Curved Domain Walls Research Articles

Semiclassical Propagation Along Curved Domain Walls

Semiclassical Propagation Along Curved Domain Walls

Extreme Domain Wall Speeds under Ultrafast Optical Excitation.

Time-resolved ultrafast EUV magnetic scattering was used to test a recent prediction of >10 km/s domain wall speeds by optically exciting a magnetic sample with a nanoscale labyrinthine domain pattern. Ultrafast distortion of the diffraction pattern was observed at markedly different timescales compared to the magnetization quenching. The diffraction pattern distortion shows a threshold dependence with laser fluence, not seen for magnetization quenching, consistent with a picture of domain wall motion with pinning sites. Supported by simulations, we show that a speed of ≈66 km/s for highly curved domain walls can explain the experimental data. While our data agree with the prediction of extreme, nonequilibrium wall speeds locally, it differs from the details of the theory, suggesting that additional mechanisms are required to fully understand these effects.

Dynamics of curved domain walls in hard ferromagnets with nonlinear dissipative and inertial effects

This article investigates the field-driven motion of curved domain walls in ferromagnetic nanostructures under the framework of modified Landau–Lifshitz–Gilbert equation with inertial effects. The considered governing equation involves the torques arising from nonlinear dissipative (viscous and dry friction) and inertial effects. We study the most relevant dynamical features in the steady dynamical regime for the considered model by employing the reductive perturbation technique. Finally, we illustrate the results numerically for the various domain wall surfaces (plane, cylinder, and sphere) and discuss their physical significance. The results obtained here agree with the recent theoretical and experimental observations.

Curved domain walls in the ferromagnetic nanostructures with Rashba and nonlinear dissipative effects

This work reveals an analytical investigation of the curved domain wall motion in ferromagnetic nanostructures in the framework of the extended Landau-Lifshitz-Gilbert equation. To be precise, the study delineates the description of curved domain wall motion in the steady-state dynamic regime for metallic and semiconductor ferromagnets. The study is done under the simultaneous action of the Rashba field and nonlinear dissipative effects described via the viscous-dry friction mechanism. By means of reductive perturbation technique and realistic assumption on the considered parameters, we establish an analytical expression of the steady domain wall velocity that depends on mean curvature of domain wall surfaces, nonlinear dissipation coefficients, Rashba parameter, external magnetic field, and spin-polarized electric current. In particular, it is observed that the domain wall velocity, mobility, threshold, and Walker breakdown can be manipulated by the combined mechanism of the Rashba field and nonlinear dissipation coefficients. Finally, the obtained analytical results are illustrated numerically for the curved domain walls through constant-curvature surfaces under-considered scenarios. The results presented herein are in qualitatively good agreement with the recent observations.

Supersymmetric black holes with spiky horizons

We use the recipe of [1] to find half-BPS near-horizon geometries in the t3 model of N = 2, D = 4 gauged supergravity, and explicitely construct some new examples. Among these are black holes with noncompact horizons, but also with spherical horizons that have conical singularities (spikes) at one of the two poles. A particular family of them is extended to the full black hole geometry. Applying a double-Wick rotation to the near-horizon region, we obtain solutions with NUT charge that asymptote to curved domain walls with AdS3 world volume. These new solutions may provide interesting testgrounds to address fundamental questions related to quantum gravity and holography.

Ferroelectric domain evolution in a Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 piezoceramic studied using piezoresponse force microscopy

Lead-free Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-0.5BCT) ceramics have drawn attention in recent years because of their outstanding dielectric and electromechanical properties, such as a piezoelectric coefficient d33 ∼ 620 pC N−1 and a large signal of d33* ∼ 1100 pm V−1 at 0.5 kV mm−1 at room temperature (RT). These particular properties are relevant to a range of applications. However, the structural origin of this high piezoelectric coefficient is still a subject of discussion. An in-depth understanding of the ferroelectric domain evolution of BZT-0.5BCT ceramics is crucial for probing the underlying mechanisms and for guiding practical applications. Using piezoresponse force microscopy (PFM), we have directly visualized the evolution of the BZT-0.5BCT domain structure using temperature and electric field stimulation on micrometer and nanometer scales. The PFM results unambiguously evidence the coexistence of wedge-shaped and lamellar domains with miniaturized nanodomain structures at RT. The temperature- and electric-field-dependent PFM study presented here highlights the critical role of wedge-shaped domains in domain evolution. Wedge-shaped domains turn into small domains with curved domain walls after the heating cycle and then become lamellar domains after the poling cycle at RT. Transitional domain structures with an increased density of nanodomains appear in both the thermal and poling cycles. More interestingly, the electric-field-dependent domain structure evolution at different temperatures shows better domain structure reversibility at high temperatures than at temperatures close to the phase boundary. This demonstrates that the BZT-0.5BCT ceramic has superior stability at medium temperatures (40 °C–60 °C), implying excellent stability for applications.

