Anisotropic Contributions Research Articles

Separating azimuthal orientation dependence in polarization measurements of anisotropic media.

Polarization imaging and Mueller polarimetry provide powerful tools for probing the microstructure of complex anisotropic media, which is a core task in material science, biomedical diagnosis and many research fields. However, Mueller matrix elements and many polarization parameters are sensitive to the spatial orientation of the sample and experimental configurations, hindering the effectiveness for distinguishing different sources of anisotropies. In this paper, we propose a set of rotation invariant parameters and corresponding orientation parameters, which are explicit functions of the Mueller matrix elements. They are valid under the condition that the illumination and detection directions are collinear with the rotation axis of the sample. More detailed examinations show that these parameters have potential applications for fast analyzing different anisotropy contributions in the media, such as birefringence, dichroism, and their coexistence. The conclusions are validated with Monte Carlo simulations and the experimental results of transparent tape samples.

The impact of bending stress on magnetic properties of Finemet type microwires and ribbons

In our research we studied the influence of bending on the total magnetic losses, the coercivity of minor hysteresis loops and the virgin magnetization curves of amorphous ribbons and microwires prepared from Finemet-type alloy wound on a mandrel of different diameters. The impact of various factors contributing on static and dynamic magnetic properties through bending stress are discussed. The factors pertain to contribution of the magnetoelastic anisotropy, the onset of the strain-induced magnetization, the changes of domain structure and the mobility of domain walls. The major contribution of hysteresis losses as-compared to the eddy-current losses have been observed on changing the mandrel diameter in wound ribbons and wires. The total specific losses of the Finemet-type ribbons are lower than that of microwires at the frequencies ranging from 50 to 400 Hz. Observed difference in the magnetic properties is attributed to different magnetization processes of amorphous microwires and ribbons. A different geometry and magnetoelastic anisotropy of studied materials are most likely to influence the formation of different domain structures.

Determination of the out-of-plane anisotropy contributions (first and second anisotropy terms) in amorphous Nd-Co thin films by micromagnetic numerical simulations

Amorphous Nd-Co thin films exhibit stripe shaped periodic magnetic domains with local out-of-plane magnetization components due to their perpendicular magnetic anisotropy. This anisotropy has been quantified in a fairly simple way by reproducing the experimental magnetization curves by means of micromagnetic numerical simulations. The simulations show that the first (K1) and second (K2) anisotropy constants must be used to properly describe the variation of the stripe domains with the in plane applied magnetic field. A strong temperature dependence of both K1 and K2 has been obtained between 10 K and room temperature. This anisotropy behavior is characteristic of two magnetically coupled 3d-4f sublattices with competing anisotropies.

Association of Heritable Cognitive Ability and Psychopathology With White Matter Properties in Children and Adolescents

Many mental disorders emerge during adolescence, which may reflect a cost of the potential for brain plasticity offered during this period. Brain dysconnectivity has been proposed as a common factor across diagnostic categories. To investigate the hypothesis that brain dysconnectivity is a transdiagnostic phenotype in adolescence with increased susceptibility and symptoms of psychiatric disease. We investigated clinical symptoms as well as cognitive function in 6487 individuals aged 8 to 21 years from November 1, 2009, to November 30, 2011, in the Philadelphia Neurodevelopmental Cohort and analyzed diffusion magnetic resonance imaging brain scans for 748 of the participants. Independent component analysis was used to derive dimensional psychopathology scores, and genome-wide complex trait analysis was used to estimate its heritability. Multimodal fusion simultaneously modeled contributions of the diffusion magnetic resonance imaging metrics fractional anisotropy, mean diffusivity, radial diffusivity, L1 (the principal diffusion tensor imaging eigen value), mode of anisotropy, as well as dominant and secondary fiber orientations, and structural connectivity density, and their association with general psychopathology and cognition. Machine learning with 10-fold cross-validation and permutation testing in 729 individuals (aged 8 to 22 years; mean [SD] age, 15.1 [3.3] years; 343 females [46%]) revealed significant association with general psychopathology levels (r = 0.24, P < .001) and cognition (r = 0.39, P < .001). A brain white matter pattern reflecting frontotemporal connectivity and crossing fibers in the uncinate fasciculus was the most associated feature for both traits. Univariate analysis across a range of clinical domains and cognitive test scores confirmed its transdiagnostic importance. Both the general psychopathology (16%; SE, 0.095; P = .05) and cognitive (18%; SE, 0.09; P = .01) factor were heritable and showed a negative genetic correlation. Dimensional and heritable general cognitive and psychopathology factors are associated with specific patterns of white matter properties, suggesting that dysconnectivity is a transdiagnostic brain-based phenotype in individuals with increased susceptibility and symptoms of psychiatric disorders.

