Tunable Anisotropy Research Articles

Holmium(iii) molecular nanomagnets for optical thermometry exploring the luminescence re-absorption effect.

Coordination complexes of lanthanide(3+) ions can combine Single-Molecule Magnetism (SMM) with thermally modulated luminescence applicable in optical thermometry. We report an innovative approach towards high performance SMM-based optical thermometers which explores tunable anisotropy and the luminescence re-absorption effect of HoIII complexes. Our concept is shown in dinuclear cyanido-bridged molecules, {[HoIII(4-pyridone)4(H2O)2][MIII(CN)6]}·nH2O (M = Co, 1; Rh, 2; Ir, 3) and their magnetically diluted analogues, {[HoIIIxYIII1–x(4-pyridone)4(H2O)2][MIII(CN)6]}·nH2O (M = Co, x = 0.11, 1@Y; Rh, x = 0.12, 2@Y; Ir, x = 0.10, 3@Y). They are built of pentagonal bipyramidal HoIII complexes revealing the zero-dc-field SMM effect. Experimental studies and the ab initio calculations indicate an Orbach magnetic relaxation with energy barriers varying from 89.8 to 86.7 and 78.7 cm−1 K for 1, 2, and 3, respectively. 1–3 also differ in the strength of quantum tunnelling of magnetization which is suppressed by hyperfine interactions, and, further, by the magnetic dilution. The YIII-based dilution governs the optical properties as 1–3 exhibit poor emission due to the dominant re-absorption from HoIII while 1@Y–3@Y show room-temperature blue emission of 4-pyridone. Within ligand emission bands, the sharp re-absorption lines of the HoIII electronic transitions were observed. Their strong thermal variation was used in achieving highly sensitive ratiometric optical thermometers whose good performance ranges, lying between 25 and 205 K, are adjustable by using hexacyanidometallates. This work shows that HoIII complexes are great prerequisites for advanced opto-magnetic systems linking slow magnetic relaxation with unique optical thermometry exploiting a luminescence re-absorption phenomenon.

Strain-tunable giant anisotropic conductivity in the polar metal of KNbO3/BaTiO3 ferroelectric superlattice

Anisotropy in crystals originates from different periodicity atomic arrangements along with different crystallographic directions, leading to different physical properties of the crystal in different directions, which is particularly pronounced in two-dimensional (2D) materials. In this work, by using first-principles calculations, we predicted the tunable giant electrical anisotropy of the KNbO3/BaTiO3 (KNO/BTO) ferroelectric superlattice (SL) between in-plane and out-of-plane directions through the in-plane strain-engineering. The anisotropy ratio of electrical conductivity and electronic thermal conductivity shows an exponentially increase when the KNO/BTO SL undergoes a strain from tensile to compressive, and their anisotropy ratio can be tuned from several times to over 105 at room temperature. In addition, the compressively strained KNO/BTO SL has a large out-of-plane Seebeck coefficient and a high anisotropic thermopower. This anisotropy of the KNO/BTO SL primarily derives from the proportion of electrons occupied by dxy and dxz/dyz orbitals and that proportion is highly dependent on the polarization intensity of the KNO/BTO SL. Moreover, the ferroelectric and ferromagnetic properties of the KNO/BTO SL can be tuned by strain due to the coexistence of polar structure and metallicity. Our results provide a road map for creating multifunctional materials at perovskite oxides interfaces, which may be favorable for designing new volatile memory devices with anisotropy-based ultrafast and low-power properties.

Architectured Lattice Materials with Tunable Anisotropy: Design and Analysis of the Material Property Space with the Aid of Machine Learning

Architectured beam lattice materials whose anisotropy can be tuned by varying the composition of their elementary cell are investigated. As an exemplary prototype of such material architecture, a regular triangular lattice with an elementary cell composed of 12 beams is considered. One out of three possible values of the elastic modulus is assigned to each beam. The structure is fully defined by a vector in the 12D composition‐structure space whose components are given by the elastic modulus values of the beams comprising the elementary cell. The elastic properties of this 2D material are represented by the compliance elasticity tensor with six independent compliance coefficients. Aiming at a specific set of properties thus involves finding the point in the 12D composition‐structure space that corresponds to a given point in the 6D property space. This is a problem of large dimensionality. To solve it, the neural network approach is used. This enables creation of architectured materials with tunable elastic anisotropy. A chiral element combining large twist with additional anisotropy requirements is presented as an example of successful machine‐learning‐based optimization of beam lattices proposed.

