Small Magnetic Field Research Articles

Field-tunable toroidal moment in a chiral-lattice magnet

Ferrotoroidal order, which represents a spontaneous arrangement of toroidal moments, has recently been found in a few linear magnetoelectric materials. However, tuning toroidal moments in these materials is challenging. Here, we report switching between ferritoroidal and ferrotoroidal phases by a small magnetic field, in a chiral triangular-lattice magnet BaCoSiO4 with tri-spin vortices. Upon applying a magnetic field, we observe multi-stair metamagnetic transitions, characterized by equidistant steps in the net magnetic and toroidal moments. This highly unusual ferri-ferroic order appears to come as a result of an unusual hierarchy of frustrated isotropic exchange couplings revealed by first principle calculations, and the antisymmetric exchange interactions driven by the structural chirality. In contrast to the previously known toroidal materials identified via a linear magnetoelectric effect, BaCoSiO4 is a qualitatively new multiferroic with an unusual coupling between several different orders, and opens up new avenues for realizing easily tunable toroidal orders.

GRMHD simulations of BH activation by small scale magnetic loops: formation of striped jets and active coronae

ABSTRACT We have performed a series of numerical experiments aimed at studying the activation of Kerr black holes (BHs) by advection of small scale magnetic fields. Such configurations may potentially give rise to the formation of quasi-striped Blandford-Znajek jets. It can also lead to enhanced dissipation and generation of plasmoids in current sheets formed in the vicinity of the BH horizon, which may constitute a mechanism to power the hard X-ray emission seen in many accreting BH systems (a la lamppost models). Our analysis suggests that formation of quasi-striped jets with significant power may be possible provided loops with alternating polarity having sizes larger than ∼10rg or so can be maintained (either form sporadically or advected from outside) at a radius ≲ 102rg. This conclusion is consistent with recent results of general relativistic force-free simulations. We also find that the accretion dynamics exhibits cyclic behaviour in Magnetically Arrested Disc states, alternating between high accretion phases and quenched accretion phases during which the magnetosphere becomes force-free out to radii ≳ 10rg. We suggest that such a behaviour should lead to notable variations of the observed luminosity and image of the inner disc (BH shadow image). Finally, we find that the transition between accreted loops on the BH gives rise to the formation of current sheets and energetic plasmoids on the jet boundary during intermittent periods when the jet becomes inactive, in addition to an equatorial current sheet that forms during peaks in the jet activity.

The On-Axis Magnetic Field for a Single, Helical Turn Coil

Science and engineering students in the second semester of a calculus-based physics sequence typically study and measure the on-axis magnetic field for a multiple, circular turn coil. There are four benefits to this approach: 1) an analytical solution is easily obtained, 2) the coil is easily constructed using tightly wound, high-gauge wire where each turn is approximately circular, 3) the magnetic field along the axis of symmetry is parallel to that axis, which simplifies the placement of a magnetic field sensor, and 4) the magnetic field strength can be increased by increasing the number of turns, which increases the signal to “noise” ratio. This approach is fine as an introduction, but it doesn’t address several practical questions. How does one calculate the magnetic field for a current-carrying wire that isn’t in the shape of a line, circle, or multiple concentric circles? Are all coils made of high-gauge wire? Aren’t coils helical? Under what conditions can a circular coil in theory be used to represent a helical coil in practice? How does one measure small magnetic fields in the presence of other magnetic fields? This paper presents a secondary approach that begins to address these questions. It determines the on-axis magnetic field for a single, helical turn coil and then details the setup and procedures used for an experimental confirmation. It is presented to students as a guided experiment.

