Weak Anisotropy Research Articles

Weak or Strong Anisotropy in Fe(Se,Te) Superconducting Thin Films Made of Layered Iron-Based Material?

We study the temperature dependence of the upper critical magnetic field Hc2 in a layered iron-based superconducting (IBS) material of the 11-family, namely the Fe(Se,Te) thin film grown on CaF2 substrate. On the basis of intrinsic anisotropy as well as system dimensionality, it turns useful to make a comparison with an ultrathin film conventional low temperature superconductors (LTS) such as NbN, mostly used for device applications. We compare the anisotropy factors as a function of temperature and magnetic field. Both materials present a peculiar behavior: The LTS behaves as a strong anisotropic system, whereas the IBS shows very weak anisotropic features. The strong NbN character can be directly ascribed to the dimensionality of the ultrathin film employed, thus revealing a geometry effect. The weak Fe(Se,Te) trend should be related to its layered crystallographic structure, thus probing an intrinsic origin of its anisotropy. These characteristics become relevant for the potential application of this material in coated conductor technology.

On Backus average in modelling guided waves

We study the Backus (1962) average of a stack of layers overlying a halfspace to examine its applicability for the quasi-Rayleigh and Love wave dispersion curves. We choose these waves since both propagate in the same model. We compare these curves to values obtained for the stack of layers using the propagator matrix. In contrast to the propagator matrix, the Backus (1962) average is applicable only for thin layers or low frequencies. This is true for both a weakly inhomogeneous stack of layers resulting in a weakly anisotropic medium and a strongly inhomogeneous stack of alternating layers resulting in a strongly anisotropic medium. We also compare the strongly anisotropic and weakly anisotropic media, given by the Backus (1962) averages, to results obtained by the isotropic Voigt (1910) averages of these media. As expected, we find only a small difference between these results for weak anisotropy and a large difference for strong anisotropy. We perform the Backus (1962) average for a stack of alternating transversely isotropic layers that is strongly inhomogeneous to evaluate the dispersion relations for the resulting medium. We compare the resulting dispersion curves to values obtained using a propagator matrix for that stack of layers. Again, there is a good match only for thin layers or low frequencies. Finally, we perform the Backus (1962) average for a stack of nonalternating transversely isotropic layers that is strongly inhomogeneous, and evaluate the quasi-Rayleigh wave dispersion relations for the resulting transversely isotropic medium. We compare the resulting curves to values obtained using the propagator matrix for the stack of layers. In this case, the Backus (1962) average performs less well, but—for the fundamental mode—remains adequate for low frequencies or thin layers.

Anisotropy of magnetic susceptibility in salt rocks from the German Zechstein Basin, Sondershausen mine

SUMMARY The anisotropy of magnetic susceptibility (AMS) of rocks reflects the alignment of certain minerals, and therefore it can be used to investigate the deformation history of rocks. However, for salt rocks, very few studies on the AMS of salt rocks and the influence of accessory minerals exist. In this study, we analysed the potential to use the AMS of salt rocks with low impurity content for fabric characterization. Samples of rock salt, sylvinite and carnallitite from a salt mine in Sondershausen (Germany) from the Late Permian (Zechstein 2, Stassfurt series) are investigated. The results of low-field AMS (LF-AMS) measurements show a very weak but significant magnetic anisotropy for sylvinite, carnallitite, and rock salt with an elevated content of accessory minerals. The AMS results are consistent in individual layers of the same rock type. In order to identify the magnetic minerals, which cause the magnetic anisotropy, the high-field AMS (HF-AMS) was measured using a torque magnetometer in order to separate ferrimagnetic and paramagnetic contributions to the AMS. A significant paramagnetic subfabrics exists, which reflects the alignment of phyllosilicates. The magnitude of the LF-AMS is considerably greater than that of the paramagnetic subfabric. This indicates the existence of a ferrimagnetic subfabric due to magnetite, which can have a different orientation than the paramagnetic subfabric. Differences in the orientation of the AMS in samples from two sites suggest a relationship of deformation history and AMS. At a site with dipping layers, the AMS orientation is independent of the bedding and shows large differences between individual lithological layers. In a tight fold, the AMS of all rock types has similar shape and orientation. We conclude that AMS in salt rocks can give meaningful information on the mineral fabric, which could be used in the analysis of the deformation history.

