Low Values Of Magnetic Field Research Articles

Electromagnetic shielding properties of LPBF produced Fe2.9wt.%Si alloy

Ferromagnetic materials are used in various applications such as rotating electrical machines, wind turbines, electromagnetic shielding, transformers, and electromagnets. Compared to hard magnetic materials, their hysteresis cycles are featured by low values of coercive magnetic field and high permeability. The application of additive manufacturing to ferromagnetic materials is gaining more and more attraction. Indeed, thanks to a wider geometrical freedom, new topological optimized shapes for stator/rotor shapes can be addressed to enhance electric machines performances. However, the properties of the laser powder bed fusion (LPBF) processed alloy compared to conventionally produced counterpart must be still addressed. Accordingly, this paper presents for the first time the use of the LPBF for the manufacturing of Fe2.9wt.%Si electromagnetic shields. The process parameter selection material microstructure and the magnetic shielding factor are characterized.

The role of plasma β in global coronal models: Bringing balance back to the force

Context. COolfluid COrona uNstrUcTured (COCONUT) is a global coronal magnetohydrodynamic (MHD) model that was recently developed and will soon be integrated into the ESA Virtual Space Weather Modelling Centre (VSWMC). In order to achieve robustness and fast convergence to a steady state for numerical simulations with COCONUT, several assumptions and simplifications were made during its development, such as prescribing filtered photospheric magnetic maps to represent the magnetic field conditions in the lower corona. This filtering leads to smoothing and lower magnetic field values at the inner boundary (i.e. the solar surface), resulting in an unrealistically high plasma β (greater than 1 in a large portion of the domain). Aims. We aim to examine the effects of prescribing such filtered magnetograms in global coronal simulations and formulate a method for achieving more realistic plasma β values and improving the resolution of electromagnetic features without losing computational performance. Methods. We made use of the newly developed COCONUT solver to demonstrate the effects of the highly pre-processed magnetic maps set at the inner boundary and the resulting high plasma β on the features in the computational domain. Then, in our new approach, we shifted the inner boundary to 2 R⊙ from the original 1.01 R⊙ and preserved the prescribed highly filtered magnetic map. With the shifted boundary, the boundary density and pressure were also naturally adjusted to better represent the considered physical location. This effectively reduces the prescribed plasma β and leads to a more realistic setup. The method was applied on a magnetic dipole, a minimum (2008) and a maximum (2012) solar activity case, to demonstrate its effects. Results. The results obtained with the proposed approach show significant improvements in the resolved density and radial velocity profiles, and far more realistic values of the plasma β at the boundary and inside the computational domain. This is also demonstrated via synthetic white light imaging (WLI) and with the validation against tomography data. The computational performance comparison shows similar convergence to a limit residual on the same grid when compared to the original setup. Considering that the grid can be further coarsened with this new setup, as its capacity to resolve features or structures is superior, the operational performance can be additionally increased if needed. Conclusions. The newly developed method is thus deemed as a good potential replacement of the original setup for operational purposes, providing higher physical detail of the resolved profiles while preserving a good convergence and robustness of the solver.

Towards Resonantly Enhanced Acoustic Phonon-Exchange Magnon Interactions at THz Frequencies

Using valid experimental parameters, we quantify the magnitude of resonantly phonon-driven precession of exchange magnons in freestanding ferromagnetic nickel thin films on their thickness L. Analytical solutions of acoustically driven equations for magnon oscillators display a nonmonotonous dependence of the peak magnetization precession on the film thickness. It is explained by different L-dependence of multiple prefactors entering in the expression for the total magnetization dynamics. Depending on the ratio of acoustic and magnetic (Gilbert) damping constants, the magnetization precession is shown to be amplified by a Q-factor of either the phonon or the magnon resonance. The increase in the phonon mode amplitude for thinner membranes is also found to be significant. Focusing on the magnetization dynamics excited by the two first acoustic eigenmodes with p=1 and p=2, we predict the optimum thicknesses of nickel membranes to achieve large amplitude magnetization precession at multi 100 GHz frequencies at reasonably low values of an external magnetic field. By extending the study to the case of Ni-Si bilayers, we show that these resonances are achievable at even higher frequencies, approaching the THz range.

