Anisotropy Axis Research Articles

Crystal field excitation in the chiral helimagnet YbNi3Al9

A crystal field level scheme of a uniaxial chiral helimagnet $\mathrm{Yb}{\mathrm{Ni}}_{3}{\mathrm{Al}}_{9}$, exhibiting a chiral magnetic soliton lattice state by Cu substitution for Ni, has been determined by inelastic neutron scattering. The ground and the first excited doublets are separated by 44 K and are simply expressed as $\ensuremath{\alpha}|\ifmmode\pm\else\textpm\fi{}7/2\ensuremath{\rangle}+\ensuremath{\beta}|\ensuremath{\mp}5/2\ensuremath{\rangle}$, with $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ nearly equal to $\ifmmode\pm\else\textpm\fi{}1/\sqrt{2}$. The easy axis of the crystal field anisotropy is the $c$ axis when the excited levels are populated at high temperatures and high magnetic fields. On the other hand, the magnetism at low temperatures and low magnetic fields, where only the ground doublet is populated, is described by an easy-plane anisotropy which may be treated as an $S=1/2$ system with an anisotropic $g$ factor, ${g}_{xy}=3.02$ and ${g}_{z}=1.14$. An orbital-dependent exchange interaction is also discussed to explain the temperature dependence of the magnetic susceptibility based on this level scheme.

The Mid‐Lithospheric Discontinuity Caused by Channel Flow in Proto‐Cratonic Mantle

AbstractGlobal geophysical observations show the presence of the enigmatic mid‐lithospheric discontinuity (MLD) at depths of ca. 80–150 km which may question the stability and internal structure of the continental lithosphere. While various mechanisms may explain the MLD, the dynamic processes leading to the seismic observations are unclear. Here we present a physical mechanism for the origin of MLD by channel flow in the cratonic mantle lithosphere, triggered by convective instabilities at cratonic margins in the Archean when the mantle was hot. Our numerical modeling shows that the top of the frozen‐in channel flow creates a shear zone at a depth comparable to the globally observed seismic MLD. Grain size reduction in the shear zone and accumulation of percolated melts or fluids along the channel top may reduce seismic wave speeds as observed below the MLD, while the channel flow itself may explain radial anisotropy of seismic wave speeds and change in direction of the seismic anisotropic fast axis. The proposed mechanism is valid for a broad range of physically realistic parameters and that MLD may have been preserved since its formation in the Archean. The intensity of the channel flow ceased with time due to secular cooling of the Earth's interior. The new mechanism may reshape our understanding of the evolution and stability of cratonic lithosphere.

Investigation of load-bearing capacity of rigid-plastic reinforced ellipsoidal shells of rotation

For ellipsoidal fiber-reinforced shells of rotation, the upper (kinematic) limit of load-bearing capacity is determined. The structures are clamped along the lower support contour and have rigid polar plate. The material of the components is rigid-plastic with asymmetry in tension and compression. The matrix is orthotropic. The principal axes of matrix anisotropy coincide with meridional, circumferential and transverse directions. Two-layer shell model and refined relations of structural model with two-dimensional stress state in components are used. Extreme problem is formulated and numerically solved for spherical and two different ellipsoidal shells. Influence of reinforcement parameters on limit load is investigated.

Tuning the Magnetic Anisotropy of Co–N‐Doped Graphene

Large magnetic anisotropy energy and out‐of‐plane easy magnetization direction are beneficial for the application of 2D ferromagnetism. Although the robust room‐temperature ferromagnetism is reported in Co–N‐doped graphene [Nat. Commun. 2021, 12, 1854], its magnetic anisotropy and easy axis of magnetization are still unclear. Herein, the magnetic anisotropy of CoN4–graphene and CoN3–graphene is systematically studied. The CoN4–graphene has an in‐plane easy magnetization direction, while the easy magnetization direction of CoN3–graphene is out‐of‐plane. The magnetic anisotropy energy can be tuned by injecting charge or applying strain. Especially, for CoN3–graphene, both charge injection and tensile strain can tune the easy magnetization direction from out‐of‐plane to in‐plane. The tunable magnetic anisotropy may promote the application of Co–N‐doped graphene in spintronic devices.