Computational alloy design of (Co1-xNix)88Zr7B4Cu1 nanocomposite soft magnets

The dependence of coercivity on composition is an important factor for establishing optimized soft magnetic properties. In this study, we have used the random anisotropy and coherent rotation models to estimate the variation of coercivity with composition in (Co1-xNix)88Zr7B4Cu1 nanocomposite alloys. Our calculations that the magnetoelastic anisotropy contribution to coercivity dominates for Ni rich compositions (x &amp;gt; 0.5). A small range of compositions (0.65 &amp;lt; x &amp;lt; 0.75) is predicted to result in low values of coercivity (&amp;lt;10 A/m). To validate this prediction, (Co1-xNix)88Zr7B4Cu1 nanocomposites in this range were prepared by melt spinning followed by 3600 s isothermal annealing at the primary crystallization peak temperature (∼673 K). Hysteresis loops were measured using vibrating sample magnetometry at room temperature and saturation magnetostriction was measured using a strain gage based magnetostrictometer. Moderately small coercivities (30-40 A/m) and magnetostrictions (3-4 ppm) were measured at for samples with 0.685 &amp;lt; x &amp;lt; 0.725. Our measured coercivity had a minimum value of 32 A/m at x = 0.725, a shift in composition of about 5 at% in the direction of higher Ni content and without the anticipated low value of coercivity. Several reasons for the inaccuracy of this approach are described, including: ignored contributions from amorphous phase (especially in magnetoealstic anisotropy), composition segregation during crystallization leading to unpredictable compositional shifts in prediction, and the general observation that the predictability of minimum coercivity from minimal combined anisotropies has unexplained deviation even in far less complicated materials.

Manipulating the magnetic anisotropy and magnetization dynamics by stress: Numerical calculation and experiment

We perform a theoretical and experimental investigation of the magnetic properties and magnetization dynamics of a ferromagnetic magnetostrictive multilayer grown onto a flexible substrate and submitted to external stress. We calculate the magnetic behavior and magnetoimpedance effect for a trilayered system from an approach that considers a magnetic permeability model for planar geometry and a magnetic free energy density which takes into account induced uniaxial and magnetoelastic anisotropy contributions. We verify remarkable modifications of the magnetic anisotropy with external stress, as well as we show that the dynamic magnetic response is strongly affected by these changes. We discuss the magnetic features that lead to modifications of the frequency limits where distinct mechanisms are responsible by the magnetoimpedance variations, enabling us to manipulate the resonance fields. To test the robustness of the approach, we directly compare theoretical results with experimental data. Thus, we provide experimental evidence to confirm the validity of the theoretical approach, as well as to manipulate the resonance fields to tune the MI response according to real applications in devices.

