Intrinsic Anisotropy Research Articles

Proximity-Coupling-Induced Significant Enhancement of Coercive Field and Curie Temperature in 2D van der Waals Heterostructures.

Magnetism in 2D has long been the focus of condensed matter physics due to its important applications in spintronic devices. A particularly promising aspect of 2D magnetism is the ability to fabricate 2D heterostructures with engineered optical, electrical, and quantum properties. Recently, the discovery of intrinsic ferromagnetisms in atomic thick materials has provided a new platform for investigations of fundamental magnetic physics. In contrast to 2D CrI3 and Cr2 Ge2 Te6 insulators, itinerant ferromagnetic Fe3 GeTe2 (FGT), which has a larger intrinsic perpendicular anisotropy, higher Curie temperature (TC ), and relatively better stability, is a promising candidate for achieving permanent room-temperature ferromagnetism through interface or component engineering. Here, it is shown that the ferromagnetic properties of FGT thin flakes can be modulated through coupling with a FePS3 . The magneto-optical Kerr effect results show that the TC of FGT is improved by more than 30 K and that the coercive field is increased by ≈100% due to the proximity coupling effect, which changes the spin textures of FGT at the interface. This work reveals that antiferromagnet/ferromagnet coupling is a promising way to engineer the magnetic properties of itinerant 2D ferromagnets, which paves the way for applications in advanced magnetic spintronic and memory devices.

Mechanical properties of kirigami phosphorene via molecular dynamics simulation

The newly discovered two-dimensional phosphorene suffers low stretchability which limits its application in flexible devices. Herein we employ kirigami technique to overcome this limitation. Molecular dynamics simulation is employed to investigate the mechanical properties of kirigami-phosphorene under shear and tensile loadings. Our simulation results show that loading type, intrinsic structural anisotropy, and the height of middle cuts are three key factors that govern the mechanical response of kirigami-phosphorene. Under the tensile loading along the armchair direction, phosphorene exhibits a considerable increase in its tensile strain. By contrast, phosphorene is too weak to stand any structural modification induced by kirigami in the zigzag direction. Under shear loading, there is merely no improvement in the shear properties of kirigami-phosphorene. Our results demonstrate the prospective applications of kirigami-phosphorene along the armchair direction in modern wearable, and stretchable electronics and optoelectronics devices.

Microstructured Multilayered Surface-Acoustic-Wave Device for Multifunctional Sensing

A multifunctional sensor based on a surface-acoustic-wave (SAW) device is fabricated. It allows independent measurements of temperature and applied magnetic field. Optimization of the multilayered device structure leads to a temperature-coefficient frequency of the Love-wave resonance reduced to zero. The sensitivity to an applied magnetic field is obtained through magnetostriction of a $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$ layer. By use of shape anisotropy, the variation of intrinsic magnetic anisotropy with temperature is strongly reduced. On one hand, interrogating the device at the Love-wave-resonance frequency allows us to extract the applied magnetic field independently of the temperature in a [130--370 K] range. On the other hand, the Rayleigh or Leaky waves are less or not sensitive to applied fields but have a high temperature coefficient of frequency. So interrogating the device at the Rayleigh resonance frequency allows us to extract the temperature independently of the magnetic field. In addition, the used resonator geometry offers the possibility for future batteryless and wireless interrogation.

Articaine interaction with phospholipid bilayers

Local anesthetics promote analgesia by interacting with excitable membranes. Articaine (ATC) has a unique composition among local anesthetics as it possesses a thiophene instead of the typical phenyl ring. Aiming to characterize the interaction of neutral articaine (nATC) with phospholipid membranes, we have employed a synergistic approach of experimental and computational techniques. Fluorescence measurements supported nATC partitioning into the membranes, since its intrinsic fluorescence anisotropy increased from 0.03 in water to 0.29 in the presence of egg phosphatidylcholine (EPC) liposomes, and the fluorescence of AHBA, a probe that monitors the water-membrane interface, was quenched by nATC. 1H NMR experiments revealed changes in the chemical shifts of articaine and EPC hydrogens after partitioning, and shorter T1 values of nATC hydrogens when inserted into the EPC vesicles. Contacts of nATC and the phospholipid polar head group were inferred from 2D-NOE. Taken together, these results indicate a superficial insertion of the nATC molecules inside EPC bilayers. This conclusion was confirmed by molecular dynamics simulations, which allowed the identification of the key interactions underlying the preferential location of nATC in the bilayer. Contrary to what is often stated (that articaine is a high lipophilic local anesthetic agent) our results place ATC among the hydrophilic ones, such as lidocaine, prilocaine, and mepivacaine, for which the water/membrane interface is the preferred location.

