Rotational Anisotropy Research Articles

Reconfigurable spin-wave dynamics in two-dimensional quasiperiodic magnonic crystals

Active control of spin-wave properties is at the heart of on-chip reconfigurable magnonics, which promise development of energy efficient and miniaturized on-chip processing and communication devices in the GHz and sub-THz frequency range. Here, we demonstrated active control of spin-wave spectra in two-dimensional nanoscale antidot lattices arranged in shifted honeycomb and octagonal symmetries by varying the antidot diameter using broadband ferromagnetic resonance. Both these lattices exhibited stark variation in the spin-wave spectra with antidot diameter, besides the variation of the strength and orientation of bias magnetic field. With increasing diameter number of spin-wave modes gradually increased, and the position of mode splitting shifted to higher magnetic field due to ensuing increase in the demagnetizing field governing the spin-wave extension or localization in the lattices. The bias-field angle variation revealed higher-order rotational symmetry characteristic of quasi-periodic structure. Micromagnetic simulations qualitatively reproduced the experimental observations, and simulated spin-wave mode profiles revealed extended and standing spin-wave modes and their mutual conversion with bias-field angle in both lattices. The contrasting variation in the amplitude of rotational symmetry of spin-wave frequency with antidot diameter was interpreted by the variation of spin-wave mode profiles as well as the simulated internal magnetic field variation. Such efficient tunability of spin-wave properties with internal structure and external control parameters and higher-order rotational anisotropy in these atypical magnonic crystals will promote them for applications in on-chip reconfigurable magnonic devices.

Molecular simulation of adsorption thermodynamics and dynamics behavior of GOs at air-water interface

ABSTRACT The adsorption of two-dimensional (2-D) graphene oxide (GO) nanosheets at the air-water interface has broad technological implications for GO-based functional material preparations. Molecular-level understanding of the adsorption mechanism and interfacial behaviour is of great significance. Here, the adsorption free energy of GO nanosheets at the air-water interface was simulated, in which the effect of functional groups and deprotonation has been investigated. It was observed that the neutral GOs are generally thermodynamically favourable for the interface adsorption, while the deprotonated GO sheet has no such interface activity. Although the carboxyl group can enhance the interface adsorption minimum in the free energy profile, thereby improving the interfacial activity of the GO sheet, the oxidisation degree on GO is the main controlling factor determining the interface affinity. The interfacial morphology and dynamics of GO nanosheets have also been simulated, in which a parallel movement with a 2-D anisotropic rotation along the interface was revealed. Our simulation results provide new insights into the adsorption mechanism and behaviour of GO at the air-water interface.

Stimulated Nucleation of Skyrmions in a Centrosymmetric Magnet.

Understanding the dynamics of skyrmion nucleation and manipulation is important for applications in spintronic devices. In this contribution, the spin textures at magnetic domain-boundaries stimulated by application of in-plane magnetic fields in a centrosymmetric kagome ferromagnet, Fe3Sn2, with thickness gradient are investigated using Lorentz transmission electron microscopy. Switching of the in-plane magnetic field is shown to induce a reversible transformation from magnetic stripes to skyrmions, or vice versa, at the interface between differently oriented domains. Micromagnetic simulations combined with experiments reveal that the rotatable anisotropy and thickness dependence of the response to the external in-plane field are the critical factors for the skyrmion formation. In addition, it is shown that the helicity of skyrmions can also be controlled using this dynamic process. The results suggest that magnetic materials with rotatable anisotropy are potential skyrmionic systems and provides a different approach for nucleation and manipulation of skyrmions in spintronic devices.

