Additional Anisotropy Research Articles

Deflected Mantle Flow and Shearing‐Aligned Lithospheric Melt Under the Strike‐Slip Dead Sea Rift

AbstractContinental rifting is one of the fundamental tectonics of the Earth evolution while our current understandings on the dynamic mechanism of the strike‐slip ones are relatively limited. Here, we have utilized three kinds of core‐refracted shear waves (including PKS, SKKS, and SKS) and employed the shear‐wave splitting technique to systematically investigate the azimuthal anisotropy of the upper mantle under the typical strike‐slip Dead Sea rift. There are a total of 1,855 well‐determined anisotropic measurements from 187 stations with dominantly N‐S fast orientations. We have proposed a new model from a joint analysis of multiple newly available geophysical observations to interpret the resulting anisotropy as mainly due to the absolute‐plate‐motion‐driven mantle flow deflected by the thick lithosphere of the eastern Arabian plate. Relatively larger splitting times are locally revealed at the rift zone and attributed to additional lithospheric anisotropy from the shearing‐oriented melt pockets whose existence further induces complex anisotropy with slight difference of fast orientations between the upper and lower layer anisotropy. The overall rift‐parallel fast orientations, when combined with the absence of low‐velocity and hot thermal anomalies in the mantle transition zone, rule out the role of mantle plume or edge‐driven convection in the rift development and further infer the Dead Sea rift to evolve in a passive mode.

Steerable current-driven emission of spin waves in magnetic vortex pairs.

The efficient excitation of spin waves is a key challenge in the realization of magnonic devices. We demonstrate current-driven generation of spin waves in antiferromagnetically coupled magnetic vortices. We use time-resolved x-ray microscopy to directly image the emission of spin waves upon the application of alternating currents flowing directly through the magnetic stack. Micromagnetic simulations allow us to identify the current-driven Oersted field as the main origin of excitation, in contrast to spin-transfer torques. In our case, these internal Oersted fields have an orders of magnitude higher spin-wave excitation efficiency than commonly used stripline antennas. For magnetostrictive materials, we furthermore demonstrate that the direction of magnon propagation can be steered by increasing the excitation amplitude, which modifies the underlying magnetization profile through an additional anisotropy. The demonstrated methods allow for the efficient and tunable excitation of spin waves, marking a substantial advance concerning the design of magnonic devices.

Highly Ordered Nanoassemblies of Janus Spherocylindrical Nanoparticles Adhering to Lipid Vesicles.

In recent years, there has been a heightened interest in the self-assembly of nanoparticles (NPs) that is mediated by their adsorption onto lipid membranes. The interplay between the adhesive energy of NPs on a lipid membrane and the membrane's curvature energy causes it to wrap around the NPs. This results in an interesting membrane curvature-mediated interaction, which can lead to the self-assembly of NPs on lipid membranes. Recent studies have demonstrated that Janus spherical NPs, which adhere to lipid vesicles, can self-assemble into well-ordered nanoclusters with various geometries, including a few Platonic solids. The present study explores the additional effect of geometric anisotropy on the self-assembly of Janus NPs on lipid vesicles. Specifically, the current study utilized extensive molecular dynamics simulations to investigate the arrangement of Janus spherocylindrical NPs on lipid vesicles. We found that the additional geometric anisotropy significantly expands the range of NPs' self-assemblies on lipid vesicles. The specific geometries of the resulting nanoclusters depend on several factors, including the number of Janus spherocylindrical NPs adhering to the vesicle and their aspect ratio. The lipid membrane-mediated self-assembly of NPs, demonstrated by this work, provides an alternative cost-effective route for fabricating highly engineered nanoclusters in three dimensions. Such structures, with the current wide range of material choices, have great potential for advanced applications, including biosensing, bioimaging, drug delivery, nanomechanics, and nanophotonics.