Theoretical Investigations of the Spin Hamiltonian Parameters for the Mononuclear Square Pyramidal [CuO5] Groups in Two Paddle Wheel Copper Complexes

The spin Hamiltonian parameters (SHPs) (g factors and hyperfine structure constants) for the mononuclear square pyramidal [CuO5] groups in two paddle wheel copper complexes {Cu2(μ2–O2CCH3)4}(OCNH2CH3) and \({}_{\infty }^{3} [{\text{Cu}}_{ 2}^{\text{I}} {\text{Cu}}_{ 2}^{\text{II}} ( {\text{H}}_{ 2} {\text{O)}}_{ 2} {\text{L}}_{ 2} {\text{Cl}}_{ 2} ]\) are theoretically investigated from the perturbation calculations of these parameters for a rhombically elongated octahedral 3d 9 group. The slightly larger anisotropy Δg (≈ g // − g⊥) of complex 1 than complex 2 is attributed to the slightly bigger deviations of the polar angles related to the ideal value 90° and relative differences between the axial and basal Cu–O distances in the former. The axiality of the EPR signals for both systems can be illustrated as the fact that the perpendicular anisotropic contributions to X and Y components of the SHPs arising from the four basal ligands with slightly distinct bond lengths and bond angles may roughly cancel one another. The signs of hyperfine structure constants are also theoretically determined for both complexes.

Interface perpendicular magnetic anisotropy in ultrathin Ta/NiFe/Pt layered structures

Interface perpendicular magnetic anisotropy (PMA) in ultrathin Ta/NiFe/Pt layered structures was investigated through magnetization measurements. Ta/NiFe/Pt films with NiFe layer thickness (t) values of 2 nm or more showed typical in-plane magnetization curves, which was presumably due to the dominant contribution of the shape magnetic anisotropy. The thickness dependence of the saturation magnetization of the entire NiFe layer (Ms) was well analyzed using the so-called dead-layer model, showing that the magnetically active part of the NiFe layer has saturation magnetization () independent of t and comparable to the bulk value. In the perpendicular direction, the saturation field Hk was found to clearly decrease with decreasing t, while the effective field of shape magnetic anisotropy due to the active NiFe saturation magnetization should be independent of t. These observations show that there exists interface PMA in the layered structures. The interface PMA energy density was determined to be ∼0.17 erg/cm2 using the dead-layer model. Motivated by the correlation observed between Ms and Hk, we also attempted to interpret the experimental results using an alternative approach beyond the dead-layer model; however, it gives only implications on the incomplete validity of the dead-layer model and no better understanding.

The Role of Texture, Cracks, and Fractures in Highly Anisotropic Shales

AbstractOrganic shales generally have low permeability unless fractures are present. However, how gas, oil, and water flows into these fractures remains enigmatic. The alignment of clay minerals and the alignment of fractures and cracks are effective means to produce seismic anisotropy. Thus, the detection and characterization of this anisotropy can be used to infer details about lithology, rock fabric, and fracture and crack properties within the subsurface. We present a study characterizing anisotropy using S wave splitting from microseismic sources in a highly anisotropic shale. We observe very strong anisotropy (up to 30%) with predominantly VTI (vertical transverse isotropy) symmetry, but with evidence of an HTI (horizontal transverse isotropy) overprint due to a NE striking vertical fracture set parallel to the maximum horizontal compressive stress. We observe clear evidence of a shear wave triplication due to anisotropy, which to our knowledge is one of only a very few observations of such triplications in field‐scale data. We use modal proportions of minerals derived from X‐ray fluorescence data combined with realistic textures to estimate the contribution of intrinsic anisotropy as well as possible contributions of horizontally aligned cracks. We find that aligned clays can explain much of the observed anisotropy and that any cracks contributing to the vertical transverse isotropy (VTI) must have a low ratio of normal to tangential compliance (ZN/ZT), typical of isolated cracks with low hydraulic connectivity. Subhorizontal cracks have also been observed in the reservoir, and we propose that their reactivation during hydraulic fracturing may be an important mechanism to facilitate gas flow.

