arXiv subjects

Vincent Sokalski

Publications and source records attributed to Vincent Sokalski.

13 recordsLinked to original sources

arXiv2022

Deterministically switching the Bloch chirality of domain walls and skyrmions: the Dzyaloshinskii astroid

We present a mechanism for deterministic control of the Bloch chirality in magnetic domain walls and skyrmions, originating from the interplay between an interfacial Dzyaloshinskii$-$Moriya interaction (DMI) and a perpendicular magnetic field. Although conventional interfacial DMI favors chiral Néel skyrmions, it does not break the energetic symmetry of the two Bloch chiralities in mixed Bloch$-$Néel skyrmions. However, the energy barrier to switching between Bloch chiralities does depend on the sense of rotation, which is dictated by the direction of the driving field. Our analytical and micromagnetic analyses of steady-state Dzyaloshinskii domain wall dynamics culminates in a switching diagram akin to the Stoner$-$Wohlfarth astroid, revealing the existence of achiral, chiral, and precessional Bloch regimes; we further show that in view of established energetic models, skyrmions should exhibit the same fundamental behavior. Finally, we discuss a recent theory of vertical Bloch line$-$mediated deterministic Bloch chirality in the precessional regime and extend these arguments to lower driving fields. This work establishes that the Bloch chirality of magnetic solitons can be switched on demand with applied fields, as indicated by the Dzyaloshinskii astroid.

cond-mat.mtrl-sci↗

arXiv2021

Towards hexagonal $C_{2v}$ systems with anisotropic DMI: Characterization of epitaxial Co$(10\bar{1}0)$/Pt$(110)$ multilayer films

Ferromagnet/heavy metal (FM/HM) multilayer thin films with $C_{2v}$ symmetry have the potential to host antiskyrmions and other chiral spin textures via an anisotropic Dzyaloshinkii-Moriya interaction (DMI). Here, we present a candidate material system that also has a strong uniaxial magnetocrystalline anisotropy aligned in the plane of the film. This system is based on a new Co/Pt epitaxial relationship, which is the central focus of this work: hexagonal closed-packed Co$(10\bar{1}0)[0001]$ $\parallel$ face-centered cubic Pt$(110)[001]$. We characterized the crystal structure and magnetic properties of our films using X-ray diffraction techniques and magnetometry respectively, including q-scans to determine stacking fault densities and their correlation with the measured magnetocrystalline anisotropy constant and thickness of Co. In future ultrathin multilayer films, we expect this epitaxial relationship to further enable an anisotropic DMI while supporting interfacial perpendicular magnetic anisotropy. The anticipated confluence of these properties, along with the tunability of multilayer films, make this material system a promising testbed for unveiling new spin configurations in FM/HM films.

cond-mat.mtrl-sci↗

arXiv2021

Dynamic symmetry breaking in chiral magnetic systems

The Dzyaloshinskii-Moriya interaction (DMI) in magnetic systems stabilizes spin textures with preferred chirality, applicable to next-generation memory and computing architectures. In perpendicularly magnetized heavy-metal/ferromagnet films, the interfacial DMI originating from structural inversion asymmetry and strong spin-orbit coupling favors chiral Néel-type domain walls (DWs) whose energetics and mobility remain at issue. Here, we characterize a new effect in which domains expand unidirectionally in response to a combination of out-of-plane and in-plane magnetic fields, with the growth direction controlled by the in-plane field strength. These growth directionalities and symmetries with applied fields cannot be understood from static treatments alone. We theoretically demonstrate that perpendicular field torques stabilize steady-state magnetization profiles highly asymmetric in elastic energy, resulting in a dynamic symmetry breaking consistent with the experimental findings. This phenomenon sheds light on the mechanisms governing the dynamics of Néel-type DWs and expands the utility of field-driven DW motion to probe and control chiral DWs.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic domain wall substructures in Pt/Co/Ni/Ir multi-layers

We examine the substructures of magnetic domain walls (DWs) in [Pt/(Co/Ni)$_M$/Ir]$_N$ multi-layers using a combination of micromagnetic theory and Lorentz transmission electron microscopy (LTEM). Thermal stability calculations of Q=$\pm$1 substructures (2-$π$ vertical Bloch lines (VBLs) and DW skyrmions) were performed using a geodesic nudged elastic band (GNEB) model, which supports their metastability at room temperature. Experimental variation in strength of the interfacial Dzyaloshinskii-Moriya interaction (DMI) and film thickness reveals conditions under which these substructures are present and enables the formation of a magnetic phase diagram. Reduced thickness is found to favor Q=$\pm$1 substructures likely due to the suppression of hybrid DWs. The results from this study provide an important framework for examining 1-D DW substructures in chiral magnetic materials.

cond-mat.mtrl-sci↗

arXiv2021

Direct observation of topological charge impacting skyrmion bubble stability in Pt/Ni/Co asymmetric superlattices

We characterize the magnetic properties and domain structure of Pt/Ni/Co asymmetric superlattices in comparison to the more established Pt/Co/Ni structure. This reversal in stacking sequence leads to a marked drop in interfacial magnetic anisotropy and the magnitude of the interfacial Dzyaloshinskii-Moriya interaction (DMI) as inferred from the DW structure, which we speculate could be related to a degradation of the Pt/Co interface when Pt is deposited on top of the Co layer. Lorentz transmission electron microscopy reveals exclusively Néel type domain walls and, with a perpendicular field, Néel skyrmions in the Pt/Co/Ni films. Conversely, the Pt/Ni/Co samples show only achiral Bloch domain walls, which leads to the formation of achiral Bloch ($Q=1$) and type II bubbles ($Q=0$) at increased perpendicular field. Combined with the reduced anisotropy leading to greater bubble densities, the latter case makes for an excellent test bed to examine the benefits of topological charge on stability. Simultaneous observation of Bloch and type II bubbles shows a roughly 50 mT larger annihilation field for the former. An in-plane component to the magnetic field is shown to both impact the structure of the formed bubbles and separately suppress the topological benefit.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic Nanohorns for measurement of the interfacial Dzyaloshinskii-Moriya interaction: A micromagnetic study