High-frequency magnetic response of superparamagnetic composites of spherical Fe65Co35 nanoparticles

The response of composites of magnetic nanoparticles (MNPs) with randomly-oriented magnetocrystalline anisotropy axes to a linearly-polarized ac field or a rotating field is studied with regard to the concept of using superparamagnetic nanocomposites as core materials for high (sub-GHz and GHz) frequency power conversion devices. We perform micromagnetic simulations of systems of spherical 5nm-diameter nanoparticles of Fe65Co35 with two different concentrations arranged periodically within a dielectric matrix, including thermal fluctuations of the magnetization within the stochastic thermal field. Because the particles are closely packed, the effect of surface anisotropy is neglected. The dependence of the amplitude of the magnetic response function on temperature is determined. We show the inapplicability of Brown’s classic theory of thermal excitations in single-domain magnetic particles for given material parameters even in the absence of the magnetostatic field. Magnetostatic interactions of MNPs are found to be necessary for stabilizing the dynamical magnetic response in the high temperature (low-energy-barrier) range. They allow for driving magnetization oscillations at room temperature with amplitude comparable to the zero-temperature case. Dynamical (hysteretic and residual) losses are discussed.

Vortex-bound solitons in topological superfluid 3He

The different superfluid phases of 3He are described by p-wave order parameters that include anisotropy axes both in the orbital and spin spaces. The anisotropy axes characterize the broken symmetries in these macroscopically coherent quantum many-body systems. The systems’ free energy has several degenerate minima for certain orientations of the anisotropy axes. As a result, spatial variation of the order parameter between two such regions, settled in different energy minima, forms a topological soliton. Such solitons can terminate in the bulk liquid, where the termination line forms a vortex with trapped circulation of mass and spin superfluid currents. Here we discuss possible soliton-vortex structures based on the symmetry and topology arguments and focus on the three structures observed in experiments: solitons bounded by spin-mass vortices in the B phase, solitons bounded by half-quantum vortices (HQVs) in the polar and polar-distorted A phases, and the composite defect formed by a half-quantum vortex, soliton and the Kibble-Lazarides-Shafi wall in the polar-distorted B phase. The observations are based on nuclear magnetic resonance (NMR) techniques and are of three types: first, solitons can form a potential well for trapped spin waves, observed as an extra peak in the NMR spectrum at shifted frequency; second, they can increase the relaxation rate of the NMR spin precession; lastly, the soliton can present the boundary conditions for the anisotropy axes in bulk, modifying the bulk NMR signal. Owing to solitons’ prominent NMR signatures and the ability to manipulate their structure with external magnetic field, solitons have become an important tool for probing and controlling the structure and dynamics of superfluid 3He, in particular HQVs with core-bound Majorana modes.

Studies of decoherence in strongly anisotropic spin triangles with toroidal or general noncollinear easy axes

Magnetic molecules are investigated with respect to their usability as units in future quantum devices. In view of quantum computing, a necessary prerequisite is a long coherence time of superpositions of low-lying levels. In this article, we investigate by means of numerical simulations whether a toroidal structure of single-ion easy anisotropy axes is advantageous as often conjectured. Our results demonstrate that there is no general advantage of toroidal magnetic molecules, but that arrangements of tilted anisotropy axes perform best in many cases.

A unified approach to three-dimensional modeling of the logging process by galvanic and induction probes in anisotropic media

The article considers a unified approach to modeling logging probes with both galvanic and induction electric field sources. This approach is based on the properties of the functional spaces used in the finite element method and allows for a single software implementation for various logging methods. As an example of the use of the proposed approach, the logging process in an anisotropic medium with a slope of the main axes of anisotropy is considered.