Angle-resolved broadband ferromagnetic resonance apparatus enabled through a spring-loaded sample mounting manipulator

Broadband ferromagnetic resonance is a useful technique to determine the magnetic anisotropy and study the magnetization dynamics of magnetic thin films. We report a spring-loaded sample loading manipulator for reliable sample mounting and rotation. The manipulator enables maximum signal, enhances system stability, and is particularly useful for fully automated in-plane-field angle-resolved measurements. This angle-resolved broadband ferromagnetic resonance apparatus provides a viable method to study anisotropic damping and weak magnetic anisotropies, both vital for fundamental research and applications.

Reflection and transmission responses for layered transversely isotropic media with vertical and horizontal symmetry axes

Reflection and transmission (R/T) responses characterize the energy distributions for incident and generated waves across the subsurface interface. The R/T coefficients are considerably influenced by the local anisotropy, and this implies the significance of the R/T responses analysis for anisotropic media. We have considered the plane interface bounded by two transversely isotropic media with, respectively, vertical and horizontal symmetry axes, and R/T coefficients normalized by the vertical energy flux are obtained in the phase domain. We define two simple anisotropic layered models characterized by fewer independent model parameters. Under the assumption of weak contrast model parameters across the interface, the R/T coefficient approximations are obtained as the perturbations from the simple models’ counterparts. The isotropic background medium is also used to obtain the approximations under an additional weak anisotropy assumption. Compared with approximations degenerated from more general cases, our approximations rely on fewer independent parameters. Numerical tests are implemented to evaluate our approximations.

Anisotropy of Ionic Conductivity of TbF3 Crystals

Temperature measurements (in the range of 375–830 K) of the ionic conductivity of terbium trifluoride single crystals (structure type β-YF3) along three unit-cell crystallographic axes (a, b, and c) have been performed. It is found that TbF3 crystals (orthorhombic system, sp. gr. Pnma) have a weak anisotropy of electrical conductivity: σ||b/σ||a = σ||b/σ||c ≈ 2. The conductivity along the b axis is σ||b = 8 × 10–6 S/cm at 500 K. The anisotropy of fluorine-ion conductivity in rare-earth fluorides with β-YF3, LaF3 (tysonite), and β-BaTm2F8 structures is discussed in the contest of the specific features of their atomic structure.

Spin injection and pumping generated by a direct current flowing through a magnetic tunnel junction

A charge flow through a magnetic tunnel junction (MTJ) leads to the generation of a spin-polarized current which exerts a spin-transfer torque (STT) on the magnetization. When the density of applied direct current exceeds some critical value, the STT excites high-frequency magnetization precession in the "free" electrode of MTJ. Such precession gives rise to microwave output voltage and, furthermore, can be employed for spin pumping into adjacent normal metal or semiconductor. Here we describe theoretically the spin dynamics and charge transport in the CoFeB/MgO/CoFeB/Au tunneling heterostructure connected to a constant-current source. The magnetization dynamics in the free CoFeB layer with weak perpendicular anisotropy is calculated by numerical integration of the Landau-Lifshitz-Gilbert-Slonczewski equation accounting for both STT and voltage controlled magnetic anisotropy associated with the CoFeB|MgO interface. It is shown that a large-angle magnetization precession, resulting from electrically induced dynamic spin reorientation transition, can be generated in a certain range of relatively low current densities. An oscillating spin current, which is pumped into the Au overlayer owing to such precession, is then evaluated together with the injected spin current. Considering both the driving spin-polarized charge current and the pumped spin current, we also describe the charge transport in the CoFeB/Au bilayer with the account of anomalous and inverse spin Hall effects. An electric potential difference between the lateral sides of the CoFeB/Au bilayer is calculated as a function of distance from the CoFeB|MgO interface. It is found that this transverse voltage signal in Au is large enough for experimental detection, which indicates significant efficiency of the proposed current-driven spin injector.

