Straight Domain Wall Research Articles

A molecular ferroelectric thin film of imidazolium perchlorate on silicon

Molecular ferroelectrics have garnered significant attention due to their structural tunability, low synthesis temperature, and high flexibility. Herein, we successfully synthesized imidazole perchlorate (ImClO4) single crystals and high-quality, highly-oriented thin films on Si substrates. These films demonstrated a high inverse piezoelectric coefficient of 55.7 pm/V. Two types of domain bands were observed: type-I bands tilted ~60° relative to the horizontal axis, and type-II bands positioned perpendicular to the horizontal axis. Under a + 20 V bias, type-I bands showed a reduction and detachment of 180° domain walls to form a needle-like domain. It extended toward the band boundary after applying −20 V bias, which grew along the boundary upon contact. In contrast, type-II bands showed straight domain wall motion and displayed a higher piezoresponse than type-I bands. The growth of high quality molecular ferroelectric thin films on Si substrates paves the way for the development of on-chip devices.

Spontaneous Topological (Anti)meron Chains in the Domain Walls of Centrosymmetric Rare‐Earth Magnet

AbstractExploration and manipulation of topological protected magnetic swirls, such as skyrmion, antiskyrmion, meron, and vortex, holds significance for fundamental research and practical applications in high‐density magnetic information storage and spintronics because of their high storage density and low driven current. This study unveils the existence of field‐free spontaneous (anti)meron chains with a topological charge of ±1/2 in the centrosymmetric rare‐earth magnet Tb6Co2.17Si2.5 via in situ real‐space observation. The spin reorientation transition from in‐plane to uniaxial anisotropy contributes to the spontaneous transformation from straight domain walls to topological (anti)meron chains and to stripe domains along the [110] zone axis during in situ cooling. The study further confirms the critical role of the noncollinear magnetic structure of Tb atoms in the formation of topological (anti)meron chains via real‐space observations, first‐principles calculations, and micromagnetic simulations. The spontaneous topological magnetic texture is strongly correlated with the 4f electrons of rare‐earth atoms, enriching and stimulating alternative generation mechanisms of topological spin textures from emerging rare‐earth magnets, and further applications in spintronics.

Nanoscopic spin-wave channeling along programmable magnetic domain walls in a CoFeB/BaTiO3 multiferroic heterostructure

We report a micromagnetic study on spin-wave propagation along magnetic domain walls in a ferromagnetic/ferroelectric bilayer. In our system, strain coupling between the two ferroic materials and inverse magnetostriction produce a fully correlated domain pattern wherein straight and narrow ferroelectric domain walls pin the magnetic domain walls. Consequently, an external magnetic field does tailor the spin structure of the magnetic domain walls instead of moving them. We use experimental parameters from a previously studied CoFeB/BaTiO3 material system to investigate the potential of artificial multiferroics for programmable nanoscopic spin-wave channeling. We show that spin waves are transported along the pinned magnetic domain walls at zero magnetic field and low frequency due to a local demagnetizing field. Further, switching of the domain wall spin structure from a head-to-tail to a head-to-head configuration abruptly changes the propagating spin-wave mode. We study the effect of magnetic field strength on the localized modes and discuss reversible control of spin-wave channeling via electric-field-driven magnetic domain wall motion. Nanoscopic guiding of propagating spin waves by an electric field, in combination with positional robustness to and mode programming by an external magnetic field, offers prospects for low-power and reconfigurable domain-wall-based magnonic devices.

A phase-field model for ferroelectrics with general kinetics, Part I: Model formulation

When subjected to electro-mechanical loading, ferroelectrics see their polarization evolve through the nucleation and evolution of domains. Existing mesoscale phase-field models for ferroelectrics are typically based on a gradient-descent law for the evolution of the order parameter. While this implicitly assumes that domain walls evolve with linear kinetics, experiments instead indicate that domain wall kinetics is nonlinear. This, in turn, is an important feature for the modeling of rate-dependent effects in polarization switching. We propose a new multiple-phase-field model for ferroelectrics, which permits domain wall motion with nonlinear kinetics, with applications in other solid–solid phase transformation problems. By means of analytical traveling wave solutions, we characterize the interfacial properties (energy and width) and the interface kinetics of straight domain walls, as furnished by the general kinetics model, and compare them to those of the classical Allen–Cahn model. We show that the proposed model propagates domain walls with arbitrarily chosen nonlinear kinetic relations, which can be tuned to differ for the different types of domain walls in accordance with experimental evidence.

