Magnetic Domain Wall Structure Research Articles

Spin-orbit torque-driven domain wall motion in the absence of Dzyaloshinskii-Moriya interactions

The fine structure and dynamics of magnetic domain walls in ultrathin films with perpendicular magnetization, in the presence of a secondary anisotropy, is analysed owing to micromagnetics. Two cases are considered, a cubic anisotropy typical for (111) oriented garnet epitaxial films, and an orthorhombic anisotropy as found in, e.g., Co/W(110) films. The statics is solved first, showing that, in general, domain walls are not of the pure Bloch type. The dynamics under the spin Hall effect induced by a current flowing in an adjacent layer is then monitored. Finite and non-negligible domain wall velocities are predicted in both cases, in the absence of Dzyaloshinskii-Moriya interactions, with distinct behaviours regarding the current density and its orientation with respect to the secondary anisotropy axes. The relevance of these results to recent reports of current-driven domain wall dynamics in insulating ultrathin garnet films, capped with platinum, is discussed.

The structure of magnetic domain walls in cylindrical nano- and microwires with inhomogeneous anisotropy

The structure of magnetic domain walls in cylindrical nano- and microwires with inhomogeneous anisotropy

Direct observation of tensile-strain-induced nanoscale magnetic hardening

Magnetoelasticity is the bond between magnetism and mechanics, but the intricate mechanisms via which magnetic states change due to mechanical strain remain poorly understood. Here, we provide direct nanoscale observations of how tensile strain modifies magnetic domains in a ferromagnetic Ni thin plate using in situ Fresnel defocus imaging, off-axis electron holography and a bimetallic deformation device. We present quantitative measurements of magnetic domain wall structure and its transformations as a function of strain. We observe the formation and dissociation of strain-induced periodic 180° magnetic domain walls perpendicular to the strain axis. The magnetization transformation exhibits stress-determined directional sensitivity and is reversible and tunable through the size of the nanostructure. In this work, we provide direct evidence for expressive and deterministic magnetic hardening in ferromagnetic nanostructures, while our experimental approach allows quantifiable local measurements of strain-induced changes in the magnetic states of nanomaterials.

Strain-coupled domains in BaTiO3(111)−CoFeB heterostructures

The coupling of ferroelectric and ferromagnetic order holds promise for the realization of low-energy voltage-controlled multiferroic devices. Here, the authors show that using a (111)-oriented BaTiO${}_{3}$ substrate it is possible to create two distinct strain-coupled regions in a deposited CoFeB film, in which the in-plane magnetic easy axis rotates by either 60 or 120 degrees. These regions, which exist side by side in a sample heterostructure, contain different magnetic domain wall structures that exhibit different dependences on magnetic field.

Magnetic Vortex Domain Wall Observation on Polycrystalline Imperfect Iron−Cobalt Alloy Nanowires Growing on 1050 Aluminum

Herein, the magnetic vortex domain wall structure of polycrystalline imperfect iron−cobalt alloy nanowires (Nws) growing on 1050 aluminum by pulsed electrodeposition is reported. The magnetic properties are analyzed using magnetometry and off‐axis electron holography. The electrodeposited Nw arrays show homogeneous elemental composition and exhibit a structure composed of piled‐up grains of small crystallites. The saturation magnetization, coercive field, and reduced remanence, measured in directions parallel and perpendicular to the Nw's axis, are studied as a function of the temperature. Although the array of Nws on anodized aluminum grows both straight and within an inclination angle, in both cases, a high shape anisotropy is noticed, which is the most predominant contribution to the magnetic behavior. Misalignments and defects of the Nws in the array, as well as the wide distribution of lengths and diameters, do not contribute significantly to coercivity dispersion. A modified model is used to explain the changes in the magnetic behavior of the Nw's arrays, which accounts for structural imperfections and magnetostatic interactions. The reversal magnetization arising from the vortex domain wall propagation via localized curling is verified by off‐axis electron holography.

Magnetoelectric properties of micromagnetic structural inhomogeneities of ferrite garnet films with uniaxial magnetic anisotropy

The method of optical polarimetry was used by us to study the magnetoelectric properties of micromagnetic structural inhomogeneities of epitaxial films of ferrite garnets. The effect of sensitivity of the magnetic structure of vertical Bloch lines and domain walls of the films to an influence of external low-frequency electric field was revealed.

Analytical studies of the magnetic domain wall structure in the presence of non-uniform exchange bias

The pinning phenomena of the domain wall in the presence of exchange bias are studied analytically. The analytic solution of the domain wall spin configuration is presented. Unlike the traditional solution, which is symmetric, our new solution could exhibit the asymmetry of the domain wall spin profile. Using the solution, the domain wall position, its width, its stability, and the depinning field are discussed analytically.

