Types Of Domain Walls Research Articles

Quasidegenerate ice manifold in a purely two-dimensional square array of nanomagnets

We investigate numerically the micromagnetic properties and the low-energy physics of an artificial square spin system in which the nanomagnets are physically connected at the lattice vertices. Micromagnetic simulations reveal that the energy stored at the vertex sites strongly depends on the type of magnetic domain wall formed by the four connected nanomagnets. As a consequence, the energy gap between the vertex types can be partially modified by varying the geometrical parameters of the nanomagnets, such as their width and thickness. Based on the energy levels given by the micromagnetic simulations, we compute the thermodynamic properties of the corresponding spin models using Monte Carlo simulations. We found two regimes, both being characterized by an extensive ground state manifold, in sharp contrast with similar lattices with disconnected nanomagnets. For narrow and thin nanomagnets, low-energy spin configurations consist of independent ferromagnetic straight lines crossing the whole lattice. The ground state manifold is thus highly degenerate, although this degeneracy is subdominant. In the limit of thick and wide nanomagnets, our findings suggest that the celebrated square ice model may be fabricated experimentally from a simple square lattice of connected elements. These results show that the micromagnetic nature of artificial spin systems involves another degree of freedom that can be finely tuned to explore strongly correlated disordered magnetic states of matter.

Lorentz TEM investigation of chiral spin textures and Néel Skyrmions in asymmetric [Pt/(Co/Ni)M/Ir]N multi-layer thin films

We examine magnetic domain patterns in symmetric [Co/Ni]$_M$ and asymmetric [Pt/(Co/Ni)$_M$/Ir]$_N$ multi-layers using Fresnel mode Lorentz transmission electron microscopy (LTEM). In the symmetric multi-layer, where the Dzyaloshinskii-Moriya Interaction is expected to be zero, we observe purely Bloch type domain walls with no preferred chirality. In the asymmetric multi-layers, where significant interfacial DMI is present, we observe domain patterns with chiral N\'eel domain walls, which evolve into sub-100nm isolated N\'eel Skyrmions with the application of a perpendicular field. The impact of layer thickness and film stack on interfacial magnetic properties is discussed in the context of developing a tunable multi-layer system for future spintronic applications.

Chiral p-wave superconducting states in mesoscopic noncircular systems at zero external magnetic field

Using Bogoliubov-de Gennes formalism, the chiral order parameters in mesoscopic p-wave superconducting noncircular systems are investigated in the absence of applied magnetic field. For the perfect square or rectangular sample, a single-domain-wall state tends to emerge, accompanied by a vertical or horizontal domain wall separating the regions with different chiralities. This type of domain walls can be effectively tuned by temperature and the aspect ratio of samples, and the multi-domain-wall state containing two domain walls can be obtained. Moreover, for a p-wave square loop with a centered hole, the diagonal-symmetric mode for orders is presented, leading to four domain walls placed at the center of four arms of the loop. In particular, several peculiar domain-wall states may show up in the asymmetric square loop system with an off-centered hole.

Domain walls in strontium titanate

Density functional theory calculations of twin-domain walls in strontium titanate are presented. The two possible domain walls are characterized. The domain wall type is set by the relative phase of the octahedral rotations in the body-centered-tetragonal SrTiO3 on either side of the domain wall. The width of the domain walls is estimated to be approximately 3 nm.

Manipulation of Conductive Domain Walls in Confined Ferroelectric Nanoislands

AbstractConductive ferroelectric domain walls—ultranarrow configurable conduction paths—have been considered as essential building blocks for future programmable domain wall electronics. For applications in high‐density devices, it is imperative to explore the conductive domain walls in small confined systems, while earlier investigations have hitherto focused on thin films or bulk single. Here, an observation and manipulation of conductive domain walls confined within small BiFeO3 nanoislands aligned in high‐density arrays are demonstrated. Using conductive atomic force microscopy, various types of conductive domain walls, including the head‐to‐head charged domain walls (CDWs), zigzag domain walls, and typical 71° head‐to‐tail neutral domain walls (NDWs), are distinctly visualized. The CDWs exhibit remarkably enhanced metallic conductivity with current of ≈nA order in magnitude and 104 times larger than that inside domains (0.01–0.1 pA), while the semiconducting NDWs allow much smaller current (≈10 pA) than the CDWs. The substantial difference in conductivity for dissimilar walls enables manipulations of various wall conduction states for individual addressable nanoislands via electrical tuning of domain structures. A controllable writing of four distinctive states in individual nanoislands can be achieved, showing application potentials for developing multilevel high‐density memories.