Supersymmetric solutions from N=6 gauged supergravity

We study supersymmetric solutions in four-dimensional $N=6$ gauged supergravity with $SO(6)$ gauge group. There is a unique $N=6$ supersymmetric $AdS_4$ vacuum with $SO(6)$ symmetry dual to an $N=6$ SCFT in three dimensions. We find a number of domain walls interpolating between this $AdS_4$ vacuum and singular geometries in the IR with $SO(2)\times SO(4)$, $U(3)$, $SO(3)$ and $SO(2)\times SO(2)\times SO(2)$ symmetries. The $SO(3)$ case admits $N=6$ or $N=2$ solutions depending on whether the pseudoscalars are present or not. On the other hand, all the remaining solutions preserve $N=6$ supersymmetry. These solutions describe RG flows from the $N=6$ SCFT to non-conformal field theories driven by mass deformations. In particular, the $SO(2)\times SO(4)$ solution is in agreement with the previously known mass deformations of the dual $N=6$ SCFT. We also give a supersymmetric Janus solution with $SO(2)\times SO(4)$ symmetry, describing two-dimensional conformal defects in the $N=6$ SCFT with unbroken $N=(4,2)$ supersymmetry. Finally, we find an $N=2$ supersymmetric $AdS_2\times H^2$ solution with $SO(2)\times SO(4)$ symmetry and the corresponding domain wall interpolating between this fixed point and the $AdS_4$ vacuum. The solution describes an $AdS_4$ black hole with a magnetic charge and is dual to a twisted compactification of the $N=6$ SCFT on a hyperbolic space $H^2$. We also give a domain wall interpolating between a locally supersymmetric $AdS_4$ and a curved domain wall with $SO(2)\times SO(2)\times SO(2)$ symmetry dual to an RG flow across dimensions from the $N=6$ SCFT to a supersymmetric quantum mechanics.

Reconstruction of moiré lattices in twisted transition metal dichalcogenide bilayers

An important step in understanding the exotic electronic, vibrational, and optical properties of the moir\'{e} lattices is the inclusion of the effects of structural relaxation of the un-relaxed moir\'{e} lattices. Here, we propose novel structures for twisted bilayer of transition metal dichalcogenides (TMDs). For $\theta\gtrsim 58.4^{\circ}$, we show a dramatic reconstruction of the moir\'{e} lattices, leading to a trimerization of the unfavorable stackings. We show that the development of curved domain walls due to the three-fold symmetry of the stacking energy landscape is responsible for such lattice reconstruction. Furthermore, we show that the lattice reconstruction notably changes the electronic band-structure. This includes the occurrence of flat bands near the edges of the conduction as well as valence bands, with the valence band maximum, in particular, corresponding to localized states enclosed by the trimer. We also find possibilities for other complicated, entropy stabilized, lattice reconstructed structures.

Prediction of tunable magnetoelectric properties in compositionally graded ferroelectric/ferromagnetic laminated nanocomposites

The advent of modern thin-film deposition approaches has ushered in a new era of designed materials with well-controlled composition distributions, e.g., compositionally graded ferroelectric (CGFE) thin films, and thereby, it is readily accessible CGFE/FM (ferromagnetic) multilayer thin films. Being recognized this emerging class of materials, in this study, we develop a phase-field model based on the Ginzburg–Landau theory that takes into account the gradient of ferroelectric (FE) compositions in order to predict material properties of CGFE/FM thin films. The developed phase-field model is applied to investigate the effect of the FE composition gradient on magnetoelectric (ME) coupling of graded Pb(1−x)SrxTiO3/CoFe2O4 laminated nanocomposites. Two types of composition gradients are considered: inward (O-type) and outward (X-type) gradients. Unusual polarization domain structures with curved domain walls are formed in CGFE layers, which are governed by the composition gradients and distinct from typical stripe domains in homogeneous counterparts. As a result, the ME effect is strongly dependent on the composition gradient. Particularly, the ME coupling in the O-type nanocomposites increases with the increasing composition gradient, while it decreases in the X-type ones. The dependence of ME coupling on the composition gradient originates from the distinguishable energy distributions in O-type and X-type nanocomposites. This work, therefore, provides a strategy to design the ME effect via the configuration of the composition gradient.