Learning spin liquids on a honeycomb lattice with artificial neural networks

Machine learning methods provide a new perspective on the study of many-body system in condensed matter physics and there is only limited understanding of their representational properties and limitations in quantum spin liquid systems. In this work, we investigate the ability of the machine learning method based on the restricted Boltzmann machine in capturing physical quantities including the ground-state energy, spin-structure factor, magnetization, quantum coherence, and multipartite entanglement in the two-dimensional ferromagnetic spin liquids on a honeycomb lattice. It is found that the restricted Boltzmann machine can encode the many-body wavefunction quite well by reproducing accurate ground-state energy and structure factor. Further investigation on the behavior of multipartite entanglement indicates that the residual entanglement is richer in the gapless phase than the gapped spin-liquid phase, which suggests that the residual entanglement can characterize the spin-liquid phases. Additionally, we confirm the existence of a gapped non-Abelian topological phase in the spin liquids on a honeycomb lattice with a small magnetic field and determine the corresponding phase boundary by recognizing the rapid change of the local magnetization and residual entanglement.

Magnetic Stiffening in 3D Cell Culture Matrices.

The mechanical environment of a cell is not constant. This dynamic behavior is exceedingly difficult to capture in (synthetic) in vitro matrices. This paper describes a novel, highly adaptive hybrid hydrogel composed of magnetically sensitive magnetite nanorods and a stress-responsive synthetic matrix. Nanorod rearrangement after application of (small) magnetic fields induces strain in the network, which results in a strong (over 10-fold) stiffening even at minimal (2.5 wt %) nanorod concentrations. Moreover, the stiffening mechanism yields a fast and fully reversible response. In the manuscript, we quantitatively analyze that forces generated by the particles are comparable to cellular forces. We demonstrate the value of magnetic stiffening in a 3D MCF10A epithelial cell experiment, where simply culturing on top of a permanent magnet gives rise to changes in the cell morphology. This work shows that our hydrogels are uniquely suited as 3D cell culture systems with on-demand adaptive mechanical properties.

Large-scale magnetic field saturation and the Elsasser number in rotating spherical dynamo models

ABSTRACT Numerical simulations are used to investigate large-scale (mean) magnetic field generation in rotating spherical dynamos. Beyond a certain threshold, we find that the magnitude of the mean magnetic field becomes nearly independent of the system rotation rate and buoyancy forcing. The analysis suggests that this saturation arises from the Malkus-Proctor mechanism in which a Coriolis-Lorentz force balance is achieved in the zonal component of the mean momentum equation. When based on the large-scale magnetic field, the Elsasser number is near unity in the saturated regime. The results show that the large and small magnetic field saturate via distinct mechanisms in rapidly rotating dynamos, and that only the axisymmetric component of the magnetic field appears to follow an Elsasser number scaling.

Weyl triplons in SrCu2(BO3)2

We propose that Weyl triplons are expected to appear in the low energy magnetic excitations in the canonical Shastry-Sutherland compound, \ce{SrCu2(BO3)2}, a quasi-2D quantum magnet. Our results show that when a minimal, realistic inter-layer coupling is added to the well-established microscopic model describing the excitation spectrum of the individual layers, the Dirac points that appear in the zero-field triplon spectrum of the 2D model split into two pairs of Weyl points along the $k_z$ direction. Varying the strength of the inter-layer DM-interaction and applying a small longitudinal magnetic field results in a range of band-topological transitions accompanied by changing numbers of Weyl points. We propose inelastic neutron scattering along with thermal Hall effect as the experimental techniques to detect the presence of Weyl node in the triplon spectrum of this material. We show that the logarithmic divergence in the second derivative in thermal Hall conductance near phase transition from regime Weyl points to a regime with topologically gapped bands as well as a finite slope in the thermal Hall conductance as a function of magnetic field at zero magnetic field are promising evidence for the presence of Weyl triplons.

Positive and Negative Muon Spin Rotation and Relaxation

Although a positive muon spin rotation and relaxation (μ+SR) technique is commonly used for materials science, its counterpart technique, μ-SR, has been less common due to its very limited availability and a very slow counting rate. However, a combination between an intense pulsed muon beam, such as, J-PARC and ISIS, and the development of a multi-channel counter system enables us to utilize μ-SR for studying a small nuclear magnetic field in solids. Here, we summarize the advantage and disadvantage of the two μSR techniques and propose μ±SR as a novel tool to identify diffusing species and clarify the ion diffusive nature in battery materials.