Observation of strong and anisotropic nonlinear optical effects through polarization-resolved optical spectroscopy in the type-II Weyl semimetal Td−WTe2

The unique bulk crossing points in the band structure of topological Weyl semimetals have been shown to enhance nonlinear optical and optoelectronic properties, even for light at optical wavelengths. Such pronounced nonlinear optical effects have been studied in type-I Weyl semimetals, yet few studies have quantified these effects in type-II Weyl semimetals. We here present an optical experimental study with polarization resolution of two nonlinear optical effects in the type-II Weyl semimetal ${T}_{\mathrm{d}}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Te}}_{2}$. We begin by investigating the bulk optical second-harmonic response of this material using the rotational anisotropy of the second-harmonic generation. This technique allows us to simultaneously investigate the dependence of the second-harmonic response on the symmetry of the crystal and to quantify the size of that response, which we compare to other nonlinear crystals. We then use polarized time-resolved optical reflectivity spectroscopy to identify the nonlinear optical effect of impulsive stimulated Raman scattering as the origin of the coherent oscillations of the 0.25-THz shear mode. We find that the strength of this response in ${T}_{\mathrm{d}}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Te}}_{2}$ is enhanced compared with the observation of other modes excited through the displacive excitation of coherent phonons. Notably, both the second-harmonic response and the coherent excitations of the phonons demonstrate strong anisotropy, displaying a clear dependence on the polarization of the light used in the experiments. We use point-symmetry analyses of the material to guide our understanding of these observations and perform fluence-dependent measurements to investigate the electron-phonon coupling.

Dynamic configurational anisotropy in Ni80Fe20 antidot lattice with complex geometry

Efficient tunability of spin-wave dynamics has been demonstrated in Ni80Fe20 diamond-shaped antidot lattices (DADLs) arranged in square and hexagonal symmetry using all-optical time-resolved magneto-optical Kerr effect (TRMOKE) microscope. A stark variation in the spin-wave spectra is obtained with the strength and orientation of the in-plane bias magnetic field. The spin-wave modes in the square lattice exhibit four-fold rotational anisotropy, which is in contrast to a strong six-fold rotational anisotropy superposed with a weak four-fold anisotropy observed in the hexagonal lattice. Micromagnetic simulations qualitatively reproduce the experimentally observed spin-wave modes and the simulated mode profiles reveal the presence of diverse genres of extended, pseudo extended and quantized standing spin-wave modes in these lattices. Additionally, some lower frequency localized edge modes are obtained for some specific bias field orientations due to the presence of demagnetizing fields at the sharp corners of the diamond antidots. The strong modifications of the demagnetizing regions, as well as the internal field strengths around the diamond-shaped antidots, can explain the observed variation of the spin-wave dynamics. These findings are important for potential applications of the DADLs in future magnonic devices, leading towards the development of on-chip microwave logic and communication systems.

Rotation of a Charged Spin Probe in Room-Temperature Ionic Liquids.

X-band electron paramagnetic resonance spectroscopy has been used to investigate the rotational diffusion of a stable, positively charged nitroxide 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (Cat-1) in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons. The rotation of Cat-1 is anisotropic with the preferential axis of rotation along the NO• moiety. The Stokes-Einstein-Debye law describes the mean rotational correlation time of Cat-1, assuming that the hydrodynamic radius is smaller than the van der Waals radius of the probe. This implies that the probe rotates freely, experiencing slip boundary condition, which is solvent-dependent. The rotational correlation time of Cat-1 in RTILs can very well be fitted to a power-law functionality with a singular temperature, which suggests that the apparent activation energy of rotation exhibits non-Arrhenius behavior. Compared to the rotation of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO), which is neutral, the rotation of Cat-1 is several times slower. The rotational anisotropy, the ratio of the rotational times of pDTO and Cat-1, and the apparent activation energy indicate the transition from a homogeneously globular structure to a spongelike structure when the alkyl chain has four carbons, which is also observed in molecular dynamics computational studies. For the first time, we have been able to show that the rotational correlation time of a solute molecule can be analyzed in terms of the Cohen-Turnbull free volume theory. The Cohen-Turnbull theory fully describes the rotation of Cat-1 in all ionic liquids in the measured temperature range.

Structural and dynamical heterogeneity of water trapped inside Na+-pumping KR2 rhodopsin in the dark state.