An efficient rock-physics workflow for modeling and inversion in anisotropic organic-shales

In this work we present a workflow for modeling and inversion of physical parameters using ultrasonic wave velocities measured on dry and saturated samples of Bakken, Bazhenov shales and Niobrara marls. The forward problem is based on the well known theory of Ciz and Shapiro (2007) combined with the anisotropic generalization of two empirical elastic models for the rock matrix: the critical porosity model Nur et al. (1998) and Krief et al. model (1990). Moreover, taking into account the high variability of the physical properties of kerogen and clay minerals, and their influence on the mechanical properties of these rocks, the procedure also involves the numerical inversion of such properties, using an efficient numerical routine. The results found for the effective elastic properties and density of clay minerals and kerogen are reasonable taking into account those published in the reference literature. In all cases the quality of fit between real and synthetic data is analysed by means of an adequate error measure. The stability of the predicted elastic coefficients is verified using strain energy restrictions. The calibrated models are also used to quantify the influence of the effective porosity on the computations and for a fluid substitution problem. Finally, we found that even for weak anisotropy the generalized Krief's model requires at least three empirical parameters. We conclude that our novel anisotropic critical porosity model is a simpler and more efficient choice for rock-physics applications.

Effect of grain orientation and local strains on void growth and coalescence in titanium

Ductile fracture has been extensively studied in metals with weak mechanical anisotropy such as copper and aluminum. The fracture of more anisotropic metals, especially those with a hexagonal crystal structure (e.g. titanium), remains far less understood. This paper investigates the ductile fracture process in commercially pure titanium (CP-Ti) with particular emphasis on the influence of grain orientation and local state of strain on void growth. An experimental approach was developed to directly relate the growth of a void in three dimensions to its underlying grain orientation. Grain orientation was obtained by electron back scattered diffraction on void-containing CP-Ti sheets prior to their diffusion bonding. Changes in void dimensions were measured during in-situ straining within an x-ray tomography system. The strong influence of the embedded grain orientation and that of its neighbors on void growth rate and coalescence has been experimentally quantified. Finite element crystal plasticity simulations that take into account both grain orientation and the local strain state were found to predict the experimental void growth. Grains where basal slip dominates show the largest void growth rates because they are closer to a plane strain condition that favors void growth and coalescence.

Stepwise inversion method for determining anisotropy parameters in a horizontal transversely isotropic formation

The anisotropy of a geologic formation can reflect the direction of fractures and ground stress, which is an important metric that guides the exploration and development of oil and gas reservoirs. Cross-dipole acoustic logging is the main method used to detect anisotropy with borehole geophysics. In this paper, a stepwise inversion method for three anisotropy parameters in a horizontal transversely isotropic (HTI) formation is proposed, which turns one 3D operation of simultaneous inversion into three 1D operations. The scheme’s stability and reliability were tested by numerically simulated data using a finite-difference method, and by field logging data. The inversion results of the simulated data show that the stepwise inversion method can stably obtain the fast shear azimuth and the anisotropy parameters in both fast and slow formations with strong and weak anisotropy, and it performed well even with noisy data. In particular, the results of the fast shear azimuth inversion were very stable and reliable. The inversion results of field logging data were consistent with those given by existing commercial software, which used simultaneous inversion, for both fast and slow formations. Where large difference was observed between our stepwise method and the commercial software, our analysis suggests that the fast shear azimuth of our inversion was more reasonable, which reinforces its superior performance and practicality.