Is the chiral magnetic effect fast enough?

It depends: While we find within holography that the lifetime of the magnetic field for collider energies like the ones achieved at RHIC is long enough to build up the chiral magnetic current, the lifetime of the magnetic field at LHC seems to be too short. We study the real time evolution of the chiral magnetic effect out-of-equilibrium in strongly coupled holographic gauge theories. We consider the backreaction of the magnetic field onto the geometry and monitor pressure and chiral magnetic current. Our findings show that generically at small magnetic field the pressure builds up faster than the chiral magnetic current whereas at strong magnetic field the opposite is true. At large charge we also find that equilibration is delayed significantly due to long lived oscillations. We also match the parameters of our model to QCD parameters and draw lessons of possible relevance to the realization of the chiral magnetic effect in heavy ion collisions. In particular, we find an equilibration time of about $\sim0.35$ fm/c in presence of the chiral anomaly for plasma temperatures of order $T\sim300-400$ MeV.

New ex-OH maser detections in the northern celestial hemisphere

Aims.Molecular masers, including methanol and hydroxyl masers, and in particular the ones in excited rotational states (ex-OH masers), are one of the most informative tools for studying star-forming regions. So, the discovery, of new maser sources in these regions is of great importance. Many studies and surveys of ex-OH maser sources have been carried out in the southern celestial hemisphere, but only a few have been done in the northern hemisphere. The specific aim of this work is to close this gap.Methods.The star-forming regions in the northern hemisphere with known active methanol masers were observed to search for new ex-OH maser sources with the 32 m and 16 m radio telescopes of the Ventspils International Radio Astronomy Centre (VIRAC).Results.Three OH maser lines in the excited state at the 6035 MHz in three northern hemisphere star-forming regions are detected. The maser 189.030+0.783 was previously known, but we suggest this maser is a possible variable. We confirm recent detections of the ex-OH masers 85.41+0.00 and 90.92+1.49 by other authors. The magnetic field strength in the masering regions is estimated by using right circular polarization and left circular polarization pair splitting. The high-velocity resolution provides us with an estimation of a comparatively small magnetic field strength for the 189.030+0.783 and 90.92+1.49 star-forming regions.

Spin dynamics of the quantum dipolar magnet Yb3Ga5O12 in an external field

We investigate ytterbium gallium garnet Yb$_{3}$Ga$_{5}$O$_{12}$ in the paramagnetic phase above the supposed magnetic transition at $T_{\lambda} \approx 54$ mK. Our study combines susceptibility and specific heat measurements with neutron scattering experiments and theoretical calculations. Below 500 mK, the elastic neutron response is strongly peaked in the momentum space. Along with that the inelastic spectrum develops flat excitation modes. In magnetic field, the lowest energy branch follows a Zeeman shift in accordance with the field-dependent specific heat data. An intermediate state with spin canting away from the field direction is evidenced in small magnetic fields. In the field of 2 T, the total magnetization almost saturates and the measured excitation spectrum is well reproduced by the spin-wave calculations taking into account solely the dipole-dipole interactions. The small positive Curie-Weiss temperature derived from the susceptibility measurements is also accounted for by the dipole spin model. Altogether, our results suggest that Yb$_{3}$Ga$_{5}$O$_{12}$ is a quantum dipolar magnet.

Shear rheology of a dilute emulsion of ferrofluid droplets dispersed in a nonmagnetizable carrier fluid under the influence of a uniform magnetic field