Generalized Phase Sensitivity of Directional Bond Breaking in the Laser-Molecule Interaction.

We establish a generalized picture of the phase sensitivity of laser-induced directional bond breaking using the H_{2} molecule as the example. We show that the well-known proton ejection anisotropy measured with few-cycle pulses as a function of their carrier-envelope phases arises as an amplitude modulation of an intrinsic anisotropy that is sensitive to the laser phase at the ionization time and determined by the molecule's electronic structure. Our work furthermore reveals a strong electron-proton correlation that may open up a new approach to experimentally accessing the laser-sub-cycle intramolecular electron dynamics also in larger molecules.

Voltage- and temperature-dependent rare-earth dopant contribution to the interfacial magnetic anisotropy

The control of magnetic materials and devices by voltages without electric currents holds the promise of power-saving nano-scale devices. Here we study the temperature-dependent voltage control of the magnetic anisotropy caused by rare-earth (RE) local moments at an interface between a magnetic metal and a non-magnetic insulator, such as Co|(RE)|MgO. Based on a Stevens operator representation of crystal and applied field effects, we find large dominantly quadrupolar intrinsic and field-induced interface anisotropies at room temperature. We suggest improved functionalities of transition metal tunnel junctions by dusting their interfaces with rare earths.

Dynamic crack modeling and analytical stress field analysis in single-crystal silicon using quantitative fractography

Single-crystal silicon is the fundamental building block enabling today’s plethora of integrated electronic components. However, complex mechanical stresses, originating from either direct mechanical loading or thermal cycling, can result in fracture of the constituent silicon, one of the leading causes of semiconductor device failure. Although phenomenological relationships to estimate the fracture strength in silicon have been proposed in the past, no quantitative fractographic method addressing the intrinsic anisotropy of crystals exists. In this work, a fractographic approach using optical confocal microscopy and atomic force microscopy is developed to identify and analyze the cleavage planes associated with dynamic instabilities in single-crystal silicon. We analytically determined the dynamic crack propagation behavior and asymptotic stress field at the crack-tip for unstable, anisotropic, circular cracks in silicon. The fractographic features predicted by the analytical model are consistent with experimental observations, and correctly predict how the {111} planes define the fractographic mirror region (including mirror constant), as well as the {112} planes associated with fractographic Wallner lines. These findings have important consequences in reducing mechanical and thermomechanical failure in semiconductor devices, where the mechanical strength is highly dependent on crystallographic anisotropy.

Enhancement of the Seebeck coefficient in ferrofluid based thermoelectric materials: A numerical study

It has been demonstrated that the thermocells, which contain ionically stabilized magnetic nanoparticles (ferrofluids) dispersed in aqueous electrolytes, show enhanced Seebeck coefficient. We have addressed this issue by deriving an analytic expression that relates the Seebeck coefficient to the chemical potential and its temperature derivative taking into account the nanoparticle’s intrinsic characteristics, i.e. magnetization and magnetic anisotropy. Mesoscopic scale modelling with the implementation of the Monte Carlo Metropolis algorithm has been performed on diluted assemblies of γ-Fe 2 O 3 and CoFe 2 O 4 nanoparticles – commonly used ferrofluid materials with different magnetic anisotropies and particle concentrations - to calculate the particle Seebeck coefficient at different temperatures. Importantly, our numerical results show enhancement of the Seebeck coefficient with the increase of the magnetic particle anisotropy and particle concentrations with a maximum that shifts to higher temperatures as the anisotropy increases. Our findings suggest that by tailoring the magnetic particle anisotropy, enhanced thermoelectric signal can be achieved in ferrofluid based thermoelectric materials.

Disorder effect on mass-asymmetry in monolayer black phosphorus

In this work, we calculated the band structure for the monolayer black phosphorus (phosphorene) from the slater-Koster tight-binding method, where the effect of s-p mixing is considered. Using the new parameters, we studied the electronic properties of phosphorene. The electronic properties were calculated using the recursive Green’s Function method. From the resistance calculation, we showed that electrons in the ZZ directions had an effective mass ten times larger than those in the AC direction, which agrees with experimental results. We investigated the effect of disorder on the anisotropy of the effective mass and we showed that the intrinsic anisotropy is robust to the presence of disorder.