Modulation of magnetic properties in self-assembled one-dimensional CoxFe3-xO4 nano-chains with Co substitution

Modulation of magnetic properties on the low dimensional material under multiple fields is a captivating area of study in material science and nano-technology. In this paper, we focus on the self-assembly of chain-like nanostructures composed of CoxFe3−xO4 nanoparticles with an average size of 150 nm. These structures, consisting of more than 20 nanoparticles, are formed at 90 °C under a 0.25 T external magnetic field. The length of chains is surprising to decrease with increasing Co content due to an obvious additional cubic magneto-crystalline anisotropy superposed on the uniaxial induced anisotropy which is induced by the synthesized magnetic field with easy axis along the length of the chain. The additional magneto-crystalline anisotropy field of Co blocks the chain growing in length with Co doping. Additionally, our X-ray magnetic circular dichroism (XMCD) measurements confirm the doping of Co2+ ions in the CoxFe3−xO4 nano-chains, where they predominantly occupy the B sites and substitute the Fe2+ ions. This finding underlines the importance of spin-orbit coupling enhancement in the magnetic behavior resulting from the Fe2+ substitution with Co2+.

Computational analysis of thermal cycling phenomena during multilayer laser powder bed fusion process for Ti6Al4V

During laser powder bed fusion (LPBF) process, single tracks overlap to form layers and layers merge to form the complete 3D object. Because of this, a recurring temperature rise and fall happens, which is called thermal cycling phenomenon. It affects the microstructure and mechanical properties of the manufactured component via multiple reheating (and remelting) cycles, resulting in an intrinsic aging type heat treatment process that invokes an additional anisotropy along the build direction. To date, this phenomenon has been rarely studied due to the computational resource constraints and difficulties in experimental temperature measurement. Hence, in this study, a computationally efficient thermal model has been developed and applied to study the multilayer LPBF process for Ti6Al4V. The model was validated against the single track single layer experimental data and then used to simulate multilayer LPBF till a height of 3 layers to study the temperature field evolution and thermal cycling phenomenon. The average temperature of the computational domain is plotted against time to show the origin of intrinsic heat treatment. It is observed that while with each next layer the temperature rises, the maximum temperature value remains constant due to material evaporation taking place. The results suggest that thermal buildup also happens in the process as the average temperature of the computational domain as well as the melt pool volume increases with each layer.

Magnetic anisotropy and ferromagnetic resonance in inhomogeneous demagnetizing fields near edges of thin magnetic films

Using local ferromagnetic resonance spectroscopy, we have studied the magnetic properties near edges of thin tangentially magnetized permalloy films, in which a well-defined uniaxial magnetic anisotropy was induced perpendicular to one of the edges. In the experiment, two samples with thicknesses of 90 and 300 nm and with slightly different compositions were examined. To explain the magnetization dynamics near edges, we propose a simple yet effective model of a film in the form of a rectangular prism, which yields the modified Kittel formula for the resonance frequency. In this formula, the locally averaged distance-dependent demagnetizing field that emerges near the edges is included as an additional uniaxial anisotropy term. The measurements reveal that at a certain distance from the edge, the resulting (apparent) anisotropy, determined from the angular dependencies of the resonance field, almost vanishes. Moreover, its easy axis reorients to become parallel to the edge. The model predictions agree well with these results, proving that the main resonance mode behavior near the film edges can be accurately described by introducing additional effective uniaxial anisotropy, provided the measuring area is relatively large. However, for the thick (300 nm) sample, additional precession modes are also observed. These modes distort the angular dependence of the main mode, thus demonstrating the limitations of the model.

Numerical evaluation of the infill pattern upon mechanical proprieties of 3D printed materials

The effect of the structure that is printed inside an object, known as infill pattern, on mechanical proprieties of ABS specimens was investigated in this paper. Numerous studies demonstrated that parts created with fused deposition molding (FDM) technology present inferior mechanical properties due to additional porosity and anisotropy caused by the nature of the manufacturing process. In this regard the influence of printing parameters may be analyzed for correct evaluation of mechanical behavior and material constants. The methodology proposed in this paper consists of a numerical simulation of the fused deposition moulding 3D printed specimen, identification of the real cross-sectional area and evaluation of the strain-stress curves of the materials. There are several infill pattern geometries, each with benefits and compromises between material usage, printing time or mechanical strength of the obtained part. The current paper analysed specimens with 100% density (infill rate) but having different infill patterns: grid 0°-90° and ±45°, triangular 60°, fast honeycomb, full honeycomb and wiggle. The results are showing the dependence of the specimen's E modulus with the infill pattern and comparison of the strain-stress curves drawn with full cross-section and numerically calculated ones.

