Magnetism

Recording systems that deal with Extra Low Frequency (ELF) data in the Schumann resonance (SR) range exhibit high sensitivity to external noise. In our effort to refine a time series by identifying and removing external disturbances from real data, we analyzed the effect of induced deliberate anthropogenic disturbances. The signals were recorded at the same time and same place by two separate systems with different designs and implementations. The main purpose of this experiment was to confirm that different systems in various observational sites could identify parasitic noises in the same way. The outcomes of this study may help ELF observers to discern intrinsic signals from artificial noise. Full article

Electro-mechanical devices incorporating rotating magnetic fields can be modelled using a wide range of analytical techniques. Choosing a modelling technique usually requires a trade off between computational efficiency and accuracy. Magnetic flux-based models aim to achieve an optimum balance between computational intensity and accuracy, as required for real time control applications. This paper demonstrates how vector-based magnetic circuit equations can be used to describe the operational characteristics of an induction motor at a more fundamental level than commonly used magnetic flux models. Doing so allows for closed form equations to be derived directly from device-specific geometry. The resultant model has advantages of numerical method-based analytical techniques while retaining the computational efficiency of closed form equations. Full article

We derive the reflection and refraction laws for an electron spin incident from a quasi-two-dimensional semiconductor region (with no spin–orbit interaction) on the metallic surface of a topological insulator (TI) when the two media are in contact edge to edge. For a given incident angle, there can generally be two different refraction angles for refraction into the two spin eigenstates in the TI surface, resulting in two different ‘spin refractive indices’ (birefringence) and the possibility of two different critical angles for total internal reflection. We derive expressions for the spin refractive indices and the critical angles, which depend on the incident electron’s energy for given effective masses in the two regions and a given potential discontinuity at the TI/semiconductor interface. For some incident electron energies, there is only one critical angle, in which case 100% spin polarized injection can occur into the TI surface from the semiconductor if the angle of incidence exceeds that critical angle. The amplitudes of reflection of the incident spin with and without spin flip at the interface, as well as the refraction (transmission) amplitudes into the two spin eigenstates in the TI, are derived as functions of the angle of incidence. Full article

By combining the two types of magnetoelastic and magnetochromatic materials in an epoxy, we can make a hybrid system that exhibits an optical response due to an elastic strain. It could be used in structural health monitoring, for real-time monitoring of crack propagation or general evaluation of the condition of a structure, both visualized by a change in color. In this study, magnetostrictive polymer composites (MPCs) with Fe₈₁Al₁₉ (Alfenol) alloy particles are evaluated to determine magneto-elastic properties in composite patches attached to a surface, prior to understanding the full hybrid magneto–elasto–optical interactions. To measure static magneto-elastic performance, a tension apparatus within a solenoid was fabricated to apply uniform strain to the MPC patch on an aluminum dog-bone substrate. It was demonstrated that, for epoxies with an elastic modulus higher than ~0.1 GPa, a tensile strain/stress applied to the composite improved magneto-elastic coupling, resulting in increased permeability values, at least up to strains of 0.1%. Composites were fabricated with both spherical and flake-shaped powders, with flake-shaped powders exhibiting better magnetic responses than those with spherical morphology. Alfenol MPCs were also measured dynamically at ultrasonic frequencies, exhibiting comparable dynamic sensing performance to Galfenol at 120 kHz using ultrasonic guided wave techniques. Full article

The quasi in-situ indentation behaviour of <110>||BD and <111>||BD-oriented grains in a FeCo alloy is studied in this investigation. The effect of build height on melt pool shape and melt pool size is also studied by finite element method simulations. As the building height increases, the aspect ratio of the elliptical melt pool increases. Correspondingly, the effect of the laser scan speed on the melt pool shape and size is studied by the finite element method, because, as the laser scan speed increases, the aspect ratio of the elliptical melt pool increases, too. The microstructural characterisation of the indentation area before and after indentation is performed by electron backscatter diffraction (EBSD). Based on the EBSD data grain reference orientation deviation (GROD), calculations are performed to describe the effect of indentations on the neighbouring grain orientations. High GROD angles are detected in the neighbouring grain region adjoining the indented grain. An in-depth slip trace analysis shows the activation of all three slip systems ({110}<111>, {112}<111> and {123}<111>) which is also confirmed by slip lines on the sample surface that are detected by laser scanning confocal microscopy. A high concentration of geometrically necessary dislocations (GNDs) are observed on the adjoining area to the indentation. Local surface topography measurements by laser scanning confocal microscopy confirmed the formation of pile-ups near the indentation. Full article

