Absence Of External Field Research Articles

Coexistence of anomalous Hall effect and weak magnetization in a nominally collinear antiferromagnet MnTe

In various material systems, an antiferromagnetic phase was found to coexist with a weak ferromagneticlike signal, while symmetry-based theoretical predictions indicate a possibility of a nonzero anomalous Hall effect (AHE) even in the absence of sample magnetization. This is the case of nominally collinear antiferromagnets, in particular, hexagonal MnTe, where the AHE and no detectable magnetization have been recently reported. To clarify the role of magnetization, we present a study of bulk MnTe samples, combining experiment and theory. We demonstrate that the existence of the AHE in the hexagonal MnTe is accompanied by the presence of a weak but detectable ferromagneticlike signal, vanishing at the Néel temperature. In contrast to thin layer samples, we find that the AHE hysteresis loop shows an opposite sign and Barkhausen-like jumps. We introduce a macrospin model involving the Dzyaloshinskii–Moriya type interaction, which explains the existence of a nonzero magnetic moment in the absence of external field and reproduces well hysteretic behavior of the AHE. Using analysis of Néel-vector-dependent Berry curvature, we show that the intrinsic AHE in hexagonal MnTe can be nonzero even when the magnetization vanishes and, also, that it changes sign depending on the Fermi energy position. Published by the American Physical Society 2024

Microwave susceptibility analysis of ultrafast moment dynamics in a multicore magnetic nanoparticle system

Room-temperature magnetization dynamics of multicore magnetic nanoparticles often account for intrinsic dipolar magnetism to behave as a single macrospin at low-frequency regime. Either magnetic particle imaging or hyperthermia benefits from the resulting superparamagnetism in terms of nonlinear magnetization response and relaxation losses at frequencies where the rotation of magnetic moments dominates over Brownian motion for a given sinusoidal field. For this situation, spontaneous thermal relaxation (in the absence of external fields) of each composing particle moment within the cluster is critical to define effective Néel time constant and may intersect with ferromagnetic resonance (FMR) at GHz range. Here, we performed broadband AC susceptometry on both immobilized single-core and multicore iron-oxide nanoparticles up to 26.5GHz under DC bias fields. For each solid sample, we confirmed FMR frequency, where large single-core nanoparticle systems demonstrated typical resonance blueshift as DC field increased. Interestingly, high DC field induced the secondary satellite peak in the imaginary part of AC susceptibility spectra for the case of multicore nanoparticle systems. We further highlighted that a synchronous precession of the polarized macrospins under nonuniform effective fields was responsible for splitting FMR peaks at nearby microwave frequencies. Upon curve fitting of the field-dependent FMR frequency spectra, the Landau-Lifshitz-Gilbert-Kittel model later elaborates on the complex moment dynamics of multicore nanoparticle systems in correlation with distribution functions. Published by the American Physical Society 2024

Spontaneous Small Biskyrmions in a Centrosymmetric Rare-Earth Kagome Ferrimagnet

Magnetic skyrmions with nontrivial topologies have great potential to serve as memory cells in novel spintronic devices. Small skyrmions were theoretically and experimentally confirmed to be generated under the influence of external fields in ferrimagnetic films via Dzyaloshinskii–Moriya interactions (DMIs). However, this topological state has yet to be verified in ferrimagnetic crystals, especially in the absence of external fields and DMIs. Here, spontaneous biskyrmions were directly observed in the Tb0.2Gd0.8Co2 ferrimagnetic crystal with a Kagome lattice using Lorentz transmission electron microscopy. The high-density biskyrmions exhibited a small size (approximately 50 nm) over a wide temperature range, were closely related to subtle magnetic interaction competition, and coexisted with some broken stripes that could be easily converted into zero-field biskyrmions by utilizing proper field-cooling manipulation. These results can be used to establish a platform for investigating functional sub-50-nm skyrmions in ferrimagnetic crystals and to facilitate advanced applications in magnetic devices.