Role of interfacial layer on exchange-coupled magnetic properties of bi-magnetic nanostructures: An experimental and theoretical approach

Bi-magnetic hetero-nanostructures exhibiting significant exchange bias effects are highly desirable for technological advancement in spin valves, memory devices, data storage, biomedicine, and hyperthermia-based cancer treatment. As the interfacial spin interaction and the resulting anisotropy play a central role in these phenomena, investigating the tailored physical properties of hetero-structures of magnetically contrasting nanoparticles in different architectures is extremely useful. In this work, we report the formation of microspheres containing nano-aggregates of ferrimagnetic Fe3O4 and antiferromagnetic NiO phases. We demonstrate that this kind of bi-magnetic hetero-structures provide a conducive atmosphere for strong interfacial coupling giving rise to an extensive exchange bias effect (∼1.6 kOe). In addition, we also observe a substantial enhancement of the coercive field, indicating the onset of an additional unidirectional anisotropy under the field-cooled condition. Our first-principle density functional theory calculations reveal the role of cation intermixing at the interface of the two phases, being the source of an additional higher magneto-crystalline anisotropy than the original phases. This cation intermixing at the interface also causes a narrowed energy band, which was demonstrated by UV–Vis spectroscopy and first principle calculations. Having these technologically essential features with a tunable band gap, the embedded hetero-nanostructures are envisioned to cater to the requirement of high-density data storage and memory devices.

Exchange Bias in Nanostructures: An Update.

Exchange bias (EB) is a unidirectional anisotropy occurring in exchange-coupled ferromagnetic/antiferromagnetic systems, such as thin films, core-shell particles, or nanostructures. In addition to a horizontal shift of the hysteresis loop, defining the exchange bias, asymmetric loops and even vertical shifts can often be found. While the effect is used in hard disk read heads and several spintronics applications, its origin is still not fully understood. Especially in nanostructures with their additional shape anisotropies, interesting and often unexpected effects can occur. Here, we provide an overview of the most recent experimental findings and theoretical models of exchange bias in nanostructures from different materials.

Magnetic coercivity control via buffer layer roughness in Pt/Co multilayers

Magnetic properties of thin metallic films are strongly dependent on the growth parameters. Therefore, fine-tuning these parameters is necessary to produce multilayers with specific physical properties required by various applications. Here, we present an efficient approach to tune the coercive field of magnetron sputtered Co/Pt multilayers using Au underlayer with variable thickness. We were able to control the interfacial roughness of the layers in the stack via the change of the Au underlayer thickness due to its island-like growth mechanism (Volmer-Weber mode). As the nominal thickness of Au increased, the islands grew in larger lateral size, resulting in higher surface roughness of the underlayer, which was transferred to subsequent interfaces. Magnetization measurements showed additional magnetic anisotropy promoted by this mechanism. Variating the thickness of the Au layer up to 20 nm, we changed the coercive field in the range from ∼ 200 Oe to ∼ 1100 Oe while maintaining a nearly constant saturation magnetization. Presented results demonstrate the possibility to precisely design magnetic properties of metallic multilayers for applications via buffer layer roughness engineering.

