In-plane Anisotropy Research Articles

Role of nitrogen and helium gases on microstructure and anisotropic mechanical properties of cold-sprayed scalmalloy deposits

Aluminum alloys (Al) produced by fusion-based additive manufacturing techniques exhibit limited strength due to undesirable grain growth, porosity, and thermal gradient. To address these limitations, Scalmalloy (Al–Mg4.6-Sc0.72-Zr0.3 wt%) deposits of 5 mm thickness using helium and nitrogen gases were manufactured using a solid-state cold spray (CS) deposition. The microstructure of these deposits revealed a bimodal distribution of coarse and fine grains in the splat interior and jetting region, respectively, characterized by a higher degree of plastic deformation in the He-deposit. These bimodal grains resulted in a nanohardness of 1.2 GPa in the jetting region, 20% higher than that in the interior. The cumulative effect of these microstructural features resulted in a microhardness of 149 HV in the He-deposit, 1.1 times compared to that in the N2-deposit. The increased hardness is translated to a yield strength of 383 MPa and ultimate strength of 487 MPa in the He-deposit, 1.17 and 1.08 times of N2-deposit, respectively, estimated by Profilometry-based indentation plastometry (PIP). Across the hierarchical layers, profilometry-based indentation plastometry captured a consistent pile-up difference of ∼2 μm in the He-deposit and ∼3 μm in the N2-deposit on the plane perpendicular to build plane, a manifestation of in-plane anisotropy. This study advances the microstructural understanding and its relationship with observed mechanical properties, deformation behavior, and anisotropic response in high-strength Scalmalloy CS deposits.

Full-field noise-correlation elastography for in-plane mechanical anisotropy imaging.

Elastography contrast imaging has great potential for the detection and characterization of abnormalities in soft biological tissues to help physicians in diagnosis. Transient shear-waves elastography has notably shown promising results for a range of clinical applications. In biological soft tissues such as muscle, high mechanical anisotropy implies different stiffness estimations depending on the direction of the measurement. In this study, we propose the evolution of a noise-correlation elastography approach for in-plane anisotropy mapping. This method is shown to retrieve anisotropy from simulation images before being validated on agarose anisotropic tissue-mimicking phantoms, and the first results on in-vivo biological fibrous tissues are presented.

In-plane magnetic properties and anisotropy of scandium substituted thulium iron garnet thin films for fluxgate magnetometer

In-plane easily magnetized scandium substituted thulium iron garnet films(Tm3Fe5-xScxO12) with soft magnetic properties have recently attracted attention for fluxgate magnetometer applications. In this paper, thin Tm3Fe5-xScxO12 films were prepared on gadolinium gallium garnet substrates by liquid phase epitaxial(LPE) method. Microstructural properties, composition, magnetic properties and the in-plane anisotropy of the films were discussed. We found that the films turned from tensile stress to compressive stress as the Sc3+ substitution decreased, and the in-plane anisotropy was mainly caused by stress reducing as Sc3+ declined. By analyzing the in-plane magnetic hysteresis loops, the magnetic performance was the best when x = 0.75. Tm3Fe4.25Sc0.75O12 film had the lowest in-plane coercive field(0.006 Oe) and the highest initial magnetic permeability(378), which was more beneficial to realize fluxgate magnetometers.

Improved numerical solution of anisotropic poroelastic wave equation in microseismicity: Graphic process unit acceleration and moment tensor implementation

