Magnetic Subsystem Research Articles

Multidomain modeling of nonlinear electromagnetic circuits using wave digital filters

SummaryAccurate models of electromagnetic systems can be derived by coupling electric and magnetic equivalent circuits together. The different nature of such physical domains constitutes a big challenge that puts a continuous strain on software simulators. To face this problem, a simulation approach based on Wave Digital Filters (WDFs) is proposed in this manuscript. The method is employed to solve nonlinear electromagnetic systems containing complex magnetic equivalent circuits while maintaining the modularity of the electric and magnetic subsystems. The nonlinearities can be locally handled, enabling the possibility to choose a dedicated model for each one of them. The proposed algorithm is a hierarchical generalization of the Scattering Iterative Method, which has shown, over the past few years, promising performance for the simulation of large nonlinear circuits. In addition, the method constitutes a further step towards the development of novel general‐purpose circuit simulators based on WDF principles. In a comparison with mainstream circuit simulation software, the proposed approach turns out to be considerably faster and thus particularly promising for parametric analyses of electromagnetic systems.

Magnetic domain wall motion driven by an acoustic wave

Dynamic interaction of acoustic and magnetic systems is of strong current interest, triggered by the promises of almost lossless new concepts of magnet-based information technology. In such concepts, a significant role is often given to domain walls (DW). Therefore, here we investigate how launching an acoustic shear wave, we can control the DW motion. Surprisingly, at sufficiently large amplitudes of the shear displacement, the speed of the forced DW motion can reach sizeable fraction of the speed of sound. This was shown to happen due to certain resonance conditions depending on the wave frequency, its angle of incidence, and shear displacement amplitudes, leading to a total reflection of the wave and maximizing the impact. Most interesting, strong nonlinearity appears in the interaction of the elastic and magnetic subsystems, expressed by the negative slope of the resonant reflection peak and the s-shaped dependence of the domain wall velocity on the shear displacement amplitude, typical for nonlinear systems.

Lead-substituted barium hexaferrite for tunable terahertz optoelectronics

Due to their outstanding dielectric and magnetic properties, hexaferrites are attracting ever-increasing attention for developing electronic components of next-generation communication systems. The complex crystal structure of hexaferrites and the critical dependences of their electric and magnetic properties on external factors, such as magnetic/electric fields, pressure, and doping, open ample opportunities for targeted tuning of these properties when designing specific devices. Here we explored the electromagnetic properties of lead-substituted barium hexaferrite, Ba1−xPbxFe12O19, a compound featuring an extremely rich set of physical phenomena that are inherent in the dielectric and magnetic subsystems and can have a significant effect on its electromagnetic response at terahertz frequencies. We performed the first detailed measurements of the temperature-dependent (5–300 K) dielectric response of single-crystalline Ba1−xPbxFe12O19 in an extremely broad spectral range of 1 Hz–240 THz. We fully analyzed numerous phenomena with a corresponding wide distribution of specific energies that can affect the terahertz properties of the material. The most important fundamental finding is the observation of a ferroelectric-like terahertz excitation with an unusual temperature behavior of its frequency and strength. We suggest microscopic models that explain the origin of the excitation and its nonstandard temperature evolution. Several narrower terahertz excitations are associated with electronic transitions between the fine-structure components of the Fe2+ ground state. The discovered radio-frequency relaxations are attributed to the response of magnetic domains. Gigahertz resonances are presumably of magnetoelectric origin. The obtained data on diverse electromagnetic properties of Ba1−xPbxFe12O19 compounds provide information that makes the entire class of hexaferrites attractive for manufacturing electronic devices for the terahertz range.