Pulse sequences for manipulating the spin states of molecular radical-pair-based electron spin qubit systems for quantum information applications.

Molecular qubits are an emerging platform in quantum information science due to the unmatched structural control that chemical design and synthesis provide compared to other leading qubit technologies. This theoretical study investigates pulse sequence protocols for spin-correlated radical pairs, which are important molecular spin qubit pair (SQP) candidates. Here, we introduce improved microwave pulse protocols for enhancing the execution times of quantum logic gates based on SQPs. Significantly, this study demonstrates that the proposed pulse sequences effectively remove certain contributions from nuclear spin effects on spin dynamics, which are a common source of decoherence. Additionally, we have analyzed the factors that control the fidelity of the SQP spin state, following the application of the controlled-NOT gate. It was found that higher magnetic fields introduce a high frequency oscillation in the fidelity. Thereupon, it is suggested that further research should be geared toward executing quantum gates at lower magnetic field values. In addition, an absolute bound of the fidelity outcome due to decoherence is determined, which clearly identifies the important factors that control gate execution. Finally, examples of the application of these pulse sequences to SQPs are described.

Tunable magnetoresistance in Li2BaSi

We present the magnetotransport behavior of Li2BaSi using Boltzmann transport theory within relaxation time approximation. The semimetallic nature and anisotropic band dispersion suggest a possible large magnetoresistance. The electrical resistivities and magnetoresistance display non-saturating behavior for low magnetic field values and get saturated at higher fields due to charge-carrier uncompensation and are almost similar with and without spin-orbit coupling. Shifting the Fermi level by 0.10 eV results in tunable magnetotransport features with improved transverse MR values up to 200% for 12 T at 10 K with non-saturating behavior. Our calculations predict that this investigated compound can be deemed a better MR material for its transverse effects.

Magnetic properties of Heusler compounds Co1+xCrAl (x = 0, 0.25, 0.5, 0.75, 1)

The compounds Co1+xCrAl (x = 0, 0.25, 0.5, 0.75 and 1) have been successfully prepared using the arc melting technique. Room temperature XRD patterns confirm that all compounds have Heusler compounds like fcc structure. Magnetic properties of all as-cast compounds have been studied and it is observed that all compounds are ferromagnetic below their respective PM-FM transition temperature (Tc). For half Heusler compound CoCrAl, the Tc is around 14 K but H/M as a function of temperature (T) curves at high magnetic field follows the modified Curie-Weiss law above 200 K. CoCrAl and Co1.25CrAl compounds violate the universal nature of paramagnetism (above Tc) for low magnetic field values due to the presence of magnetic clusters within the paramagnetic matrix. For the other three compounds Co1.5CrAl, Co1.75CrAl and Co2CrAl universal nature of paramagnetism and Curie-Weiss law have been satisfied properly above Tc. Except for the full Heusler compound Co2CrAl, the value of Tc increases with increasing Co concentration because lattice parameter and distance between magnetic ions decrease with increasing Co concentration. The saturation magnetization (MS) of the compounds increases with increasing Co concentration within the compounds. The saturation magnetization (MS) does not follow the well-known Slater-Pauling rule for any compounds except for CoCrAl. M−H curves for all compounds below Tc suggest the soft magnetic nature of the compounds.

Pathways to Green Perspectives: Production and Characterization of Polylactide (PLA) Nanocomposites Filled with Superparamagnetic Magnetite Nanoparticles.