Size effects of the magnetic anisotropy of fcc cobalt nanoparticles embedded in copper

Cobalt nanoparticles embedded in copper matrices show strong size effects in the magnetic anisotropy with a non-monotonous dependence on the particle diameter. In this article we discuss quantitative values of the magnetic anisotropy in the frame of two models: in small clusters the surface anisotropy contribution dominates whereas larger particles ( >3 nm diameter) have an elliptic shape leading to increased shape anisotropy. The crystalline structure of the particles is shown to be face-centered cubic, justifying that the magneto-crystalline anisotropy can be neglected.

Impact of organic capping layer on the magnetic anisotropy of ultrathin Co films

Using combined magnetometry and ferromagnetic resonance studies, it is shown that introduction of a hydrocarbon cover layer leads to an increase of the contribution of the surface anisotropy to the effective magnetic anisotropy energy of hexagonal close-packed (0 0 0 1) cobalt films, largely enhancing the perpendicular anisotropy. Due to a weak electronic interaction of the organic molecules with the Co atoms, an increase of surface anisotropy could be explained by the presence of an electric field at the organic-material/Co interface and by modification of surface atoms charge configuration.

Acoustic frequency filter based on anisotropic topological phononic crystals

We present a design of acoustic frequency filter based on a two-dimensional anisotropic phononic crystal. The anisotropic band structure exhibits either a directional or a combined (global + directional) bandgap at certain frequency regions, depending on the geometry. When the time-reversal symmetry is broken, it may introduce a topologically nontrivial bandgap. The induced nontrivial bandgap and the original directional bandgap result in various interesting wave propagation behaviors, such as frequency filter. We develop a tight-binding model to characterize the effective Hamiltonian of the system, from which the contribution of anisotropy is explicitly shown. Different from the isotropic cases, the Zeeman-type splitting is not linear and the anisotropic bandgap makes it possible to achieve anisotropic propagation characteristics along different directions and at different frequencies.

Magnetic properties of the anion-radical salts [M(bipy)3](TCNQ)4·X, where M = Co, Ni, Zn and X = H2O or (CH3)2CO

An anion-radical salt (ARS) [Co(bipy)3](TCNQ)4·H2O was synthesized and its crystal structure was resolved. The experimental investigation of [Co(bipy)3](TCNQ)4·H2O and two ARS with similar crystal structure [Zn(bipy)3](TCNQ)4·H2O and [Ni(bipy)3](TCNQ)4·(CH3)2CO by the measurement of the temperature dependence of magnetic susceptibility at temperature from 1.8 K to 300 K, the field dependence of magnetization up to 7 T at different temperatures and EPR spectra in temperature range from 2 K to 300 K was performed. [Zn(bipy)3](TCNQ)4·H2O was used to estimate the contribution of the TCNQ radicals to the magnetic moment. In result, the compound [Co(bipy)3](TCNQ)4·H2O was characterized by the presence of Co2+ ions in spin-admixed state and low-temperature susceptibility, magnetization and specific heat of [Ni(bipy)3](TCNQ)4·(CH3)2CO was shown to be dominated by the contribution of single-ion anisotropy of Ni2+ ions.

FMR investigations of exchange biased NiFe/IrMn/NiFe trilayers with high and low Ni relative content

Magnetic properties of trilayer double-exchange biased Si/Ta(30 nm)/NixFe(1−x)(10 nm)/IrMn(tAF)/NixFe(1−x(10 нм)/Ta(30 нм) thin film structures with x = 0.4 and x = 0.75 were investigated. The exchange bias, noncollinearity of unidirectional and uniaxial anisotropy and magnetization in both bottom and top ferromagnetic layers as a function of the antiferromagnetic (AF) layer thickness in the range from 2 to 50 nm were investigated. The obtained effects support the model of ferromagnetic coupling through the antiferromagnetic spacer and the model of rotatable anisotropy contribution at small AF thickness. It was found that usage of low-nickel permalloy in exchange biased systems has advantages of higher magnetization, a higher magnetic ordering at the interface and smaller critical thickness of antiferromagnetic layer to set the exchange bias, though provides a lower exchange bias compared with normal permalloy composition.

