Range Of Magnetic Fields Research Articles

Detecting dipolarity of the geomagnetic field in the paleomagnetic record

Recovering the geomagnetic field strength in the past is key to understanding deep Earth dynamics and detecting potential geodynamo regimes throughout the history of Earth. To better constrain the predictive power of the paleomagnetic record, we propose an approach based on the analysis of the dependency between geomagnetic field strength and inclination (angle made by the horizontal with the field lines). Based on the outcomes of statistical field models, we show that these two quantities should correlate for a wide range of Earth-like magnetic fields, even with enhanced secular variation, persistent nonzonal components, and severe noise contamination. Focusing on the paleomagnetic record, we show that the correlation is not significant for the Brunhes polarity chron, what we ascribe to inadequate spatiotemporal sampling. In contrast, the correlation is significant for the 1 to 130Ma interval, whereas it only marginally succeeds prior to 130Ma when strict filters on both paleointensities and paleodirections are applied. As we cannot detect significant variations in the strength of the correlation over the 1 to 130Ma interval, we conclude that the Cretaceous Normal Superchron may not be associated with enhanced dipolarity of the geodynamo. The strong correlation obtained prior to 130Ma when strict filters are applied indicates that the ancient field may not be on average so different from the present-day field. If long-term fluctuations nevertheless existed, detecting potential geodynamo regimes during the Precambrian is currently impeded by the sparsity of high-quality data passing strict filters in both paleointensities and paleodirections.

Magnetic bubbles with alternating chirality in domain walls

In magnetic multilayers with perpendicular anisotropy, the competition of short-range and long-range interactions gives rise to the stability of cylindrical magnetic domains, also known as magnetic bubbles. The presence of Dzyaloshinsky-Moriya interaction induced by asymmetric interfaces between magnetic and nonmagnetic layers may lead to the formation of cylindrical bubble domains with Neel-type domain walls across the whole thickness of the multilayer. Such domain walls produce no contrast in Lorentz TEM under the normal incidence of the electron beam to the film. The latter is often used as an argument for the presence of Dzyaloshinskii-Moriya interaction in the system. Here we show that in magnetic multilayers, the absence of the Lorentz TEM contrast might also have another origin. In particular, in the absence of interfacial Dzyaloshinskii-Moriya interaction and weak interlayer exchange coupling, the magnetic bubbles might have Bloch-type domain walls of alternate chirality in adjacent layers. Such domain walls also do not produce magnetic contrast in Lorentz TEM at normal incidence of the electron beam. We show that, in the absence of interlayer exchange coupling, the magnetic bubble domains with the domain walls of fixed and alternate chirality have nearly identical energies and can coexist in the same range of magnetic fields. Using the geodesic nudged elastic band method, we prove that these states are separated by finite energy barriers. Furthermore, we demonstrate that magnetic multilayers with only dipolar coupling, besides the magnetic bubbles with nontrivial topology in all layers, can accommodate solutions with trivial topology within the internal layers.

Magnetic-field-induced valence change in Eu(Co1−xNix)2P2 up to 60 T

The solid solution 122 compounds, $\mathrm{Eu}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{P}}_{2}$, show a valence transition between divalent state and intermediate valence states at Eu, which is firmly correlated to multiple degrees of freedom in the solid such as the isostructural transition between the collapsed tetragonal (cT) and uncollapsed tetragonal (ucT) structures, $3d$ magnetism, and the formation of P-P dimers. To gain insights into the correlated behavior, we investigate the effect of high magnetic fields on the samples of $x=0.4$ and 0.5 using magnetostriction and magnetization measurements up to 60 T. The samples are in the Eu valence-fluctuating regime, where the possible structural transition from cT to ucT may be induced by the Eu valence change under the magnetic fields. For both samples, magnetostriction smoothly increases with increasing magnetic fields. The behavior is in good agreement with the calculated results using the interconfigurational fluctuation (ICF) model that describes the Eu valence change. This indicates that $\mathrm{\ensuremath{\Delta}}L$ represents the change of the Eu valence state in these compounds. Magnetization curves for both compounds show good agreement with the ICF model at high magnetic fields. In contrast, in the low-magnetic-field region, magnetization curves do not agree with the ICF model. These results indicate that the Eu valence changes manifest themselves in the magnetization curves at high magnetic fields and that the magnetism of the $3d$ electrons manifests itself in the magnetization at low magnetic fields. Hence, we conclude that the valence change occurs within the Eu valence fluctuation regime coupled with the cT structure. Thereby, we believe that the transition to ucT structure, which is firmly coupled with the divalent Eu state, does not occur within the magnetic field range of the present study. Even higher magnetic fields or pulsed magnetic fields with slower pulse durations may induce such large state changes in the present material, which is triggered by the Eu valence change under high magnetic fields.