In the category of biopolymers, polylactide or polylactic acid (PLA) is one of the most promising candidates considered for future developments, as it is not only biodegradable under industrial composting conditions, but it is produced from renewable natural resources. The modification of PLA through the addition of nanofillers is considered as a modern approach to improve its main characteristic features (mechanical, thermal, barrier, etc.) and to obtain specific end-use properties. Iron oxide nanoparticles (NPs) of low dimension (10–20 nm) such as magnetite (Fe3O4), exhibit strong magnetization in magnetic field, are biocompatible and show low toxicity, and can be considered in the production of polymer nanocomposites requiring superparamagnetic properties. Accordingly, PLA was mixed by melt-compounding with 4–16 wt.% magnetite NPs. Surface treatment of NPs with a reactive polymethylhydrogensiloxane (MHX) was investigated to render the nanofiller water repellent, less sensitive to moisture and to reduce the catalytic effects at high temperature of iron (from magnetite) on PLA macromolecular chains. The characterization of nanocomposites was focused on the differences of the rheology and morphology, modification, and improvements in the thermal properties using surface treated NPs, while the superparamagnetic behavior was confirmed by VSM (vibrating sample magnetometer) measurements. The PLA−magnetite nanocomposites had strong magnetization properties at low magnetic field (values close to 70% of Mmax at H = 0.2 T), while the maximum magnetic signal (Mmax) was mainly determined by the loading of the nanofiller, without any significant differences linked to the surface treatment of MNPs. These bionanocomposites showing superparamagnetic properties, close to zero magnetic remanence, and coercivity, can be further produced at a larger scale by melt-compounding and can be designed for special end-use applications, going from biomedical to technical areas.

Cold magnetized quark matter at finite density in a nonlocal chiral quark model

We study the behavior of two-flavor dense quark matter under the influence of an external magnetic field in the framework of a nonlocal chiral quark model with separable interactions. The nonlocality is incorporated in the model by using a Gaussian form factor. It is found that for low and moderate values of magnetic field there is a decrease of the critical chiral restoration chemical potential $$\mu _c$$ , i.e. an inverse magnetic catalysis effect is observed. For larger values of eB the behavior of $$\mu _c$$ becomes more or less flat, depending on the parametrization. Within the considered parametrization range we do not find a significant growth of the critical chemical potential for large magnetic fields, as occurs in the case of the local NJL model.

Crystal field effect on the magneto caloric properties of the mixed spin-7/2 and spin-3/2 3D-ising ferrimagnet: A Monte Carlo simulation

It has been observed by the Monte Carlo simulation that the Magneto Caloric effect (MCE) of the ferrimagnetic mixed spin-7/2 and spin-3/2 3D Ising system is influenced by the change of crystal field coefficients. The peak value of the magnetic entropy change ΔSP has a second order polynomial dependency on the reduced crystal field coefficient De. This relation is only valid for the low magnetic field and low De values. The inverse of the coefficients of the polynomials has a linear dependency with the 4/3 power of the magnetic field. Full width half maximum (FWHM) of temperature dependence ΔS curve increases with the increment of De. The relative cooling power (RCP) increases with De and at De = 4 a large percentage change of ΔRCP % (~55%) is observed for the reduced field h = 5.

An anomalously high Seebeck coefficient and power factor in ultrathin Bi2Te3 film: Spin–orbit interaction

The present study reports a strong thickness-dependence and anomalously large enhancement in the values of the Seebeck coefficient and electrical conductivity in Bi2Te3 films at ultralow thickness. An opposite sign of the Hall coefficient (negative) and Seebeck coefficient (positive) is observed in an ultrathin Bi2Te3 film (65 nm) as compared to the normally observed identical sign in the case of Bi2Te3 thin films (520 nm). A simultaneous enhancement in the values of electrical conductivity and the Seebeck coefficient results in a giant enhancement in the value of power factor from 1.86 mW/m K2 to 18.0 mW/m K2 at 416 K, with a reduction in thickness. X-ray photoelectron spectroscopy investigation reveals the absence of any significant change in stoichiometry and chemical bonding upon reduction of thickness. Magnetoresistance vs magnetic field data show a sharp dip at the lower magnetic field values, indicating a weak antilocalization effect in the case of the ultrathin film sample suggesting the role of strong spin–orbit coupling toward the carrier filtering effect resulting in enhancement of thermoelectric properties. Observation of the large Seebeck coefficient and the power factor at lower thickness values and its relationship with spin–orbit coupling is an important result, both for practical applications and for better understanding of the thermoelectric properties.