AbstractThe accuracy and computational efficiency of full waveform forward modelling in poroelastic media are crucial for microseismic monitoring. It enables intuitive, precise and efficient simulation of subsurface responses, thereby improving the reliability of moment tensor inversion and seismic source mechanism interpretation. Additionally, it reflects the role of fluid effects in waveform evolution. In this paper, based on the Biot mechanism, we derived the first‐order velocity–stress equation of poroelastic media and discretized the model using a rotated staggered grid. The rotated staggered grid can well adapt to anisotropic media with high contrast parameters. We provide the finite difference formula based on graphic process unit–acceleration and moment tensor and also provide the graphic process unit workflow for forward modelling of anisotropic poroelastic media. First, two models with different grid sizes were run based on single graphic process unit, 1‐thread Central Processing Unit (CPU) and 16‐thread CPU. The results show that the speedup factors are approximately 14.3 and 3.7, respectively, compared with the running time of 1‐thread CPU and 16‐thread CPU. Then, we compare and evaluate the response of three basic source mechanisms (isotropic expansion, double couple and compensated linear vector dipole) in the model. The comparison of analytical and numerical results verifies the effectiveness of the method. Furthermore, wavefield snapshots of two typical anisotropic media (vertical transversely isotropic and horizontal transversely isotropic) are analysed to correspond to different moment tensor sources. The results showed that the source mechanism does not change the isotropic and anisotropic plane and the wave travel time, but it does change the polarization amplitude of the velocity component. The attenuation of slow qP‐wave increases along with the increase of the value of viscosity. The effect of permeability on wavefield appears with the opposite effect of viscosity. Finally, the seismic waveform differences between multi‐layer elastic media and poroelastic media are compared and analysed. The results showed that the seismic wavefield and waveform of poroelastic media are more complex, the propagation speed of seismic waves is faster, but the attenuation of seismic waves is stronger.

Magnetospheric flows in X-ray pulsars – I. Instability at super-Eddington regime of accretion

ABSTRACT Within the magnetospheric radius, the geometry of accretion flow in X-ray pulsars is shaped by a strong magnetic field of a neutron star. Starting at the magnetospheric radius, accretion flow follows field lines and reaches the stellar surface in small regions located close to the magnetic poles of a star. At low mass accretion rates, the dynamics of the flow is determined by gravitational attraction and rotation of the magnetosphere due to the centrifugal force. At the luminosity range close to the Eddington limit and above it, the flow is additionally affected by the radiative force. We construct a model simulating accretion flow dynamics over the magnetosphere, assuming that the flow strictly follows field lines and is affected by gravity, radiative, and centrifugal forces only. The magnetic field of a neutron star is taken to be dominated by the dipole component of arbitrary inclination with respect to the accretion disc plane. We show that accretion flow becomes unstable at high mass accretion rates and tends to fluctuate quasi-periodically with a typical period comparable to the free-fall time from the inner disc radius. The inclination of a magnetic dipole with respect to the disc plane and strong anisotropy of X-ray radiation stabilize the mass accretion rate at the poles of a star, but the surface density of material covering the magnetosphere fluctuates even in this case.

Hyperbolic phonon polaritons-induced photonic spin Hall effect in an α -MoO3 thin film

Photonic spin Hall effect (PSHE) can be achieved by using the patterned structures or metal/dielectric multilayers in the nanophotonic systems; however, the complicated structures of these devices hinder their further applications. Herein, we demonstrate that highly directional PSHE can be realized through the excitation of hyperbolic phonon polaritons (HPPs) in a comparably simple architecture based on an anisotropic α-MoO3 thin film. It is shown that the propagation of the HPP modes of α-MoO3 in the reststrahlen (RS) bands exhibits topological transitions between open hyperbola and closed ellipse in both real space and momentum space (k-space) due to the extreme in-plane anisotropy. Specifically, larger dispersion angle possesses larger figure of merit (FoM), and high k mode of HPPs exhibits robust propagation properties at the maximum dispersion angle. Spin-selected propagation with asymmetric ratio of intensity equal to ±0.94 can be realized by changing the handedness of the dipole emitters. By changing the incident wavelengths of the RS bands, the propagation angle of the HPP modes can be dynamically tuned in wide angular and wavelength ranges.

Near-field detection of gate-tunable anisotropic plasmon polaritons in black phosphorus at terahertz frequencies

Polaritons in two-dimensional layered crystals offer an effective solution to confine, enhance and manipulate terahertz (THz) frequency electromagnetic waves at the nanoscale. Recently, strong THz field confinement has been achieved in a graphene-insulator-metal structure, exploiting THz plasmon polaritons (PPs) with strongly reduced wavelength (λp ≈ λ0/66) compared to the photon wavelength λ0. However, graphene PPs propagate isotropically, complicating the directional control of the THz field, which, on the contrary, can be achieved exploiting anisotropic layered crystals, such as orthorhombic black-phosphorus. Here, we detect PPs, at THz frequencies, in hBN-encapsulated black phosphorus field effect transistors through THz near-field photocurrent nanoscopy. The real-space mapping of the thermoelectrical near-field photocurrents reveals deeply sub-wavelength THz PPs (λp ≈ λ0/76), with dispersion tunable by electrostatic control of the carrier density. The in-plane anisotropy of the dielectric response results into anisotropic polariton propagation along the armchair and zigzag crystallographic axes of black-phosphorus. The achieved directional subwavelength light confinement makes this material system a versatile platform for sensing and quantum technology based on nonlinear optics.