МОДЕЛЮВАННЯ ПРОЦЕСУ ІНДУКЦІЙНОГО НАГРІВУ ДЛЯ СИСТЕМ МАГНІТНОЇ ГІПЕРТЕРМІЇ

The paper is devoted to the challenges of applying the induction heating (IH) for magnetic hyperthermia. The analysis of the results of previous studies has shown that within the biologically safe range of AC magnetic fields, insufficient induction heating power still appears to be one of the key problems for the successful clinical application of magnetic hyperthermia. In this paper, several possible effective circuit design solutions for the IH system are proposed, and their influence on the parameters of the heating processes of ferrites and ferromagnets is investigated. The model of the induction heater created in COMSOL Multiphysics allowes to simulate the distribution of current density and temperature in the heater. The developed model ensures better assessment of the processes occurring in living tissues and enables to simulate the impact of the magnetic particle material type and size on the temperature of heating and power consumption of the device.

Characterization of Terfenol-D and Comparison With Predictions of a Thermodynamically Consistent Two-Input, Two-Output Model of Hysteresis

Since its discovery more than forty years ago, Terfenol-D has promised much but has not yet been fully exploited in applications.The main reason for this is that it shows significant hysteresis and other nonlinearities, such as saturation, which need to be modeled correctly in order to maximize the application potential. In this article, we focus on the characterization of a Terfenol-D sample with the intention of developing a predictive two-input (magnetic field and stress) and two-output (magnetic flux density and strain) model of magnetostriction that exhibits hysteresis and saturation and is compatible with the laws of thermodynamics. Compatibility with thermodynamics is essential to ensure that numerical simulations do not exhibit unphysical behavior. The model is based on the Preisach hysteresis operator and its storage function and may be interpreted as a two-input, two-output neural net with elementary hysteresis operators as the neurons. In order to estimate the parameters, it is necessary to collect data over a wide range of magnetic fields (-300 kA/m to 300 kA/m) and compressive stresses (up to 60 MPa). Using the rate-independent memory evolution properties of the Preisach operator, we split the parameter estimation problem into three numerically well-conditioned, linear least squares problems with constraints. We show that the model is able to fit experimental data for strain and magnetization over a wide range of magnetic fields and stress.

H-T magnetic phase diagram of CuCrO2: A Monte Carlo study based on a realistic 3D classical Heisenberg model

In the multiferroic geometrically frustrated triangular antiferromagnet CuCrO2, we investigate the magnetic phases as function of temperature under a magnetic field 70 T ≤ B ≤ 120 T along the [001] axis using the importance sampling Monte Carlo method. We consider a realistic three-dimensional classical Heisenberg model, which takes into consideration the distortion and exchange interactions up to the third-nearest neighbors in the ab planes and an interplane interaction, in addition to hard and easy anisotropy axes along the [110] and the [001] directions, respectively. For 70 T ≤ B < 106 T, our calculations show that two phase transitions occur when decreasing the temperature. The first one, which takes place around 30 K, is a “paramagnetic ↔ up-up-down (UUD)” transition and the second one, at lower temperatures, is an “UUD ↔ commensurate Y (CY)” transition. Thus, the colinear UUD phase is stable in an intermediate temperature range. For B = 90 T, our estimates of the critical exponents indicate that the two transitions belong to the universality class of the two-dimensional Ising model. For 106 T ≤ B < 110 T, since the CY phase is identical to the UUD one, only a “paramagnetic ↔ up-up-down (UUD)” transition occurs. For 110 T ≤ B ≤ 120 T, only a “paramagnetic ↔ V” transition is observed at ∼27 K. Our results are in good agreement with the existing experimental data on the stable phases in this magnetic field range. Moreover, we have determined the temperature range where the CY and UUD phases are stable which was not done previously.