Mesozoic Acid Magmatites of Southeastern Transbaikalia: Petrogeochemistry and Relationship with Metasomatism and Ore Formation

The characteristic features of the manifestations of acid magmatism and metasomatic hydrothermal alteration processes in Southeastern Transbaikalia, an important mining region in Russia, are discussed. The zoned or beltlike distribution of the ores of various metals, uranium in particular, and other mineral deposits is due to the general evolutionary trends of the geodynamic regimes of the respective blocks of the territory and the subsequent magmatic differentiation trends. The highly differentiated and fluorine-rich granitoids of Southeastern Transbaikalia are located only in the zones of the consolidated crust, which are characterized by low gravity and negative quiet magnetic field values. In other zones, i.e., volcanic belts, volcanogenic troughs, and the regions dominated by the intrusive massifs of the Shakhtama complex, rare metal manifestations are absent. It was demonstrated that there is no correlation between the localization of large uranium deposits and leucocratic rare metal granite domains and that these deposits are concentrated in large volcanic calderas or volcanic troughs, controlled by long-lived fault systems. In addition to uranium, these volcanogenic structures control base metal, molybdenum, gold, fluorite, and some other mineral deposits. The results of a comparative analysis of the geochemical features of the acid volcanics that host the uranium mineralization and are classified as the last differentiation products of the contrasting latite volcanism, on the one hand, and the subvolcanic and hypabyssal derivatives of the rare metal granites, on the other, are presented. The analysis revealed the signatures of their fundamental difference from each other and that they belong to different branches of magmatic differentiation. A comparative analysis of the metasomatic aureoles of the altered wallrocks in the Late Mesozoic volcanics and the Shakhtama granitoid domains, on the one hand, and the leucocratic rare metal granite domain, on the other, is given. The data presented attest to the existence of a mixed geodynamic regime within the study area of the Central Asian mobile belt during the Jurassic and the earliest Cretaceous time: residual collisions with strong deformations in a compressional setting, accompanied by the development of late acid differentiation products of the magmatic latite series, and the incipient rifting associated with the intrusion of the leucogranites of the Kukul’bei complex in a more quiet extensional setting.

Spatio-temporal behavior of magnetohydrodynamic fluctuations with cross-helicity and background magnetic field

We study the spatiotemporal behavior of the Elsässer variables describing magnetic and velocity field fluctuations, using direct numerical simulations of three-dimensional magnetohydrodynamic turbulence. We consider cases with relatively small, intermediate, and large values of a mean background magnetic field and with null, small, and high cross-helicity (correlations between the velocity and the magnetic field). Wavenumber-dependent time correlation functions are computed for the different simulations. From these correlation functions, the decorrelation time is computed and compared with different theoretical characteristic times: the local nonlinear time, the random-sweeping time, and the Alfvénic time. It is found that decorrelation times are dominated by sweeping effects for low values of the mean magnetic field and for low values of the cross-helicity, while for large values of the background field or of the cross-helicity and for wave vectors sufficiently aligned with the guide field, decorrelation times are controlled by Alfvénic effects. Finally, we observe counterpropagation of Alfvénic fluctuations due to reflections produced by inhomogeneities in the total magnetic field. This effect becomes more prominent in flows with large cross-helicity, strongly modifying the propagation of waves in turbulent magnetohydrodynamic flows.

Properties and Geoeffectiveness of Solar Wind High‐Speed Streams and Stream Interaction Regions During Solar Cycles 23 and 24

AbstractWe study the properties and geoeffectiveness of solar wind high‐speed streams (HSSs) emanating from coronal holes and associated with stream interaction regions (SIRs). This paper presents a statistical study of 588 SIR/HSS events with solar wind speed at 1 AU exceeding 500 km/s during 1995–2017, encompassing the decline of solar cycle 22 to the decline of cycle 24. Events are detected using measurements of the solar wind speed and the interplanetary magnetic field. Events misidentified as or interacting with interplanetary coronal mass ejections are removed by comparison with an existing interplanetary coronal mass ejection list. Using this SIR/HSS event catalog (list given in the supporting information), a superposed epoch analysis of key solar wind parameters is carried out. It is found that the number of SIR/HSSs peaks during the late declining phase of solar cycle (SC) 23, as does their velocity, but that their geoeffectiveness in terms of the AE and SYM‐H indices is low. This can be explained by the anomalously low values of magnetic field during the extended solar minimum. Within SC23 and SC24, the highest geoeffectiveness of SIR/HSSs takes place during the early declining phases. Geoeffectiveness of SIR/HSSs continues to be up to 40% lower during SC24 than SC23, which can be explained by the solar wind properties.