Fabrication of single-crystalline YFeO3 films with large antiferromagnetic domains

The antiferromagnetic orthoferrite YFeO3 possesses fascinating magnetic properties for spintronics, such as terahertz spin dynamics, ultrafast domain wall motion, and long magnon decay length. YFeO3 belongs to a special family of antiferromagnets that show an unusually strong non-trivial Kerr response due to its weak ferromagnetism. The highly stable antiferromagnetic domains without any spontaneous spin rotation transitions below the 645 K Néel temperature may be useful for nanoscale device applications. We report the successful fabrication of high-quality twinning-free (110)-oriented YFeO3 films by pulsed laser deposition. Detailed structural and magnetic characterization revealed that the crystal structure and magnetic properties of the YFeO3 films are comparable to bulk single crystals. We show that the spin rotation under high magnetic fields follows the two-sublattice approximation model. The film surface is atomically flat with step-terrace surface morphology. A longitudinal magneto-optic Kerr (MOKE) rotation of 10 mdeg was observed at room temperature, which is consistent with earlier reports on bulk single crystals. The in-plane anisotropy of the Kerr response corresponds to the obtained magnetic anisotropy from the SQUID measurement. The large MOKE signal enables the imaging of antiferromagnetic domains and their reversal. The domain size was found to be larger than 100 μm. These high-quality YFeO3 thin films facilitate the fabrication of antiferromagnetic spintronic devices and provide a convenient platform for studying various spin-related phenomena in thin films and at interfaces.

Unexpected versatile electrical transport behaviors of ferromagnetic nickel films

Perpendicular magnetic anisotropy (PMA) of magnets is paramount for electrically controlled spintronics due to their intrinsic potentials for higher memory density, scalability, thermal stability and endurance, surpassing an in-plane magnetic anisotropy (IMA). Nickel film is a long-lived fundamental element ferromagnet, yet its electrical transport behavior associated with magnetism has not been comprehensively studied, hindering corresponding spintronic applications exploiting nickel-based compounds. Here, we systematically investigate the highly versatile magnetism and corresponding transport behavior of nickel films. As the thickness reduces within the general thickness regime of a magnet layer for a memory device, the hardness of nickel films’ ferromagnetic loop of anomalous Hall effect increases and then decreases, reflecting the magnetic transitions from IMA to PMA and back to IMA. Additionally, the square ferromagnetic loop changes from a hard to a soft one at rising temperatures, indicating a shift from PMA to IMA. Furthermore, we observe a butterfly magnetoresistance resulting from the anisotropic magnetoresistance effect, which evolves in conjunction with the thickness and temperature-dependent magnetic transformations as a complementary support. Our findings unveil the rich magnetic dynamics and most importantly settle down the most useful guiding information for current-driven spintronic applications based on nickel film: The hysteresis loop is squarest for the ∼8 nm-thick nickel film, of highest hardness with Rxy r /Rxy s ∼ 1 and minimum Hs −Hc , up to 125 K; otherwise, extra care should be taken for a different thickness or at a higher temperature.

Effects of Buffer and Capping Layers on Thermal Stability of CoFeB/MgO Frames at Various Temperatures

The utilization of CoFeB thin films in spintronic devices has attracted significant attention due to their exceptional magnetic properties, which include high saturation magnetization and spin polarization. However, the effect of ambient temperature on the magnetic properties of CoFeB/MgO frames, particularly those with different buffer and capping layers, remains unexplored. Therefore, in this study, the magnetostatic and dynamic properties of CoFeB/MgO frames were investigated at various temperatures. Using vibrating sample magnetometry and ferromagnetic resonance spectroscopy, changes in key parameters such as saturation magnetization, the Gilbert damping constant, magnetic anisotropy field, in-plane uniaxial magnetic anisotropy energy, and thermal stability factor were investigated. Furthermore, the thermal stabilities of CoFeB/MgO frames with Ta buffer and capping layers were compared with those of CoFeB/MgO frames with W buffer and capping layers by examining the changes in the key parameters at various temperatures. These results reveal that the thermal stability of the latter surpassed that of the former. This study provides significant insights for the development of thermally robust spintronic devices capable of operating above room temperature.