Investigations of the Crystallographic Orientation on the Martensite Variant Reorientation of the Single-Crystal Ni-Mn-Ga Cube and Its Composites for Actuator Applications

High-speed actuators are greatly required in this decade due to the fast development of future technologies, such as Internet-of-Things (IoT) and robots. The ferromagnetic shape memory alloys (FSMAs), whose shape change could be driven by applying an external magnetic field, possess a rapid response. Hence, these materials are considered promising candidates for the applications of future technologies. Among the FSMAs, the Ni-Mn-Ga-based materials were chosen for their large shape deformation strain and appropriate phase transformation temperatures for near-room temperature applications. Nevertheless, it is widely known that both the intrinsic brittleness of the Ni-Mn-Ga alloy and the constraint of shape deformation strain due to the existence of grain boundaries in the polycrystal inhibit the applications. Therefore, various Ni-Mn-Ga-based composite materials were designed in this study, and their shape deformation behaviors induced by compressive or magnetic fields were examined by the in situ micro CT observations. In addition, the dependence of the martensite variant reorientation (MVR) on the crystallographic directions was also investigated. It was found that most of the MVRs are active within the magnetic field range applied in the regime of the &lt;100&gt;p, &lt;110&gt;p, and &lt;111&gt;p of the single-crystal {100}p Ni-Mn-Ga cubes.

Influence of the modifiers in polyol method on magnetically induced hyperthermia and biocompatibility of ultrafine magnetite nanoparticles

Magnetite nanoparticles (Fe3O4 NPs) are widely tested in various biomedical applications, including magnetically induced hyperthermia. In this study, the influence of the modifiers, i.e., urotropine, polyethylene glycol, and NH4HCO3, on the size, morphology, magnetically induced hyperthermia effect, and biocompatibility were tested for Fe3O4 NPs synthesized by polyol method. The nanoparticles were characterized by a spherical shape and similar size of around 10 nm. At the same time, their surface is functionalized by triethylene glycol or polyethylene glycol, depending on the modifiers. The Fe3O4 NPs synthesized in the presence of urotropine had the highest colloidal stability related to the high positive value of zeta potential (26.03 ± 0.55 mV) but were characterized by the lowest specific absorption rate (SAR) and intrinsic loss power (ILP). The highest potential in the hyperthermia applications have NPs synthesized using NH4HCO3, for which SAR and ILP were equal to 69.6 ± 5.2 W/g and 0.613 ± 0.051 nHm2/kg, respectively. Their application possibility was confirmed for a wide range of magnetic fields and by cytotoxicity tests. The absence of differences in toxicity to dermal fibroblasts between all studied NPs was confirmed. Additionally, no significant changes in the ultrastructure of fibroblast cells were observed apart from the gradual increase in the number of autophagous structures.

Giant nonlocal edge conduction in the axion insulator state of MnBi2Te4

The recently discovered antiferromagnetic (AFM) topological insulator (TI) MnBi2Te4 represents a versatile material platform for exploring exotic topological quantum phenomena in nanoscale devices. It has been proposed that even-septuple-layer (even-SL) MnBi2Te4 can host helical hinge currents with unique nonlocal behavior, but experimental confirmation is still lacking. In this work, we report transport studies of exfoliated MnBi2Te4 flakes with varied thicknesses down to the few-nanometer regime. We observe giant nonlocal transport signals in even-SL devices when the system is in the axion insulator state but vanishingly small nonlocal signal in the odd-SL devices at the same magnetic field range. In conjunction with theoretical calculations, we demonstrate that the nonlocal transport is via the helical edge currents mainly distributed at the hinges between the side and top/bottom surfaces. The helical edge currents in the axion insulator state may find unique applications in topological quantum devices.