Cryogenic magnetocaloric effect in Gd2O3 nanoparticles studied by heat capacity

Structural characteristics and magnetic properties with respect to magnetocaloric effect was studied in the nanocomposite systems consisting of Gd2O3 nanoparticles embedded in the regular pores of amorphous silica matrix. The particles of size 2–16 nm were prepared by nanocasting method within the pores of periodic silica matrices of hexagonal (SBA15 type) and cubic (SBA16 type) symmetries. Magnetocaloric response of the systems was examined in the applied fields up to 9 T in the low temperature region ranging from 0.4 K to 55 K. The data obtained from the measurements of the heat capacity Cp(H,T) revealed a peak at temperature below 10 K. Taking into account the superposition principle, the contributions corresponding to the lattice and the magnetic subsystem heat capacities were analyzed separately. The high value of magnetic entropy change |ΔSM| = ~27 JK−1kg−1 and |ΔSM| = 37 JK−1kg−1 for the systems with hexagonal and cubic matrices, respectively were observed at the temperature close to 2 K under field change from 0 to 9 T. The evidence on considerable high |ΔSM| values along with the large surface area of mesoporous silica, easy and reproducible preparation manifest some extra advantages over the conventional refrigeration technology.

Coherent GHz lattice and magnetization excitations in thin epitaxial Ag/Fe/Cr/Fe films

We excited an epitaxial magnetic Ag/Fe/Cr/Fe multilayer nonthermally and nonoptically with very short (1 ps) electromagnetic pulses. We detected the synchronous phononic-magnetic response by time-resolved magneto-optical Kerr effect measurements. The Ag/Fe/Cr/Fe multilayer was patterned into a coplanar waveguide transmission line, and the electromagnetic pulses were generated by pulsed-laser illumination of an integrated GaAs photoconductive switch (PCS). The detected magnetic excitations comprise up to four narrow-band high-order modes with the highest frequency reaching 30 GHz. The mode frequencies are independent of both temperature in the range from 16 to 300 K and the applied external magnetic field up to 120 mT. Our analysis shows that the origin of the rigidity of these high-frequency modes is the strong coupling of the magnetic subsystem with the lattice of the Ag/Fe/Cr/Fe multilayer. The exciting electromagnetic pulse generated by the PCS induces, via magnetoelastic coupling, long-lived (ns) standing GHz acoustic waves normal to the Ag/Fe/Cr/Fe film plane. These lattice oscillations in turn couple back and drive the magnetization oscillations via the magnetoelastic coupling. The temperature and field dependence of the damping of the oscillations can be described by inelastic phonon-phonon and phonon-magnon scattering. Our study opens up a possibility of using coherent lattice and magnetization dynamics in ferromagnetic films for spintronic devices at GHz clock rates.

Uncompensated magnetic moment and surface and size effects in few-nanometer antiferromagnetic NiO particles

-The analysis of the M(H) magnetization curves of antiferromagnetic nanoparticles yields information about magnetic subsystems formed in these objects, which are characterized by a large fraction of surface atoms. However, in the conventionally investigated experimental magnetic field range of up to 60–90 kOe, this analysis often faces the ambiguity of distinguishing the Langevin function-simulated contribution of uncompensated magnetic moments μun of particles against the background of a linear-in-field dependence (the antiferromagnetic susceptibility and other contributions). Here, this problem has been solved using a pulsed technique, which makes it possible to significantly broaden the range of external fields in which the μun contribution approaches the saturation. Nanoparticles of a typical NiO antiferromagnet with an average size of <d> ~ 4.5 nm have been investigated. Based on the thorough examination of the M(H) magnetization curves measured in pulsed fields of up to 250 kOe, a model of the magnetic state of NiO nanoparticles of such a small size has been proposed. The average moment is ~130 μB (μB is the Bohr magneton) per particle, which corresponds to 60–70 decompensated spins of nickel atoms localized, according to the Néel hypothesis (μun~<d>3/2), both on the surface and in the bulk of a particle. A part of the surface spins unrelated to the antiferromagnetic core form another subsystem, which behaves as free paramagnetic atoms. Along with the antiferromagnetic core, an additional linear-in-field contribution has been detected, which is apparently related to superantiferromagnetism, i.e., the size effect inherent to small antiferromagnetic particles.