Anisotropic mechanical characterization of human skin by in vivo multi-axial ring suction test

Human skin is a soft tissue behaving as an anisotropic material. The anisotropy emerges from the alignment of collagen fibers in the dermis, which causes the skin to exhibit greater stiffness in a certain direction, known as Langer’s line. The importance of determining this anisotropy axis lies in assisting surgeons in making incisions that do not produce undesirable scars. In this paper, we introduce an open-source numerical framework, MARSAC (Multi-Axial Ring Suction for Anisotropy Characterization: https://github.com/aflahelouneg/MARSAC), adapted to a commercial device CutiScan CS 100® that applies a suction load on an annular section, causing a multi-axial stretch in the central zone, where in-plane displacements are captured by a camera. The presented framework receives inputs from a video file and converts them into displacement fields through Digital Image Correlation (DIC) technique. From the latter and based on an analytical model, the method assesses the anisotropic material parameters of human skin: Langer’s line ϕ, and the elastic moduli E1 and E2 along the principal axes, providing that the Poisson’s ratio is fixed. The pipeline was applied to a public data repository, https://search-data.ubfc.fr/femto/FR-18008901306731-2021-08-25_In-vivo-skin-anisotropy-dataset-for-a-young-man.html, containing 30 test series performed on a forearm of a Caucasian subject. As a result, the identified parameter averages, ϕˆ=40.9±8.2∘ and the anisotropy ratio E1ˆ/E2ˆ=3.14±1.60, were in accordance with the literature. The intra-subject analysis showed a reliable assessment of ϕ and E2. As skin anisotropy varies from site to site and from subject to subject, the novelty of the method consists in (i) an optimal utilization of CutiScan CS 100® probe to measure the Langer’s line accurately and rapidly on small areas with a minimum diameter of 14mm, (ii) validation of an analytical model based on deformation ellipticity.

Transition in vortex skyrmion structures in superfluid He3−A driven by an analog of the zero-charge effect

In quantum electrodynamics, the zero-charge effect originates from the logarithmic dependence of the coupling constant in the action of the electromagnetic field on the ratio of the ultraviolet and infrared energy cutoffs. An analog of this effect in chiral Weyl superfluid $^{3}\mathrm{He}\text{\ensuremath{-}}A$ is the logarithmic divergence of the bending energy of the orbital anisotropy axis at low temperatures, where temperature plays the role of the infrared cutoff, and the vector of the orbital anisotropy plays the role of the vector potential of the synthetic electromagnetic field for Weyl fermions. We calculate numerically the spatial distribution of the order parameter in rotating $^{3}\mathrm{He}\text{\ensuremath{-}}A$ as a function of temperature. At temperatures close to the superfluid transition, we observe formation of vortex skyrmions known as the double-quantum vortex and the vortex sheet. These structures include alternating circular and hyperbolic merons as a bound pair or a chain, respectively. As temperature lowers towards absolute zero, we find a continuous transition in the vortex structures towards a state where the vorticity is distributed in thin tubes around the circular merons. For the vortex sheet, we present a phase diagram of the transition in the temperature--angular velocity plane and calculations of the nuclear magnetic resonance response.

Wellbore Stability in the Bazhenov Formation Considering the Anisotropy of Elastic Properties of Composing Strata

This paper offers a general approach to the modelling of the wellbore stress state in the Bazhenov formation. In modelling, strata are presented as transversely isotropic objects with a set direction of the anisotropy axis that does not coincide with the vertical line. The paper also presents results of the laboratory Bazhenov formation core analysis. The analysis of core materials yielded correlation dependences for reconstructing stress-strain parameters of a transversely isotropic medium in selected wells. Parameters of the rock anisotropy were analyzed on the basis of standard and non-standard laboratory tests. The solution of the stress state problem in a deformation bounded medium was used to suggest an algorithm of calculating the components of the regional stress state along the well path. RN-SIGMA software tools were used to construct borehole stability models for wells penetrating anisotropic rocks. The effect of the anisotropy of elasticity on a safe drilling mud density window is demonstrated. The results of the calculations show that the anisotropy effects are crucial for calculating a safe drilling window. Examples are given to demonstrate potential effects of the anisotropy in image interpretation as well as in selecting horizontal wellbore drilling directions.