Parameter identification for a model for multi-functional materials with hysteresis and thermodynamic compatibility

Multifunctional materials have tremendous potential for engineering applications as they are able to convert mechanical to electromagnetic energy and vice-versa. One of the features of this class of materials is that they show significant hysteresis, which needs to be modeled correctly in order to maximize their application potential. A method of modeling multifunctional materials that exhibit the phenomenon of hysteresis and is compatible with the laws of thermodynamics was developed recently. The model is based on the Preisach hysteresis operator and its storage function and may be interpreted as a two-input, two-output neural net with elementary hysteresis operators as the neurons. The difficulty is that the parameters in the model appear in a non-linear fashion, and there are several constraints that must be satisfied by the parameters for thermodynamic compatibility. In this article, we present a novel methodology that uses the rate-independent memory evolution properties of the Preisach operator to split the parameter estimation problem into three numerically well-conditioned, linear least squares problems with constraints. The alternative direction method of multipliers (ADMM) algorithm and accelerated proximal gradient method are used to compute the Preisach weights. Numerical results are presented over data collected from experiments on a Galfenol sample. We show that the model is able to fit not only experimental data for strain and magnetization over a wide range of magnetic fields and stress but also able to predict the response for stress and magnetic fields not used in the parameter estimation.

Enhancement of spin–flop-induced magnetic hysteresis in van der Waals magnet (Fe1−xCox)5GeTe2

We have systematically studied magnetotransport properties in van der Waals (vdW) magnetic materials, (Fe1−xCox)5GeTe2, where the magnetic phase changes from the ferromagnetic with the perpendicular magnetic anisotropy (PMA; x = 0 , 0.05) or with the in-plane magnetic anisotropy (IMA; x = 0.19) to the antiferromagnetic (x = 0.46) with the PMA. We have demonstrated that such magnetic properties seen in bulk still remain even in thin film devices. An anomalous Hall resistance with magnetic hysteresis was clearly observed in the low Co substitution ( x = 0 , 0.05). The anomalous Hall effect was still observable for x = 0.19, but the magnetic hysteresis vanishes because of the IMA. In the antiferromagnetic region, there was no anomalous Hall effect in the low magnetic field range, but a clear hysteresis was observed at 2.5 T where the spin–flop transition takes place. This hysteresis can be seen only below 30 K and monotonically decreases with increasing temperature. We argue that the defects at a specific site in this system and also the resistance upturn below 30 K could be related to the hysteric behavior at the spin–flop transition. Our findings provide a recipe for the use of (Fe1−xCox)5GeTe2 with different Co substitutions to construct vdW magnetic devices.

Hysteresis loss and field dependence of magnetic entropy change of Zn-doped Mn5Ge3 system

In this work, a series of Mn5Ge3-xZnx samples were prepared by the arc-melting method. The Rietveld structure refinement of the powder XRD spectrum was performed using the FullProf program. The lattice parameters and cell volume increase with the doping of Zn atoms. From the magnetization curve at 5 K, the compounds show a decreased magnetic moment of the Mn atom with increasing Zn content, resulting from mixing the 3d electronic state of Mn atoms and the 3d electronic state of Zn atoms. The small hysteresis loss and soft ferromagnetism of Mn5Ge3-xZnx indicate that it is suitable for magnetic refrigeration. Thermomagnetic M(T) curves show an increase in thermal hysteresis concurrent with increasing Zn concentration, which is verified by the DSC curve. It is found that the magnetic entropy change and effective magnetic moment decrease with the increase of Zn content, which can be attributable to the doping of Zn atoms resulting in the weakening of the exchange interaction between Mn-Mn atoms and the decrease of magnetic entropy change. In addition, the specific power-law relationships between the magnetic entropy change, the maximum magnetic entropy change, the relative cooling capacity, and the magnetic field of this system were determined, which provides a tool for evaluating the performance of the magnetic refrigerant in the entire magnetic field range. Finally, the specific heat was calculated by the magnetic entropy change obtained by the experiment, and the specific heat curve proves that the sample undergoes a second-order phase transition, consistent with the results of Arrott plots. The TC obtained by the specific heat -ΔCP−T curve is in good agreement with the TC obtained by the magnetic measurement, revealing that TC obtained by the specific heat is also an effective method.