Complete integrability and equilibrium thermodynamics of biaxial nematic systems with discrete orientational degrees of freedom

We study a discrete version of a biaxial nematic liquid crystal model with external fields via an approach based on the solution of differential identities for the partition function. In the thermodynamic limit, we derive the free energy of the model and the associated closed set of equations of state involving four order parameters, proving the integrability and exact solvability of the model. The equations of state are specified via a suitable representation of the orientational order parameters, which imply two-order parameter reductions in the absence of external fields. A detailed exact analysis of the equations of state reveal a rich phase diagram where isotropic versus uniaxial versus biaxial phase transitions are explicitly described, including the existence of triple and tricritical points. Results on the discrete models are qualitatively consistent with their continuum analogue. This observation suggests that, in more general settings, discrete models may be used to capture and describe phenomena that also occur in the continuum for which exact equations of state in closed form are not available.

Penta graphene: Investigating the role of external fields and electron/hole doping in enhancing transport performance

This comprehensive study utilizes the full tight-binding model and Green's function approach, employing the linear response theory, to investigate the electronic and thermal conductance of penta-graphene (PG) under various external parameters such as bias voltage, magnetic field, and doping. The results reveal that these parameters significantly enhance the thermoelectric properties of PG through modifications in its band structure and increased charge carrier density. By applying electric and magnetic fields, the wide band gap of PG, can be substantially reduced, with the magnetic field inducing subband splitting and modifying the band edges. The thermal properties of PG are initially negligible below 1500 K in the absence of external fields, but their magnitude increases remarkably by external fields, with the magnetic field exhibiting a more pronounced effect than the electric field. Interestingly, the chemical potential exhibits a surprising influence on the temperature dependence of PG's thermal properties, surpassing even the impact of the magnetic field at higher temperatures. The Lorenz number of PG complies with the Wiedemann-Franz law, remaining constant at low temperatures and reaching a peak value at higher temperatures and external fields alters this behavior by reducing the constant region and shifting the peak to lower temperatures. Moreover, the presence of electron-hole impurities in PG leads to peaks in the power factor (PF) and figure of merit (ZT), with the electric and magnetic fields causing these peaks to shift towards lower impurity concentrations. These findings highlight the potential of PG for various device applications and open up new avenues for further research and development in this promising field.

Фермион с тремя массовыми параметрами во внешнем магнитном поле

Recently, models for a spin 1/2 particle with two (or three)
 mass parameters were developed. Specific features of
 these models are as follows. For corresponding two (or
 three) bispinors in absence of external fields, separate Diraclike
 equations are derived, they differ in masses. However,
 in presence of external electromagnetic or gravitational
 fields with non-vanishing Ricci scalar, the wave equation for
 bispinors does not split into separated equations but makes
 quite a definite mixing of two (or three) equations arises. In
 the present paper, the model of a fermion with three mass parameters
 is studied in presence of the external uniform magnetics
 field. After performing a diagonalizing transformation,
 three separate equations are obtained for particles with different
 anomalous magnetic moments. Their exact solutions
 and generalized energy spectra are found

Can the energy of a particle be negative in the absence of external fields?

Can the energy of a particle be negative in the absence of external fields?

Skyrmionic spin structures in layered Fe5GeTe2 up to room temperature

The role of the crystal lattice, temperature and magnetic field for the spin structure formation in the 2D van der Waals magnet Fe5GeTe2 with magnetic ordering up to room temperature is a key open question. Using Lorentz transmission electron microscopy, we experimentally observe topological spin structures up to room temperature in the metastable pre-cooling and stable post-cooling phase of Fe5GeTe2. Over wide temperature and field ranges, skyrmionic magnetic bubbles form without preferred chirality, which is indicative of centrosymmetry. These skyrmions can be observed even in the absence of external fields. To understand the complex magnetic order in Fe5GeTe2, we compare macroscopic magnetometry characterization results with microscopic density functional theory and spin-model calculations. Our results show that even up to room temperature, topological spin structures can be stabilized in centrosymmetric van der Waals magnets.

