Types Of Domain Walls Research Articles

Lateral domain wall oscillations in IMA/PMA bilayered nano-strips driven by a perpendicular current: A type of domain wall based oscillators

Spin transfer torque-driven dynamic properties of spins in a bilayered nano-strip system composed of a layer with in-plane magnetic anisotropy (IMA) and a layer with perpendicular magnetic anisotropy (PMA) were studied using a micro-magnetic simulation method. It is demonstrated that a domain wall (DW) can nucleate in the IMA layer at the end of the nano-strips and propagate with a large speed toward the other end under a perpendicular current. With DW annihilation and nucleation at either end of the nano-strips, their back and forth motion induces a magnetization oscillation whose amplitude and frequency can be easily manipulated by the current density, the dimensions and material of the nano-strip, and a perpendicularly applied magnetic field. Considering that the oscillation amplitude can be greatly amplified by the simultaneous oscillations of a large number of nano-strips in an array and the simple structure of the each element, this kind of IMA/PMA bilayered nano-strip may have important applications in magnetic nano-oscillators.

Study of length of domain walls in cylindrical magnetic microwires

Dynamic property of elliptic and spiral domain structures has been studied in magnetic cylindrical microwires. The investigations have been performed using the magneto-optical Kerr-effect and the modified Sixtus-Tonks technique. The direct relation between the domain wall mobility and the domain wall length has been established for two types of domain walls. The domain wall length was controlled by the reversible change of the angle of domain wall inclination caused in turn by such type of the external parameters as axial-circular magnetic field and mechanical torsion stress.The domain wall mobility increases with domain wall length for elliptic and spiral domain walls. For the elliptic domain wall the dependence of the mobility on the domain wall length is linear.

Structural and thermal transport properties of ferroelectric domain walls in GeTe from first principles

Ferroelectric domain walls are boundaries between regions with different polarization orientations in a ferroelectric material. Using first-principles calculations, we characterize all different types of domain walls forming on ($11\overline{1}$), (111), and ($1\overline{1}0$) crystallographic planes in thermoelectric GeTe. We find large structural distortions in the vicinity of most of these domain walls, which are driven by polarization variations. We show that such strong strain-order parameter coupling will considerably reduce the lattice thermal conductivity of GeTe samples containing domain walls with respect to a single crystal. Our results thus suggest that domain engineering is a promising path for enhancing the thermoelectric figure of merit of GeTe.

Twenty-Vertex Model with Domain Wall Boundaries and Domino Tilings

We consider the triangular lattice ice model (20-Vertex model) with four types of domain-wall type boundary conditions. In types 1 and 2, the configurations are shown to be equinumerous to the quarter-turn symmetric domino tilings of an Aztec-like holey square, with a central cross-shaped hole. The proof of this statement makes extensive use of integrability and of a connection to the 6-Vertex model. The type 3 configurations are conjectured to be in same number as domino tilings of a particular triangle. The four enumeration problems are reformulated in terms of four types of Alternating Phase Matrices with entries $0$ and sixth roots of unity, subject to suitable alternation conditions. Our result is a generalization of the ASM-DPP correspondence. Several refined versions of the above correspondences are also discussed.

Inhomogeneous domain walls in spintronic nanowires

In case of a spin-polarized current, the magnetization dynamics in nanowires are governed by the classical Landau–Lifschitz equation with Gilbert damping term, augmented by a typically non-variational Slonczewski term. Taking axial symmetry into account, we study the existence of domain wall type coherent structure solutions, with focus on one space dimension and spin-polarization, but our results also apply to vanishing spin-torque term. Using methods from bifurcation theory for arbitrary constant applied fields, we prove the existence of domain walls with non-trivial azimuthal profile, referred to as inhomogeneous. We present an apparently new type of domain wall, referred to as non-flat, whose approach of the axial magnetization has a certain oscillatory character. Additionally, we present the leading order mechanism for the parameter selection of flat and non-flat inhomogeneous domain walls for an applied field below a threshold, which depends on anisotropy, damping, and spin-transfer. Moreover, numerical continuation results of all these domain wall solutions are presented.