Domain Structure of Hexaferrite BaFe12O19 Near the Curie Temperature

The features of the structure of domain walls in hexaferrite BaFe12O19 have been studied near the Curie temperature TC = 724 K. The studies have been performed on the single-crystal samples in ac magnetic field with frequencies of 73 MHz and 71 MHz, and in dc magnetic field in the range 0–0.25 kOe. In BaFe12O19 at temperature T1 = 716 K, the magnetic structure of domain walls is changed: the Bloch walls change into linear walls as temperature increases. The value of ΔT = TC – T1 ≈ 8 K. The physical mechanism considered in this work and determining the difference of temperature ranges of existence a magnetically ordered state and the temperature range of existence of the Bloch domain walls agrees with the experimental results. Estimation of the criterion of the Bloch domain wall transition to a linear wall at T1 = 716 K agrees with the obtained experimental data.

Stabilization of coupled Dzyaloshinskii domain walls in fully compensated synthetic anti-ferromagnets

We examine the combined effects of interlayer exchange coupling (IEC) and the interfacial Dzyaloshinskii-Moriya Interaction (DMI) on the structure of magnetic domain walls in fully compensated synthetic anti-ferromagnets (SAFs). Ir-based SAFs with ferromagnetic (FM) layers based on [Pt/(Co/Ni)M]N were characterized by Lorentz transmission electron microscopy (LTEM). The multi-layer design of the individual ferromagnetic layers enables control of the interfacial Dzyaloshinskii-Moriya interaction (via ‘M’) and, in turn, the structure and chirality of domain walls (DWs). We compare the Fresnel-mode LTEM images in SAF designs with only a change in the purported strength of the DMI. The existence of anti-ferromagnetically coupled Dzyaloshinskii domain walls (DWs) in a high DMI SAF is confirmed through application of in-situ perpendicular magnetic field and sample tilt. This conclusion is based on a unique set of conditions required to observe contrast in Fresnel-mode LTEM, which we outline in this document.

Magnetic domain wall skyrmions

It is well established that the spin-orbit interaction in heavy metal/ferromagnet heterostructures leads to a significant interfacial Dzyaloshinskii-Moriya Interaction (DMI) that modifies the internal structure of magnetic domain walls (DWs) to favor N\'{e}el over Bloch type configurations. However, the impact of such a transition on the structure and stability of internal DW defects (e.g., vertical Bloch lines) has not yet been explored. We present a combination of analytical and micromagnetic calculations to describe a new type of topological excitation called a DW Skyrmion characterized by a $360^\circ$ rotation of the internal magnetization in a Dzyaloshinskii DW. We further propose a method to identify DW Skyrmions experimentally using Fresnel mode Lorentz TEM; simulated images of DW Skyrmions using this technique are presented based on the micromagnetic results.

Magnetic domain structure and domain wall analysis of ferromagnetic MnAs nanodisks selectively-grown on Si (111) substrates for spintronic applications

We report on the experimental and analytic results on magnetic domain and domain wall structures of MnAs nanodisks on AlGaAs nanopillar buffers selectively grown on Si (111) substrates partially covered with dielectric SiO2 thin film mask patterns using selective-area metal-organic vapor phase epitaxy. The results on the size dependence of the magnetic domain structure in MnAs nanodisks investigated by magnetic force microscopy show that a single domain is predominant in the MnAs nanodisks with an area of approximately 3 × 104 nm2 or less. It is also indicated that in the nanodisks with an area of approximately 6 × 104 nm2 or more, multiple domains, in particular, two magnetic domain structures with a 180° domain wall, are predominant. In addition, in the case of nanodisks with multiple domains, not only Néel walls but also Bloch walls are possibly formed, according to the detailed analyses of the magnetic force microscope images obtained. These results suggest that the magnetic domains and domain walls can be tuned by the control of the MnAs nanodisk size making them interesting nanostructures for spintronic applications.

Effect of surface self-nanocrystallization and Si infiltration on Si diffusion behavior, hardness and magnetic properties of pure Fe

Surface nanocrystallization of pure Fe was performed using an improved surface treatment process. The phase transformation and Si infiltration depth of the pure Fe before and after surface mechanical attrition treatment (SMAT) were compared by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results indicated that nanocrystallization of Fe surface was achieved using SMAT, which resulted in deeper penetration of Si. Prolonging time of SMAT and Si infiltration also resulted in increasing microhardness, with the hardness first increasing with increasing distance from the surface and then decreasing. Furthermore, longer Si infiltration time, nanocrystallization of Si and longer SMAT time resulted in higher saturation magnetization (MS). The greatest Si penetration depth (150 μm), maximum hardness (280 HV), and maximum MS (1.849 × 106 A/m) were achieved after SMAT for 45 min and Si infiltration for 9 h. The interaction between adjacent grains after surface nanocrystallization leads to a region of the magnetic domain wall structure located at the grain boundary, which causes the remanence enhancement effect.

The Influence of Magnetic Interaction on Magnetic Structure of Domain Walls in Metallic Ferromagnetic Nanofilms

Using transmission electron microscopy, the structure of domain walls is investigated in Fe0.2–Ni0.8 alloy and nickel nanofilms. It is shown that it is controlled not only by the film thickness but also by the angle between the magnetization vector I s and the easy magnetization axis. Applying an external magnetic field along the hard magnetization axis, the field values at which the domain wall structure varies are considered. As the domain wall density becomes higher, the field interval, where magnetization variations occur, is found to increase sharply.