Dynamics of domain formation in a ferromagnetic fluid

Ferrofluids are colloidal suspensions of magnetic nanoparticles. Ferromagnetic ordering – with spontaneous net magnetization in the absence of external fields – has been described for a system of permanently magnetic nanoplatelets suspended in n-butanol, concentrated enough to undergo the transition to the nematic phase. Such material shows magnetic domain structures analogous to those of solid-state ferromagnets. Through optimization of fundamental interactions, the threshold concentration for nematic ordering was reduced. The optimized material allows reproducible and controllable analysis of domain structures and their formation dynamics, with domains forming on the time scale of the order of seconds. Finally, the effect of an external applied magnetic field on the nematic director fundamental fluctuations inside a domain has been studied with differential dynamic microscopy. Interestingly, the results provide an estimation of the ferrofluid's spontaneous magnetization in the absence of external magnetic fields.

Thermoelectric performance of biased silicene nanoribbon in the presence of magnetic field

In this study with the tight binding model, we investigated the electronic and thermal properties of biased Armchair Silicene nanoribbon (AN-SiNR) in the presence of the perpendicular magnetic field. The band gap of AN-SiNRs decreases and increases with applying the vertical magnetic field and bias voltage, respectively. In the presence of magnetic field the DOS spectrum shows significant modifications such as splitting the DOS peak and changing the number of DOS peaks. The thermal properties of unbiased A16-SiNR in absence of external fields is negligible in low temperature and increases rapidly with temperature because the thermal energy increases with T. The bias voltage and magnetic field have opposite effects on the electronic and thermal properties of SiNRs. The increasing rate of thermal properties in terms of temperature decreases (increases) with bias voltage U (magnetic field Π) and the stronger U (Π) has smaller (larger) thermal properties due to increasing (decreasing) the band gap. The Lorenz coefficient has peak for each AN-SiNRs and it decreases by increasing the magnetic field and their peak positions move to lower temperature range. By comparing the thermal properties of A16-SiNR and A19-SiNR, we found that A19-SiNR has higher thermal properties because it has a smaller band gap in the present magnetic field and bias voltage and its charge carriers can be excited to conduction bands by lower thermal energy.

About the force of electrostatic interaction between two spheroidal macroparticles in the Poisson-Boltzmann model

Electrostatic interaction of two charged spheroidal macroparticles is analyzed within the linearized Poisson-Botzmann model.Interparticle forces are calculated through finite-element method in the regimes of weak and moderate screening with constant surface potentials in the absence of external field. Keywords: linearized Poisson-Botzmann model, two charged macroparticles, spheroidal macroparticles, colloidal particles, dusty plasmas.

О силе электростатического взаимодействия двух сфероидальных макрочастиц в модели Пуассона-Больцмана

Еlectrostatic interaction of two charged spheroidal macroparticles is analyzed within the linearized Poisson–Botzmann model.Interparticle forces are calculated through finite-element method in the regimes of weak and moderate screening with constant surface potentials in the absence of external field.

Thermodynamic Investigation of Droplet-Droplet and Bubble-Droplet Equilibrium in an Immiscible Medium.

In the absence of external fields, interfacial tensions between different phases dictate the equilibrium morphology of a multiphase system. Depending on the relative magnitudes of these interfacial tensions, a composite system made up of immiscible fluids in contact with one another can exhibit contrasting behavior: the formation of lenses in one case and complete encapsulation in another. Relatively simple concepts such as the spreading coefficient (SC) have been extensively used by many researchers to make predictions. However, these qualitative methods are limited to determining the nature of the equilibrium states and do not provide enough information to calculate the exact equilibrium geometries. Moreover, due to the assumptions made, their validity is questionable at smaller scales where pressure forces due to curvature of the interfaces become significant or in systems where a compressible gas phase is present. Here we investigate equilibrium configurations of two fluid drops suspended in another fluid, which can be seen as a simple building block of more complicated systems. We use Gibbsian composite-system thermodynamics to derive equilibrium conditions and the equation acting as the free energy (thermodynamic potential) for this system. These equations are then numerically solved for an example system consisting of a dodecane drop and an air bubble surrounded by water, and the relative stability of distinct equilibrium shapes is investigated based on free-energy comparisons. Quantitative effects of system parameters such as interfacial tensions, volumes, and the scale of the system on geometry and stability are further explored. Multiphase systems similar to the ones analyzed here have broad applications in microfluidics, atmospheric physics, soft photonics, froth flotation, oil recovery, and some biological phenomena.