A Comprehensive Study of the Thermal and Magnetic Properties of Two-Harmonically Interacting Electrons Confined in a Parabolic GaAs Quantum Dot in a Magnetic Field

The thermal and magnetic properties of a parabolic GaAs quantum dot for two-Harmonically interacting electrons when it exposed to an external magnetic field, taking into account the spin-Zeeman energy are investigated using the canonical ensemble approach. The effect of spin on these properties is also investigated. With the possibility of a basic and physically sensible model of electron-electron interaction, the issue is precisely soluble. We found a Schottky-like anomaly in the heat capacity at low temperature, while it saturates to the 4kB value as the temperature increases. Also it is noted that entropy enhances with temperature as expected. However as a function of a magnetic field, a peak structure is observed in heat capacity at very low values of magnetic field, while it saturates to the 2kB value as magnetic field increases. Also we noticed that these peaks are not presented in the spinless case. Moreover magnetic field does not show a significant effect on the entropy at high temperatures, but at relatively lower temperatures, the entropy shows a monotonic increase with magnetic field. As a function of the Lande g* factor, we found a local minima and a double peak-structure in the susceptibility and in the heat capacity at g*=0. It is demonstrated that the favored state for both magnetization and susceptibility is the diamagnetic state. The significant effect of the spin on the magnetic properties of quantum dot is seen at low values of temperature and magnetic field. Moreover, our results showed a very good agreement with reported previous works.

Large magnetic thermal conductivity induced by frustration in low-dimensional quantum magnets

We study the magnetic field-dependence of the thermal conductivity due to magnetic excitations in frustrated spin-1/2 Heisenberg chains. Near the saturation field, the system is described by a dilute gas of weakly-interacting fermions (free-fermion fixed point). We show that in this regime the thermal conductivity exhibits a non-monotonic behavior as a function of the ratio $\alpha= J_2/J_1$ between second and first nearest-neighbor antiferromagnetic exchange interactions. This result is a direct consequence of the splitting of the single-particle dispersion minimum into two minima that takes place at the Lifshitz point $\alpha=1/4$. Upon increasing $\alpha$ from zero, the inverse mass vanishes at $\alpha=1/4$ and it increases monotonically from zero for $\alpha \geq 1/4$. By deriving an effective low-energy theory of the dilute gas of fermions, we demonstrate that the Drude weight $K_{\rm th}$ of the thermal conductivity exhibits a similar dependence on $\alpha$ near the saturation field. Moreover, this theory predicts a transition between a two-component Tomonaga-Luttinger liquid and a vector-chiral phase at a critical value $\alpha=\alpha_c$ that agrees very well with previous density matrix renormalization group results. We also show that the resulting curve $K_{\rm th}(\alpha)$ is in excellent agreement with exact diagonalization (ED) results. Our ED results also show that $K_{\rm th}(\alpha)$ has a pronounced minimum at $\alpha\simeq 0.7$ and it decreases for sufficiently large $\alpha$ at lower magnetic field values. We also demonstrate that the thermal conductivity is significantly affected by the presence of magnetothermal coupling.

Heat capacity and entropy of two electrons quantum dot in a magnetic field with parabolic interaction

The thermodynamic properties of two electrons in two dimensional parabolic GaAs quantum dot are studied where both the magnetic field and the e–e interaction are fully considered. The e–e interaction has been treated by a model potential which makes the Hamiltonian exactly solvable. The energy spectrum is used to calculate the canonical partition function, and then we obtain the thermodynamic properties; mean energy, heat capacity and entropy as a function of temperature (T) and magnetic field (B).A steep transition from zero to 4kB is observed in the heat capacity as a function of temperature for small values of magnetic field and saturates within a small temperature range, also the heat capacity has a peak-like structure at low temperature, while for high magnetic field heat capacity develops a shoulder at 2kB then it approaches the saturation value with further increase in temperature. The entropy increases with increasing temperature, but at higher temperature, it remains almost independent of the magnetic field. It is shown that, at low magnetic field values, the effect of magnetic field on heat capacity is tangible and it attains a constant value with further increase in magnetic field. Entropy is almost linearly proportional with increasing magnetic field strength.