Dyakonov surface polaritons in antiferromagnet film

Dyakonov surface polaritons (DSPs) in a single antiferromagnetic (AF) film are investigated in the parallel geometry where the AF anisotropic axis is parallel to the film and no external magnetic field is applied. The analytical dispersion relation is obtained and four surface modes are found, among which one symmetric mode and an anti-symmetric mode are exist in the AF reststrahlen frequency band, and another symmetric and anti-symmetric modes are situated out the reststrahlen band. The existence of symmetric and anti-symmetric modes is caused by the mirror symmetry and finite thickness of the AF film. A pair of modes in the band have magnetostatic limits but the other pair out the band do not. The dispersion properties, field and energy-flux density distributions of each DSP are numerically calculated based on the FeF2 film for various propagating directions and film thicknesses. It is very interesting that the energy flux of each DSP can seriously deviates from its propagation direction, and the largest walk-off angle can reach 200°. In addition, we briefly discuss dispersion properties of Dyakonov odd and even guided modes, they also are symmetric and antisymmetric and have multiple solutions.

Bound states in the continuum in asymmetric one-dimensional photonic crystal systems guided by anisotropy.

Bound states in the continuum (BICs) have been widely observed in many symmetric geometries in the optical system during the last decade. Here, we consider a scenario in which the structure is designed asymmetrically with anisotropic birefringent material embedded in one-dimensional photonic crystals. This kind of new shape opens the possibility of obtaining symmetry-protected BICs (SP-BICs) and Friedrich-Wintgen BICs (FW-BICs) form in tunable anisotropy axis tilt. Interestingly, these BICs can be observed as high-Q resonances by variation of the system's parameters, such as the incident angle, which means the structure without being injected at Brewster's angle can also achieve BICs. Our findings might achieve active regulation and are easy to manufacture.

Trapped mode control in metasurfaces composed of particles with the form birefringence property.

Progress in developing advanced photonic devices relies on introducing new materials, discovered physical principles, and optimal designs when constructing their components. Optical systems operating on the principles of excitation of extremely high-quality factor trapped modes (also known as the bound states in the continuum, BICs) are of great interest since they allow the implementation of laser and sensor devices with outstanding characteristics. In this paper, we discuss how one can utilize the anisotropic properties of novel materials (transition metal dichalcogenides, TMDs), particularly, the bulk molybdenum disulfide (MoS2), to realize the excitation of trapped modes in dielectric metasurfaces. The bulk MoS2 is a thin-film structure in which the light wave behaves the same way as that in the uniaxial anisotropic material with the form birefringence property. Our metasurface is composed of an array of disk-shaped nanoparticles (resonators) made of the MoS2 material under the assumption that the anisotropy axis of MoS2 can be tilted to the rotation axis of the disks. We perform a detailed analysis of eigenwaves and scattering properties of such anisotropic resonators as well as the spectral features of the metasurface revealing dependence of the excitation conditions of the trapped mode on the anisotropy axis orientation of the MoS2 material used.

On the Single-Molecule Magnetic Behavior of Linear Iron(I) Arylsilylamides.