The effect of a spatially uniform magnetic field on the shear rheology of a dilute emulsion of monodispersed ferrofluid droplets, immersed in a nonmagnetizable immiscible fluid, is investigated using direct numerical simulations. The direction of the applied magnetic field is normal to the shear flow direction. The droplets' extra-stress tensor arising from the presence of interfacial forces of magnetic nature is modeled on the basis of the seminal work of G. K. Batchelor [J. Fluid Mech. 41, 545–570 (1970)] under the assumptions of a linearly magnetizable ferrofluid phase and negligible inertia. The results show that even relatively small magnetic fields can have significant consequences on the rheological properties of the emulsion due to the magnetic forces that contribute to deform and orient the droplets toward the direction of the applied magnetic vector. In particular, we have observed an increase in the effective (bulk) viscosity and a reversal of the sign of the two normal stress differences with respect to the case without magnetic field for those conditions where the magnetic force prevails over the shearing force. Comparisons between the results of our model with a direct integration of the viscous stress have provided an indication of its reliability to predict the effective viscosity of the suspension. Moreover, this latter quantity has been found to behave as a monotonic increasing function of the applied magnetic field for constant shearing flows (“magneto-thickening” behavior), which allowed us to infer a simple constitutive equation describing the emulsion viscosity.

Terahertz magneto-optical response of bismuth-gadolinium-substituted rare-earth garnet film.

We report the magneto-optical Faraday response of bismuth-gadolinium-substituted rare-earth iron garnet at terahertz frequencies ranging from 100 GHz to 1.2 THz. The maximum transmittance of ±45° component is about 60% near the frequency point of 0.63 THz. When the external magnetic field change from -100 mT to +100 mT, the Faraday rotation angle is between -6° and +7.5°. The overall change of ellipticity is relatively small. The maximum value of the Verdet constant is about 260 °/mm/T at 0.1 THz and then gradually decreases to 80 °/mm/T at 1.2 THz. Within the considered frequency range, the thick film exhibits magnetically tunable, non-reciprocal characters and a strong magneto-optical effect within a small external magnetic field at room temperature, which will be widely used for the terahertz isolators, circulators, nonreciprocal phase shifters, and magneto-optical modulators.

QCD phase diagram in a magnetized medium from the chiral symmetry perspective: the linear sigma model with quarks and the Nambu–Jona-Lasinio model effective descriptions

We review the main features of the QCD phase diagram description, at finite temperature, baryon density and in the presence of a magnetic field, from the point of view of effective models, whose main ingredient is chiral symmetry. We concentrate our attention on two of these models: The linear sigma model with quarks and the Nambu--Jona-Lasinio model. We show that a main ingredient to understand the characteristics of the phase transitions is the inclusion of plasma screening effects that capture the physics of collective, long-wave modes, and thus describe a prime property of plasmas near transition lines, namely, long distance correlations. Inclusion of plasma screening makes possible to understand the inverse magnetic catalysis phenomenon even without the need to consider magnetic field-dependent coupling constants. Screening is also responsible for the emergence of a critical end point in the phase diagram even for small magnetic field strengths. Although versatile, the NJL model is also a more limited approach since, being a non-renormalizable model, a clear separation between pure vacuum and medium effects is not always possible. The model cannot describe inverse magnetic catalysis unless a magnetic field dependent coupling is included. The location of the critical end point strongly depends on the choice of the type of interaction and on the magnetic field dependence of the corresponding coupling. Overall, both models provide sensible tools to explore the properties of magnetized, strongly interacting matter. However, a cross talk among them as well as a consistent physical approach to determine the model parameters is much needed.

Symmetry breaking in ideal magnetohydrodynamics: the role of the velocity

The ideal magnetohydrodynamic equations are, roughly speaking, a quasi-linear symmetric hyperbolic system of PDEs, but not all the unknowns play the same role in this system. Indeed, in the regime of small magnetic fields, the equations are close to the incompressible Euler equations. In the present paper, we adopt this point of view to study questions linked with the lifespan of strong solutions to the ideal magnetohydrodynamic equations. First of all, we prove a continuation criterion in terms of the velocity field only. Secondly, we refine the explicit lower bound for the lifespan of 2-D flows found in Cobb and Fanelli (arXiv, 2020), by relaxing the regularity assumptions on the initial magnetic field.