Structure and Dynamics of Curcumin Encapsulated Lecithin Micelles: A Molecular Dynamics Simulation Study

Curcumin is a natural product with potential pharmaceutical applications that can be augmented by drug delivery technology such as nano emulsion. Our study focuses on microscopic structural and dynamics response of curcumin encapsulation in micellar system with lecithin as a natural surfactant under variations of composition and temperature using molecular dynamics (MD) simulations. The results highlight the self-assembly of lecithin micelle, with curcumin encapsulated inside, from initial random configurations in the absence of external field. The variation of composition shows that lecithin can aggregate into spherical and rod-like micelle with the second critical micelle concentration lies between 0.17-0.22 mol dm−3. The radial local density centering at the micelle center of mass shows that the effective radius of micelle is indeed defined by the hydrophilic groups of lecithin molecule and theencapsulated curcumin molecules are positioned closer to these hydrophilic groups than the innermost part of the micelle. The spherical micelle is shown to be thermally stable within the temperature range of 277-310 K without a perceivable change in the spherical eccentricity. The dynamics of micelle are enhanced by the temperature, but it is shown to be insensitive to the variation of lecithin-curcumin composition within the studied range. Simulation results are in agreement with the pattern obtained from experimental results based on particle size, polydispersity index, and encapsulation efficiency.

Synthesis and aging behaviour study of lead-free piezoelectric BCZT ceramics

This present work involves the synthesis of lead - free (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3– BCZT ceramics through solid-state process and its aging behaviour effect on piezoelectric and dielectric properties. Aging phenomenon occurs in ferroelectric state and associated with change in polarization with time. Aging behaviour occurs, below the Curie temperature, in the form of stress relaxation in absence of external field, mechanical stress and temperature. During the processing of the BCZT ceramics mechanical, thermal and electrical stresses were generated which subsequently relaxed in the process of aging that was confirmed by time dependent polarization vs. electric field (hysteresis behaviour) measurements. Aging phenomenon in BCZT ceramics were studied by measuring the piezoelectric and dielectric properties with time and is correlated with the variations in crystal structure using the x-ray diffraction and Raman spectroscopy data.

Bound state solution and thermodynamic properties of the screened cosine Kratzer potential under influence of the magnetic field and Aharanov–Bohm flux field

In this article, approximate analytical bound state solutions of the Schrödinger equation in two-dimensional space for screened cosine Kratzer potential (SCKP) under the influence of the magnetic field and Aharanov–Bohm flux field have been investigated. We obtained energy eigenvalues and wave functions for SCKP with external fields (magnetic field and Aharanov–Bohm flux field) via parametric Nikiforov–Uvarov (pNU) method using the approximation method suggested by Greene–Aldrich for handling centrifugal barriers. We deduced energy eigenvalues for screened Kratzer potential (SKP) and Hellmann potential (HP) from the obtained energy spectrum of SCKP with an external field. We extended our results in D dimensions for Hellmann potential in the absence of external fields. Thermodynamic properties such as partition function Z(B→,ΦAB,β), mean energy U(B→,ΦAB,β), mean free energy F(B→,ΦAB,β), entropy S(B→,ΦAB,β), specific heat capacity Cs(B→,ΦAB,β), magnetization at finite temperature (B→,ΦAB,β) and magnetic susceptibility χm(B→,ΦAB,β) at finite temperature are presented. The obtained numerical resultsfor SKP and Hellmann potential are in very good agreementwith numerical results available in the literature with and without external fields respectively.

Spin 3/2 particle: Puali – Fierz and Fradkin models, interaction with external fields