The rational design of 3d-metal-based single-molecule magnets (SMM) requires a fundamental understanding of their intrinsic electronic and structural properties and how they translate into experimentally observable features. Here, we determined the magnetic properties of the linear iron(I) silylamides K{crypt}[FeL2] and [KFeL2] (L = -N(Dipp)SiMe3; crypt = 4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo[8.8.8]hexacosan). For the former, slow-relaxation of the magnetization with a spin reversal barrier of Ueff = 152 cm-1 as well as a closed-waist magnetic hysteresis and magnetic blocking below 2.5 K are observed. For the more linear [KFeL2], in which the potassium cation is encapsulated by the aryl substituents of the amide ligands, the relaxation barrier and the blocking temperature increase to Ueff = 184 cm-1 and TB = 4.5 K, respectively. The increase is rationalized by a more pronounced axial anisotropy in [KFeL2] determined by dc-SQUID magnetometry. The effective relaxation barrier of [KFeL2] is in agreement with the energy spacing between the ground and first-excited magnetic states, as obtained by field-dependent IR-spectroscopy (178 cm-1), magnetic measurements (208 cm-1), as well as theoretical analysis (212 cm-1). In comparison with the literature, the results show that magnetic coercivity in linear iron(I) silylamides is driven by the degree of linearity in conjunction with steric encumbrance, whereas the ligand symmetry is a marginal factor.

Zero-Field Splitting in Cyclic Molecular Magnet {Cr8Y8}: A High-Frequency ESR Study

Cyclic 3d-4f molecular magnets have received considerable attention owing to their potential applications in high-density data storage and quantum information processing. As a rare example of ferromagnetic polynuclear Cr rings, {Cr8Y8} represents a valuable test bed to directly study the magnetic interaction between Cr3+ ions in large hexadecametallic {Cr8Ln8} (Ln = 4f metal) molecules. We have proposed a “single-J” model to approximate the low-temperature spin dynamics of {Cr8Y8} in our earlier study, while a zero-field splitting (ZFS) of the quantum levels was also suggested by the heat capacity data. In order to have a deeper understanding of the magnetism of {Cr8Y8}, it is necessary to verify the ZFS by means of high-resolution spectral methods and identify its origin. In this work, we present a high-frequency electron spin resonance (HF-ESR) study on the ZFS of {Cr8Y8}. The X-band ESR spectra consists of multi-peak structure, indicative of magnetic anisotropy that breaks the degeneracy between spin states in the absence of a magnetic field. HF-ESR spectra are collected to extract the ZFS parameters. We observed a sharp resonance peak due to the transitions between the S = 11 quantum levels and a broadband corresponding to a distribution of resonance peaks due to the ZFS of the S = 12 quantum levels. By analyzing HF-ESR spectra, we confirm the expected S = 12 ground state and determine its ZFS parameter D as −0.069 K, and, furthermore, we reproduce the spectra recorded at 154 GHz. The macrospin model proves to still be valid. The ZFS is attributed to the axial magnetic anisotropy, as found in some other Cr-based molecular wheels. The detailed HF-ESR investigation presented in this paper will benefit the studies on other {Cr8Ln8} wheels with magnetic Ln3+ ions and highlights the importance of the HF-ESR method as a high-resolution probe in determining the ZFS parameters with very small magnitude.

Magnetoimpedance effect in electrodeposited NiFe/Cu wire using trisodium citrate additive in plating bath

Magnetoimpedance (MI) and magnetic properties of the electroplated NiFe/Cu thin films with a steadily rising concentration of trisodium citrate (TSC) additive in the plating bath is systematically examined in a wide frequency range of 10 kHz–8 MHz. The addition of TSC to the plating bath reduces the crystalline size and coercivity of the NiFe film, resulting in increased magnetic softness and MI ratio. The grain refining action of TSC additive is responsible for the random distribution of magnetic anisotropy axis in individual tiny crystallite which enhance circumferential permeability of deposited film and its MI ratio. The optimum concentration of TSC additive is found to be 40 g/l and sample electrodeposited at that TSC concentration boosts the MI ratio to 113% as compared to 28% when no TSC is used. When the film is deposited without any additive, a typical double peak behavior of the MI response curve is exhibited at higher frequency. This feature is eradicated with slightest inclusion of TSC additive in the electrodeposition bath which is an indication that inclusion of trisodium citrate weakens the circumferential anisotropy in plated film. Based on the analytical formulation of circumferential permeability the results of MI measurements are mapped with the theoretical framework, and the circumferential permeability of the NiFe magnetic film is reconstructed by curve fitting. Through the investigation of the fitting parameters, it is found that the addition of TSC to the bath completely annihilates the circumferential anisotropy found in as deposited permalloy film which is responsible for the transition from double to single peak behavior of MI response at higher frequencies.