Maxwell’s equations and the Lorentz force equation form the foundation of classical electromagnetic theory and their discovery led to the development of special relativity. Despite this achievement, their universal compatibility with the conservation of momentum and relativistic energy transformations is still debated. Incorporating effects of hidden momentum with the Lorentz force equation or using the Einstein–Laub formula are two common approaches to address some of these concerns. Which method to use, or if a change to classical electromagnetism is even required, remains controversial. A new theoretical approach is presented in this paper to address this using relativistic electromagnetic energy inertial frame transformations. These transformations identify a situation where an apparent violation of conservation laws could occur and how to consolidate this with electromagnetic theory. An explanation regarding the elementary nature of magnetism and the relationship between inertia and electromagnetic energy is also commented on. Full article

The rare earth-free Mn₅Ge₃ compound shows magnetocaloric properties similar to those of pure Gd; therefore, it is a good candidate for magnetic refrigeration technology. In this work, we investigate the influence of chemical substitution on the crystal structure and the magnetic, thermodynamic, and magnetocaloric properties of a polycrystalline Mn₅Ge₃ compound prepared by induction melting. For this purpose, we replaced 5% of the Mn with Cr or Co and 5% of the Ge with B or Al. The additional chemical elements were shown not to change the crystal structure of the parent compound (space group P6₃/mcm, No. 193). In the case of the magnetic properties, all samples remained ferromagnetic with the ordering temperature (T_C) lower than for the original compound (T_C = 295(1) K). The exception was the sample with B, where we observed an increase in T_C by 3 K. The maximum value of the magnetic entropy change, |∆S_m|^MAX (for a magnetic field change of 5 T), decreased from 7.1(1) for Mn₅Ge₃ to 6.2(1), 6.8(1), 4.8(1), and 5.8(1) J kg⁻¹ K⁻¹ for the alloys with B, Al, Cr, and Co, respectively. The adiabatic temperature change (∆T_ad) (for a magnetic field change of 1 T) was determined from the specific heat measurements and was equal to 1.1(1), 1.2(1), 1.2(1), 0.8(1), and 0.8(1) K for Mn₅Ge₃, Mn₅Ge_2.85B_0.15, Mn₅Ge_2.85Al_0.15, Mn_4.75Cr_0.25Ge₃, and Mn_4.75Co_0.25Ge₃, respectively. The obtained data were compared with those from the literature. It was found that the substitution allowed for tuning of the ordering temperature in a wide temperature range. At the same time, the reduction in the magnetocaloric parameters’ values was relatively small. Therefore, the produced Mn₅Ge₃-based alloys allow for the expansion of the operation temperature range of the parent compound as a magnetocaloric material. Full article

Stoichiometric compositions of NiO were prepared by the standard chemical co-precipitation method to inspect the effect of the calcination temperature on structures, morphology, and physical properties. The samples were calcined at three different temperatures viz. 350 °C, 550 °C, and 650 °C for 5 h. X-ray diffraction analysis confirmed the cubic (Fm-3m) structure of the prepared samples. The average crystalline size increases from 41 nm to above 100 nm as the calcination temperature increases in the same time period. In Fourier transform infrared spectra, the spectral absorption bands were observed at ~413, 434, and 444 cm^–1. The bandgap energy of NiO particles is decreased from 3.6 eV to 3.41 eV as the calcination temperature increases. The magnetic analysis confirms that the magnetization value of NiO is invariably decreased with a rise in the calcination temperature. Full article