Univariate Gauss quadrature for structural modelling of tissues and materials with distributed fibres

This work presents a method for computing the averaged free energy and constitutive relations in hyperelastic material models with distributed fibres, as they apply to soft fibre-reinforced materials and biological tissues. While these models are currently implemented through either spherical cubature of the fibre free energy or its Taylor series, we here propose a new method based on a univariate Gauss quadrature rule with integration points and weights informed by the statistical moments of the distribution of fibre stretch. As an intrinsic property, the new approach separates the integration of the fibre constitutive law from the integration of the orientation distribution, the latter leading to structural tensors of even order. Provided the latter 2n−1 tensors are computed accurately up to tensorial order 2(2n−1), the method integrates exactly any polynomial of order 2n−1 that agrees with the fibre law at the n integration points. After formally introducing the quadrature method for generally non-affine fibre deformations and arbitrary order, we focus on the important special case of affine fibre kinematics and discuss the rules with n≤3 integration points, for which the corresponding positions and weights are determined analytically. At a computational cost comparable to the existing approaches, the new method does not require the fibre law to be analytic and can thus robustly deal with piece-wise definitions of the fibre energy, in contrast to Taylor-series approaches, and it does not induce additional anisotropy as it can occur with spherical cubature rules. The 3-point rule is further investigated and illustrated in numerical examples relating to soft collagenous tissues based on a Fortran implementation of the method suitable for use in finite element analyses.

Tilt and Anisotropy of the Dirac Spectrum Caused by the Overlapping of Bloch Functions

It has been shown that the overlapping of bands belonging to equivalent representation of the symmetry group is possible in systems with Dirac points appearing at the crossing of these bands. This overlapping results in the tilt and additional anisotropy of the Dirac spectrum, as well as in the renormalization of the velocity. At the same time, overlapping does not violate the general conditions of existence of the stable band crossing point. The effective Dirac Hamiltonian in the presence of band overlapping is pseudo-Hermitian and corresponds to the effective action of a massless spinor field in the curved spacetime.

Fe-Gd Ferromagnetic Cyclic Coordination Cluster [FeIII4GdIII4(teaH)8(N3)8(H2O)] with Magnetic Anisotropy─Theory and Experiment.

The synthesis, structural, and magnetic characterization of [FeIII4LnIII4(teaH)8(N3)8(H2O)] (Ln = Gd and Y) and the previously reported isostructural Dy analogue are discussed. The commonly held belief that both FeIII and GdIII can be regarded as isotropic ions is shown to be an oversimplification. This conclusion is derived from the magnetic data for the YIII analogue in terms of the zero-field splitting seen for FeIII and from the fact that the magnetic data for the new GdIII analogue can only be fit employing an additional anisotropy term for the GdIII ions. Furthermore, the Fe4Gd4 ring shows slow relaxation of magnetization. Our analysis of the experimental magnetic data employs both density functional theory as well as the finite-temperature Lanczos method which finally enables us to provide an almost perfect fit of magnetocaloric properties.

Integrated intravoxel incoherent motion tensor and diffusion tensor brain MRI in a single fast acquisition.

The acquisition of intravoxel incoherent motion (IVIM) data and diffusion tensor imaging (DTI) data from the brain can be integrated into a single measurement, which offers the possibility to determine orientation-dependent (tensorial) perfusion parameters in addition to established IVIM and DTI parameters. The purpose of this study was to evaluate the feasibility of such a protocol with a clinically feasible scan time below 6 min and to use a model-selection approach to find a set of DTI and IVIM tensor parameters that most adequately describes the acquired data. Diffusion-weighted images of the brain were acquired at 3 T in 20 elderly participants with cerebral small vessel disease using a multiband echoplanar imaging sequence with 15 b-values between 0 and 1000 s/mm2 and six non-collinear diffusion gradient directions for each b-value. Seven different IVIM-diffusion models with 4 to 14 parameters were implemented, which modeled diffusion and pseudo-diffusion as scalar or tensor quantities. The models were compared with respect to their fitting performance based on the goodness of fit (sum of squared fit residuals, chi2 ) and their Akaike weights (calculated from the corrected Akaike information criterion). Lowest chi2 values were found using the model with the largest number of model parameters. However, significantly highest Akaike weights indicating the most appropriate models for the acquired data were found with a nine-parameter IVIM-DTI model (with isotropic perfusion modeling) in normal-appearing white matter (NAWM), and with an 11-parameter model (IVIM-DTI with additional pseudo-diffusion anisotropy) in white matter with hyperintensities (WMH) and in gray matter (GM). The latter model allowed for the additional calculation of the fractional anisotropy of the pseudo-diffusion tensor (with a median value of 0.45 in NAWM, 0.23 in WMH, and 0.36 in GM), which is not accessible with the usually performed IVIM acquisitions based on three orthogonal diffusion-gradient directions.