Anomalous domain wall dynamics in Ir50Mn50/Fe2CoSi bilayers

We report on the evidenced anomalous saw-tooth and curved domains in the Si/SiO2/IrMn/Fe2CoSi/Ta bilayers. The existence of saw-tooth domains is attributed to higher magnetic charge density at the domain boundary induced by the IrMn layer, while curved domains are apparent due to Dzyaloshinskii-Morya interaction (DMI). On the other hand, straight domain walls are apparent as a result of uni–axial anisotropy and four-fold anisotropy along the easy axis. The curvature for the domain walls decreases upon changing the angle between sample edge and the magnetic field. We believe that the present results would indeed be helpful for the development of the domain wall motion devices.

Plasmon-induced transparency in a reconfigurable composite valley photonic crystal.

We propose a new kind of reconfigurable topological valley photonic crystal (TVPC), and a novel topological waveguide can be formed by constructing a domain wall between two TVPCs with opposite valley-Chern indices. The topological waveguide mode in the composite TVPC has large group refractive index. A topologically protected coupled waveguide cavity system is then designed by introducing a hexagonal ring cavity at the center of the straight domain wall of a combined TVPC, in which a narrow plasmon induced transparency window rises at 3.8848 GHz with a Q-factor of 1387 and a maximum group refractive index as high as 186. We propose a notch filter with a resonant frequency of 3.8852 GHz and a very high Q-factor of 10224. By changing the refractive index of liquid crystals via an external voltage applied between two parallel metal plates, the filter can be switched between band-pass and band-stop based on the reconfigurable topological interface state.

Magnetic Domain With Straight Domain Walls Passing Through a Ferromagnetic Insulator for a Continuously Looped Racetrack Memory

We demonstrate via micromagnetic simulations that a magnetic domain, which has two straight domain walls (DWs) and is formed in a perpendicularly magnetized ferromagnetic metal (FM), moves through a perpendicularly magnetized ferromagnetic insulator (FI) because of the spin-transfer torque arising from a spin-polarized current flowing in the FM. This phenomenon could be used for realizing racetrack memory with a continuous loop structure built from a nanowire comprising an FM/FI junction. We employ two nanowire structures in which two FMs are separated by a rectangular or a zigzag FI. In the system with a rectangular FI, the domain passes through the insulator when the length of the insulator is shorter than the DW length. However, the domain transforms into an unstable structure with current-induced domain motion after it passes through the insulator. In the system with a zigzag FI, the domain passes through the insulator irrespective of the length of the insulator. When passing through the zigzag FI, the velocity of the domain increases.

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.

Analytical computation of the demagnetizing energy of thin-film domain walls

Due to its non-local nature, calculating the demagnetizing field remains the biggest challenge in understanding domain structures in ferromagnetic materials. Analytical descriptions of demagnetizing effects typically approximate domain walls as uniformly magnetized ellipsoids, neglecting both the smooth rotation of magnetization from one domain to the other and the interaction between the two domains. Here, instead of the demagnetizing field, we compute analytically the demagnetizing energy of a straight domain wall described by the classical $\tanh$ magnetization profile in a thin film with perpendicular magnetic anisotropy. We then use our expression for the demagnetizing energy to derive an improved version of the 1D model of field-driven domain wall motion, resulting in accurate expressions for important properties of the domain wall such as the domain wall width and the Walker breakdown field. We verify the accuracy of our analytical results by micromagnetic simulations.

Structure and Dynamics of Ferroelectric Domains in Polycrystalline Pb(Fe1/2Nb1/2)O3.

A complex domain structure with variations in the morphology is observed at ambient temperature in monoclinic Pb(Fe1/2Nb1/2)O3. Using electron microscopy and piezoresponse force microscopy, it is possible to reveal micrometre-sized wedge, lamellar-like, and irregularly shaped domains. By increasing the temperature, the domain structure persists up to 80 °C, and then starts to disappear at around 100 °C due to the proximity of the ferroelectric–paraelectric phase transition, in agreement with macroscopic dielectric measurements. In order to understand to what degree domain switching can occur in the ceramic, the mobility of the domain walls was studied at ambient temperature. The in situ poling experiment performed using piezoresponse force microscopy resulted in an almost perfectly poled area, providing evidence that all types of domains can be easily switched. By poling half an area with 20 V and the other half with −20 V, two domains separated by a straight domain wall were created, indicating that Pb(Fe1/2Nb1/2)O3 is a promising material for domain-wall engineering.