Controlling asymmetric transmission in layered natural hyperbolic crystals

We explore the electromagnetic properties of a structure composed of two optically active materials—each layer contains a hyperbolic crystal with its anisotropy axis rotated with respect to the crystal surface. Through this, we can control the transmission spectra where at one frequency, light with a positive incidence angle is transmitted while it is absorbed for a negative incidence angle and the reverse occurs at a second frequency. Using a Gaussian beam analysis, we determine in which material layer the absorption occurs. In a radiating line current source study, we obtain tunable output collimated beams. From these discoveries, our structure can be applied as an efficient frequency or angle selector, demultiplexer, or filter.

The influence of anisotropic electrical resistivity on surface magnetotelluric responses and the design of two new anisotropic inversions

Using the 3-D axial anisotropy, the dipping anisotropy, and the azimuthal anisotropy as case studies, we investigated the influence of each anisotropic resistivity element on the magnetotelluric surface responses. To justify the strong and weak influence and edge effect, we have introduced the influence indices for the impedance components, and the edge effect indices for the tipper components. Interestingly, for decoupled modes, we found that {rho }_{xx} has a strong influence on Zxy, Zyy, and Ty, while {rho }_{yy} strongly affects Zyx, Zxx, and Tx. The three elements {rho }_{zz}, {rho }_{xz}, and {rho }_{yz} have only a very weak influence on all types of responses. For the coupled mode, {rho }_{xx}, {rho }_{yy}, and {rho }_{xy} display a strong influence on all responses. Based on our studies on the influence of the anisotropic resistivity elements, we design and propose two practical processes to replace the conventional axial, dipping, azimuthal, and general anisotropic inversions. First, the axial or dipping inversion can be approximately decoupled into {rho }_{x}-mode and {rho }_{y}-mode inversions. The decoupled mode inversions can be performed either independently and in parallel, or as a joint inversion. Second, since the three resistivity elements always show a weak influence, the general anisotropic inversion can be simplified to just the reduced coupled azimuthal anisotropic inversion with only three resistivity elements as outputs. Both proposed techniques can save a lot of the computational resources. Criteria to choose either the decoupled or coupled modes depend greatly on the magnitudes and distributions of the Zxx and/or Zyy, and Tx and/or Ty responses.Graphical

Polarized neutron scattering study of hollow Fe3O4 submicron spherical particles

We report results of polarized small-angle and wide-angle neutron scattering experiments at T = 10 and 300 K for 420 nm-sized hollow Fe3O4 spherical particles. Each hollow particle is a mesocrystal, which is composed of small nanoparticles with nearly the same crystallographic orientation. Polarized neutron experiments allow us to evaluate magnetic correlations of parallel and perpendicular magnetization components with respect to magnetic field during magnetization process. Small-angle neutron scattering reveals that as the magnetic field decreases from a saturation field of 10 kOe, the perpendicular magnetization component maximizes around zero applied field, whereas the parallel component minimizes. This behavior was observed below and above Verwey transition temperature of ∼ 120 K. Calculations of neutron intensities for vortex structures suggest the reorientation of the vortex core towards the magnetocrystalline anisotropy axis from the magnetic field direction at low applied fields. Moreover, a magnetic domain length obtained from the wide-angle scattering is of the order of 30-40 nm and comparable to the size of the small nanoparticles forming a hollow sphere, suggesting that magnetic correlations within the small nanoparticles always retain during magnetization process.