Intrinsic microstructural anisotropy in tape cast ceramics by ice templating

AbstractIce templating concepts based on tape cast processing technology are capable of creating continuous columnar‐graded pore structures using a wide variety of ceramic oxides with thicknesses from 100 µm up to 1 cm for applications including fuel cells, compositional grading templates, filtration/separation membranes, and catalyst supports. The merging of the tape casting process allows for the preparation of large area flexible green tapes as well as the long‐range alignment of the acicular ice crystals transverse to the cross‐section, yielding an additional degree of microstructure control beyond traditional freeze cast processing. Moreover, optimization of the freeze tape casting processing parameters has proven effective in tailoring porosity in aqueous cast ceramics. Magnetic resonance microscopy (MRM) combined with scanning electron microscopy (SEM) has revealed inherent microstructural anisotropy in the morphology of pores related to the direction of the cast tape. Due to the ability of MRM to image noninvasively with volume averaging over a range of slice thicknesses, the MRM images provide complementary information to the higher resolution single plane SEM images. Morphological variance as observed at different orientations through the cross‐sections of cast yttria stabilized zirconia is reported.

Mechanical detwinning device for anisotropic resistivity measurements in samples requiring dismounting for particle irradiation.

Uniaxial stress is used to detwin the samples of orthorhombic iron based superconductors to study their intrinsic electronic anisotropy. Here, we describe the development of a new detwinning setup enabling variable-load stress-detwinning with easy sample mounting/dismounting without the need to re-solder the contacts. It enables the systematic study of the anisotropy evolution as a function of an external parameter when the sample is modified between the measurements. In our case, the external parameter is the dose of 2.5 MeV electron irradiation at low temperature. We illustrate the approach by studying resistivity anisotropy in single crystals of Ba1-xKxFe2As2 at x = 0.25, where the much discussed unusual re-entrance of the tetragonal C4 phase, C4 → C2 → C4, is observed on cooling. With the described technique, we found a significant anisotropy increase in the C2 phase after electron irradiation with a dose of 2.35 C/cm2.

First-Principles Prediction of Enhanced Magnetic Anisotropy of α″-Phase Fe₁₆N₂ With B and C Impurities

Intrinsic magnetic properties and uniaxial magnetic anisotropy (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> ) of α"-phase ordered Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloys have been investigated through first-principles calculations. Being in agreement with experimental results, the lattice parameters, magnetic moments, and K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> of α"-Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> have been determined. Small addition of B or C dopant atoms enhances tetragonal distortion and K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> up to 0.75 MJ·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for both Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.75</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.75</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> compounds, which is more than 20% larger compared with that (0.6 MJ·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) of α"-Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . Furthermore, the presence of B and C reduces magnetic moments of its neighboring Fe atoms at the 4e and 8h sites, resulting in reduced magnetization, due to less electron transfer from Fe to B and C than N. The underlying mechanism for the enhancement in K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> is discussed in connection with the Jahn-Teller lattice distortion, atom-decomposed magnetocrystalline anisotropy, and orbital magnetism.

Tuning the Intrinsic Anisotropy with Disorder in the CaKFe4As4 Superconductor

We report on the anisotropy of the London penetration depth of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$, discussing how it relates to its electronic structure and how it modifies under introduction of disorder, both chemically induced (by $\mathrm{Ni}$ substitution) and irradiation induced (by 3.5-MeV protons). Indeed, ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$ is particularly suitable for the study of fundamental superconducting properties due to its stoichiometric composition, exhibiting clean-limit behavior in the pristine samples and having a fairly high critical temperature, ${T}_{c}\ensuremath{\approx}35$ K. The London penetration depth ${\ensuremath{\lambda}}_{L}$ is measured with a microwave-coplanar-resonator technique that allows us to deconvolve the anisotropic contributions ${\ensuremath{\lambda}}_{L,ab}$ and ${\ensuremath{\lambda}}_{L,c}$ and obtain the anisotropy parameter ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}={\ensuremath{\lambda}}_{L,c}/{\ensuremath{\lambda}}_{L,ab}$. The ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(T)$ found for the undoped pristine sample is in good agreement with previous literature and is here compared to ab initio density-functional-theory and Eliashberg calculations. The dependence of ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(T)$ on both chemical and irradiation-induced disorder is discussed to highlight which method is more suitable to decrease the direction dependence of the electromagnetic properties while maintaining a high critical temperature. Lastly, the relevance of an intrinsic anisotropy such as ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}$ on application-related anisotropic parameters (critical current, pinning) is discussed in light of the recent employment of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$ in the production of wires.