Phase-field simulation of superconductor vortex clustering in the vicinity of ferromagnetic domain bifurcations* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51972028 and 12004036) and the National Key Research and Development Program of China (Grant No. 2019YFA0307900).

Superconductors and ferromagnets are highly non-compatible materials due to the natures of their respective electronic states. But when artificially brought together, they develop interesting characteristics, one of which, vortex clustering, is discussed here in this paper. Phase-field and micromagnetic simulations are performed to investigate the superconductor and ferromagnet bilayer, respectively. The ferromagnet with uniaxial anisotropy is observed to develop the maze domain, whereas the superconductor subjected to the influence of the ferromagnetic stray field displays a vortex pattern. Clustered vortices in superconductors at certain locations are observed to be precisely located over magnetic domain bifurcations. The enhanced out-of-plane stray field at bifurcations around the curved domain walls and the convergent Lorentz force due to screening currents in superconductor are attributed to the formation of clusters at bifurcation sites. Segregation of the inter-vortex spacing between straight and bifurcated domain is clearly observed. More importantly, inter-vortex spacing is predicted to serve as a precise tool to map local ferromagnet domain shapes.

Chiral Magnonics: Reprogrammable Nanoscale Spin Wave Networks Based on Chiral Domain Walls.

SummarySpin waves offer promising perspectives as information carriers for future computational architectures beyond conventional complementary metal-oxide-semiconductor (CMOS) technology, owing to their benefits for device minimizations and low-ohmic losses. Although plenty of magnonic devices have been proposed previously, scalable nanoscale networks based on spin waves are still missing. Here, we demonstrate a reprogrammable two-dimensional spin wave network by combining the chiral exchange spin waves and chiral domain walls. The spin-wave network can be extended to two dimensions and offers unprecedented control of exchange spin waves. Each cell in the network can excite, transmit, and detect spin waves independently in the chiral domain wall, and spin-wave logics are also demonstrated. Our results open up perspectives for integrating spin waves into future logic and computing circuits and networks.

Twisted compactifications of 6D field theories from maximal 7D gauged supergravity

We study supersymmetric AdS_ntimes Sigma ^{7-n}, n=2,3,4,5 solutions in seven-dimensional maximal gauged supergravity with CSO(p,q,5-p-q) and CSO(p,q,4-p-q) gauge groups. These gauged supergravities are consistent truncations of eleven-dimensional supergravity and type IIB theory on H^{p,q}circ T^{5-p-q} and H^{p,q}circ T^{4-p-q}, respectively. Apart from recovering the previously known AdS_ntimes Sigma ^{7-n} solutions in SO(5) gauge group, we find novel classes of AdS_5times S^2, AdS_3times S^2times Sigma ^2 and AdS_3times CP^2 solutions in non-compact SO(3, 2) gauge group together with a class of AdS_3times CP^2 solutions in SO(4, 1) gauge group. In SO(5) gauge group, we extensively study holographic RG flow solutions interpolating from the SO(5) supersymmetric AdS_7 vacuum to the AdS_ntimes Sigma ^{7-n} fixed points and singular geometries in the form of curved domain walls with Mkw_{n-1}times Sigma ^{7-n} slices. In many cases, the singularities are physically acceptable and can be interpreted as non-conformal phases of (n-1)-dimensional SCFTs obtained from twisted compactifications of N=(2,0) SCFT in six dimensions. In SO(3, 2) and SO(4, 1) gauge groups, we give a large number of RG flows between the new AdS_ntimes Sigma ^{7-n} fixed points and curved domain walls while, in CSO(p,q,4-p-q) gauge group, RG flows interpolating between asymptotically locally flat domain walls and curved domain walls are given.

Higher dimensional curved domain walls on Kähler surfaces

In this paper we study some aspects of curved BPS-like domain walls in higher dimensional gravity theory coupled to scalars where the scalars span a complex Kähler surface with scalar potential turned on. Assuming that a fake superpotential has a special form which depends on Kähler potential and a holomorphic function, we prove that BPS-like equations have a local unique solution. Then, we analyze the vacuum structure of the theory including their stability using dynamical system and their existence in ultraviolet-infrared regions using renormalization group flow.

Photodrive of magnetic bubbles via magnetoelastic waves

Precise control of magnetic domain walls continues to be a central topic in the field of spintronics to boost infotech, logic, and memory applications. One way is to drive the domain wall by current in metals. In insulators, the incoherent flow of phonons and magnons induced by the temperature gradient can carry the spins, i.e., spin Seebeck effect, but the spatial and time dependence is difficult to control. Here, we report that coherent phonons hybridized with spin waves, magnetoelastic waves, can drive magnetic bubble domains, or curved domain walls, in an iron garnet, which are excited by ultrafast laser pulses at a nonabsorbing photon energy. These magnetoelastic waves were imaged by time-resolved Faraday microscopy, and the resultant spin transfer force was evaluated to be larger for domain walls with steeper curvature. This will pave a path for the rapid spatiotemporal control of magnetic textures in insulating magnets.