Ba8MnNb6O24 : A model two-dimensional spin- 52 triangular lattice antiferromagnet

We successfully synthesized and characterized the triangular lattice anitferromagnet Ba$_8$MnNb$_6$O$_{24}$, which comprises equilateral spin-5/2 Mn$^{2+}$ triangular layers separated by six non-magnetic Nb$^{5+}$ layers. The detailed susceptibility, specific heat, elastic and inelastic neutron scattering measurements, and spin wave theory simulation on this system reveal that it has a 120 degree ordering ground state below T$_N$ = 1.45 K with in-plane nearest-neighbor exchange interaction ~0.11 meV. While the large separation 18.9 A between magnetic layers makes the inter-layer exchange interaction virtually zero, our results suggest that a weak easy-plane anisotropy is the driving force for the k$_m$ = (1/3 1/3 0) magnetic ordering. The magnetic properties of Ba$_8$MnNb$_6$O$_{24}$, along with its classical excitation spectra, contrast with the related triple perovskite Ba$_3$MnNb$_2$O$_9$, which shows easy-axis anisotropy, and the iso-structural compound Ba$_8$CoNb$_6$O$_{24}$, in which the effective spin-1/2 Co$^{2+}$ spins do not order down to 60 mK and in which the spin dynamics shows sign of strong quantum effects.

Functional properties of randomly mixed and layered BaTiO3 - CoFe2O4 ceramic composites close to the percolation limit

Di-phase ferroelectric-ferrite composites with the same composition close to the percolation limit (0.66BaTiO3-0.33CoFe2O4) and with different phase arrangements (randomly mixed phases and tri-layer structures) have been consolidated by using spark plasma sintering and their properties were comparatively analyzed. The intrinsic effective dielectric constant has been estimated by using finite element method and a value almost ten times higher was predicted for the randomly mixed composite, as result of different field distributions inside the ferrite and ferroelectric phases. At room temperature, experimental permittivity contains an intrinsic behavior overlapped onto a very strong extrinsic component determined by the interfaces and by local charge inhomogeneity. The extrinsic dielectric behavior has been discussed in terms of thermally activated relaxation mechanisms developed in both types of investigated structures, as revealed by the temperature-dependence impedance spectroscopy. The magnetic properties are derived as “sum property” from the ferrimagnetic character of CoFe2O4 and show the lowest magnetization for the randomly mixed composites and a weak magnetic anisotropy when the magnetic field was applied in-plane or out-of-plane in the layered structure. Due to the different levels of interface doping specific to the different types of interfaces, the magnetic Curie temperature of the two composites ranges from 720 K in the randomly mixed structure to 746 K for the layered one. The ferroelectric P(E) loops and a modest ME coefficient of 14 mVOe−1cm−1 at 100 Hz were determined only for the layered structure, due to the high leakage and to the small grain size (below 200 nm) in the ferroelectric BaTiO3 component.

Thermal transport properties in monolayer GeS

Utilizing the first-principle techniques combined with the Boltzmann transport equation, we investigate the thermal transport properties of monolayer GeS. The calculated lattice thermal conductivity κL is fairly low, e.g., 11.76 (12.53) Wm−1K−1 at 300 K, along the x (y) axis, which is much lower than the values for phosphorene, monolayer MoS2, graphene, etc. Our analysis reveals that the low lattice thermal conductivity is resulted from the strong three-phonon scattering process and the low phonon group velocity. Due to the weak anisotropy of GeS, we study the contribution of each phonon branch to the heat transport along x and y axis, and find that the longitudinal acoustic (LA) mode contributes most of the total κL along x axis, while the transverse acoustic (TA) branch has the largest contribution to the total κL along y axis. Additionally, the size dependence of thermal conductivity with respect to the nanoribbon width is also studied for the design of nanometer devices.

Magnetohydrodynamic Particle-in-cell Simulations of the Cosmic-Ray Streaming Instability: Linear Growth and Quasi-linear Evolution