Current-induced motion of bubble domains stabilised in nanowires with edge defects

Bubble domain (BD) motion is expected to be useful for domain motion magnetic memory. We investigate the current-induced motion of BDs produced in a perpendicularly magnetised thin wire with edge defects by using micro-magnetic simulation. We produce a stable BD in the wire and find that its size increases with the wire width. Upon application of a spin-polarised current, the BD both with and without Bloch points moves with a velocity similar to the theoretical velocity of domain wall motion while maintaining its structure. In the presence of edge defects in the wire, the motion of the BD is slightly affected by the magnetisation of the uneven edge structure, whereas a straight domain wall is pinned strongly by the edge defects. The effect of edge defects on BD motion is much smaller than that in straight domain wall motion.

Millimeter-sized magnetic domains in perpendicularly magnetized ferrimagnetic Mn4N thin films grown on SrTiO3

The use of epitaxial layers for domain wall-based spintronic applications is often hampered by the presence of pinning sites. Here, we show that when depositing Mn4N (10 nm) epitaxial films, the replacement of MgO(001) by SrTiO3(001) substrates allows minimizing the misfit, and to obtain an improved crystalline quality, a sharper switching, a full remanence, a high anisotropy and remarkable millimeter-sized magnetic domains, with straight and smooth domain walls. In a context of rising interest for current-induced domain wall motion in rare-earth ferrimagnets, we show that Mn4N/SrTiO3, which is rare-earth free, constitutes a very promising ferrimagnetic system for current-induced domain wall motion.

Local ferroelectricity at the domain walls and stripe domain heads in iron garnet films

ABSTRACTThe concept of local ferroelectricity discussed in this paper belongs to the emerging field of ultrathin multiferroics. The idea of the ferroelectric polarization associated with the domain walls in magnets proposed by V.G. Bar'yakhtar et al. more than thirty years ago has recently got experimental evidence including electric field driven magnetic domain wall motion and electric field induced Stark shift in the single molecule microscopy. The special attention is paid here to the magnetoelectric properties of dislocation in stripe domain structure (i.e. the stripe domain heads) that differ considerably from the ones of straight domain wall segments. It is shown that the electric field induced displacement of the domain wall can be increased by an order of value with external constant magnetic field.

Direct Observation of Ferroelectric Domain Walls in LiNbO3: Wall‐Meanders, Kinks, and Local Electric Charges

Direct observations of the ferroelectric domain boundaries in LiNbO3 are performed using high‐resolution high‐angle annular dark field scanning transmission electron microscopy imaging, revealing a very narrow width of the domain wall between the 180° domains. The domain walls demonstrate local side‐way meandering, which results in inclinations even when the overall wall orientation follows the ferroelectric polarization. These local meanders contain kinks with “head‐to‐head” and “tail‐to‐tail” dipolar configurations and are therefore locally charged. The charged meanders are confined to a few cation layers along the polarization direction and are separated by longer stretches of straight domain walls.

Domain wall generated by graded interlayer coupling in Co/Pt/Co film with perpendicular anisotropy

A magnetic multilayer of the structure substrate/Pt-15 nm/Co-0.8 nm/Pt-wedge(0-7 nm)/Co-0.6 nm/Pt-2 nm is characterized by perpendicular anisotropy of both Co layers. For a Pt spacer thickness tPt≤2.6nm, the magnetization reversal of the Co-layers occurs cooperatively, while for larger tPt, it occurs sequentially. The Co-layer with 0.6 nm thickness (CoS) is magnetically softer than the second one (CoH). In the 2.6≤tPt≤3.0 nm range, there are significant changes of the switching field due to a strong gradient of the interlayer coupling. In this region, the magnetization reversal in the CoS layer takes place reversibly by the propagation of a single, straight domain wall. This specific nature of magnetization reversal is explained by a decelerated motion of the domain wall observed both for the direction corresponding to the increasing, as well as decreasing coupling energy.