Domain imaging in Fe-doped KNbO3 single crystal via trinocular microscopy and scanning electron microscopy

Abstract Trinocular microscopy with a high spatial resolution is a promising technique for studying the surface morphology of materials. The cleavage face of ferroelectric Fe-doped K niobate (KNbO3) crystals was chemically etched by using HNO3. Trinocular microscopy was used to study the surface morphology of the grown ferroelectric KNbO3 single crystals. The grown surface of the crystal with a 60° domain wall revealed the same interaction and orientations. The results indicate that the type of domain walls that occur in ferroelectric crystals depend on the symmetry of both the nonferroelectric and ferroelectric phases of the crystal.

Simultaneous Control of Interlayer Exchange Coupling and the Interfacial Dzyaloshinskii–Moriya Interaction in Ru-Based Synthetic Antiferromagnets

We present a Ru-based synthetic antiferromagnet (SAF) made with asymmetric ferromagnetic building blocks (Pt/Co/Ir or Ir/Co/Pt) to couple their interfacial Dzyaloshinskii-Moriya interaction (DMI) with strong interlayer exchange coupling (IEC). This combination may lead to new types of antiferromagnetic domain walls (DWs), which are conceptually presented in this letter. These include an energetically frustrated configuration and an “isolated” spin chain where a net magnetization exists only at the DW. While most SAF designs incorporate a Co/Ru/Co IEC layer, chiral SAFs require Pt and/or Ir at the Ru/Co interface. In all cases except Pt/Ru/Pt, a significant IEC field is preserved over a range of Ru thicknesses, with the largest occurring for Ir/Ru/Ir. These results demonstrate that ferromagnetic film stacks with purportedly strong DMI can be incorporated into SAFs without compromising IEC.

Charged domain walls in improper ferroelectric hexagonal manganites and gallates

Ferroelectric domain walls are attracting broad attention as atomic-scale switches, diodes and mobile wires for next-generation nanoelectronics. Charged domain walls in improper ferroelectrics are particularly interesting as they offer multifunctional properties and an inherent stability not found in proper ferroelectrics. Here we study the energetics and structure of charged walls in improper ferroelectric YMnO$_3$, InMnO$_3$ and YGaO$_3$ by first principles calculations and phenomenological modeling. Positively and negatively charged walls are asymmetric in terms of local structure and width, reflecting that polarization is not the driving force for domain formation. The wall width scales with the amplitude of the primary structural order parameter and the coupling strength to the polarization. We introduce general rules for how to engineer $n$- and $p$-type domain wall conductivity based on the domain size, polarization and electronic band gap. This opens the possibility of fine-tuning the local transport properties and design $p$-$n$-junctions for domain wall-based nano-circuitry.

Topological Defects in Helical Magnets

Helical magnets which violated space inversion symmetry have rather peculiar topological defects. In isotropic helical magnets with exchange and Dzyaloshinskii-Moriya interactions there are only three types of linear defects: $\pm\pi$ and $2\pi$-disclinations. Weak crystal anysotropy suppresses linear defects on large scale. Instead planar defects appear: domain walls that separate domains with different preferential directions of helical wave vectors. The appearance of such domain walls in the bulk helical magnets and some of their properties were predicted in the work \cite{Li 2012}. In a recent work by an international team of experimenters and theorists \cite{Schoenherr 2018} the existence of new types of domain walls on crystal faces of helical magnet FeGe was discovered. They have many features predicted by theory \cite{Li 2012}, but display also unexpected properties, one of them is the possibility of arbitrary angle between helical wave vectors. Depending on this angle the domain walls observed in \cite{Schoenherr 2018} can be divided in two classes: smooth and zig-zag. This article contains a mini-review of the existing theory and experiment. It also contains new results that explain why in a system with continuos orientation of helical wave vectors domain walls are possible. We discuss why and at what conditions smooth and zig-zag domain walls appear, analyze spin textures associated with helical domain walls and find the dependence of their width on angle between helical wave vectors.

Domain walls in a non-linear {mathbb{S}}^2 -sigma model with homogeneous quartic polynomial potential

In this paper the domain wall solutions of a Ginzburg-Landau non-linear {mathbb{S}}^2 -sigma hybrid model are exactly calculated. There exist two types of basic domain walls and two families of composite domain walls. The domain wall solutions have been identified by using a Bogomolny arrangement in a system of sphero-conical coordinates on the sphere {mathbb{S}}^2 . The stability of all the domain walls is also investigated.

Induced Formation of Structural Domain Walls and Their Confinement on Phase Dynamics in Strained Manganite Thin Films.