Photoisomerization in the retinal leads to a channel opening in rhodopsins that triggers translocation or pumping of ions/protons. Crystal structures of rhodopsins contain several structurally conserved water molecules. It has been suggested that water plays an active role in facilitating the ion pumping/translocation process by acting as a lubricant in these systems. In this paper, we systematically investigate the localization, structure, dynamics, and energetics of the water molecules along the channel for the resting/dark state of KR2 rhodopsin. By employing several microseconds long atomistic molecular dynamics simulation of this trans-membrane protein system, we demonstrate the presence of five distinct water containing pockets/cavities separated by gateways controlled by protein side-chains. There exists a strong hydrogen bonded network involving these buried water molecules and functionally important key residues. We present evidence of significant structural and dynamical heterogeneity in the water molecules present in these cavities, with very rare exchange between them. The exchange time scale of such buried water with the bulk has an extremely wide range, from tens of nanoseconds to >1.5µs. The translational and rotational dynamics of buried water are found to be strongly dependent on the protein cavity size and local interactions with a classic signature of trapped diffusion and rotational anisotropy.

Interactions and Dynamics in Aqueous Solutions of pH-Responsive Polymers: A Combined Fluorescence and Dielectric Relaxation Study.

Interaction and dynamics of aqueous solutions of pH-responsive smart polymers are investigated via steady-state, time-resolved fluorescence emission spectroscopy with the help of external local reporter coumarin 153 (C153), while MHz to GHz dielectric relaxation spectroscopic (DRS) measurement reports the intrinsic medium relaxation features. A series of pH-responsive random copolymers (DPL-DP60) comprising of a pH-responsive moiety 2-((leucinyl)oxy)ethyl methacrylate (l-Leu-HEMA) and hydrophobic methyl methacrylate (MMA) are synthesized and characterized. A balance between the pH-responsive (l-Leu-HEMA) and the hydrophobic (MMA) content dictates the phase transition pH, which is found to be ∼5-7 for these aqueous copolymer solutions (1 mg/mL). Dynamic light scattering measurements in aqueous solutions of these polymers reflect a small particle size (∼2-8 nm) at solution pH below their individual phase transition pH, while a large particle size (∼140-340 nm) forms beyond their phase transition pH. No signature of a phase transition pH-driven abrupt change in static and dynamic properties of aqueous polymer solutions has been registered from pH-dependent dielectric relaxation as well as solute (C153)-centric fluorescence measurements. A significant impact of varying the l-Leu-HEMA/MMA segment ratio on steady-state fluorescence emission and rotational anisotropy decay of the fluorophore solute (C153) has been observed. MHz to GHz DRS in aqueous solutions of these pH-responsive polymers reflects bulk water-like dielectric features.

Non-volatile voltage control of in-plane and out-of-plane magnetization in polycrystalline Ni films on ferroelectric PMN–PT (001)pc substrates

We identify room-temperature converse magnetoelectric effects (CMEs) that are non-volatile by using a single-crystal substrate of PMN–PT (001)pc (pc denotes pseudocubic) to impart voltage-driven strain to a polycrystalline film of Ni. An appropriate magnetic-field history enhances the magnetoelectric coefficient to a near-record peak of ∼10−6 s m−1 and permits electrically driven magnetization reversal of substantial net magnetization. In zero magnetic field, electrically driven ferroelectric domain switching produces large changes of in-plane magnetization that are non-volatile. Microscopically, these changes are accompanied by the creation and destruction of magnetic stripe domains, implying the electrical control of perpendicular magnetic anisotropy. Moreover, the stripe direction can be rotated by a magnetic field or an electric field, the latter yielding the first example of electrically driven rotatable magnetic anisotropy. The observed CMEs are associated with repeatable ferroelectric domain switching that yields a memory effect. This memory effect is well known for PMN–PT (110)pc but not PMN–PT (001)pc. Given that close control of the applied field is not required as for PMN–PT (110)pc, this memory effect could lead the way to magnetoelectric memories based on PMN–PT (001)pc membranes that switch at low voltage.

Thickness-dependent magnetic properties of Ni65Fe28Ga7 films prepared by magnetron co-sputtering

In this paper, we investigated the structure and magnetic properties of Ni65Fe28Ga7 films with different thicknesses prepared by magnetron co-sputtering. We found the grain size is less than 8 nm for all films and increases with the thickness. For the thinner films with the thickness lower than the critical value, the effective saturated magnetization is about 8.4 kGs, while the saturated field and coercivity are, respectively, lower than 25 Oe and 15 Oe. The films exhibit good soft magnetic properties and the Gilbert damping increases almost linearly with the thickness. Above the critical thickness, the stripe domains are observed. As the thickness increases further, the stripe domains widen and the rotatable anisotropy strengthens.