In the frame of the general Gel’fand – Yaglom formalism, the Fradkin theory for a spin 3/2 particle in presence of external fields is investigated. Applying the standard requirements of relativistic invariance, P-symmetry, existence of a Lagrangian for the model, we derive a set of spinor equations, first in absence of external fields. The wave function consists of a bispinor and a vector-bispinor. It is shown that in absence of external fields the Fradkin model is reduced to the Pauli – Fierz theory. Taking into account the presence of external electromagnetic fields, the Fradkin theory can be turned to the minimal form of the equation for the main bispinor. This equation contains an additional interaction term governed by the electromagnetic tensor F αβ . Meanwhile, there appears a parameter in the Fradkin equation related to any characteristic of the particle additional to its charge. The theory is generalized for taking into account the pseudo-Riemannian space-time geometry. In this case, the Fradkin equation contains an additional interaction term, governed by the Ricci tensor R αβ . If the electric charge of the particle is zero, the Fradkin model remains correct and describes a neutral spin 3/2 particle of the Majorana type interacting nonminimally with the geometrical background through the Ricci tensor. To clarify the meaning of the additional physical characteristics underlying the Fradkin model in contrast to the Pauli – Fierz one we have considered nonrelativistic approximation for both theories in presence of an external uniform magnetic field, and found respective energy spectra. The structure of the ninrelativistic Fradkin equation permits to consider such an additional parameter as polarizability.

Effects of external electric and magnetic fields on the linear and nonlinear optical properties of InAs cylindrical quantum dot with modified Pöschl-Teller and Morse confinement potentials

The theoretical investigation of linear and third-order nonlinear absorption coefficients and refractive index changes in the cylindrical quantum dot with two models of confinement potentials in axial direction, namely, modified Pöschl-Teller and Morse potentials, and parabolic potential in radial direction, have been considered in the presence of external electric and magnetic fields. The selection rules for intraband transitions have been obtained for both potentials in the presence and absence of external fields. The behavior of linear, nonlinear and total absorption spectra have been observed for different values of temperatures for both potentials taking into account electron population on energy levels. The effect of change in the direction of electric field on the absorption spectra have been investigated, and the asymmetric and symmetric characters of these potentials manifest themselves differently: the peak positions of absorption lines for Morse potential undergo blue and red shifts compared to the case when the external electric field is absent, while for the case of modified Pöschl-Teller potential the absorption lines have only red shift. The same dependencies have been obtained for refractive index changes for both potentials. The second and third harmonic generation coefficients as a function of the photon energy have been plotted both in the absence and presence of external fields.

Electric-field-induced reorientation of ferroelectric micro- and nano-platelets in the nematic liquid crystal

ABSTRACT We demonstrate that ferroelectric layered perovskite Bi3.8Nd0.2Ti3O12 micro-platelets can be reoriented in the nematic liquid crystal by the linear coupling to an external electric field. The electric dipole moment of platelets is perpendicular to the plane of platelets and provides torque that rotates the platelets. The experiments were made in dispersions of ferroelectric platelets in a zero dielectric anisotropy nematic liquid crystal (LC), which excludes the reorientation of the dispersion via the dielectric coupling of a LC. Typical reorientation times for micrometre-size ferroelectric platelets are of the order of 100 ms for 0.5 V/μm applied electric field. The electric dipole moment in the absence of external field is very small and does not contribute to the force between the platelets. This could be either due to very narrow hysteresis loop of the ferroelectric material, or screening of dipolar electric field by ions in the LC. A stable and homogenous dispersion of ferroelectric nano-platelets could not be observed in any range of platelets’ concentration. To create a ferroelectric LC the shape and thickness of the nano-platelets has to be controlled, and the electrostatic screening of dipolar field should be minimised.

Electric current and field control of vortex structures in cylindrical magnetic nanowires

Magnetization dynamics in a cylindrical Permalloy nanowire under simultaneously applied electric current and field is investigated by means of micromagnetic simulations. The reversal process starts with the creation of open vortex structures with different rotation senses at the nanowire ends. Our results conclude that the current alone enlarges or reduces the size of these vortex structures according to the rotational sense of the associated Oersted field. Large current intensity creates a vortex structure which covers the whole nanowire surface. At the same time the magnetization in the nanowire core remains the same, i.e., no complete magnetization reversal is possible in the absence of external field. The simultaneous action of the current and field allows for the complete control of the vortex structures in terms of setting the polarity and vorticity. The state diagram for the minimum field and current required for the vorticity and axial magnetization switching is presented. This control is essential for future information technologies based on three-dimensional vertical structures, and the presented state diagram will become very useful for future experiments on current-induced domain wall dynamics in cylindrical magnetic nanowires.