Spin-reorientation transition induced magnetic skyrmion in Nd2Fe14B magnet

The easy axis of Nd2Fe14B is known to deviate from the c-axis when the temperature decreases to under the spin reorientation point TSR, 135 K. In this work, magnetic domain evolution in Nd2Fe14B was in situ observed by using Lorentz transmission electron microscopy at variable temperatures and magnetic fields. It appears that most inverse domains shrink to stripes and disappear suddenly to achieve the saturation state under a magnetic field, and the saturation field increases with the decreasing temperature due to the increased anisotropy. Magnetic bubbles with zero topological number are formed at temperatures higher than TSR, whereas magnetic skyrmions are found at temperatures around TSR due to the spin reorientation. The tunable anisotropy and saturation magnetization at TSR are the main causes of forming magnetic skyrmions. This finding exhibits the feasibility of generating skyrmions in the ordinary rare-earth permanent magnetic materials.

Advances in Electrospun Scaffolds Towards Meniscus: From Tissue Engineering to Repair and Regeneration

The meniscus plays a critical role in maintaining the homeostasis, biomechanics, and structural stability of the knee joint. Unfortunately, it is predisposed to damages either from sports-related trauma or age-related degeneration. The meniscus has an inherently limited capacity for tissue regeneration. Self-healing of injured adult menisci only occurs in the peripheral vascularized portion, while the spontaneous repair of the inner avascular region seems never happens. Repair, replacement, and regeneration of menisci through tissue engineering strategies are promising to address this problem. Recently, many scaffolds for meniscus tissue engineering have been proposed for both experimental and preclinical investigations. Electrospinning is a feasible and versatile technique to produce nano- to micro-scale fibers that mimic the microarchitecture of native extracellular matrix and is an effective approach to prepare nanofibrous scaffolds for constructing engineered meniscus. Electrospun scaffolds are reported to be capable of inducing colonization of meniscus cells by modulating local extracellular density and stimulating endogenous regeneration by driving reprogramming of meniscus wound microenvironment. Electrospun nanofibrous scaffolds with tunable mechanical properties, controllable anisotropy, and various porosities have shown promises for meniscus repair and regeneration and will undoubtedly inspire more efforts in exploring effective therapeutic approaches towards clinical applications. In this article, we review the current advances in the use of electrospun nanofibrous scaffolds for meniscus tissue engineering and repair and discuss prospects for future studies.

Protecting Spin Coherence in a Tunable Heisenberg Model.

Using an ensemble of atoms in an optical cavity, we engineer a family of nonlocal Heisenberg Hamiltonians with continuously tunable anisotropy of the spin-spin couplings. We thus gain access to a rich phase diagram, including a paramagnetic-to-ferromagnetic Ising phase transition that manifests as a diverging magnetic susceptibility at the critical point. The susceptibility displays a symmetry between Ising interactions and XY (spin-exchange) interactions of the opposite sign, which is indicative of the spatially extended atomic system behaving as a single collective spin. Images of the magnetization dynamics show that spin-exchange interactions protect the coherence of the collective spin, even against inhomogeneous fields that completely dephase the noninteracting and Ising systems. Our results underscore prospects for harnessing spin-exchange interactions to enhance the robustness of spin squeezing protocols.

Folding at the Microscale: Enabling Multifunctional 3D Origami-Architected Metamaterials.