Probing axionlike particles via cosmic microwave background polarization

Axion-like particles (ALPs) rotate the linear polarization of photons through the ALP-photon coupling and convert the cosmic microwave background (CMB) $E$-mode to the $B$-mode. We derive the relation between the ALP dynamics and the rotation angle by assuming that the ALP $\phi$ has a quadratic potential, $V=m^2\phi^2/2$. We compute the current and future sensitivities of CMB observations to the ALP-photon coupling $g$, which can reach $g=4\times 10^{-21}\,\mathrm{GeV}^{-1}$ for $10^{-32}\,\mathrm{eV}\lesssim m\lesssim 10^{-28}\,\mathrm{eV}$ and extensively exceed the other searches for any mass $m\lesssim 10^{-25}\,\mathrm{eV}$. We find that the fluctuation of the ALP field at the observer, which has been neglected in previous studies, can induce significant isotropic rotation of the CMB polarization. The measurements of isotropic and anisotropic rotation allow us to put bounds on relevant quantities such as the ALP mass $m$ and the ALP density parameter $\Omega_\phi$. In particular, if LiteBIRD detects anisotropic rotation, we obtain the lower bound on the tensor-to-scalar ratio as $r > 5 \times 10^{-9}$.

Exchange-bias and magnetic anisotropy fields in core\u2013shell ferrite nanoparticles

Exchange bias properties of MnFe_2O_4@gamma–Fe_2O_3 core–shell nanoparticles are investigated. The measured field and temperature dependencies of the magnetization point out a well-ordered ferrimagnetic core surrounded by a layer with spin glass-like arrangement. Quasi-static SQUID magnetization measurements are presented along with high-amplitude pulse ones and are cross-analyzed by comparison against ferromagnetic resonance experiments at 9 GHz. These measurements allow one to discern three types of magnetic anisotropies affecting the dynamics of the magnetic moment of the well-ordered ferrimagnetic NP’s core viz. the easy-axis (uniaxial) anisotropy, the unidirectional exchange-bias anisotropy and the rotatable anisotropy. The uniaxial anisotropy originates from the structural core–shell interface. The unidirectional exchange-bias anisotropy is associated with the spin-coupling at the ferrimagnetic/spin glass-like interface; it is observable only at low temperatures after a field-cooling process. The rotatable anisotropy is caused by partially-pinned spins at the core/shell interface; it manifests itself as an intrinsic field always parallel to the external applied magnetic field. The whole set of experimental results is interpreted in the framework of superparamagnetic theory, i.e., essentially taking into account the effect of thermal fluctuations on the magnetic moment of the particle core. In particular, it is found that the rotatable anisotropy of our system is of a uniaxial type.

Spatial dispersion contribution to second harmonic generation in inversion-symmetric materials

It is well known that second harmonic generation (SHG) from dipoles within the bulk vanishes in inversion-symmetric semiconductor materials as a consequence of parity symmetry. Hence SHG is then ascribed in the form of either surface dipole effects or bulk-related electric quadrupole or magnetic dipole effects. By incorporating the redefined spatial dispersion into the simplified bond-hyperpolarizability model, we show that the SHG spatial dispersion contribution for Si(001) and Si(111) facet orientations can be reformulated by a third-rank tensor containing one independent parameter, namely, the complex SHG spatial dispersion hyperpolarizability. Our results show that certain unexplained rotational anisotropy SHG intensity features for different incoming light wavelengths or optical penetration depths can be well reproduced only if the contribution from spatial dispersion is incorporated.