Coexistence of phases and interphase boundaries in BaTiO3

A theoretical analysis of the internal structure of interphase boundaries separating domains of coexisting phases is presented for the perovskite ferroelectric BaTiO3. The temperature dependence of interphase boundary widths and surface energies are calculated and compared with the corresponding parameters for different types of domain walls existing within the ferroelectric phases of BaTiO3.

Linear and symmetric conductance response of magnetic domain wall type spin-memristor for analog neuromorphic computing

We developed a three-terminal spintronic memristor named spin-memristor for the artificial synapse of neuromorphic devices. Current-induced domain wall (DW) motion type magnetoresistance was used to generate variable analog conductance changes. Magnetic tunnel junction with a perpendicularly magnetized DW layer was fabricated on a silicon substrate. Due to the forward and backward DW motions, a linear and symmetric conductance response was achieved. The impact of memristive behavior on neural networks was evaluated using numerical simulation of hand-written digit recognition. This spin-memristor is identified as one of the promising candidates for artificial synapses in analog neuromorphic devices.

Origin of sawtooth domain walls in ferroelectrics

Domains and domain walls are among the key factors that determine the performance of ferroelectric materials. In recent years, a unique type of domain walls, i.e., the sawtooth-shaped domain walls, has been observed in BiFeO$_{3}$ and PbTiO$_{3}$. Here, we build a minimal model to reveal the origin of these sawtooth-shaped domain walls. Incorporating this model into Monte-Carlo simulations shows that (i) the competition between the long-range Coulomb interaction (due to bound charges) and short-range interaction (due to opposite dipoles) is responsible for the formation of these peculiar domain walls and (ii) their relative strength is critical in determining the periodicity of these sawtooth-shaped domain walls. Necessary conditions to form such domain walls are also discussed.

Ultralow-loss domain wall motion driven by a magnetocrystalline anisotropy gradient in an antiferromagnetic nanowire

Searching for new methods controlling antiferromagnetic (AFM) domain wall is one of the most important issues for AFM spintronic device operation. In this work, we study theoretically the domain wall motion of an AFM nanowire, driven by the axial anisotropy gradient generated by external electric field, allowing the electro control of AFM domain wall motion in the merit of ultra-low energy loss. The domain wall velocity depending on the anisotropy gradient magnitude and intrinsic material properties is simulated based on the Landau-Lifshitz-Gilbert equation and also deduced using the energy dissipation theorem. It is found that the domain wall moves at a nearly constant velocity for small gradient, and accelerates for large gradient due to the enlarged domain wall width. The domain wall mobility is independent of lattice dimension and types of domain wall, while it is enhanced by the Dzyaloshinskii-Moriya interaction. In addition, the physical mechanism for much faster AFM wall dynamics than ferromagnetic wall dynamics is qualitatively explained. This work unveils a promising strategy for controlling the AFM domain walls, benefiting to future AFM spintronic applications.

Gapped topological kink states and topological corner states in honeycomb lattice

Based on the tight-binding calculations on honeycomb lattice and photonic experimental visualization on artificial graphene (AG), we report the domain-wall-induced gapped topological kink states and topological corner states. In honeycomb lattice, domain walls (DWs) with gapless topological kink states could be induced either by sublattice symmetry breaking or by lattice deformation. We find that the coexistence of these two mechanisms will induce DWs with gapped topological kink states. Significantly, the intersection of these two types of DWs gives rise to topological corner state localized at the crossing point. Through the manipulation of the DWs, we show AG with honeycomb lattice structure not only a versatile platform supporting multiple topological corner modes in a controlled manner, but also possessing promising applications such as fabricating topological quantum dots composed of gapped topological kink states and topological corner states.