Mechanical metamaterials inspired by the Japanese art of paper folding have gained considerable attention because of their potential to yield deployable and highly tunable assemblies. The inherent foldability of origami structures enlarges the material design space with remarkable properties such as auxeticity and high deformation recoverability and deployability, the latter being key in applications where spatial constraints are pivotal. This work integrates the results of the design, 3D direct laser writing fabrication, and in situ scanning electron microscopic mechanical characterization of microscale origami metamaterials, based on the multimodal assembly of Miura-Ori tubes. The origami-architected metamaterials, achieved by means of microfabrication, display remarkable mechanical properties: stiffness and Poisson's ratio tunable anisotropy, large degree of shape recoverability, multistability, and even reversible auxeticity whereby the metamaterial switches Poisson's ratio sign during deformation. The findings here reported underscore the scalable and multifunctional nature of origami designs, and pave the way toward harnessing the power of origami engineering at small scales.

Structural Evolution and Optical Property Tunability by Halogen Substitution in [N(CH3)4]MX2 (M = Ga+, In+, X = Cl, Br): A Family of Organically Templated Metal Halides.

Structural transformation in hybrid halides is an attractive way to modulate and improve their optical properties. Following newly reported monovalent-metal-based organic-inorganic chlorides [N(CH3)4]MCl2 (M = Ga+, In+), their bromide analogues [N(CH3)4]MBr2 (M = Ga+, In+) were subsequently synthesized. Impressively, their structure transforms from noncentrosymmetric P4̅21m space group for chlorides to centrosymmetric P42/m space group for bromides. As the halogen changes, the (MBr2)- units are partially rotated along the c-axis, which can be attributed to the difference in the anion radius and H-bond interactions between host networks and guest molecules. In addition, the tunable electronic structure and optical anisotropy in this family are also presented. This work provides a typical example for exploring new asymmetric and symmetric organic-inorganic hybrid halides as appliable photoelectric functional materials.

Continuous Formation of Ultrathin, Strong Collagen Sheets with Tunable Anisotropy and Compaction.

The multiscale organization of protein-based fibrillar materials is a hallmark of many organs, but the recapitulation of hierarchal structures down to fibrillar scales, which is a requirement for withstanding physiological loading forces, has been challenging. We present a microfluidic strategy for the continuous, large-scale formation of strong, handleable, free-standing, multicentimeter-wide collagen sheets of unprecedented thinness through the application of hydrodynamic focusing with the simultaneous imposition of strain. Sheets as thin as 1.9 μm displayed tensile strengths of 0.5–2.7 MPa, Young’s moduli of 3–36 MPa, and modulated the diffusion of molecules as a function of collagen nanoscale structure. Smooth muscle cells cultured on engineered sheets oriented in the direction of aligned collagen fibrils and generated coordinated vasomotor responses. The described biofabrication approach enables rapid formation of ultrathin collagen sheets that withstand physiologically relevant loads for applications in tissue engineering and regenerative medicine, as well as in organ-on-chip and biohybrid devices.

Universal Design Platform for an Extended Class of Photonic Dirac Cones

Band engineering near Dirac points is an emerging topic in condensed matter physics and photonics, enabling multifaceted devices of record-high conductivity and zero refractive index in both electronic and photonic structures. Recently, an extended class of Dirac cone, type I, II, or III, has attracted much attention with its controlled directionality of singular wave behaviors near the Dirac points. However, despite the significance of the discovery and applications of each type of Dirac cone, the theoretical framework and unified design structure bridging different types of Dirac cones have not been clearly elucidated. Here, we propose a universal design platform for all types of photonic Dirac cones and reveal the key parameter determining their types and transformations. Starting from the accidentally induced photonic Dirac cone, we show that the inverse design of an optical potential landscape enables the deterministic control of the spectral ordering, which leads to designer construction of each type of Dirac cone. This approach will pave the way for the design of artificial degeneracies with tunable anisotropy for general wave systems.

Structural, magnetic, redox and theoretical characterization of seven-coordinate first-row transition metal complexes with a macrocyclic ligand containing two benzimidazolyl N-pendant arms.