Low-temperature peculiarities of thermal properties of EuF2.16 europium fluoride and (SrF2)0.5(EuF2.16)0.5 solid solution

The heat capacity and lattice parameters of europium fluoride EuF2.16 and (SrF2)0.5(EuF2.16)0.5 solid solution are experimentally investigated at temperatures 2–300 K. The revealed anomalies in the heat capacity are due to magnetic phase transition and disorder in fluorides crystal structures. The dependencies studied were analyzed using the Debye–Einstein approximation, taking into account the influence of the magnetic phase transition, two-level systems, and anharmonicity of lattice vibrations on their thermodynamic properties. We determined the parameters of the Debye and Einstein models – the characteristic temperatures as well as the magnitudes their respective contributions to the overall thermodynamic characteristics, the parameters characterizing crystal structure disorder, and the exchange parameters characterizing interaction in magnetic subsystems of fluorides. Dependences studied for solid solution deviate from the additive values. We further established that the mechanisms causing these are based on the disorder in the crystal structures of europium fluoride and its solid solution.

Hydrothermal Synthesis and a Composite Crystal Structure of Na6Cu7BiO4(PO4)4[Cl,(OH)]3 as a Candidate for Quantum Spin Liquid

A novel sodium bismuth oxo-cuprate phosphate chloride, Na6Cu7BiO4(PO4)4[Cl2.23(OH)0.77], containing square-kagomé layers of Cu2+ has been synthesized by hydrothermal techniques. The compound crystallizes in the tetragonal space group P4/nmm, a = 10.0176(4), c = 10.8545(6), Z = 2, V = 1089.3(1) Å3, R1 = 0.021, wR = 0.053, S = 1.32. Its composite crystal structure includes [O4Cu6Bi]7+ layers, which are formed by the clusters of oxygen-centered tetrahedra [OCu3Bi]. These positively charged two periodic fragments are intercalated in a negatively charged [CuNa6Cl3(PO4)4]7- matrix built by Na-centered polyhedra, PO4 tetrahedra, and CuO4Cl pyramids. The composite character of the crystal structure of Na6Cu7BiO4(PO4)4[Cl2.23(OH)0.77], as well as the way of its self-assembly, are discussed in close connection with the sulfohalite Na6ClF(SO4)2 salt. It is shown that the "host-guest" model of the formation of the tetragonal Na6Cu7BiO4(PO4)4[Cl2.23(OH)0.77] phase is due to the group-subgroup symmetry relation with the cubic crystal structure of mineral sulfohalite and is also supported by the crystallization condition in excess sodium chloride. The magnetic subsystem of Na6Cu7BiO4(PO4)4[Cl2.23(OH)0.77] is represented by a dense square-kagomé network of 2Cu1 and 4Cu2 ions, decorated with weakly bonded Cu3 ions. Measurements of magnetization and heat capacity indicate the absence of long-range order up to 2 K, which makes this compound a candidate for a highly demanded spin liquid.

Exploring a synergistic-type magnetostructural transformation in Ni-Mn-Ga-X (X = Sn, Sb) Heusler alloys

In this paper, the magnetostructural transformation from paramagnetic austenite to ferromagnetic martensite is described as a synergistic-type phase transformation because the structural and magnetic subsystems exhibit two unique synergistic effects: (1) lattice vibration and magnetic entropy changes synergistically contribute to the total entropy change; (2) the structure and magnetism synergistically response to external cross-fields. Such synergistic-type phase transformation has been explored in Ni-Mn-Ga-X (X = Sn, Sb) Heusler alloys. By the substitution of Sn/Sb for Ga, a temperature window of 50 K for the synergistic-type phase transformation has been established in the phase diagram of Ni-Mn-Ga-X (X = Sn, Sb). Associated with the synergistic contribution of lattice vibration and magnetic entropy changes, the samples exhibit high phase transformation entropy changes (≥1.2 J/molK) within the temperature window. The synergistic effects of the magnetostructural transformation make the Ni-Mn-Ga-X (X = Sn, Sb) alloys to be potential multicaloric materials.