Imaging the kinetics of anisotropic dissolution of bimetallic core\u2013shell nanocubes using graphene liquid cells

Chemical design of multicomponent nanocrystals requires atomic-level understanding of reaction kinetics. Here, we apply single-particle imaging coupled with atomistic simulation to study reaction pathways and rates of Pd@Au and Cu@Au core-shell nanocubes undergoing oxidative dissolution. Quantitative analysis of etching kinetics using in situ transmission electron microscopy (TEM) imaging reveals that the dissolution mechanism changes from predominantly edge-selective to layer-by-layer removal of Au atoms as the reaction progresses. Dissolution of the Au shell slows down when both metals are exposed, which we attribute to galvanic corrosion protection. Morphological transformations are determined by intrinsic anisotropy due to coordination-number-dependent atom removal rates and extrinsic anisotropy induced by the graphene window. Our work demonstrates that bimetallic core-shell nanocrystals are excellent probes for the local physicochemical conditions inside TEM liquid cells. Furthermore, single-particle TEM imaging and atomistic simulation of reaction trajectories can inform future design strategies for compositionally and architecturally sophisticated nanocrystals.

Formation of Fe(III)-phosphonate Coatings on Barium Hexaferrite Nanoplatelets for Porous Nanomagnets.

Amorphous coatings formed with mono-, di-, and tetra-phosphonic acids on barium hexaferrite (BHF) nanoplatelets using various synthesis conditions. The coatings, synthesized in water with di- or tetra-phosphonic acids, were thicker than that could be expected from the ligand size and the surface coverage, as determined by thermogravimetric analysis. Here, we propose a mechanism for coating formation based on direct evidence of the surface dissolution/precipitation of the BHF nanoplatelets. The partial dissolution of the nanoplatelets was observed with atomic-resolution scanning transmission electron microscopy, and the released Fe(III) ions were detected with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy in amorphous coating. The strong chemical interaction between the surface Fe(III) ions with phosphonic ligands induces the dissolution of BHF nanoplatelets and the consequent precipitation of the Fe(III)-phosphonates that assemble into a porous coating. The so-obtained porous nanomagnets are highly responsive to a very weak magnetic field (in the order of Earth’s magnetic field) at room temperature, which is a major advantage over the classic mesoporous nanomaterials and metal–organo-phosphonic frameworks with only a weak magnetic response at a few kelvins. The combination of porosity with the intrinsic magneto-crystalline anisotropy of BHF can be exploited, for example, as sorbents for heavy metals from contaminated water.

Electron spin resonance and ferromagnetic resonance spectroscopy in the high-field phase of the van der Waals magnetCrCl3

We report a comprehensive high-field/high-frequency electron spin resonance (ESR) study on single crystals of the van der Waals magnet CrCl$_3$. This material, although being known for quite a while, has received recent significant attention in a context of the use of van der Waals magnets in novel spintronic devices. Temperature-dependent measurements of the resonance fields were performed between 4 and 175 K and with the external magnetic field applied parallel and perpendicular to the honeycomb planes of the crystal structure. These investigations reveal that the resonance line shifts from the paramagnetic resonance position already at temperatures well above the transition into a magnetically ordered state. Thereby the existence of ferromagnetic short-range correlations above the transition is established and the intrinsically two-dimensional nature of the magnetism in the title compound is proven. To study details of the magnetic anisotropies in the field-induced effectively ferromagnetic state at low temperatures, frequency-dependent ferromagnetic resonance (FMR) measurements were conducted at 4 K. The observed anisotropy between the two magnetic-field orientations is analyzed by means of numerical simulations based on a phenomenological theory of FMR. These simulations are in excellent agreement with measured data if the shape anisotropy of the studied crystal is taken into account, while the magnetocrystalline anisotropy is found to be negligible in CrCl$_3$. The absence of a significant intrinsic anisotropy thus renders this material as a practically ideal isotropic Heisenberg magnet.