A structurally new heptadentate derivative of a 15-membered pyridine-based macrocycle containing two benzimidazol-2-yl-methyl N-pendant arms (L = 3,12-bis((1H-benzimidazol-2-yl)methyl)-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),14,16-triene) was synthesized and its complexes with the general formula [M(L)](ClO4)2·1.5CH3NO2 (M = MnII (1), FeII (2), CoII (3) and NiII (4)) were thoroughly investigated. X-ray crystal structures confirmed that all complexes are seven-coordinate with axially compressed pentagonal bipyramidal geometry having the largest distortion for NiII complex 4. FeII, CoII and NiII complexes 2, 3 and 4 show rather large magnetic anisotropy manifested by moderate to high obtained values of the axial zero-field splitting parameter D (7.9, 40.3, and -17.2 cm-1, respectively). Magneto-structural correlation of the FeII, CoII and NiII complexes with L and with previously studied structurally similar ligands revealed a significant impact of the functional group in pendant arms on the magnetic anisotropy especially that of the CoII and NiII complexes and some recommendations concerning the ligand-field design important for anisotropy tuning in future. Furthermore, complex 3 showed field-induced single-molecule magnet behavior described with the Raman (C = 507 K-n s-1 for n = 2.58) relaxation process. The magnetic properties of the studied complexes were supported by theoretical calculations, which very well correspond with the experimental data of magnetic anisotropy. Electrochemical measurements revealed high positive redox potentials for M3+/2+ couples and high negative redox potentials for M2+/+ couples, which indicate the stabilization of the oxidation state +ii expected for the σ-donor/π-acceptor ability of benzimidazole functional groups.

Tunable In-Plane Anisotropy in Amorphous Sm–Co Films Grown on (011)-Oriented Single-Crystal Substrates

Abstract Amorphous Sm–Co films with uniaxial in-plane anisotropy have great potential for application in information-storage media and spintronic materials. The most effective method to produce uniaxial in-plane anisotropy is to apply an in-plane magnetic field during deposition. However, this method inevitably requires more complex equipment. Here, we report a new way to produce uniaxial in-plane anisotropy by growing amorphous Sm–Co films onto (011)-cut single-crystal substrates in the absence of an external magnetic field. The tunable anisotropy constant, kA, is demonstrated with variation in the lattice parameter of the substrates. A kA value as high as about 3.3 × 104 J·m−3 was obtained in the amorphous Sm–Co film grown on a LaAlO3(011) substrate. Detailed analysis indicated that the preferential seeding and growth of ferromagnetic (FM) domains caused by the anisotropic strain of the substrates, along with the formed Sm–Co, Co–Co directional pair ordering, exert a substantial effect. This work provides a new way to obtain in-plane anisotropy in amorphous Sm–Co films.

Mechanical characteristics of tunable uniaxial aligned carbon nanotubes induced by robotic extrusion technique for hydrogel nanocomposite

The utilisation of additive manufacturing technique for producing soft hydrogel nanocomposites could enable extraordinary advances in controlling of the material’s structure at microscale. A soft gel-printing based on 2-hydroxyethyl methacrylate (HEMA)-based gelation system with embedded uniaxial aligned carbon nanotubes (CNTs), was proposed in this paper. Shear-induced alignment determines the uniform orientation of CNTs during the gel-printing process, thereby introducing tunable mechanical anisotropy into the printed nanocomposite products. Herein, mechanical characterisation in terms of uniaxial tension, dynamic cyclic tension, and numerical simulations have been implemented on the printed samples. The numerical modelling findings, verified by the experimental observations, confidently confirm that the orientation of CNTs significantly impacts the mechanical properties of the nanocomposite, i.e. the ultimate strength, degree of viscoelasticity and strain-rate-sensitivity. Both the gel-printing technique and the anisotropic PHEMA-CNTs nanocomposite will ultimately assist to the rapid manufacture of diverse applications such as flexible electronics or tissue engineering.

