across a vast range of frequencies up to hundreds of GHz. Such spin wave engineering is a key area of ongoing research in the fields of magnonics and spintronics, which are expected to enable the next generation of wireless communication technology and information processing. Analogous nonreciprocity is expected to occur in other wave systems with gradient properties.

Prevention of type-1 human immunodeficiency virus (HIV) infections has primarily been through condom usage, abstinence, and behavioral and risk awareness programs. Pre-exposure prophylaxis (PrEP) medications became approved for infection prevention in 2012. Mathematical models exploring the projections and impacts of PrEP on the spread of HIV are sparse. This study develops an epidemiological model for type-1 HIV infection accounting for both PrEP and condom usage. We utilized CDC surveillance data on HIV/AIDS statistics to develop a susceptible, infected, AIDS numerical model containing PrEP, condoms, and the combination of both. We explore projections with the percentages of PrEP usage held constant and the impact of average annual sexual partners. We find when 70% of the high-risk population uses PrEP properly with an average of 2 sexual partners per year, the eventual infected percentage drops to about 15%. In contrast, when PrEP is not used correctly and with 4 partners per year the total eventual infected percentage rises to 59%. PrEP can be a highly effective mitigator for the spread of HIV. However, an increased average number of sexual partners per year with improper PrEP usage can minimize the long-term effectiveness of PrEP.

Purpose One standard method, proton resonance frequency shift, for measuring temperature using magnetic resonance imaging (MRI), in MRI-guided surgeries, fails completely below the freezing point of water. Because of this, we have developed a new methodology for monitoring temperature with MRI below freezing. The purpose of this paper is to show that a strong temperature dependence of the nuclear relaxation time T-1 in soft silicone polymers can lead to temperature-dependent changes of MRI intensity acquired with T-1 weighting. We propose the use of silicone filaments inserted in tissue for measuring temperature during MRI-guided cryoablations. Methods The temperature dependence of T-1 in bio-compatible soft silicone polymers was measured using nuclear magnetic resonance spectroscopy and MRI. Phantoms, made of bulk silicone materials and put in an MRI-compatible thermal container with dry ice, allowed temperature measurements ranging from -60 degrees C to + 20 degrees C. T-1-weighted gradient echo images of the phantoms were acquired at spatially uniform temperatures and with a gradient in temperature to determine the efficacy of using these materials as temperature indicators in MRI. Ex vivo experiments on silicone rods, 4 mm in diameter, inserted in animal tissue were conducted to assess the practical feasibility of the method. Results Measurements of nuclear relaxation times of protons in soft silicone polymers show a monotonic, nearly linear, change with temperature (R-2 > 0.98) and have a significant correlation with temperature (Pearson's r > 0.99, p < 0.01). Similarly, the intensity of the MR images in these materials, taken with a gradient echo sequence, are also temperature dependent. There is again a monotonic change in MRI intensity that correlates well with the measured temperature (Pearson's r p < 0.01). The MRI experiments show that a temperature change of 3 degrees C can be resolved in a distance of about 2.5 mm. Based on MRI images and external sensor calibrations for a sample with a gradient in temperature, temperature maps with 3 degrees C isotherms are created for a bulk phantom. Experiments demonstrate that these changes in MRI intensity with temperature can also be seen in 4 mm silicone rods embedded in ex vivo animal tissue. Conclusions We have developed a new method for measuring temperature in MRI that potentially could be used during MRI-guided cryoablation operations, reducing both procedure time and cost, and making these surgeries safer.

Magnetic particles have been evaluated for their biomedical applications as a drug delivery system to treat asthma and other lung diseases. In this study, ferromagnetic barium hexaferrite (BaFe12O19) and iron oxide (Fe3O4) particles were suspended in water or glycerol, as glycerol can be 1000 times more viscous than water. The particle concentration was 2.50 mg/mL for BaFe12O19 particle clusters and 1.00 mg/mL for Fe3O4 particle clusters. The magnetic particle cluster cross-sectional area ranged from 15 to 1000 μμm2, and the particle cluster diameter ranged from 5 to 45 μμm. The magnetic particle clusters were exposed to oscillating or rotating magnetic fields and imaged with an optical microscope. The oscillation frequency of the applied magnetic fields, which was created by homemade wire spools inserted into an optical microscope, ranged from 10 to 180 Hz. The magnetic field magnitudes varied from 0.25 to 9 mT. The minimum magnetic field required for particle cluster rotation or oscillation in glycerol was experimentally measured at different frequencies. The results are in qualitative agreement with a simplified model for single-domain magnetic particles, with an average deviation from the model of 1.7 ± 1.3. The observed difference may be accounted for by the fact that our simplified model does not include effects on particle cluster motion caused by randomly oriented domains in multi-domain magnetic particle clusters, irregular particle cluster size, or magnetic anisotropy, among other effects.