Domain walls in neutron P23 superfluids in neutron stars

We work out domain walls in neutron $^{3}P_{2}$ superfluids realized in the core of neutron stars. Adopting the Ginzburg-Landau (GL) theory as a bosonic low-energy effective theory, we consider configurations of domain walls interpolating ground states, i.e., the uniaxial nematic (UN), ${\mathrm{D}}_{2}$-biaxial nematic (${\mathrm{D}}_{2}$-BN), and ${\mathrm{D}}_{4}$-biaxial nematic $({\mathrm{D}}_{4}$-BN) phases in the presence of zero, small and large magnetic fields, respectively. We solve the Euler-Lagrange equation from the GL free energy density, and calculate surface energy densities of the domain walls. We find that one extra Nambu-Goldstone mode is localized in the vicinity of a domain wall in the UN phase while a U(1) symmetry restores in the vicinity of one type of domain wall in the ${\mathrm{D}}_{2}$-BN phase and all domain walls in the ${\mathrm{D}}_{4}$-BN phase. Considering a pile of domain walls in the neutron stars, we find that the most stable configurations are domain walls perpendicular to the magnetic fields piled up in the direction along the magnetic fields in the ${\mathrm{D}}_{2}$-BN and ${\mathrm{D}}_{4}$-BN phases. We estimate the energy released from the deconstruction of the domain walls in the edge of a neutron star, and show that it can reach an astrophysical scale such as glitches in neutron stars.

(2 + 1)-dimensional Boiti–Leon–Pempinelli equation – Domain walls, invariance properties and conservation laws

We analyze domain wall type solutions, symmetry/invariance properties and conservation laws of the (2+1)-dimensional Boiti–Leon–Pempinelli system which describes the evolution of the horizontal velocity component of water waves propagating in two directions. It turns out that the equation is extensive in these properties. Some domain wall properties are also investigated.

Evidence for dispersing 1D Majorana channels in an iron-based superconductor.

The possible realization of Majorana fermions as quasiparticle excitations in condensed-matter physics has created much excitement. Most studies have focused on Majorana bound states; however, propagating Majorana states with linear dispersion have also been predicted. Here, we report scanning tunneling spectroscopic measurements of crystalline domain walls (DWs) in FeSe0.45Te0.55 We located DWs across which the lattice structure shifts by half a unit cell. These DWs have a finite, flat density of states inside the superconducting gap, which is a hallmark of linearly dispersing modes in one dimension. This signature is absent in DWs in the related superconductor, FeSe, which is not in the topological phase. Our combined data are consistent with the observation of dispersing Majorana states at a π-phase shift DW in a proximitized topological material.

A possible propagating Majorana

Topological Matter Majorana states in solid-state systems may one day form a basis for topological quantum computing. Most of the candidates identified so far have been Majorana bound states, but theorists have predicted that propagating Majorana states may exist as well. Wang et al. looked for such a state on the surface of the material FeSe0.45Te0.55 (see the Perspective by Tewari and Stanescu). Using scanning tunneling spectroscopy, the researchers measured a flat, bias-independent density of states along a particular type of domain wall, which was consistent with a theoretical prediction for a propagating Majorana state in this material. Although topologically trivial origins of this finding are difficult to completely rule out, the work is likely to stimulate interest in iron-based superconductors as hosts of Majorana states. Science , this issue p. [104][1]; see also p. [23][2] [1]: /lookup/doi/10.1126/science.aaw8419 [2]: /lookup/doi/10.1126/science.aaz6961

Investigation of the pinning and mobility of domain walls in cobalt micro- and nanowires by the nuclear spin echo method under the additional influence of a magnetic video pulse

The dynamics of domain walls in magnetic cobalt microwires synthesized using electroless technology was studied by the nuclear spin echo method of 59Co nuclei located at the centers of the domain walls of the FCC cobalt phase under the influence of an additional magnetic video pulse. The measured dependences of the intensity of the echo signals on the amplitude of the magnetic video pulse allow one to estimate the pinning and mobility of the domain walls in micro- and nanowires, the anisotropy of its effect during rotation of the sample containing transversely polarized magnetic wires, and also to establish the possible type of domain walls moving in them.