A Comparative Study on DMSO-Induced Modulation of the Structural and Dynamical Properties of Model Bilayer Membranes

Modulating the structures and properties of biomembranes via permeation of small amphiphilic molecules is immensely important, having diverse applications in cell biology, biotechnology, and pharmaceuticals, because their physiochemical and biological interactions lead to new pathways for transdermal drug delivery and administration. In this work, we have elucidated the role of dimethyl sulfoxide (DMSO), broadly used as a penetration-enhancing agent and cryoprotective agent on model lipid membranes, using a combination of fluorescence microscopy and time-resolved fluorescence spectroscopy. Spatially resolved fluorescence lifetime imaging microscopy (FLIM) has been employed to unravel how the fluidity of the DMSO-induced bilayer regulates the structural alteration of the vesicles. Moreover, we have also shown that the dehydration effect of DMSO leads to weakening of the hydrogen bond between lipid headgroups and water molecules and results in faster solvation dynamics as demonstrated by femtosecond time-resolved fluorescence spectroscopy. It has been gleaned that the water dynamics becomes faster because bilayer rigidity decreases in the presence of DMSO, which is also supported by time-resolved rotational anisotropy measurements. The enhanced diffusivity and increased membrane fluidity in the presence of DMSO are further ratified at the single-molecule level through fluorescence correlation spectroscopy (FCS) measurements. Our results indicate that while the presence of DMSO significantly affects the 1,2-dimyristoyl-rac-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-rac-glycero-3-phosphatidylcholine (DPPC) bilayers, it has a weak effect on 1,2-dimyristoyl-sn-glycero-3-phospho-rac-glycerol (DMPG) vesicles, which might explain the preferential interaction of DMSO with the positively charged choline group present in DMPC and DPPC vesicles. The experimental findings have also been further verified with molecular dynamics simulation studies. Moreover, it has been observed that DMSO is likely to have a differential effect on heterogeneous bilayer membranes depending on the structure and composition of their headgroups. Our results illuminate the importance of probing the lipid structure and composition of cellular membranes in determining the effects of cryoprotective agents.

Generation, reflection and transmission of nonlinear harmonic waves by direct superposition of anharmonic dipoles

It is well known that the linear and nonlinear light reflection and transmission formula can be obtained from the standard macroscopic Maxwell equation. In the past, a microscopic approach based on the Ewald–Oseen extinction theorem to derive the reflection and transmission formula by direct dipole summation has been performed by Fearn et al where they successfully derived the Fresnel formula in linear optics. In this work, we extend their work for the first time to derive the nonlinear reflection and transmission formula by direct summation of anharmonically oscillating dipoles for the case of a normal incidence wave, yielding similar results with coupled-mode-theory (CMT). We demonstrate for a decaying incoming field that dipolar radiation is no longer forbidden inside the bulk of a centrosymmetric material. Using the simplified bond hyperpolarizability model (SBHM), we find that these additional bulk dipole contribution must be accounted along with the surface dipoles and bulk quadrupolar contribution to explain recent rotational anisotropy SHG intensity experimental data of Si(111) with high precision.

Orbital rotations induced by charges of polarons and defects in doped vanadates

We explore the competiton of doped holes and defects that leads to the loss of orbital order in vanadate perovskites. In compounds such as La$_{1-{\sf x}}$Ca$_{\,\sf x}$VO$_3$ spin and orbital order result from super-exchange interactions described by an extended three-orbital degenerate Hubbard-Hund model for the vanadium $t_{2g}$ electrons. Long-range Coulomb potentials of charged Ca$^{2+}$ defects and $e$-$e$ interactions control the emergence of defect states inside the Mott gap. The quadrupolar components of the Coulomb fields of doped holes induce anisotropic orbital rotations of degenerate orbitals. These rotations modify the spin-orbital polaron clouds and compete with orbital rotations induced by defects. Both mechanisms lead to a mixing of orbitals, and cause the suppression of the asymmetry of kinetic energy in the $C$-type magnetic phase. We find that the gradual decline of orbital order with doping, a characteristic feature of the vanadates, however, has its origin not predominantly in the charge carriers, but in the off-diagonal couplings of orbital rotations induced by the charges of the doped ions.