Suppressed Plastic Anisotropy via Sigma-Phase Precipitation in CoCrFeMnNi High-Entropy Alloys.

The effect of sigma-phase precipitation on plastic anisotropy of the equiatomic CoCrFeMnNi high-entropy alloy was investigated. Annealing at 700 °C after cold-rolling leads to the formation of the Cr-rich σ phase with a fraction of 2.7%. It is experimentally found that the planar anisotropy (∆r = -0.16) of the CoCrFeMnNi alloy annealed at 700 °C is two times lower than that of the alloy annealed at 800 °C (∆r = -0.35). This observation was further supported by measuring the earing profile of cup specimens after the deep drawing process. The plastic strain ratio, normal anisotropy, and planar anisotropy were also predicted using the visco-plastic self-consistent model. The results indirectly indicated that the reduction of plastic anisotropy in alloy annealed at 700 °C can be attributed to the formation of the σ phase.

Exploring van der Waals materials with high anisotropy: geometrical and optical approaches

The emergence of van der Waals (vdW) materials resulted in the discovery of their high optical, mechanical, and electronic anisotropic properties, immediately enabling countless novel phenomena and applications. Such success inspired an intensive search for the highest possible anisotropic properties among vdW materials. Furthermore, the identification of the most promising among the huge family of vdW materials is a challenging quest requiring innovative approaches. Here, we suggest an easy-to-use method for such a survey based on the crystallographic geometrical perspective of vdW materials followed by their optical characterization. Using our approach, we found As2S3 as a highly anisotropic vdW material. It demonstrates high in-plane optical anisotropy that is ~20% larger than for rutile and over two times as large as calcite, high refractive index, and transparency in the visible range, overcoming the century-long record set by rutile. Given these benefits, As2S3 opens a pathway towards next-generation nanophotonics as demonstrated by an ultrathin true zero-order quarter-wave plate that combines classical and the Fabry–Pérot optical phase accumulations. Hence, our approach provides an effective and easy-to-use method to find vdW materials with the utmost anisotropic properties.

Unraveling High Thermal Conductivity with In-Plane Anisotropy Observed in Suspended SiP2.

The anisotropic thermal transport properties of low-symmetry two-dimensional materials play an important role in understanding heat dissipation and optimizing thermal management in integrated devices. Examples of efficient energy dissipation and enhanced power sustainability have been demonstrated in nanodevices based on materials with anisotropic thermal transport properties. However, the exploration of materials with high thermal conductivity and strong in-plane anisotropy remains challenging. Herein, we demonstrate the observation of anisotropic in-plane thermal conductivities of few-layer SiP2 based on the micro-Raman thermometry method. For suspended SiP2 nanoflake, the thermal conductivity parallel to P-P chain direction (κ∥b) can reach 131 W m-1 K-1 and perpendicular to P-P chain direction (κ⊥b) is 89 W m-1 K-1 at room temperature, resulting in a significant anisotropic ratio (κ∥b/κ⊥b) of 1.47. Note that such a large anisotropic ratio mainly results from the higher phonon group velocity along the P-P chain direction. We also found that the thermal conductivity can be effectively modulated by increasing the SiP2 thickness, reaching a value as high as 202 W m-1 K-1 (120 W m-1 K-1) for κ∥b (κ⊥b) at 111 nm thickness, which is the highest among layered anisotropic phosphide materials. Notably, the anisotropic ratio always remains at a high level between 1.47 and 1.68, regardless of the variation of SiP2 thickness. Our observation provides a new platform to verify the fundamental theory of thermal transport and a crucial guidance for designing efficient thermal management schemes of anisotropic electronic devices.