We report magnetism of tetragonal β-Fe₃Se₄ nanoplates controllably synthesized by thermal decomposition at 603 K of inorganic–organic (β-Fe₂Se₃)₄[Fe(tepa)] hybrid nanoplates (tepa = tetraethylenepentamine). (β-Fe₂Se₃)₄[Fe(tepa)] hybrid precursor and β-Fe₃Se₄ nanoplates are in single crystal features as characterized by selected area electron diffraction. Rietveld refinements reveal that ordered inorganic–organic (β-Fe₂Se₃)₄[Fe(tepa)] hybrid nanoplates are in a tetragonal layered crystal structure with a space group of I4cm (108) and room-temperature lattice parameters are a = 8.642(0) Å and c = 19.40(3) Å, while the as-synthetic tetragonal β-Fe₃Se₄ nanoplates have a layered crystal structure with the P4/nmm space group, and room-temperature lattice parameters are a = 3.775(8) Å and c = 5.514(5) Å. Magnetic measurements show the weak ferrimagnetism for (β-Fe₂Se₃)₄[Fe(tepa)] hybrid nanoplates at room temperature, while the as-synthetic β-Fe₃Se₄ nanoplates are antiferromagnetic in a temperature range between 120 and 420 K but in a ferrimagnetic feature below ~120 K. The as-synthetic β-Fe₃Se₄ nanoplates are thermally instable, which are transformed to ferrimagnetic β-Fe₃Se₄ nanoplates by annealing at 623 K (a little higher than the synthetic temperature). There is an irreversible change from antiferromagnetism of the as-synthetic β-Fe₃Se₄ phase to the ferrimagnetism of the as-annealed β-Fe₃Se₄ phase in a temperature between 420 and 470 K. Above 470 K, the tetragonal β-Fe₃Se₄ phase transforms to monoclinic Fe₃Se₄ phase with a Curie temperature (T_C) of ~330 K. This discovery highlights that crystal structure and magnetism of Fe-Se binary compounds are highly dependent on both their phase compositions and synthesis procedures. Full article

An analytical approach for computing the coefficient of refrigeration performance (CRP) was described for materials that exhibited a giant inverse magnetocaloric effect (MCE), and their governing thermodynamics were reviewed. The approach defines the magnetic work input using thermodynamic relationships rather than isothermal magnetization data discretized from the literature. The CRP was computed for only cyclically reversible temperature and entropy changes in materials that exhibited thermal hysteresis by placing a limit on their operating temperature in a thermodynamic cycle. The analytical CRP serves to link meaningful material properties in first-order MCE refrigerants to their potential work and efficiency and can be employed as a metric to compare the behaviors of dissimilar alloy compositions or for materials design. We found that an optimum in the CRP may exist that depends on the applied field level and Clausius–Clapeyron (CC) slope. Moreover, through a large literature review of NiMn-based materials, we note that NiMn(In/Sn) alloys offer the most promising materials properties for applications within the bounds of the developed framework. Full article

Thin films of magnetic topological insulators (TIs) are expected to exhibit a quantized anomalous Hall effect when the magnetizations on the top and bottom surfaces are parallel and a quantized topological magnetoelectric effect when the magnetizations have opposite orientations. Progress in the observation of these quantum effects was achieved earlier in the films with modulated magnetic doping. On the other hand, the molecular-beam-epitaxy technique allowing the growth of stoichiometric magnetic van der Waals blocks in combination with blocks of topological insulator was developed. This approach should allow the construction of modulated heterostructures with the desired architecture. In the present paper, based on the first-principles calculations, we study the electronic structure of symmetric thin film heterostructures composed of magnetic MnBi${}_2$Se${}_4$ blocks (septuple layers, SLs) and blocks of Bi${}_2$Se${}_3$ TI (quintuple layers, QLs) in dependence on the depth of the magnetic SLs relative to the film surface and the TI spacer between them. Among considered heterostructures we have revealed those characterized by nontrivial band topology. Full article