Fractional derivative in the elastic-viscoplastic stress-strain state model describing anisotropic creep of soft clays

A fractional elastic-viscoplastic (F-EVP) creep model describing the anisotropy of soft soil is established. A nonlinear viscoplastic creep expression is obtained by a fractional derivative and the Laplace transformation, which is extended by the definition of the reference yield surface. On this basis, the anisotropy of soil is considered in the model by a rotating hardening criterion, and the effect of anisotropic parameters on the stress-dilatancy ratio is discussed. The scalar multiplier in the form of stress ratio is described, and a critical friction angle is used to unify the modeling parameter. The validity of the model is examined by the existing triaxial creep results. The results indicate that the rotational anisotropy induces coaxial rotation as the result of the stress-dilatation relationship. The fractional viscoplastic model can reasonably describe the plastic behavior of soil before yielding, and accurately characterize the creep behavior of soil. From the comparison between the test results and the model calculation results, it is found that the model calculation results are basically consistent with the test results.

The effects on CMB power spectra and bispectra from the polarization rotation and its correlations with temperature and E-polarization

The Chern-Simons term, through which the cosmic Axion-like field couples to the electromagnetic field, has the effect to rotate CMB polarization directions and to break the CPT symmetry. This rotation will change the CMB power spectra, no matter isotropic or anisotropic the rotation angle is. In this paper we revisit this issue by further considering the correlations between the anisotropic rotation angle α and the CMB temperature and CMB E polarization fields. These correlations could be generated in the Axion-like models with nonzero potential under the adiabatic initial condition. We first investigate how these correlations contribute further modifications to the CMB power spectra, then calculate the CMB bispectra for the temperature and rotated polarization fields. These bispectra would vanish if the Tα and Eα correlations are absent. So, they are useful in searching for CPT violation and the Tα and Eα correlations arisen in the Axion-like models.

Angular control of multi-mode resonance frequencies in obliquely deposited CoZr thin films with rotatable stripe domains* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51871117 and 51671099), the Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT-16R35), and the Gansu Provincial Science Foundation for Distinguished Young Scholars, China (Grant No. 20JR10RA649).

We investigate the angular-dependent multi-mode resonance frequencies in CoZr magnetic thin films with a rotatable stripe domain structure. A variable range of multi-mode resonance frequencies from 1.86 GHz to 4.80 GHz is achieved by pre-magnetizing the CoZr films along different azimuth directions, which can be ascribed to the competition between the uniaxial anisotropy caused by the oblique deposition and the rotatable anisotropy induced by the rotatable stripe domain. Furthermore, the regulating range of resonance frequency for the CoZr film can be adjusted by changing the oblique deposition angle. Our results might be beneficial for the applications of magnetic thin films in microwave devices.

Solid-state synthesis, rotatable magnetic anisotropy and characterization of Co1-xPtx phases in 50Pt/50fccCo(001) and 32Pt/68fccCo(001) thin films

We reported the phase formation sequences in 50Pt/50fcc-Co(001) and 32Pt/68fcc-Co(001) thin films after annealing up to 850 °C. In both cases, the ordered L10 phase formed first on the Pt/Co interface at ∼400 °C and as the annealing temperature increased the L10 phase transformed into the chemically disordered fcc A1 phase in 50Pt/50fcc-Co(001) at 750 °C and in 32Pt/68fcc-Co(001) films at 550 °C. Based on the analysis of solid-state reactions in thin films, a phase transition at ∼ 400 °C is predicted in Co-Pt systems with a 32–72% Pt composition. Torque measurements of the 50Pt/50fcc-Co(001) samples showed that the rotatable magnetic anisotropy coexisted with the three variants of L10 in a temperature range of 400–750 °C. An analysis of the torque curves revealed that the L10 films consist of a soft magnetic layer epitaxially intergrown to the substrate MgO(001) and a top layer having rotatable magnetic anisotropy. It showed that the magnetically hard properties of L10 films are associated with a rotatable magnetic anisotropy layer. A model of rotatable magnetic anisotropy is reasoned, which is founded on some identical mechanisms of rotatable magnetic anisotropy and magnetic-field-induced strains, explaining the ferromagnetic shape-memory effect in Heusler alloys. Our results suggested that the rotatable magnetic anisotropy phenomena may have an important role in the origin of perpendicular anisotropy in hard magnetic L10 structures.