Controlling the domain structure of ferroelectric nanoparticles using tunable shells

The possibility of controlling the domain structure in spherical nanoparticles of uniaxial and multiaxial ferroelectrics using a shell with tunable dielectric properties is studied in the framework of Landau-Ginzburg-Devonshire theory. Finite element modeling and analytical calculations are performed for Sn2P2S6 and BaTiO3 nanoparticles covered with polymer, temperature dependent isotropic paraelectric strontium titanate, or anisotropic liquid crystal shells with a strongly temperature dependent dielectric permittivity tensor. It appeared that the “tunable” paraelectric shell with a temperature dependent high dielectric permittivity (∼300 – 3000) provides much more efficient screening of the nanoparticle polarization than the polymer shell with a much smaller (∼10) temperature-independent permittivity. The tunable dielectric anisotropy of the liquid crystal shell (∼ 1 – 100) adds a new level of functionality for the control of ferroelectric domains morphology (including a single-domain state, domain stripes and cylinders, meandering and labyrinthine domains, and polarization flux-closure domains and vortexes) in comparison with isotropic paraelectric and polymer shells. The obtained results indicate the opportunities to control the domain structure morphology of ferroelectric nanoparticles covered with tunable shells, which can lead to the generation of new ferroelectric memory and advanced cryptographic materials.

Metallic stacked origami cellular materials: Additive manufacturing, properties, and modelling

Stacking layers of Miura-folded origami sheets has been shown to create a cellular material with attractive properties. It has a rich array of geometric parameters that allow its elastic properties and compressive collapse response to be varied over a wide range, at fixed relative density. It displays tunable anisotropy and auxetic behaviour, and can also be easily conformed to curved surfaces. Because of these attributes, metallic stacked origami materials have shown promise for applications such as impact energy absorption and blast mitigation. However, to date, manufacturability challenges have yet to be adequately solved. Sensitivity to dimensional tolerance, and challenges of layer interfaces, make the material difficult to manufacture via sheet metal forming routes. However, additive manufacturing (AM) techniques offer a route to overcoming these challenges. In this investigation, we use a selective laser melting (SLM) process to allow metallic stacked origami materials to be assessed experimentally for the first time. The as-manufactured characteristics of these cellular materials were investigated via X-ray tomography and microstructural inspection. For comparison, tensile coupons were also fabricated in different orientations with respect to the build plane, in order to investigate material anisotropy resulting from the AM process. Scaling effects, both in terms of manufacturability and mechanical response, were investigated with cellular specimens fabricated with different cell sizes and cell wall thicknesses. The origami specimens were tested under quasi-static compression to provide an assessment of their as-manufactured energy absorbing properties. Finally, a finite element analysis is carried out, in order to assess its predictive accuracy and sensitivity to key modelling decisions.

Strong and Tunable Electrical Anisotropy in Type-II Weyl Semimetal Candidate WP2 with Broken Inversion Symmetry.

A transition metal diphosphide, WP2 , is a candidate for type-II Weyl semimetals (WSMs) in which spatial inversion symmetry is broken and Lorentz invariance is violated. As one of the prerequisites for the presence of the WSM state in WP2 , spatial inversion symmetry breaking in this compound has rarely been investigated. Furthermore, the anisotropy of the WP2 electrical properties and whether its electrical anisotropy can be tuned remain elusive. Angle-resolved polarized Raman spectroscopy, electrical transport, optical spectroscopy, and first-principle studies of WP2 are reported. The energies of the observed Raman-active phonons and the angle dependences of the detected phonon intensities are consistent with results obtained by first-principle calculations and analysis of the proposed crystal symmetry without spatial inversion, showing that spatial inversion symmetry is broken in WP2 . Moreover, the measured ratio (Rc /Ra ) between the crystalline c-axis and a-axis electrical resistivities exhibits a weak dependence on temperature (T) in the temperature range from 100 to 250 K, but increases abruptly at T ≤ 100 K, and then reaches the value of ≈8.0 at T = 10 K, which is by far the strongest in-plane electrical resistivity anisotropy among the reported type-II WSM candidates with comparable carrier concentrations. Optical spectroscopy study, together with the first-principle calculations on the electronic band structure, reveals that the abrupt enhancement of the electrical resistivity anisotropy at T ≤ 100 K mainly arises from a sharp increase in the scattering rate anisotropy at low temperatures. More interestingly, the Rc /Ra of WP2 at T = 10 K can be tuned from 8.0 to 10.6 as the magnetic field increases from 0 to 9 T. The so-far-strongest and magnetic-field-tunable electrical resistivity anisotropy found in WP2 can serve as a degree of freedom for tuning the electrical properties of type-II WSMs, which paves the way for the development of novel electronic applications based on type-II WSMs.