Manipulation of domain wall mobility by oxygen vacancy ordering in multiferroic YMnO3

The mobility of the ferroelectric domain phases and the local conductivity of ferroelectric domain walls in multiferroic YMnO3 crystals grown in air and reduced atmosphere were studied by piezoresponse force microscopy (PFM), tip-enhanced Raman spectroscopy (TERS) and conductive atomic force microscopy (CAFM). Oxygen vacancies were found to reduce the strength of 4d-2p (Y(3+)-O(2-)) hybridization and structural trimerization distortion, leading to the disappearance of the six wedge-shaped ferroelectric domain phases in oxygen deficient YMnO3-δ crystals. We observed anisotropic domain wall motion such that the wedge-shaped domain configuration joined at one point could be changed to the stripe domain configuration by applying high electric fields in oxygen deficient YMnO3-δ single crystals. The local conductivity of the domain walls increased significantly in poled YMnO3-δ single crystals. The straight conductive domain walls in YMnO3-δ, instead of the twisted insulating ones in the stoichiometric crystal, are induced by the ordered oxygen vacancies which are verified by TERS measurements.

Oxygen-vacancy effect on structural, magnetic, and ferroelectric properties in multiferroic YMnO3 single crystals

We have investigated the structural, magnetic, and ferroelectric properties of magnetically frustrated multiferroic YMnO3 single crystals. The ferroelectric domain structures of YMnO3 samples were studied by piezoresponse force microscopy. Instead of domain vortex structure in stoichiometric crystals, YMnO3−δ exhibits a random domain configuration with straight domain walls. In magnetic measurements, the YMnO3−δ crystal shows typical antiferromagnetic behavior with higher Néel temperature and lower magnetization compared to the stoichiometric sample. The ordered oxygen vacancies dominate multiferroicity through tailoring the domain wall structure.

Stability of topological defects in chiral superconductors: London theory

This paper examines thermodynamic stability of chiral domain walls and vortices - topological defects which can exist in chiral superconductors. Using London theory it is demonstrated that at sufficiently small applied and chiral fields the existence of domain walls and vortices in the sample is not favored and the sample's configuration is a single domain. The particular chirality of the single-domain configuration is neither favored nor disfavored by the applied field. Increasing the field leads to an entry of a domain wall loop or a vortex into the sample. Formation of a straight domain wall is never preferred in equilibrium. Values of the entry (critical) fields for both types of defects, as well as the equilibrium size of the domain wall loop, are calculated. We also consider a mesoscopic chiral sample and calculate its zero-field magnetization, susceptibility and a change in the magnetic moment due to a vortex or a domain wall entry. We show that in a case of a soft domain wall whose energetics is dominated by the chiral current (and not by the surface tension) its behavior in mesoscopic samples is substantially different from that in the bulk case and can be used for a controllable transfer of edge excitations. The applicability of these results to Sr2RuO4 - a tentative chiral superconductor - is discussed.

Domain-wall and reverse-domain superconducting states of a Pb thin-film bridge on a ferromagnetic BaFe12O19single crystal

We report on imaging of the nonuniform superconducting states in a Pb thin film bridge on top of a ferromagnetic BaFe_{12}O_{19} single crystal with a single straight domain wall along the center of the bridge by low-temperature scanning laser microscopy. We have visualized domain wall superconductivity (DWS) close to the critical temperature of Pb, when the Pb film above the domain wall acts as a superconducting path for the current. The evolution of the DWS signal with temperature and the external-field-driven transition from DWS to reverse domain superconductivity was visualized.

Unique vortex and stripe domain structures in PbTiO 3 epitaxial nanodots

The domain structures of PbTiO 3 epitaxial nanodots under the influences of depolarization fields and mismatch strains have been studied using three dimensional phase field simulations. The single-vortex structure and mixed domain configuration, which consisted of zigzag stripe domain and closure dipole flux near the interfaces, were found to be effective in annihilating the depolarization fields in the isotropically tensile and compressive ferroelectric nanodots, respectively. These domain structures were produced by the combined effect of electrostatic and mismatch elastic energies. The width of stripe domain was found to be related to the volume percentage of polarization dipoles along the z-axis, which varied remarkably with the change of compressive mismatch strain. In the case of nanodots under anisotropic mismatch strains, double-vortex domain patterns and stripe domains with nearly straight domain walls were formed. Moreover, the domain structures with electrostatic energy neglected were also studied.