Tricritical Phenomena and Cascades of Temperature Phase Transitions in a Ferromagnetic Liquid Crystal Suspension

We consider temperature-driven phase transitions occurring in a liquid crystal suspension of ferromagnetic particles within the Landau–de Gennes theory. The temperature dependences of the order parameters in the uniaxial model with a vector order parameter for the magnetic subsystem are obtained. The dimensionless expression for the free energy density of the suspension has been used for the study of the phase behavior general regularities of the system. Phase state diagrams of the suspension and temperature dependences of the order parameters of the liquid crystal and the ensemble of magnetic particles for different values of the phenomenological expansion coefficients are constructed. It is shown that the considered model admits the existence of a cascade of temperature phase transitions: isotropic phase–superparamagnetic nematic phase–ferromagnetic nematic phase. We have shown that in the mesomorphic state of the liquid crystal, the spontaneous magnetization can appear in a continuous way or by a jump with decreasing temperature, which corresponds to the tricritical behavior. The values of temperature and expansion coefficients corresponding to the tricritical and triple points are numerically found.

Probing the interplay between lattice dynamics and short-range magnetic correlations in CuGeO3 with femtosecond RIXS

Investigations of magnetically ordered phases on the femtosecond timescale have provided significant insights into the influence of charge and lattice degrees of freedom on the magnetic sub-system. However, short-range magnetic correlations occurring in the absence of long-range order, for example in spin-frustrated systems, are inaccessible to many ultrafast techniques. Here, we show how time-resolved resonant inelastic X-ray scattering (trRIXS) is capable of probing such short-ranged magnetic dynamics in a charge-transfer insulator through the detection of a Zhang–Rice singlet exciton. Utilizing trRIXS measurements at the O K-edge, and in combination with model calculations, we probe the short-range spin correlations in the frustrated spin chain material CuGeO3 following photo-excitation, revealing a strong coupling between the local lattice and spin sub-systems.

Improving the Force and Time Response of a DC Solenoid Electromagnetic Actuator by Changing the Lower Core Angle

The aim of almost any electromagnetic actuator development is to increase the electromagnetic force with which an actuator acts on a plunger with as fast a time response as possible while maintaining the dimensions as small as possible. This paper presents research on the impact of the lower core angle on the force and time response of a DC solenoid electromagnetic actuator. The research method is based on the analytical analysis of the magnetic path of the DC solenoid electromagnetic actuator and a comparison with the numerical simulation results. A transient numerical simulation was performed on a 2D axial-symmetric model of the electromagnetic actuator and included simultaneously solving time-dependent partial differential equations of the electromagnetic actuator’s magnetic, electrical, and mechanical subsystems. The magnetic subsystem was analyzed by the finite element method (FEM) using the ANSYS Electronics software package. The three prototype models with different lower core angles were produced and tested in the accredited Laboratory Center of KONČAR Electrical Engineering Institute. The obtained measurements are compared with the analytical results and numerical simulation results.

Observing the Suppression of Superconductivity in RbEuFe_{4}As_{4} by Correlated Magnetic Fluctuations.

In this Letter, we describe quantitative magnetic imaging of superconducting vortices in RbEuFe_{4}As_{4} in order to investigate the unique interplay between the magnetic and superconducting sublattices. Our scanning Hall microscopy data reveal a pronounced suppression of the superfluid density near the magnetic ordering temperature in good qualitative agreement with a recently developed model describing the suppression of superconductivity by correlated magnetic fluctuations. These results indicate a pronounced exchange interaction between the superconducting and magnetic subsystems in RbEuFe_{4}As_{4}, with important implications for future investigations of physical phenomena arising from the interplay between them.