Abstract The gyro-resonant cosmic-ray (CR) streaming instability is believed to play a crucial role in CR transport, leading to the growth of Alfvén waves at small scales that scatter CRs, and impacts the interaction of CRs with the interstellar medium (ISM) on large scales. However, extreme scale separation (λ ≪ pc), low CR number density (n CR/n ISM ∼ 10−9), and weak CR anisotropy (∼v A/c) pose strong challenges for proper numerical studies of this instability on a microphysical level. Employing the recently developed magnetohydrodynamic particle-in-cell method, which has unique advantages to alleviate these issues, we conduct 1D simulations that quantitatively demonstrate the growth and saturation of the instability in the parameter regime consistent with realistic CR streaming in the large-scale ISM. Our implementation of the δf method dramatically reduces Poisson noise and enables us to accurately capture wave growth over a broad spectrum equally shared between left- and right-handed Alfvén modes. We are also able to accurately follow the quasi-linear diffusion of CRs subsequent to wave growth, which is achieved by employing phase randomization across periodic boundaries. Full isotropization of the CRs in the wave frame requires the pitch angles of most CRs to efficiently cross 90° and can be captured in simulations with relatively high wave amplitude and/or spatial resolution. We attribute this crossing to nonlinear wave–particle interaction (rather than mirror reflection) by investigating individual CR trajectories. We anticipate that our methodology will open up opportunities for future investigations that incorporate additional physics.

Preferential orientation of tracer spheroids in turbulent channel flow

ABSTRACT Axis-symmetric spheroids, such as rod-like and disk-like particles, have been found to orient preferentially in near-wall turbulence by both experiment and numerical simulation. In current work we examined the orientation of inertialess spheroids in a turbulent channel flow at medium friction Reynolds number R eτ = 1000 given based on the half of channel height. Both elongated prolate spheroid and flat oblate spheroid are considered and further compared with the reference case of spherical particle. The statistical results show that in near wall region the prolate spheroids tend to align in the streamwise direction while the oblate spheroids prefer to orient in the wall-normal direction, which are consistent with earlier observation in low Reynolds number (R eτ = 180) wall turbulence. Around the channel center we found that the orientation of spheroids is not fully isotropic, even though the fluid vorticity are almost isotropic. The mechanism that gives rise to such particle orientations in wall-turbulence has been found to be related to fluid Lagrangian stretching and compression (Zhao and Andersson 2016). Therefore, we computed the left Cauchy-Green strain tensor along Lagrangian trajectories of tracer spheroids in current flow field and analyzed the fluid Lagrangian stretching and compression. The results indicated that, similar to the earlier observations, the directions of the Lagrangian stretching and compression in near-wall region are in the streamwise and wall-normal directions, respectively. Furthermore, cross over the channel the prolate spheroids aligned with the direction of Lagrangian stretching but oblate spheroids oriented with the direction of Lagrangian compression. The weak anisotropy of orientations of fluid Lagrangian stretching and compression observed at the channel center could be the reason for the aforementioned modest anisotropic orientation of spheroids in channel central region.

Reflection and transmission responses in a layered transversely isotropic medium with horizontal symmetry axis

Seismic wave reflection and transmission (R/T) responses characterize the subsurface local property, and the widely spread anisotropy has considerable influences even at small incident angles. We have considered layered transversely isotropic media with horizontal symmetry axes (HTI), and the symmetry axes were not restricted to be aligned. With the assumption of weak contrast across the interface, linear approximations for R/T coefficients normalized by vertical energy flux are derived based on a simple layered HTI model. We also obtain the approximation with the isotropic background medium under an additional weak anisotropy assumption. Numerical tests illustrate the good accuracy of the approximations compared with the exact results.

Current-induced switching of YIG/Pt bilayers with in-plane magnetization due to Oersted fields

We report on the switching of the in-plane magnetization of thin yttrium iron garnet (YIG)/Pt bilayers induced by an electrical current. The switching is either field-induced and assisted by a dc current, or current-induced and assisted by a static magnetic field. The reversal of magnetization occurs at a current density as low as 105 A/cm2 and magnetic fields of ∼40 μT, two orders of magnitude smaller than in ferromagnetic metals, and consistent with the weak uniaxial anisotropy of the YIG layers. We use the transverse component of the spin Hall magnetoresistance to sense the magnetic orientation of YIG while sweeping the current. Our measurements and simulations reveal that the current-induced effective field responsible for switching is due to the Oersted field generated by the current flowing in the Pt layer rather than by spin–orbit torques, and that the switching efficiency is influenced by pinning of the magnetic domains.