Charge and spin textures of Ising quantum Hall ferromagnet domain walls

We investigate the charge and spin structures associated with arbitrary smooth polarization textures in Ising (integer) quantum Hall ferromagnets. We consider the case where the two polarizations (denoted "pseudospin" up and down) correspond to states with opposite physical spin and different Landau level indices, $n\uparrow$ and $m\downarrow$. We derive analytic expressions for the charge and spin densities, as functions of the underlying pseudospin texture, and use these results to investigate different types of linear domain walls, both analytically and numerically. We find that any smooth domain wall between two oppositely polarized domains carries a universal quantized charge dipole density proportional to the difference of Landau level indices, $n-m$. Additionally, non-uniformities in the domain wall may give rise to excess net charge localized at the domain wall. Interestingly, the physical spin density associated with the domain wall generally exhibits a much more complex multipolar structure than that of the pseudospin texture. These results should for example help to elucidate the mechanisms underlying nuclear electric resonance and nuclear polarization oscillations in Ising quantum Hall systems.

Dynamics and manipulation of ferroelectric domain walls in bismuth ferrite thin films.

Ferroelectric domain walls differ from domains not only in their crystalline and discrete symmetry, but also in their electronic, magnetic, and mechanical properties. Although domain walls provide a degree of freedom to regulate the physical properties at the nanoscale, the relatively lower controllability prevents their practical applications in nano-devices. In this work, with the advantages of 3D domain configuration detection based on piezoresponse force microscopy, we find that the mobility of three types of domain walls (tail-to-tail, head-to-tail, head-to-head) in (001) BiFeO3 films varies with the applied electrical field. Under low voltages, head-to-tail domain walls are more mobile than other domain walls, while, under high voltages, tail-to-tail domain walls become rather active and possess relatively long average lengths. This is due to the high nucleation energy and relatively low growth energy for charged domain walls. Finally, we demonstrate the manipulation of domain walls through successive electric writings, resulting in well-aligned conduction paths as designed, paving the way for their application in advanced spintronic, memory and communication nano-devices.

Study of Domain Wall Propagation in FePt and FePd Nanowires Driven by Sub-nanosecond Pulse of Magnetic Field by Micromagnetic Simulation

In this study, we investigated the domain wall propagation behavior in FePt and FePd nanowires by micromagnetic simulation based on LLG equation. A rectangular shape nanowire model was generated with a fixed length of 2000 nm and varied thicknesses of 2.5 nm and 5 nm. The wire widths were varied of 50 nm, 100 nm, and 150 nm. The 500 ps length of magnetic-pulse was applied in-plane to a transverse type domain wall. We observed the Walker Breakdown field (HWB), which means the maximum domain wall velocity before it was abruptly decreased. The transverse DW structure was maintained below HWB while vortex/antivortex wall structure was formed in the higher magnetic field. Interestingly, the HWB value of both FePt and FePd were decreased as the width and thickness of the nanowires increased. We have also analyzed the changing of the demagnetization and exchange energy for the domain wall structure transition from transverse wall to vortex/anti-vortex wall.

Ferroelectric Domain Walls in PbTiO3 Are Effective Regulators of Heat Flow at Room Temperature.

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using "phonon currents". With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the room-temperature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10-9 K m2 W-1), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO3. This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics.

Unidirectional spin-wave channeling along magnetic domain walls of Bloch type

From the pioneering work of Winter [Phys. Rev. 124, 452 (1961)], a magnetic domain wall of Bloch type is known to host a special wall-bound spin-wave mode, which corresponds to spin-waves being channeled along the magnetic texture. Using micromagnetic simulations, we investigate spin-waves travelling inside Bloch walls formed in thin magnetic media with perpendicular-to-plane magnetic anisotropy and we show that their propagation is actually strongly nonreciprocal, as a result of dynamic dipolar interactions. We investigate spin-wave non-reciprocity effects in single Bloch walls, which allows us to clearly pinpoint their origin, as well as in arrays of parallel walls in stripe domain configurations. For such arrays, a complex domain-wall-bound spin-wave band structure develops, some aspects of which can be understood qualitatively from the single-wall picture by considering that a wall array consists of a sequence of up/down and down/up walls with opposite non-reciprocities. Circumstances are identified in which the non-reciprocity is so extreme that spin-wave propagation inside individual walls becomes unidirectional.