Chirping instabilities produced by a runaway electron beam at a spherical tokamak

Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges. The first type arises at frequencies of an order of 1 MHz in ohmic plasmas at relatively low density in a wide range of toroidal magnetic fields and plasma currents. This type of instability was identified as compressional Alfvén waves, driven by electrons, accelerated during a sawtooth crush. It was found that the mode frequency is sweeping in time, according to the Berk–Breizman hole–clump nonlinear chirping model. The second type of wave arises in a specific single-swing regime of the central solenoid current with a very narrow plasma column, when the plasma tends to decay at extremely low density and, in fact, is an instability of the runaway electron beam. The exited modes cover the whole observed frequency range and are divided into several (two or three) frequency regions: approximately 0–30 MHz, 60–120 MHz and sometimes 30–60 MHz. Reconnection of the branches was also observed. Single chirps are more rapid than for 1 MHz Alfvén instability and follow an exponential law. This paper, to our knowledge, is the first report of frequency chirping instabilities excited by accelerated electrons at a spherical tokamak.

Ultrafast heat-assisted magnetization dynamics in a ferrimagnetic insulator

Ultrafast laser-induced heating of ferrimagnetic iron garnet in an external magnetic field triggers magnetization precessional dynamics with a large amplitude. The dynamics is studied as a function of magnetic field, laser fluence, and sample temperature. Exploring the three-dimensional space of these parameters experimentally and computationally, we identify the conditions for which the amplitude of the precession is the largest and even achieves values sufficient for magnetic recording. We found that the range of external magnetic fields and temperatures, which corresponds to the magnetic recording, is rather narrow. Modeling the dynamics, using magnetization as a macroscopic parameter, reveals that this range of parameters is defined by the optimal height of the potential barrier separating two stable states. The barrier needs to be low enough to allow the switching but not so low that the stability of the states is lost.

3D two-electron double quantum dot: Comparison between the behaviour of some physical quantities under two different confinement potentials in the presence of a magnetic field

We have considered a system consisting of two coupled quantum dots containing two electrons, i.e., two quantum dots next to each other with one excess electron each, subjected to an uniform magnetic field perpendicular to the quantum dots plane. The effect of different confinement potential profiles under which the electrons are subjected is studied. This study has been performed in the light of interest in fundamental logical quantum-gate operations: we have been concerned in analysing the behaviour of the main physical quantities which should be involved in a two-qubit quantum-gate operation for two different profiles of confinement found in literature. Our purpose was to establish how sensitive the physical quantities are to the confinement profile and in which range of magnetic field this issue can be critical.

Engineering of Advanced Materials for High Magnetic Field Sensing: A Review

Advanced scientific and industrial equipment requires magnetic field sensors with decreased dimensions while keeping high sensitivity in a wide range of magnetic fields and temperatures. However, there is a lack of commercial sensors for measurements of high magnetic fields, from ∼1 T up to megagauss. Therefore, the search for advanced materials and the engineering of nanostructures exhibiting extraordinary properties or new phenomena for high magnetic field sensing applications is of great importance. The main focus of this review is the investigation of thin films, nanostructures and two-dimensional (2D) materials exhibiting non-saturating magnetoresistance up to high magnetic fields. Results of the review showed how tuning of the nanostructure and chemical composition of thin polycrystalline ferromagnetic oxide films (manganites) can result in a remarkable colossal magnetoresistance up to megagauss. Moreover, by introducing some structural disorder in different classes of materials, such as non-stoichiometric silver chalcogenides, narrow band gap semiconductors, and 2D materials such as graphene and transition metal dichalcogenides, the possibility to increase the linear magnetoresistive response range up to very strong magnetic fields (50 T and more) and over a large range of temperatures was demonstrated. Approaches for the tailoring of the magnetoresistive properties of these materials and nanostructures for high magnetic field sensor applications were discussed and future perspectives were outlined.