Magnetic properties and theoretical calculations of mononuclear lanthanide complexes with a Schiff base coordinated to Ln(III) ion in a monodentate coordination mode

Three new mononuclear rare earth (RE) complexes [RE(HL)2(NO3)3(CH3OH)] (HL = 1-[N-(4-bromophenyl)]aminomethylidene-2(1H)naphthalenone; RE = Y (1), Dy (2), Er (3)) have been synthesized. The structures of complexes 1–3 were determined by single-crystal X-ray diffraction analyses. Each complex contains two HL ligands which bind to the RE(III) ion in a monodentate coordination mode, three NO3– and one coordinated methanol ligands, exhibiting a distorted tricapped trigonal prism (D3h) configuration. Slow relaxation of the magnetization under 1 kOe can be observed for complex 2. Further ab initio calculations show that the orientation of the ground magnetic easy axis is parallel with the line of O(HL)-O(HL) with the deviation angle being 4.29°. In addition, the ground gz value of complex 2 is 19.6858, which approximates the limit value of 20. However, the gz value of the low-lying ground state of complex 3 deviates severely from the limit value of 18, indicating weak axial anisotropy. Further measurements of ac susceptibilities confirmed the weak magnetic behavior in complex 3.

Magnetization reorientation due to the superconducting transition in heavy-metal heterostructures

Recent theoretical and experimental work has demonstrated how the superconducting critical temperature ($T_c$) can be modified by rotating the magnetization of a single homogeneous ferromagnet proximity-coupled to the superconducting layer. This occurs when the superconductor and ferromagnet are separated by a thin heavy normal metal that provides an enhanced interfacial Rashba spin-orbit interaction. We consider the reciprocal effect: magnetization reorientation driven by the superconducting phase transition. We solve the tight-binding Bogoliubov-de Gennes equations on a lattice self-consistently and find that the relative angle between the spin-orbit field and the magnetization gives rise to a contribution in the free energy even in the normal state due to band-structure effects. For temperatures below $T_c$, superconductivity gives rise to a competing contribution. By lowering the temperature, in addition to reorientation of the favored magnetization direction from in-plane to out-of-plane, a $\pi/4$ in-plane rotation for thicker ferromagnetic layers is possible. Furthermore, computation of $T_c$ of the structure in the ballistic limit shows a dependence on the in-plane orientation of the magnetization, in contrast to our previous result on the diffusive limit. This finding is relevant with respect to thin-film heterostructures since these are likely to be in the ballistic regime of transport. Finally, we discuss the experimental feasibility of observing the magnetic anisotropy induced by the superconducting transition when other magnetic anisotropies, such as the shape anisotropy for a ferromagnetic film, are taken into account. Our work suggests that the superconducting condensation energy in principle can trigger a reorientation of the magnetization of a thin-film ferromagnet upon lowering the temperature below $T_c$, in particular for ferromagnets with weak magnetic anisotropies.

Magnetically Tuned Kondo Effect in a Molecular Double Quantum Dot: Role of the Anisotropic Exchange

We investigate theoretically and experimentally the singlet-triplet Kondo effect induced by a magnetic field in a molecular junction. Temperature dependent conductance, $G(T)$, is calculated by the numerical renormalization group, showing a strong imprint of the relevant low energy scales, such as the Kondo temperature, exchange and singlet-triplet splitting. We demonstrate the stability of the singlet-triplet Kondo effect against weak spin anisotropy, modeled by an anisotropic exchange. Moderate spin anisotropy manifests itself by lowering the Kondo plateaus, causing the $G(T)$ to deviate from a standard temperature dependence, expected for a spin-half Kondo effect. We propose this scenario as an explanation for anomalous $G(T)$, measured in an organic diradical molecule coupled to gold contacts. We uncover certain new aspects of the singlet-triplet Kondo effect, such as coexistence of spin-polarization on the molecule with Kondo screening and non-perturbative parametric dependence of an effective magnetic field induced by the leads.