Spin Berry points as crucial for ultrafast demagnetization

Laser-induced ultrafast demagnetization has puzzled researchers around the world for over two decades. Intrinsic complexity in electronic, magnetic and phononic subsystems is difficult to understand microscopically. So far, it is not possible to explain demagnetization using a single mechanism, which suggests a crucial piece of information still missing. In this paper, we return to a fundamental aspect of physics: spin and its change within each band in the entire Brillouin zone. We employ face-centered cubic (fcc) Ni as an example and use an extremely dense k mesh to map out spin changes for every band close to the Fermi level along all the high symmetry lines. To our surprise, spin angular momentum at some special k points abruptly changes from [Formula: see text] to [Formula: see text] simply by moving from one crystal momentum point to the next. This explains why intraband transitions, which the spin superdiffusion model is based upon, can induce a sharp spin moment reduction, and why electric current can change spin orientation in spintronics. These special k points, which are called spin Berry points [M. V. Berry, Proc. R. Soc. Lond. A 393 (1984) 45], are not random and appear when several bands are close to each other, so the Berry potential of spin majority states is different from that of spin minority states. Although within a single band, spin Berry points jump, when we group several neighboring bands together, they form distinctive smooth spin Berry lines. It is the band structure that disrupts those lines. Spin Berry points are crucial to laser-induced ultrafast demagnetization and spintronics.

Quench Protection Study of Superconducting Magnets for the Materials Plasma Exposure Experiment

To advance the understanding of plasma material interactions, the Material Plasma Exposure eXperiment (MPEX) is a new linear plasma device that will generate and deliver plasma relevant to future fusion reactor divertors. The operation of MPEX is planned to be steady-state in order to facilitate high fluence exposures of plasma facing materials and components. The desire for steady-state operation along with the magnetic field requires the utilization of superconducting coils. The superconducting magnet system for MPEX has been developed. The baseline model has six superconducting magnet and one room-temperature magnet subsystems. In order to protect multiple superconducting magnet systems, quench analysis was carried out to determine the best protection approach for each magnet type. Because the mutual inductance accounts for approximately 35% of the stored energy in the entire system, this must be considered when determining the peak voltages and temperatures during a quench. Two approaches for passive quench protection are considered: (1) self-protecting magnets and (2) use of diodes to sub-divide the coils. For both approaches, active quench detection will be used to ensure all coils are de-energized in the event of a quench. Results of the quench analysis for several quench scenarios are presented.

Cryogenic Considerations for Superconducting Magnet System Design for the Material Plasma Exposure eXperiment (MPEX)

The Material Plasma Exposure eXperiment (MPEX) has been proposed as a facility to address plasma material interaction knowledge gaps to qualify and develop materials and technologies that surround plasma environments for future fusion reactors. Utilizing different radio-frequency (rf) heating technologies, MPEX is a linear plasma device that will generate fusion reactor-like plasmas with energies and particle fluxes at the target materials with electron temperatures of 1 to 15 eV, electron densities of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , and ion fluxes greater than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">24</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Starting with the MPEX requirements with respect to magnetic fields between 0.1 and 2.5 T and warm bores of either 0.65 m or 1.56 m, conceptual designs for a superconducting magnet system have been developed that utilize multiple NbTi windings distributed across seven cryostats to accommodate rf heating, water cooling, and vacuum systems needed for MPEX. While the cryogenic and magnet technologies relative to the field and space requirements are mature, the integration of these technologies across multiple cryostats presents several technical and logistical challenges. An analysis of the preferred refrigeration approach, modular recondensing liquid helium cryocoolers, was performed. Utilizing a design margin of a factor of two, this approach is feasible within the current design requirements for MPEX with some considerations related to its implementation within the thermal shields and the magnet subsystem geometries.

Mesostructure of Composite Materials Based on Segmented Poly(Urethane Imide) Containing Ferrite Nanoparticles

The mesostructure of segmented poly(urethane imide) containing 4–10 wt % CoFe2O4 or Al3Fe5O12 nanoparticles was experimentally studied by small-angle polarized neutron scattering, which can analyze the nuclear and magnetic subsystems of the obtained composites. The transformation of the domain structure of the magnetic elastomers based on poly(urethane imide) was investigated at various concentrations, chemical compositions, and localizations of ferrite nanoparticles in polymer chains. The dynamics of the change in the segmental mobility of soft blocks during heating of samples was characterized by magnetic–nuclear interference scattering. The experiments detected changes in the supramolecular structure of the multiblock copolymer, which depended on the percentage and type of nanoparticles. The correlation radii were determined in the magnetic subsystem of ferrite nanoparticles embedded in hard aromatic blocks of poly(urethane imide) as nodes of intermolecular crosslinkers and chain extenders. The temperature dependence was analyzed for the intensity of small-angle nuclear scattering by poly(urethane imide) and magnetic elastomers based on it, under the conditions of the transition of soft aliphatic segments from the glassy to the highly elastic state.