Non-SUSY fractional branes

We consider a simplified Ansatz for supergravity solutions describing fractional p-brane solutions (throat geometries supported by fluxes) for various p. For p = 3 the Ansatz captures the Klebanov-Tseytlin (KT) solution. The equations of motion can be derived from an effective action by performing a dimensional reduction to a flat domain wall geometry in p + 2 dimensions. We find an interesting deformation of the known superpotential defining the SUSY domain wall flow. The deformation parameter breaks supersymmetry but still preserves the property that a test Dp brane feels no force inside the throat. The new solutions come in two classes. Both classes have the same UV asymptotics as the KT solution and one class has also similar IR behavior, which makes them potentially interesting holographic backgrounds for studying cascading gauge theories with broken SUSY. We explain furthermore how the curved domain wall solutions of the same (p + 2)-dimensional theories are expected to lift to new AdS compactifications of type IIA/B supergravity.

Curved domain walls dynamics driven by magnetic field and electric current in hard ferromagnets

The propagation of curved domain walls in hard ferromagnetic materials is studied by applying a reductive perturbation method to the generalized Landau–Lifshitz–Gilbert equation. The extended model herein considered explicitly takes into account the effects of a spin-polarized current as well as those arising from a nonlinear dissipation.Under the assumption of steady regime of propagation, the domain wall velocity is derived as a function of the domain wall curvature, the nonlinear damping coefficient, the magnetic field and the electric current. Threshold and Walker-like breakdown conditions for the external sources are also determined. The analytical results are evaluated numerically for different domain wall surfaces (planes, cylinders and spheres) and their physical implications are discussed.

Micromagnetism in (001) magnetite by spin-polarized low-energy electron microscopy

Spin-polarized low-energy electron microscopy was used to image a magnetite crystal with (001) surface orientation. Sets of spin-dependent images of magnetic domain patterns observed in this surface were used to map the direction of the magnetization vector with high spatial and angular resolution. We find that domains are magnetized along the surface <110> directions, and domain wall structures include 90° and 180° walls. A type of unusually curved domain walls are interpreted as Néel-capped surface terminations of 180° Bloch walls.

Domain wall structure of weak ferromagnets according to Raman

Visualizing the domain structure and the fine structure of domain walls in orthoferrites based on Raman was proposed. The Raman mapping imaging was obtained for the straight and curved domain wall at line 221cm−1. The parameters of the domain structure and wall obtained by Raman are consistent with magnetooptical measurements.

Symmetry of domains in morphotropic PbZr1−xTixO3ceramics

PbZr${}_{1\ensuremath{-}x}$Ti${}_{x}$O${}_{3}$ (PZT) ceramic samples with compositions over the morphotropic phase boundary (MPB) ranging from 45/55 to 60/40 [$(1\ensuremath{-}x)/x$] are studied by transmission electron microscopy combined with convergent-beam electron diffraction (CBED). All point-group symmetries debated for morphotropic PZT, $4mm$, $m$, and $3m$, are identified. Up to PZT 54/46 the symmetry is mainly tetragonal within lamellar domain configurations. From PZT 52/48 to PZT 54/46 nanodomains evolve within the tetragonal microdomains and monoclinic symmetry is identified within the same domain configurations, together with tetragonal symmetry for PZT 54/46. For PZT 55/45 up to PZT 60/40 rhombohedral symmetry is observed. Rhombohedral domains, as identified by CBED, are characterized by their width, curved domain walls, and grainy inner contrast. This contrast, together with anisotropic first-order Laue-zone intensities, implies that local deviations from $3m$ symmetry may exist within rhombohedral domains. Between PZT 54/46 and PZT 55/45 the morphology of the microdomains changes abruptly. For PZT 54.5/45.5 both types of domain configurations coexist within the same grain. For PZT 56/44 a tetragonal inclusion within a rhombohedral matrix is identified by CBED. On the other side of the MPB, domains with a rhombohedral appearance are found in PZT 53.5/46.5. These observation were less frequent but show that coexistence is present in ceramic samples around the MPB.

Construction of curved domain walls

The evolution of the domain wall is reduced to the dynamics of the membrane. The rigorous construction of curved membranes in the real scalar field models with domain-wall solutions is presented. An example of the curved membrane is discussed in detail. The curved domain-wall solutions are constructed.