Fourfold magnetic anisotropy induced in CoFeB/IrMn bilayers by interfacial exchange coupling

Exchange bias (EB) occurring in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers conventionally can lead to a unidirectional magnetic anisotropy () as well as an accompanied uniaxial magnetic anisotropy (). We observed an additional fourfold magnetic anisotropy () induced by interfacial exchange coupling in amorphous CoFeB/epitaxial IrMn bilayers with an EB. Because of the combined effect of the three kinds of magnetic anisotropies, one- and two-step magnetic switching processes were observed at different magnetic field orientations, which usually appear in single-crystal FM layer with an intrinsic magnetocrystalline anisotropy but not in amorphous FM layer. The angular dependent magnetic switching fields can be nicely fitted by a phenomenological model based on domain wall nucleation and propagation with the in-plane along <100>. The ferromagnetic resonance measurements indicate that the specific strength of for EB along [100] is larger than that for EB along [110]. The induced can be understood by considering two types of AFM domains caused by both monatomic steps and defects and their induced net uncompensated spins along the in-plane <100> axes. The different dependence of on the EB direction are because of the different effects of growth magnetic field on the presence of AFM domains.

Optical absorption and dichroism of single melanin nanoparticles.

Melanin nanoparticles (NPs) have important biological functions including photoprotection and colouration, and artificial melanin-like NPs are relevant for catalysis, drug delivery, diagnosis and therapy. Despite their importance, the optical properties of single melanin NPs have not been measured. We combine quantitative differential interference contrast (qDIC) and extinction microscopy to characterise the optical properties of single NPs, both naturally sourced from cuttlefish ink, as well as synthetic NPs using polydopamine (PDA) and L-3,4-dihydroxyphenylalanine (L-DOPA). Combining qDIC with extinction, we determine the absorption index of individual NPs. We find that on average the natural melanin NPs have a higher absorption index than the artificial melanin NPs. From the analysis of the polarisation-dependent NP extinction, the NP aspect ratio is determined, with mean values at 405 nm wavelength in agreement with transmission electron microscopy. At longer wavelengths, we observe an additional optical anisotropy which is attributed to dichroism by structural ordering of the melanin. Our quantitative analysis yields a dichroism of 2-10% of the absorption index, increasing with wavelength from 455 nm to 660 nm for L-DOPA and PDA. Such an in-depth quantification of the optical properties of single melanin NPs is important for the design and future application of these ubiquitous bionanomaterials.

Spontaneous and Electrically Induced Anisotropy of Composite Agarose Gels.

Agarose gels containing and not bacteriorhodopsin purple membranes (incorporated before gelling) manifest spontaneous optical anisotropy. The dependencies of the anisotropy on the agarose concentration and time have been studied. The rise in the anisotropy is explained by the predominant orientation of the agarose fibers during the gelling and subsequent deformation of the gel net. In the electric field, additional optical anisotropy rises, which is caused by the orientation of the membranes. A procedure has been developed to separate electrically induced and spontaneous anisotropy in composite gels. The isoelectric points and surface electric potential of bacteriorhodopsin trimer and purple membranes are calculated by the method of protein electrostatics to explain their electric asymmetry, which leads to perpendicular orientation in the direct electric field and longitudinal in the kilohertz sinusoidal field. The results allow for an increase in the separation capability of composite gels of electrophoresis for macromolecules with different sizes by applying an appropriate electric field to modulate the effective pore size.