In this paper, a unique approach to the imaging of non-metallic media using capacitive sensing is presented. By using customized sensor plates in single-ended and differential configurations, responses to hidden objects can be captured over a cylindrical aperture surrounding the inspected medium. Then, by processing the acquired data using a novel imaging technique based on the convolution theory, Fourier and inverse Fourier transforms, and exact low resolution electromagnetic tomography (eLORETA), images are reconstructed over multiple radial depths using the acquired sensor data. Imaging hidden objects over multiple depths has wide range of applications, from biomedical imaging to nondestructive testing of the materials. Performance of the proposed imaging technique is demonstrated via experimental results. Full article

Polymer bonded magnets based on thermoplastics are economically produced by the injection molding process for applications in sensor and drive technology. Especially the lack of orientation in the edge layer, as well as the chemical resistance and the creep behavior limit the possible implementations of thermoplastic based polymer bonded magnets. However, thermoset based polymer bonded magnets have the opportunity to expand the applications by complying with the demands of the chemical industry or pump systems through to improved chemical and thermal resistance, viscosity and creep behavior of thermosets. This paper investigates the influence of hard magnetic particles on the flow and curing behavior of highly filled thermoset compounds based on an epoxy resin. The basic understanding of the behavior of those highly filled hard magnetic thermoset systems is essential for the fabrication of polymer bonded magnets based on thermosets in the injection molding process. It is shown that several factors like the crystal structure, the particle shape and size, as well as the thermal conductivity and the adherence between filler and matrix influence the flow and curing behavior of highly filled thermoset compounds based on epoxy resin. However, these influencing factors can be applied to any filler system with respect to a high filler amount in a thermoset compound, as they are based on the material behavior of particles. Further, the impact of the flow and curing behavior on the magnetic properties of polymer bonded magnets based on thermosets is shown. With that, the correlation between material based factors and magnetic properties within thermosets are portrayed. Full article

Ferroelectric samples Sr_1−xBa_xTiO₃ (BST), where x = 0, 0.2, 0.4, 0.6, 0.8 and 1, were prepared using the tartrate precursor method and annealed at 1200 °C for 2 h. X-ray diffraction, “XRD”, pattern analysis verified the structure phase. The crystallite size of the SrTiO₃ phase was calculated to be 83.6 nm, and for the TiO₂ phase it was 72.25 nm. The TEM images showed that the crystallites were agglomerated, due to their nanosize nature. The AC resistivity was measured as temperature dependence with different frequencies 1 kHz and 10 kHz. The resistivity was decreased by raising the frequency. The dielectric properties were measured as the temperature dependence at two frequencies, 1 kHz and 10 kHz. The maximum amount of dielectric constant corresponded to the Curie temperature and the transformation from ferroelectric to paraelectric at 1 kHz was sharp at 10 kHz. Polarization–electric field hysteresis loops for BST samples were measured using a Sawer–Tawer modified circuit. It was shown that the polarization decreased with increasing temperature for all samples. Full article

Due to lower magnetic properties of polymer-bonded magnets compared to sintered magnets, a complete redesign of the multipolar soft magnetic flux barriers in rotors with alignment guidelines was carried out to eliminate the frequently used rare-earth magnets, causing a new influence of the outer magnetic field on the cavity by using soft magnetic inserts. Within this new process, the main influencing factors on the magnetic flux density such as filler content, tool temperature, holding pressure and injection velocity were analysed and correlated. The studies were based on the compound of Polyamide 12 and up to a 60 vol.-% of the hard magnetic filler, strontium ferrite. Based on the study, the injection moulding of multipolar-bonded magnets into soft magnetic inserts for rotors and, in turn, into complex geometries can be optimized in terms of the orientation of the filler, the microstructure and the magnetic flux density. The investigations show no significant influence of the process parameters known from the literature such as the mass temperature T_m, which affects the magnetic flux density, as well as the orientation and the microstructure similar to tool temperature Tt, but is less efficient. The main influencing factors identified during the investigations are the tool temperature Tt, the injection velocity v_in and the holding pressure p_h. As known influencing factors are only based on simple geometries such as ring structures or plates, new factors were determined for complex rotor geometries. Full article