Математическая модель электродинамического сейсмоприёмника

Electrodynamic geophones are applied in seismic prospecting, seismology and security systems. Devices with a similar design can be used as a generator in devices for energy storage, in active vibration control systems, and for detection of shallow-buried objects, including mines. To control the quality of geophones and verify the constancy of their characteristics during the manufacture, it is necessary to measure their frequency response on a shake table. The mathematical model of the geophone developed by JSC Scientific Research Engineering Institute (Balashikha, Moscow region) installed on a shake table is considered. The geophone schematic design is given, and the sequence of its operation is described. The approach of dividing an electromechanical system into several subsystems with subsequently uniting them into a resulting model is used for developing the geophone overall mathematical model. Detailed descriptions of the electrical, magnetic, and mechanical subsystems are presented. The assumptions used in compiling the overall mathematical model describing the geophone operation on the shake table are listed. A system of equations describing the interaction of the subsystems is compiled. Detailed descriptions of the resulting mathematical model, each of its element, and the interface included in its composition are presented. To estimate the developed mathematical model, the results obtained from the calculations on it are compared with the results from testing two experimental samples in the geophone operating frequency band from 10 to 100 Hz. For better clarity, the obtained results are compared in graphical form. The comparison has shown that the discrepancy between the results does not exceed 5% by the output signal amplitude. From the viewpoint of practical implementation, the developed mathematical model can be used in designing new geophones with other parameters, for example, with another natural frequency, or with a higher value of the output signal. It can also be used to develop more complex mathematical models containing a geophone. The accomplished study became a basis for elaborating a more complex mathematical model of a geophone with two natural frequencies in the operating band.

Magnetic properties and magnetocaloric effect in Dy100-xYx solid solutions

Dysprosium possesses the highest magnetic moment among the rare-earth elements and undergoes a second-order magnetic phase transition to a complex helical antiferromagnetic ordering at Néel temperature TN = 180 K which is followed by a transformation to the ferromagnetic ordering at a lower temperature. Superior magnetic properties and tunability of the magnetic phase transition temperatures make Dy and its solid solutions perspective for the use in technology of magnetic refrigeration at cryogenic temperatures, specifically for the liquefaction of natural gases. In this work we report on the evolution of magnetic properties in solid solutions of Dy100-xYx (x = 0 – 15). It was found that the dilution of Dy magnetic subsystem by a non-magnetic Y has rather a weak impact on the magnetization saturation, which decreases from Ms ∼280 A⋅m2/kg in the x = 0 sample to Ms ∼259 A⋅m2/kg in the x = 15 sample. As a consequence, the isothermal magnetic entropy change was found to decrease slightly in the series of Dy100-xYx (x = 0 – 15) solid solutions.

Strongly coupled charge, orbital, and spin order in TbTe3

We report a ground state with strongly coupled magnetic and charge density wave orders mediated via orbital ordering in the layered compound \tbt. In addition to the commensurate antiferromagnetic (AFM) and charge density wave (CDW) orders, new magnetic peaks are observed whose propagation vector equals the sum of the AFM and CDW propagation vectors, revealing an intricate and highly entwined relationship. This is especially interesting given that the magnetic and charge orders lie in different layers of the crystal structure where the highly localized magnetic moments of the Tb$^{3+}$ ions are netted in the Tb-Te stacks, while the charge order is formed by the conduction electrons of the adjacent Te-Te layers. Our results, based on neutron diffraction and resonant x-ray scattering reveal that the charge and magnetic subsystems mutually influence each other via the orbital ordering of Tb$^{3+}$ ions.