Effects of intrinsic anisotropy on the dynamic behaviour of undisturbed loess

To investigate the effects of the relative orientation between a sample's principal axes of anisotropy and the principal stress axes on the dynamic behaviour of undisturbed loess, samples with axial inclinations 30° and 90° relative to the horizontal were prepared. The effects of confining pressure and depth of soil also were examined to investigate variations in the dynamic behaviour of loess. The results showed an obvious effect of intrinsic anisotropy on the dynamic behaviour of loess. With all other conditions constant, the higher the confining pressure, the greater the dynamic stress to generate the same dynamic strain. The dynamic stresses of the 90° samples were always greater than those of the 30° samples. The effect of intrinsic anisotropy on the backbone curves was more obvious for the 2 m depth samples than for the 6 m depth samples. The damping ratio was within a certain range and initially increased as the dynamic strain increased and then gradually levelled off. The 2 m samples exhibited higher damping ratios than the 6 m samples. Finally it was found that the intrinsic anisotropy of loess might weaken the sample during the loading process and a confining pressure of 300 kPa was sufficient to nearly eliminate the effects of intrinsic anisotropy.

Layer-dependence of macroscopic and atomic magnetic correlations in Co/Pd multilayers

The development of multilayered materials with engineered magnetic properties compels a deep knowledge of physical properties at the atomic scale. The magnetic anisotropy is a key property in these materials. This work accounts for the magnetic anisotropy energy and its correlation with atomic properties of Co/Pd multilayers with the number of Co/Pd repetitions. Magnetometry measurements confirm stronger perpendicular magnetic anisotropy energies as the number of repetitions increases up to 40. However, the intrinsic anisotropy, related to the Co–Pd orbital hybridization and spin–orbit coupling, saturates at 15 repetitions. This finding is supported by x-ray magnetic circular dichroism analysis that reveals a direct correlation of the atomic Co and Pd orbital magnetic moments and the effective anisotropy of the system. The proximity effect that accounts for the Pd induced magnetization, along with the increasing Co moment, provides a suitable mechanism for the observed anisotropy energy layer dependence.

Electromagnetic instabilities of low-beta alpha/proton beams in space plasmas

Relative drifts between different species or particle populations are characteristic to solar plasma outflows, e.g., in the fast streams of the solar winds, coronal mass ejections and interplanetary shocks. This paper characterizes the dispersion and stability of the low-beta alpha/proton drifts in the absence of any intrinsic thermal anisotropies, which are usually invoked in order to stimulate various instabilities. The dispersion relations derived here describe the full spectrum of instabilities and their variations with the angle of propagation and plasma parameters. The results unveil a potential competition between instabilities of the electromagnetic proton cyclotron and alpha cyclotron modes. For conditions specific to a low-beta solar wind, e.g., at low heliocentric distances in the outer corona, the instability operates on the alpha cyclotron branch. The growth rates of the alpha cyclotron mode are systematically stimulated by the (parallel) plasma beta and/or the alpha-proton temperature ratio. One can therefore expect that this instability develops even in the absence of temperature anisotropies, with potential to contribute to a self-consistent regulation of the observed drift of alpha particles.

Multigrid preconditioners for anisotropic space-fractional diffusion equations

We focus on a two-dimensional time-space diffusion equation with fractional derivatives in space. The use of Crank-Nicolson in time and finite differences in space leads to dense Toeplitz-like linear systems. Multigrid strategies that exploit such structure are particularly effective when the fractional orders are both close to 2. We seek to investigate how structure-based multigrid approaches can be efficiently extended to the case where only one of the two fractional orders is close to 2, i.e., when the fractional equation shows an intrinsic anisotropy. Precisely, we design a multigrid (block-banded–banded-block) preconditioner whose grid transfer operator is obtained with a semi-coarsening technique and that has relaxed Jacobi as smoother. The Jacobi relaxation parameter is estimated by using an automatic symbol-based procedure. A further improvement in the robustness of the proposed multigrid method is attained using the V-cycle with semi-coarsening as smoother inside an outer full-coarsening. Several numerical results confirm that the resulting multigrid preconditioner is computationally effective and outperforms current state of the art techniques.