Tailoring the formability and planar anisotropy of Al-Mg-Si-Cu-Zn alloys via cross hot rolling and two-stage cold rolling

Al-Mg-Si-(Cu) alloys have shown great potential in responding to the growing demand for automotive lightweight, but the limited formability and high planar anisotropy have always hindered their application. The present study designs a novel method for simultaneously improving the formability and planar anisotropy of Al-Mg-Si-Cu-Zn alloys by coupling cross hot rolling and two-stage cold rolling. The results have shown that the multi-scale distribution of secondary particles and strong Brass{011}<112> and S{123}<634> texture are induced by the designed cross hot rolling process, which weakens the recrystallization texture after solution treatment, especially Cube{001}<100> orientation. Meanwhile, the designed two-stage cold rolling process (intermediate cold rolling process with high reduction + final cold rolling process with low reduction) promotes the precipitation of unstable precipitates (Mg2Si and Q phase), and inhibits the formation of Copper{112}<111> orientation, which further weakens the recrystallization texture and results in the dispersion distribution of recrystallization texture. Accordingly, the formability and planar anisotropy of the Al-Mg-Si-Cu-Zn alloy are simultaneously improved without strength loss by coupling the designed cross hot rolling with two-stage cold rolling, and the influence mechanism of texture on r value has been analyzed by Visco-Plastic Self-Consistent (VPSC) model.

Effect of metal insertion on the magnetic properties and anomalous Hall effect in MgO/CoFeB/Ta/MgO films

CoFeB-based nano-magnetic multilayers can be applied in magnetic sensors, magnetic random access memory and other logic devices, which have attracted wide attention. The magnetic properties and interface structure of the magnetic multilayers still need further research for future applications. The effects of Pt insertion on the perpendicular magnetic anisotropy (PMA) and the anomalous Hall effect (AHE) of MgO/CoFeB/Pt/Ta/MgO multilayers were studied. It is found that the sample with 0.2 nm Pt insertion at the CoFeB/Ta interface and annealed at 250 °C can transform the films from in-plane magnetic anisotropy to PMA. The effective magnetic anisotropy K eff can reach 1.82 × 106 erg cm−3. The Pt insertion and annealing heat treatment can promote the migration of oxygen from MgO at the top layer to CoFeB/Ta interface and combine with Fe to generate iron oxide, and improve the orbital hybridization of Fe 3d and O 2p, thus successfully inducing the PMA of the film samples. In addition, Pt insertion and annealing treatment can improve the side-jump mechanism and the skew scattering mechanism which contribute to the AHE. This work provides guidance for the application of PMA materials in magnetic sensor and memory devices.

Isotopic effects on in-plane hyperbolic phonon polaritons in MoO3

Abstract Hyperbolic phonon polaritons (HPhPs), hybrids of light and lattice vibrations in polar dielectric crystals, empower nanophotonic applications by enabling the confinement and manipulation of light at the nanoscale. Molybdenum trioxide (α-MoO3) is a naturally hyperbolic material, meaning that its dielectric function deterministically controls the directional propagation of in-plane HPhPs within its reststrahlen bands. Strategies such as substrate engineering, nano- and hetero-structuring, and isotopic enrichment are being developed to alter the intrinsic dielectric functions of natural hyperbolic materials and to control the confinement and propagation of HPhPs. Since isotopic disorder can limit phonon-based processes such as HPhPs, here we synthesize isotopically enriched 92MoO3 (92Mo: 99.93 %) and 100MoO3 (100Mo: 99.01 %) crystals to tune the properties and dispersion of HPhPs with respect to natural α-MoO3, which is composed of seven stable Mo isotopes. Real-space, near-field maps measured with the photothermal induced resonance (PTIR) technique enable comparisons of in-plane HPhPs in α-MoO3 and isotopically enriched analogs within a reststrahlen band (≈820 cm−1 to ≈972 cm−1). Results show that isotopic enrichment (e.g., 92MoO3 and 100MoO3) alters the dielectric function, shifting the HPhP dispersion (HPhP angular wavenumber × thickness vs. IR frequency) by ≈−7 % and ≈+9 %, respectively, and changes the HPhP group velocities by ≈±12 %, while the lifetimes (≈3 ps) in 92MoO3 were found to be slightly improved (≈20 %). The latter improvement is attributed to a decrease in isotopic disorder. Altogether, isotopic enrichment was found to offer fine control over the properties that determine the anisotropic in-plane propagation of HPhPs in α-MoO3, which is essential to its implementation in nanophotonic applications.