Domain walls (DWs) in strongly correlated materials have provided fertile ground for the discovery of exotic phenomena, and controlling the formation of DWs is still a challenge. Here, it is demonstrated that a new type of structural DW can be induced in a series of manganite thin films, which are engineered to achieve a robust charge-ordering insulating (COI) ground state by selecting various films and substrates. The monoclinic domains are somewhat irregular in shape, and the corresponding DWs, taking the shape of closed loops, are ferromagnetic and metallic (FMM) at low temperatures. Remarkably, the DWs exhibit little dependence on temperature or magnetic field, due to the structural origins of the domains. Additionally, using magnetic force microscopy, the role played by DWs in the dynamics of the COI and FMM phases at the mesoscopic scale is revealed. They function as barriers, strictly confining the phase dynamics within each domain, reflecting the strong coupling of electronic phases with the lattice. Each domain exhibits binary occupation by a single pure phase, resulting in a quasi-periodic phase separation. The universal behaviors of the multiple engineered films elucidate the possibility of controlling the formation of DWs and tuning phase dynamics through DW design.

Domain orientation relationship of orthorhombic and coexisting monoclinic phases of YO1.5-doped HfO2 epitaxial thin films

Domain structures with ferroelectric and ferroelastic characteristics of the epitaxial thin films of the polar orthorhombic phase of 7 mol % YO1.5-doped HfO2 (YHO) were investigated by aberration-corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF)-STEM images of the YHO thin film reveal that the individual domain size is approximately 10 nm. Multislice simulation demonstrated that STEM imaging can detect individual domains on the nanoscale avoiding superposition with other domains in the projection direction. The STEM analysis of the domain structure of the polar orthorhombic phase elucidated that the orientation relationship between neighboring domains was affected by two factors: (i) tilt or twist relationship between the longest b-axis in the domains, and (ii) parallel or perpendicular relationship between the rotation axis and the longest b-axis. Corresponding to the longest b-axis orientation, two types of domain walls (DWs) exist: the strained DWs and the less strained ones. The latter should move more easily with less strain, and are of the tilt type with the longest b-axis parallel to the domain rotation axis. STEM imaging also reveals the coexistence of a nonpolar monoclinic phase, which deforms the orthorhombic phase around the monoclinic phase. This factor should also diminish the DW mobility. We propose that the formation of the monoclinic phase should be inhibited and that the longest b-axis should be oriented in the in-plane direction for improving the ferroelectric properties. The two-dimensional tensile stress is expected to be preferred for preparing a-/c-oriented YHO epitaxial thin films. The elastic constraint plays an important role for these purposes for controlling the ferroelectric properties of HfO2 thin films.

Interactions of charged domain walls and oxygen vacancies in BaTiO3: a first-principles study

Ferroelectric domain walls have been promised for some potential applications due to their unique properties. In particular, the electrical conductivity of charged domain walls (DWs) allows a new dimension to ferroelectric functionalities. In this work, we construct two representative types of charged DWs, i.e. head-to-head (HH) wall and tail-to-tail (TT) wall, and employ the first-principles method to study the electronic structure of these charged walls in BaTiO3 and the interactions between them and oxygen vacancies. It is revealed that the HH walls show the n-type conductivity, but the TT walls show the p-type conductivity. While embedded oxygen vacancies attract the TT wall and repel the HH wall, the interaction between the walls and oxygen vacancies depends on the vacancy occupation. This interaction enhances the conductivity of HH walls and reduces the conductivity of TT walls, and in particular a TT wall in binding with oxygen vacancies will drive the transition of p-type wall conductivity into n-type wall conductivity. The interaction of these walls with oxygen vacancies is discussed using the electrostatic model. This work represents a comprehensive understanding of electrical transport of charged DWs in ferroelectrics and possible roadmaps for manipulation.

Localized charge in various configurations of magnetic domain wall in a Weyl semimetal

We numerically investigate the electronic properties of magnetic domain walls formed in a Weyl semimetal. Electric charge distribution is computed from the electron wave functions, by numerically diagonalizing the Hamiltonian under several types of domain walls. We find a certain amount of electric charge localized around the domain wall, depending on the texture of the domain wall. This localized charge stems from the degeneracy of Landau states under the axial magnetic field, which corresponds to the curl in the magnetic texture. The localized charge enables one to drive the domain wall motion by applying an external electric field without injecting an electric current, which is distinct from the ordinary spin-transfer torque and is free from Joule heating.

Magnetic properties of artificially designed magnetic stray field landscapes in laterally confined exchange-bias layers

Magnetic stray fields generated by domain walls (DWs) have attracted significant attention as they might be employed for precise positioning and active control of micro- and nano-sized magnetic objects in fluids or in the field of magnonics. The presented work intends to investigate the near-field response of magnetic stray field landscapes above generic types of charged DWs as occurring in thin films with in-plane anisotropy and preferential formation of Néel type DWs when disturbed by external magnetic fields. For this purpose, artificial magnetic stripe domain patterns with three defined domain configurations, i.e. head-to-head (tail-to-tail), head-to-side, and side-by-side, were fabricated via ion bombardment induced magnetic patterning of an exchange-biased IrMn/CoFe bilayer. The magnetic stray field landscapes as well as the local magnetization reversal of the various domain configurations were analyzed in an external magnetic field by scanning magnetoresistive microscopy and compared to micromagnetic simulations.