Dzyaloshinskii-Moriya interaction mediated by spin-polarized band with Rashba spin-orbit coupling

Dzyaloshinskii-Moriya interaction (DMI) plays a central role in breaking chiral symmetry of magnetic domain wall structure. The recently observed chiral dependence of domain wall structures in ultrathin magnetic films with perpendicular anisotropy indicates the presence of a strong DMI. We calculate the indirect exchange interaction between magnetic ions mediated by spin-polarized conduction electrons with a Rashba spin-orbit coupling. We find the resulting DMI increases with the spin-orbit coupling strength, but decreases with the spin-polarization of the conduction electrons. The estimated DMI magnitude is comparable to the experimental results.

Domain wall engineering through exchange bias

The control of the structure and position of magnetic domain walls is at the basis of the development of different magnetic devices and architectures. Several nanofabrication techniques have been proposed to geometrically confine and shape domain wall structures; however, a fine tuning of the position and micromagnetic configuration is hardly achieved, especially in continuous films. This work shows that, by controlling the unidirectional anisotropy of a continuous ferromagnetic film through exchange bias, domain walls whose spin arrangement is generally not favored by dipolar and exchange interactions can be created. Micromagnetic simulations reveal that the domain wall width, position and profile can be tuned by establishing an abrupt change in the direction and magnitude of the exchange bias field set in the system.

Structure of Magnetic Domain Wall in Cylindrical Microwire

The magnetization in the domain walls (DWs) formed inside the inner core of the amorphous ferromagnetic microwire is studied within a simple analytical model proposed. The influence of the ordering in the internal-stress created outer shell of the wire is included via an effective Dzyaloshinskii-Moriya-like anisotropy and resulting DW textures have been classified. The model is applicable to periodically constricted nanowires (whose core-shell magnetic structure follows from the magnetostatics) as well, which has enabled the verification of the predictions on DW structure with the micromagnetic simulations.

Micromagnetic Simulation of Magnetic Structure in an Exchange-Coupled Trilayer

The reversal process of an exchange spring trilayer was studied by micromagnetic simulation, simulating the hysteresis loop and magnetic domain wall structure of a soft/hard/soft ferromagnetic exchange spring. The exchange spring effect was observed, determining the chirality of its spiral magnetization configuration. By simulation of the domain wall structure, we find that reversal nucleation emerge simultaneously in either surface of two soft layers and the magnetic moments of hard layer start rotation at reversible stage.

Engineering domain structures in nanoscale magnetic thin films via strain

We study the strain effects on magnetic domain stability and dynamics in nanoscale magnetic thin films using phase-field simulations. Numerous strain-stabilized single-/multi-domain states are discovered, including various magnetic vortices with circular in-plane domains. Furthermore, a strain-domain stability map was constructed, displaying the stable magnetic domain and domain wall structures as a function of biaxial isotropic and anisotropic in-plane strains at room temperature. The present work provides useful guidelines for a precise engineering and experimental observation of domain structures in nanoscale magnetic thin films and a promising scheme towards a low-power and local control over magnetic domain structures.

Three-dimensional Visualization of the Magnetic Microstructure in Bulk Fe-6.6 Pct Si

Magnetic materials such as Fe-Si alloys are magnetized by the rearrangement of their magnetic microstructure and domain wall motion. Understanding magnetic microstructures in the interior of a ferromagnet is essential for the control and reduction of energy losses in electrical devices. The three-dimensional magnetic microstructure of solids is still unknown due to the lack of an appropriate observation technique, and the magnetic domain wall structure inside a ferromagnet has never been observed with sufficiently high resolution. The first observation of the 3D magnetic microstructure of an Fe-6.6 pct Si alloy with a high spatial resolution is reported. The domain walls known to exist inside positive anisotropy cubic materials, i.e., the 180-deg domain walls and the 90-deg domain walls, were analyzed for the first time. The structure and orientation of the domain walls were found to be very different from the prediction of current theoretic models.

First-principles study of nanometer-sharp domain walls in ferromagnetic Fe monolayers under in-plane strain

We investigated a nanometer-sharp magnetic domain wall (DW) structure in a free-standing Fe(110) monolayer and studied the crucial role of in-plane strain using fully unconstrained noncollinear ab initio spin-density-functional theory calculations within the generalized gradient approximation. The DW width is calculated to be 0.86 nm. A precise vector-field description of the magnetization density revealed that a noncollinear character in the DW was spatially confined between atoms, whereas a collinear and high magnetization density was localized around each atom. In the rapid rotation of magnetic moments in the DW, we found an electron rearrangement from the dzx and dx2−y2 states to the dxy, dyz and dz2 states due to a shift of band structures. Applied tensile and compressive in-plane strains both bring about narrower DWs in the monolayer except when the strain is small. The strain dependence of the DW width is discussed in terms of both exchange interaction and magnetocrystalline anisotropy.