Numerical Modeling and Material Characterization for Multilayer Magnetically Shielded Room Design

In this paper, a method for the assessment of magnetic properties of ferromagnetic materials at low magnetic field values is presented. The knowledge of these properties is crucial for the design of magnetically shielded rooms that must be effective against low-frequency external magnetic field disturbances on the order of 0.1– $1~\mu \mathrm {T}$ . The considered shielding structure is composed by a number of layers made of different high-permeability ferromagnetic sheets as grain-oriented electric steel and nickel–iron soft magnetic alloy. The ferromagnetic materials in this region follow the Rayleigh law that is a quadratic law between magnetic flux density and the applied field. Two material-dependent parameters of this law are obtained by a characterization of the material behaviour performed on a scaled prototype of a magnetically shielded room. The procedure is validated on a multilayer shield structure.

Magnetic characteristics and AC losses of DC type-II superconductors under oscillating magnetic fields

Remarkable features on the magnetic moment of type-II superconducting (SC) wires of cylindrical shape, subjected to direct current conditions (DC) and transverse oscillating (AC) magnetic fields, are reported. We show how for relatively low amplitudes of the applied magnetic field, Ba, the superconducting wire rapidly develops a saturation state, , characterizing the limits of magnetization loops that exhibit a Boolean-like behaviour. Regardless of the premagnetization state of the SC wire, we show how after two cycles of magnetic relaxation, boolean-like ±Mp states can be measured during the entire period of time from which the external magnetic field B0 ranges from 0 to ±Ba, with the signs rule defined by the sign of the slope ΔB0y(t). In addition, for the practical implementation of SC DC wires sharing the right of way with AC lines, we report that for relatively low values of magnetic field, , being BP the analytical value for the full penetration field in absence of transport current, Itr, the use of semi-analytical approaches for the calculation of AC losses leads to a significant underestimation of the actual contribution of the induction losses. This phenomena is particularly relevant at dimensionless fields , being ba = Ba/BP and, ia = Ia/Ic the amplitude of an AC or DC transport current, due to the local motion of flux front profiles being dominated by the occurrence of transport current. On the other hand, we have found that regardless of the nature of the transport current, either be DC or AC, when a transverse oscillating magnetic field greater than the classical limit ba = (1 − ia) is applied to the SC wire, the difference between the obtained AC losses in both situations results to be negligible indistinctly of the approach used, semi-analytical or numerical. Thus, the actual limits from which the estimation of the AC losses can be used as an asset for the deployment of DC SC wires sharing the right of way with AC lines, against the sole use of SC wires for the transmission of AC transport current, are established.

Composite Zn1−xCdxGeAs2 semiconductors: structural and electrical properties

We present the studies of structural, transport and magnetotransport properties of CdxGeAs2 crystals with the chemical content changing from 0 to 1. The structural studies indicate that this alloy exists as a composite two-phase material in almost the entire range of average chemical compositions. The two phase nature of our samples does have a significant influence on the carrier transport and magnetotransport of the composite alloy. The change of the conductivity type is observed at room temperature, from p-type for to n-type for x > 0.18, respectively. The Hall carrier mobility measured at room temperature decreases as a function of x from about 35 cm2 (V · s)−1 for the sample with x = 0 down to 3 cm2 (V · s)−1 for the sample with x = 0.233. For x > 0.233 the Hall carrier mobility shows an increase with x, up to the highest value around 875 cm2 (V · s)−1 observed for the sample with x = 1. Temperature dependent resistivity measurements indicate the presence of thermal activation of carriers with activation energy, Ea, with values from 20 to 30 meV for all the studied samples. The temperature dependent Hall effect data show that the grain boundary limited transport is strong in all our samples. For the samples with the negative MR is observed at temperatures lower than 100 K and at low magnetic field values, T. This effect is interpreted as a weak localization phenomenon with low values of phase coherence length, nm.

Sharp transitions in low-number quantum dots Bayesian magnetometry.

We consider Bayesian estimate of static magnetic field, characterized by a prior Gaussian probability distribution, in systems of a few electron quantum dot spins interacting with infinite temperature spin environment via hyperfine interaction. Sudden transitions among optimal states and measurements are observed. Usefulness of measuring occupation levels is shown for all times of the evolution, together with the role of entanglement in the optimal scenario. For low values of magnetic field, memory effects stemming from the interaction with environment provide limited metrological advantage.