Magnetic Susceptibility of a Nanocomposite Based on an Opal Matrix with Yb2Ti2O7 Particles

The DC and AC magnetic susceptibilities of an opal matrix-based nanocomposite with pyrochlore-structured ytterbium titanate particles up to 60 nm in size have been studied in the range of magnetic fields up to 30 kOe. The measurements were performed at temperatures from 2 to 200 K. The temperature dependence of the nanocomposite Yb2Ti2O7 has been found to deviate significantly from the Curie–Weiss law. From the frequency dependence of the AC susceptibility measured in the range from 10 Hz to 10 kHz, the spin relaxation times have been determined, and two relaxation times have been found to be required for the description of the frequency dependence of the susceptibility. The field dependence of the AC susceptibility has been measured. This dependence is proved to be described by the modified Cole–Cole formula. The characteristic fields of the magnetic field dependence of the real part of the susceptibility are determined, the value of the characteristic field being found to increase with increasing temperatures.

A 220 GHz-1.1THz continuous frequency and polarization tunable quasi-optical electron paramagnetic resonance spectroscopic system.

Here, we describe a custom-designed quasi-optical system continuously operating in the frequency range 220GHz to 1.1THz with a temperature range of 5-300K and magnetic fields up to 9T capable of polarization rotation in both transmitter and receiver arms at any given frequency within the range through a unique double Martin-Puplett interferometry approach. The system employs focusing lenses to amplify the microwave power at the sample position and recollimate the beam to the transmission branch. The cryostat and split coil magnets are furnished with five optical access ports from all three major directions to the sample sitting on a two-axes rotatable sample holder capable of performing arbitrary rotations with respect to the field direction, enabling broad accessibility to experimental geometries. Initial results from test measurements on antiferromagnetic MnF2 single crystals are included to verify the operation of the system.

Hybridization‐Induced Spin‐Wave Transmission Stop Band within a 1D Diffraction Grating

AbstractSpin wave propagation is studied through a diffraction grating in a 200 nm thick YIG film by using scanning time resolved magneto‐optic Kerr microscopy (TR‐MOKE) and supported by micromagnetic simulations. Caustic‐like spin wave emission and the hybridization of Damon Eshbach (DE) type spin wave modes within the grating region, depending on the magnetic field and the dimensions of the grating, are observed. Hybridization leads to an increased attenuation length for propagating spin waves and consequently to a transmission stop‐band for spin waves at the grating for a certain magnetic field range.

Giant Density of States Enhancement Driven by a Zero-Mode Landau Level in Semimetallic Black Phosphorus under Pressure.

Dirac fermion systems form a unique Landau level at the Fermi level-the so-called zero mode-whose observation itself will provide strong evidence of the presence of Dirac dispersions. Here, we report the study of semimetallic black phosphorus under pressure by ^{31}P-nuclear magnetic resonance measurements in a wide range of magnetic field up to 24.0T. We have found a field-induced giant enhancement of 1/T_{1}T, where 1/T_{1} is the nuclear spin lattice relaxation rate: 1/T_{1}T at 24.0T reaches more than 20 times larger than that at 2.0T. The increase in 1/T_{1}T above 6.5T is approximately proportional to the squared field, implying a linear relationship between the density of states and the field. We also found that, while 1/T_{1}T at a constant field is independent of temperature in the low-temperature region, it steeply increases with temperature above 100K. All these phenomena are well explained by considering the effect of Landau quantization on three-dimensional Dirac fermions. The present study demonstrates that 1/T_{1} is an excellent quantity to probe the zero-mode Landau level and to identify the dimensionality of the Dirac fermion system.