We evaluate the micromagnetic energy of a domain wall contained in a thin, horn-shaped nanostructure and propose a method to extract the strength of the interfacial Dzyaloshinskii-Moriya interaction based on the equilibrium position of the wall. An inplane magnetic field coupled with the interfacial DMI balances a competing driving force related to the "nanohorn" tapering and consequent variation in length. It is evident that the strength of the the DMI is directly correlated to the equilibrium position of the DW. Moreover, the range of measurable DMI values can be controlled by varying the degree of tapering in the nanohorn. Numerical and semi-analytical calculations were separately performed and show broad agreement. The calculations provided here offer an alternative framework for future measurement of DMI, which has been notoriously difficult to do experimentally.

cond-mat.mtrl-sci↗

arXiv2020

Formation of zero-field Skyrmion arrays in asymmetric superlattices

We demonstrate the formation of metastable Néel-type skyrmion arrays in Pt/Co/Ni/Ir multi-layers at zero-field following \textit{ex situ} application of an in-plane magnetic field using Lorentz transmission electron microscopy. The resultant skyrmion texture is found to depend on both the strength and misorientation of the applied field as well as the interfacial Dzyaloshinskii-Moriya interaction. To demonstrate the importance of the applied field angle, we leverage bend contours in the specimens which coincide with transition regions between skyrmion and labyrinth patterns. Subsequent application of a perpendicular magnetic field near these regions reveals the unusual situation where skyrmions with opposite magnetic polarities are stabilized in close proximity.

cond-mat.mtrl-sci↗

arXiv2019

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.

cond-mat.mtrl-sci↗

arXiv2019

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 Néel 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.

cond-mat.mes-hall↗

arXiv2018

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 Néel domain walls, which evolve into sub-100nm isolated Néel 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.

cond-mat.mtrl-sci↗

arXiv2018

Disentangling factors governing Dzyaloshinskii domain wall creep in Co/Ni thin films using Pt$_x$Ir$_{1-x}$ seedlayers

We characterize asymmetric growth of magnetic bubble domains in perpendicularly magnetized Co/Ni multi-layers grown on Pt$_x$Ir$_{1-x}$ seedlayers by application of perpendicular and in-plane magnetic fields. Using a refined model of domain wall creep that incorporates contributions from the anisotropic elastic energy, $\varepsilon$, and a chirality-dependent prefactor, $v_0$, we elucidate factors that govern the mobility of Dzyaloshinskii domain walls as a function of seedlayer composition. The interfacial Dzyaloshinskii-Moriya Interaction magnitude is found to decrease monotonically with $x_{Ir}$, which is independently confirmed by Brillouin light scattering (BLS). Moreover, the persistence of significant asymmetry in velocity curves across the full composition range supports previous assertions that a chirality-dependent attempt frequency akin to chiral damping could play a critical role in the observed trends. This work helps resolve fundamental questions about the factors governing Dzyaloshinskii DW creep and demonstrates varying Pt-Ir seedlayer composition as a method to tune DMI.

cond-mat.mtrl-sci↗

arXiv2017

Dispersive Stiffness of Dzyaloshinskii Domain Walls

It is well documented that subjecting perpendicular magnetic films which exhibit the interfacial Dzyaloshinskii-Moriya interaction (DMI) to an in-plane magnetic field results in a domain wall (DW) energy, $σ$, that is highly anisotropic with respect to the orientation of the DW in the film plane, $Θ$. We demonstrate that this anisotropy has a profound impact on the elastic response of the DW as characterized by the surface stiffness, $\tildeσ(Θ) = σ(Θ) + σ"(Θ)$, and evaluate its dependence on the length scale of deformation. The influence of the stiffness on DW mobility in the creep regime is assessed, with analytic and numerical calculations showing trends in $\tildeσ$ that better represent experimental measurements of domain wall velocity in magnetic thin films compared to $σ$ alone. Our treatment provides experimental support for theoretical models of the mobility of anisotropic elastic manifolds and makes progress toward a more complete understanding of magnetic domain wall creep.

cond-mat.mtrl-sci↗

arXiv2016

Energetic molding of chiral magnetic bubbles

Topologically protected magnetic structures such as skyrmions and domain walls (DWs) have drawn a great deal of attention recently due to their thermal stability and potential for manipulation by spin current, which is the result of chiral magnetic configurations induced by the interfacial Dzyaloshinskii-Moriya Interaction (DMI). Designing devices that incorporate DMI necessitates a thorough understanding of how the interaction presents and can be measured. One approach is to measure growth asymmetry of chiral bubble domains in perpendicularly magnetized thin films, which has been described elsewhere by thermally activated DW motion. Here, we demonstrate that the anisotropic angular dependence of DW energy originating from the DMI is critical to understanding this behavior. Domains in Co/Ni multi-layers are observed to preferentially grow into non-elliptical teardrop shapes, which vary with the magnitude of an applied in-plane field. We model the domain profile using energetic calculations of equilibrium shape via the Wulff construction, which explains both the teardrop shape and the reversal of growth symmetry at large fields.

cond-mat.mtrl-sci↗