Phase-field simulation of domain walls in rhombohedral ferroelectric single crystals

Domain engineered ferroelectric relaxor-PbTiO3 single crystals exhibit ultrahigh piezoelectric coefficients. A 10th order Landau-Devonshire energy function for rhombohedral PIN-PMN-PT ferroelectric single crystals was implemented in a phase-field model to study the behavior of domain walls under external electric field. The domain formation and domain wall evolution were simulated. A new way to apply periodic boundary conditions was implemented to accommodate nonzero strain during the domain formation and evolution. It was found that 71 and 109 domain walls reacted differently to external electric field along the [110] direction. A domain wall broadening effect was observed in the 71 domain walls when the electric field was below the coercive field. When the electric field exceeded the coercive field, homogeneous polarization switching occurred with no motion of the 71 domain walls. Heterogeneous polarization switching occurred by the sweeping of the 109 domain walls. The evolution mechanism of the two types of domain walls helps explain the behavior of engineered domain structures subjected to external electric field.

Domain-wall resistance in CoFeB-based heterostructures with interface Dzyaloshinskii–Moriya interaction

We studied the domain-wall resistance in W/Ta/CoFeB/MgO heterostructures. The Ta layer thickness was varied to control the type of domain walls (DWs) via changes in the interfacial Dzyaloshinskii–Moriya interaction. The domain-wall resistance was nearly constant with changes in the Ta layer thickness. The anisotropic magnetoresistance, spin Hall magnetoresistance, and anomalous Hall effect contributed to the domain-wall resistance of the thick-Ta layer films. However, a discrepancy remains for the thin-Ta layer films, wherein chiral Néel-like DWs were observed. These results show the difficulty of studying the domain-wall type via resistance measurements.

Experimental Evidence of Chiral Ferrimagnetism in Amorphous GdCo Films.

Inversion symmetry breaking has become a vital research area in modern magnetism with phenomena including the Rashba effect, spin Hall effect, and the Dzyaloshinskii-Moriya interaction (DMI)-a vector spin exchange. The latter one may stabilize chiral spin textures with topologically nontrivial properties, such as Skyrmions. So far, chiral spin textures have mainly been studied in helimagnets and thin ferromagnets with heavy-element capping. Here, the concept of chirality driven by interfacial DMI is generalized to complex multicomponent systems and demonstrated on the example of chiral ferrimagnetism in amorphous GdCo films. Utilizing Lorentz microscopy and X-ray magnetic circular dichroism spectroscopy, and tailoring thickness, capping, and rare-earth composition, reveal that 2 nm thick GdCo films preserve ferrimagnetism and stabilize chiral domain walls. The type of chiral domain walls depends on the rare-earth composition/saturation magnetization, enabling a possible temperature control of the intrinsic properties of ferrimagnetic domain walls.

Direct imaging of antiferromagnetic domains in Mn2Au manipulated by high magnetic fields

In the field of antiferromagnetic (AFM) spintronics, information about the N\'eel vector, AFM domain sizes, and spin-flop fields is a prerequisite for device applications but is not available easily. We have investigated AFM domains and spin-flop induced changes of domain patterns in Mn2Au(001) epitaxial thin films by X-ray magnetic linear dichroism photoemission electron microscopy (XMLD-PEEM) using magnetic fields up to 70 T. As-prepared Mn$_2$Au films exhibit AFM domains with an average size $\sim$ 1 $\mu$m. Application of a 30T field, exceeding the spin-flop field, along a magnetocrystalline easy axis, dramatically increases the AFM domain size with N\'eel vectors perpendicular to the applied field direction. The width of N\'eel type domain walls (DW) is below the spatial resolution of the PEEM and therefore can only be estimated from an analysis of the DW profile to be smaller than 80 nm. Furthermore, using the values for the DW width and the spin-flop field, we evaluate an in-plane anisotropy constant ranging between 1 and 17 $\mu$eV/f.u.

As-grown domain structure in lithium tantalate with spatially nonuniform composition

ABSTRACTWe investigated the as-grown domain structure in LiTaO3 with different nonuniform spatial distributions of deviation from stoichiometry. The as-grown structure formed due to cooling below Curie temperature after vapor transport equilibration (VTE) process. The formation of wide charged domain boundary consisting of smooth head-to-head and zig-zag tail-to-tail charged domain walls and flat neutral domain walls was observed. The mechanisms of domain structure formation due to phase transition and subsequent cooling were explained. The different type of charged domain walls in ferroelectrics attracts interest due to possibilities of its applications.