Anisotropic phonon properties in SiP2 monolayer: A first-principles study

Recently, the micromechanical exfoliation method has effectively separated the two-dimensional (2D) SiP2 flake, which exhibits superior performance in field-effect transistors and photodetectors. In this paper, first-principles calculations were utilized to investigate the phononic properties of a SiP2 monolayer, which would be significant to explain the thermal deformation of 2D SiP2-based devices. We found that the acoustic phonon branches of SiP2 monolayer demonstrate significant in-plane anisotropy. The anisotropic ratios for the group velocities are 2.41 and 1.66 for the longitudinal and transverse acoustic modes, respectively. Meanwhile, the anisotropic ratio of the negative thermal expansion coefficient only considering the contribution from acoustic phonons is 0.14, while this ratio is ∼0.279 occurs at 120 K when both acoustic and optical phonons are taken into account. For the Raman-active optical phonons, the largest Grüneisen constant of these Raman-active phonons reaches 22.76, while the largest anisotropic ratio of phonon frequency is up to 2.16. To interpret this significant in-plane anisotropy, we calculated the electron density distribution, crystal orbital Hamilton populations (COHP) and interatomic force constants (IFCs), and found the maximal IFC in the x-direction is 5.79 times greater than y-direction, suggesting it as a major origin of the phononic anisotropy.

Realization of low potential barrier in MoS2/rGO heterojunction with enhanced electrical conductivity for thin film thermoelectric applications

Two-dimensional (2D) van der Waals materials in-plane anisotropy, caused by a low-symmetric lattice structure, has considerably increased their applications, particularly in thermoelectric. MoS2 and MoS2/reduced graphene oxide (rGO) thin films were grown on SiO2/Si substrate by atmospheric chemical vapor deposition technique to study the thermoelectric performance. Few layered MoS2 was confirmed by the vibrational analysis and the composition elements are confirmed by the x-ray photoelectron spectroscopy technique. The continuous grains lead to reduced phonon life time in A1g and low activation energy assists to enhance the electrical property. The MoS2/rGO has achieved the highest σ of 22 622 S m−1 at 315 K due to an electron-rich cloud around the electrons in S atoms near the adjacent layer of rGO.

Highly Sensitive Broadband Polarized Photodetector Based on the As0.6P0.4/WSe2 Heterostructure toward Imaging and Optical Communication Application.

Polarization-sensitive photodetectors based on two-dimensional anisotropic materials still encounter the issues of narrow spectral coverage and low polarization sensitivity. To address these obstacles, anisotropic As0.6P0.4 with a narrow band gap has been integrated with WSe2 to construct a type-II heterostructure, realizing a high-performance polarization-sensitive photodetector with broad spectral range from 405 to 2200 nm. By operating in photovoltaic mode at zero bias, the device shows a very low dark current of ∼0.02 picoampere, high responsivity of 492 m A/W, and high photoswitching ratio of 6 × 104, yielding a high specific detectivity of 1.4 × 1012 Jones. The strong in-plane anisotropy of As0.6P0.4 endows the device with a capability of polarization-sensitive detection with a high polarization ratio of 6.85 under a bias voltage. As an image sensor and signal receiver, the device shows great potential in imaging and optical communication applications. This work develops an anisotropic vdW heterojunction to realize polarization-sensitive photodetectors with wide spectral coverage, fast response, and high sensitivity, providing a new candidate for potential applications of polarization-resolved electronics and photonics.

Chiral Magnetic Phases in Moire Bilayers of Magnetic Dipoles

AbstractIn magnetic insulators, the sense of rotation of the magnetization is associated with novel phases of matter and exotic transport phenomena. Aimed to find new sources of chiral magnetism rooted in intrinsic fields and geometry, twisted square bilayers of magnetic dipoles with easy plane anisotropy are studied. For no twist, each lattice settles in the zig‐zag magnetic state and orders antiferromagnetically to the other layer. The moire patterns that result from the mutual rotation of the two square lattices influence such zig‐zag order, giving rise to several phases that depict non‐collinear magnetic textures with chiral motifs that break both time and inversion symmetry. For certain moire angles, helical and toroidal magnetic orders arise. Changing the vertical distance between layers can further manipulate these novel phases. It is shown that the dipolar interlayer interaction induces an emergent twist‐dependent chiral magnetic field orthogonal to the direction of the zig‐zag chains, which is responsible for the internal torques conjugated to the toroidal orders.