Laser‐Induced Transient Anisotropy and Large Amplitude Magnetization Dynamics in a Gd/FeCo Multilayer

AbstractUltrafast laser‐induced dynamics in a ferrimagnetic gadolinium iron cobalt (Gd/FeCo) multilayer with a magnetization compensation temperature of TM = 320 K is studied at room temperature as a function of laser‐fluence and strength of the applied magnetic field. The dynamics is found to be substantially different from that in archetypical GdFeCo alloys, and depending on the laser fluence one can distinguish two different regimes. At low laser fluence (⩽1.6 mJ cm‐2), ultrafast laser excitation of the medium triggers spin precession of an extraordinary large amplitude reaching over 30°. At high laser fluence (⩾2.2 mJ cm‐2), the pump heats the medium over the magnetization compensation point, spin precession reduces significantly in amplitude and the process of field‐assisted reversal of magnetization of Gd and FeCo is launched. It is argued that such a distinctly different laser‐induced magnetization dynamics in the multilayers compared to the alloys is due to the symmetry breaking at the numerous interfaces, giving rise to additional surface anisotropy. The temperature dependence of the latter is found to be the key ingredient in the mechanism of ultrafast laser‐induced magnetization dynamics in ferrimagnetic multilayers. Controlling the amount and properties of interfaces in multilayers can thus serve as a mean to achieve efficient ultrafast all‐optical control of magnetism.

Dipole-exchange spin waves in two-dimensional van der Waals ferromagnetic films and stripes

A spin-wave (SW) theory that includes the long-range dipole–dipole interactions is presented for monolayers of van der Waals (vdW) ferromagnets for which the magnetic ions lie on a two-dimensional honeycomb lattice. The dipolar interactions provide an additional anisotropy in these materials, along with the Ising exchange interaction and/or single-ion anisotropies that typically stabilize the two-dimensional magnetic ordering. Analytical results for the linearized SW energies are obtained for the ferromagnets in two geometries: complete films and finite-width stripes (or ribbons). In both cases it is found that the inclusion of the dipole–dipole interactions leads to a shift and sometimes a splitting of the magnetic modes in the vdW structure. Also, in the latter case, where the edges are assumed to be along the zigzag lattice directions, the dipole–dipole interactions are found to play a role, as well as the exchange interactions, in modifying the localized edge SWs. Numerical examples are given, including applications to the ferromagnet CrI3.

Intragranularly misoriented grain boundary evolution affected by local constraints and grain size in micro-scale deformation of ultra-thin metallic sheets

Multi-stage micro-scale deformation-based processing of ultra-thin metallic sheets has been widely used in the manufacturing of light-weight parts with complicated structures. Because only a few grains are distributed along the thickness direction of ultra-thin sheets, the grain size effect on the in-grain microstructure evolution must be understood. Moreover, the intragranular orientation mismatch induced by prior processing needs to be studied to accurately describe the additional deformation resistance and anisotropy in the step-by-step fabrication. In this study, 0.1-mm-thick SS 316L sheet samples with different mean grain sizes were deformed to two strain levels, with the electron backscatter diffraction (EBSD) used to characterize the microstructures of the deformed samples. Based on the EBSD data and crystal plasticity finite element simulations, the effects of local constraints and grain size on intragranularly misoriented grain boundary (IMGB) evolution was statistically analyzed. The IMGB patterns were organized into four categories, and the characteristics of local constraints near the grain boundary triple junctions were delineated. The formation of IMGBs was found to be induced by the concentration of high local strain, which was governed by local constraints, including the geometrical and deformation-related factors. Because of the higher intragranular strain and longer dislocation moving distance before annihilation, the IMGBs accumulate more easily inside large grains. The collected local constraint characteristics and the linear correlation between IMGB length and grain area were applied in a proposed three-step procedure for estimating the IMGB patterns and magnitudes of the micro-scale deformation of ultra-thin sheets. This comprehensive and quantitative description of IMGB evolution provides a fundamental basis for understanding the grain-level deformation mechanisms and for the constitutive modelling of the mechanical responses of ultra-thin sheets in complex deformation.