Manipulation of Magnetization Reversal by Electric Field in a FePt/(011)PMN-PT/Au

Electric field manipulation of magnetism and 180° magnetization reversal are crucial for realizing magnetic storage devices with low-power consumption. Here, we demonstrate that electric-field manipulation of magnetic anisotropy rotation is achieved by the strain-mediated magnetoelectric effect in a Fe50Pt50/(011)0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3/Au. The remanent magnetization and magnetic coercivity of the Fe50Pt50 film exhibit an obvious response with the change of the electric fields. Moreover, the reversible in-plane 180° magnetization reversal can be controlled by alternating on or off the electric field under a small bias magnetic field. These results suggest a promising application for realizing magnetoelectric random access memory (MeRAM) devices with low-power consumption.

Collective modes of polarizable holographic media in magnetic fields

We consider a neutral holographic plasma with dynamical electromagnetic interactions in a finite external magnetic field. The Coulomb interactions are introduced via mixed boundary conditions for the Maxwell gauge field. The collective modes at finite wave-vector are analyzed in detail and compared to the magneto-hydrodynamics results valid only at small magnetic fields. Surprisingly, at large magnetic field, we observe the appearance of two plasmon-like modes whose corresponding effective plasma frequency grows with the magnetic field and is not supported by any background charge density. Finally, we identify a mode collision which allows us to study the radius of convergence of the linearized hydrodynamics expansion as a function of the external magnetic field. We find that the radius of convergence in momentum space, related to the diffusive transverse electromagnetic mode, increases quadratically with the strength of the magnetic field.

Battery Pot Plants with Magnets and Adjacent Balloon as Substitutes for Light

In a future of clean technologies and innovations, it is important with knowledge of new Power sources in terms of the impact on living plants and surrounding life. Organic battery plants with electric components are constructed and analysed. The purpose is to study how illumination i.e. light energy, can be replaced with electricity and matter. The novelty of the paper is the constructions; namely a Balloon capacitor next to a current amplifier in the lines of a pot battery with a plant that should prosper, and the related analysis. Also certain aspects of small magnetic fields alone are invoked. Doi: 10.28991/HEF-2021-02-02-05 Full Text: PDF

Magnetism-induced ideal Weyl state in bulk van der Waals crystal MnSb2Te4

We have unveiled a magnetic exchange-induced topological phase transition in a bulk natural van der Waals crystal MnSb2Te4, based on magnetization and magnetotransport measurements and first principles calculations. At the A-type antiferromagnetic ground state, MnSb2Te4 is a topologically trivial insulator with a bandgap of ∼ 42 meV at the Γ point of the Brillouin zone. A small magnetic field less than 1.4 T along the c axis can drive the system into a spin fully polarized state, which hosts only a single pair of Weyl points setting near the Γ point at the Fermi level without other band mixing, supported by both the first principles calculations and the measured anomalous Hall effect. The results would setup an excellent paradigm for the study of interplay between magnetism and nontrivial topology of the electronic band structure.

A Magneto-Mechanical Piezoelectric Energy Harvester Designed to Scavenge AC Magnetic Field from Thermal Power Plant with Power-Line Cables

Piezoelectric energy harvesters have attracted much attention because they are crucial in portable industrial applications. Here, we report on a high-power device based on a magneto-mechanical piezoelectric energy harvester to scavenge the AC magnetic field from a power-line cable for industrial applications. The electrical output performance of the harvester (×4 layers) reached an output voltage of 60.8 Vmax, an output power of 215 mWmax (98 mWrms), and a power density of 94.5 mWmax/cm3 (43.5 mWrms/cm3) at an impedance matching of 5 kΩ under a magnetic field of 80 μT. The multilayer energy harvester enables high-output performance, presenting an obvious advantage given this improved level of output power. Finite element simulations were also performed to support the experimental observations. The generator was successfully used to power a wireless sensor network (WSN) for use on an IoT device composed of a temperature sensor in a thermal power station. The result shows that the magneto-mechanical piezoelectric energy harvester (MPEH) demonstrated is capable of meeting the requirements of self-powered monitoring systems under a small magnetic field, and is quite promising for use in actual industrial applications.