Tunable magnetization relaxation of Fe2Cr1−xCoxSi half-metallic Heusler alloys by band structure engineering

We report a systematic investigation on the magnetization relaxation properties of iron-based half-metallic Heusler alloy ${\mathrm{Fe}}_{2}{\mathrm{Cr}}_{1\ensuremath{-}x}\mathrm{Co}{}_{x}\mathrm{Si}$ (FCCS) thin films using broadband angular-resolved ferromagnetic resonance. Band structure engineering through Co doping ($x$) demonstrated by first-principles calculations is shown to tune the intrinsic magnetic damping over an order of magnitude, namely $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}--8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Notably, the intrinsic damping constants for samples with high Co concentration are among the lowest reported for Heusler alloys and even comparable to magnetic insulator yttrium iron garnet. Furthermore, a significant reduction of both isotropic and anisotropic contributions of extrinsic damping of the FCCS alloys was found in the FCCS films with $x=0.5--0.75$, which is of particular importance for applications. These results demonstrate a practical recipe to tailor functional magnetization for Heusler alloy-based spintronics at room temperature.

Reentrant behaviors in the phase diagram of spin-1 planar ferromagnet with single-ion anisotropy

We used the two-time Green function framework to investigate the role played by the easy-axis single-ion anisotropy on the phase diagram of (d>2)-dimensional spin-1planar ferromagnets, which exhibit a magnetic field induced quantum phase transition. We tackled the problem using two different kind of approximations: the Anderson-Callen decoupling scheme and the Devlin approach. In the latter scheme, the exchange anisotropy terms in the equations of motion are treated at the Tyablikov decoupling level while the crystal field anisotropy contribution is handled exactly. The emerging key result is a reentrant structure of the phase diagram close to the quantum critical point, for certain values of the single-ion anisotropy parameter. We compare the results obtained within the two approximation schemes. In particular, we recover the same qualitative behavior. We show the phase diagram, close to the field-induced quantum critical point and the behavior of the susceptibility for different values of the single-ion anisotropy parameter, enhancing the differences between the two different scenarios (i.e. with and without reentrant behavior).

Innovative soft magnetic multilayers with enhanced in-plane anisotropy and ferromagnetic resonance frequency for integrated RF passive devices

We present an innovative, economical method for manufacturing soft magnetic materials that may pave the way for integrated thin film magnetic cores with dramatically improved properties. Soft magnetic multilayered thin films based on the Fe-28%Co20%B (at.%) and Co-4.5%Ta4%Zr (at.%) amorphous alloys are deposited on 8” bare Si and Si/200nm-thermal-SiO2 wafers in an industrial, high-throughput Evatec LLS EVO II magnetron sputtering system. The multilayers consist of stacks of alternating 80-nm-thick ferromagnetic layers and 4-nm-thick Al2O3 dielectric interlayers. Since in our dynamic sputter system the substrate cage rotates continuously, such that the substrates face different targets alternatively, each ferromagnetic sublayer in the multilayer consists of a fine structure comprising alternating CoTaZr and FeCoB nanolayers with very sharp interfaces. We adjust the thickness of these individual nanolayers between 0.5 and 1.5 nm by changing the cage rotation speed and the power of each gun, which is an excellent mode to engineer new, composite ferromagnetic materials. Using X-ray reflectometry (XRR) we reveal that the interfaces between the FeCoB and CoTaZr nanolayers are perfectly smooth with roughness of 0.2-0.3 nm. Kerr magnetometry and B-H looper measurements for the as-deposited samples show that the coercivity of these thin films is very low, 0.2-0.3 Oe, and gradually scales up with the thickness of FeCoB nanolayers, i.e. with the increase of the overall Fe content from 0 % (e.g. CoTaZr-based multilayers) to 52 % (e.g. FeCoB-based multilayers). We explain this trend in the random anisotropy model, based on considerations of grain size growth, as revealed by glancing angle X-ray diffraction (GAXRD), but also because of the increase of magnetostriction with the increase of Fe content as shown by B-H looper measurements performed on strained wafers. The unexpected enhancement of the in-plane anisotropy field from 18.3 Oe and 25.8 Oe for the conventional CoTaZr- and FeCoB-based multilayers, respectively, up to ∼48 Oe for the nanostructured multilayers with FeCoB/CoTaZr nano-bilayers is explained based on interface anisotropy contribution. These novel soft magnetic multilayers, with enhanced in-plane anisotropy, allow operation at higher frequencies, as revealed by broadband (between 100 MHz and 10 GHz) RF measurements that exhibit a classical Landau-Lifschitz-Gilbert (LLG) behavior.