Spin Pumping in Asymmetric Fe50Pt50/Cu/Fe20Ni80 Trilayer Structure

Herein, spin transport dynamics across asymmetric Fe50Pt50/Cu/Fe20Ni80 soft‐magnetic trilayer structure is reported and thereby modulation of magnetic parameters including damping and effective field is determined by means of the angular dependence of broadband ferromagnetic resonance measurements. At distinct precession of individual magnetic layer, spin‐pumping is found to be prevalent with expected linewidth increase. Mutual precession for wide range of resonance configuration reveals a collective reduction in anisotropy field of around 200 mT for both Fe50Pt50 and Fe20Ni80 systems. Subsequent observation of no‐excess interface damping shows the possible control of spin‐pumping effect by tuning the net flow of spin‐current in a multilayer structure. These experimental findings have significance for microwave devices that require tunable anisotropy field in magnetic multilayers.

Tunable Mechanical Anisotropy, Crack Guiding, and Toughness Enhancement in Two‐Stage Reactive Polymer Networks

Using simple and inexpensive processing methodologies afforded by two‐stage reactive polymer networks (TSRPs) tunable mechanical anisotropy is displayed, defect‐independent guiding of cohesive fracture paths through soft material is demonstrated for the first time, and bio‐inspired microstructures are shown to enable performance enhancement beyond what is anticipated by the rule‐of‐mixtures in composites. The ability to pattern rubbery (stage I) and glassy (stage II) domains within a TSRP using photomasks and UV light is investigated through atomic force microscope (AFM) nanomechanical mapping techniques. AFM modulus mapping shows that the resulting stiffness anisotropy between stage I and stage II regions is length scale dependent. A gradient interface in elastic modulus between stage I and stage II materials is observed and, when patterned with an angled stage I pathway, the gradient interface exhibits remarkable resilience during failure, repeatedly deflecting cracks away from stage II regions, even while turning cracks at angles up to 135° When stage I and stage II domains are patterned in a nacre‐inspired microstructure, toughening beyond rule‐of‐mixtures’ prediction is observed.

Regulating wrinkling patterns by periodic surface stiffness in film-substrate structures

Surface wrinkling of materials holds promise for important applications in diverse fields such as multifunctional surfaces and biomedical engineering. For these applications, it is of interest to attain various surface wrinkles with tunable wavelengths and amplitudes. Through a combination of experiments and numerical simulations, we here propose a method to regulate the wrinkling patterns in a film-substrate system by introducing periodic surface stiffness, which is generated through sequential specified ultraviolet-ozone (UVO) treatments. Both experiments and numerical simulations demonstrate that the proposed technique can produce various patterns with wide, tunable geometrical features and anisotropy. The effects of surface stiffness distribution, the exposure durations of UVO-treatments, and the loading biaxiality are examined on the generated surface patterns.

Near-Field Thermal Radiation of Nanopatterned Black Phosphorene Mediated by Topological Transitions of Phosphorene Plasmons

ABSTRACTEmerging black phosphorene (BP) has unique advantages in mediating near-field thermal radiation due to its strong and tunable in-plane anisotropy, but relative researches are rarely reported in stark contrast to its gained tremendous attention in other fields. Here, we investigate near-field thermal radiation of nanopatterned BP considering different ways of patterning and electronic doping. Appropriate doping increases free carrier density, enabling the transition of BP from dielectrics to hyperbolic materials and the excitation of plasmon resonances. Nanopatterned BP is found to possess a higher radiative transfer rate by as high as 65% compared with plane counterparts due to topological transition of phosphorene ribbon plasmons from quasi-ellipses to quasi-hyperbolas. This work opens alternative routes to mediate and enhance near-field thermal radiation, which has promising applications in efficient thermal management and energy conversion.