Spacetime anisotropy affects cosmological entanglement

Most existing cosmological entanglement studies are focused on the isotropic Robertson–Walker (RW) spacetime. Here we go beyond this limitation and study the influence of anisotropy on entanglement generated by dynamical spacetime. Since the isotropic spacetime is viewed as a background medium and the anisotropy is incorporated as perturbation, we decompose entanglement entropy into isotropic and anisotropic contributions. The latter is shown to be non-negligible by analyzing two cosmological models with weak and conformal coupling. We also show the possibility of using entanglement to infer about universe features.

Study of anisotropy, magnetization reversal and damping in ultrathin Co films on MgO (0 0 1) substrate

Ultrathin Co films of 3nm thickness have been prepared on MgO (001) substrate in presence or absence of substrate pre-annealing. Uniaxial anisotropy is induced in the samples due to the deposition under oblique angle of incidence. Along with the oblique deposition induced anisotropy, another uniaxial anisotropy contribution has been observed due to pre-annealing. However, no cubic anisotropy has been observed here as compared to the thicker films. Angle dependent ferromagnetic resonance (FMR) measurement confirms the presence of two anisotropies in the pre-annealed sample with ∼18° misalignment with each other. The two anisotropy constants were calculated from both superconducting quantum interference device (SQUID) magnetometry and FMR spectroscopy. The magnetization reversal is governed by nucleation dominated aftereffect followed by domain wall motion for the pre-annealed sample. Branched domains are observed for the sample prepared without pre-annealing which indicates grain disorientation of Co. However, in the thicker (25nm) Co films ripple domains were observed in contrary to ultrathin (3nm) films.

Direct observation of temperature-driven magnetic symmetry transitions by vectorial resolved MOKE magnetometry

Angle- and temperature-dependent vectorial magnetometry measurements are necessary to disentangle the effective magnetic symmetry in magnetic nanostructures. Here we present a detailed study on an Fe(1 0 0) thin film system with competing collinear biaxial (four-fold symmetry) and uniaxial (two-fold) magnetic anisotropies, carried out with our recently developed full angular/broad temperature range/vectorial-resolved magneto-optical Kerr effect magnetometer, named TRISTAN. The data give direct views on the angular and temperature dependence of the magnetization reversal pathways, from which characteristic axes, remanences, critical fields, domain wall types, and effective magnetic symmetry are obtained. In particular, although the remanence shows four-fold angular symmetry for all investigated temperatures (15 K–400 K), the critical fields show strong temperature and angular dependencies and the reversal mechanism changes for specific angles at a given (angle-dependent) critical temperature, showing signatures of an additional collinear two-fold symmetry. This symmetry-breaking is more relevant as temperature increases to room temperature. It originates from the competition between two anisotropy contributions with different symmetry and temperature evolution. The results highlight the importance of combining temperature and angular studies, and the need to look at different magnetic parameters to unravel the underlying magnetic symmetries and temperature evolutions of the symmetry-breaking effects in magnetic nanostructures.