Magnetic Texture Research Articles

Large topological Hall effect and in situ observation of magnetic domain structures in the Mn2FeSn compound

The search for novel topological magnetic domain structures such as magnetic skyrmions, bubbles and vortices, and relative physical effect has always been the focus of spintronics devices. Here, the magnetism, transport properties and magnetic domain structures of the polycrystalline hexagonal Mn2FeSn compound are studied in detail. We find the strong texture along c-axis in this bulk compound owing to large temperature gradient between the bottom and top of the ingot during the preparation process. The resistivity resulting from topological Hall effect reaches about 5.19 μΩ cm at 50 K in the sample of compound obtained by cutting along perpendicular texture direction. Spontaneous nanoscale magnetic bubbles have directly been observed at low temperature due to the competition among the magnetocrystalline anisotropy, antiferromagnetic, and ferromagnetic exchange interactions as decreasing temperature. It is the formation of non-collinear or non-coplanar spin configuration that causes the obvious topological Hall effect. Additionally, the biskyrmion-like state can be obtained with evolution of stripe domains under an appropriate magnetic field. Based on these results, this study of Mn2FeSn magnet may be beneficial to the development for exploring novel magnetic spin textures.

Dynamical mass generation for ferromagnetic skyrmions in two dimensions

Magnetic skyrmions are topological magnetization textures that are characterized by the homotopy group of two dimensional spheres. Despite years of intensive research on skyrmions, the fundamental problem of the inertia of a skyrmion in driven motion remains unresolved. By properly taking into account a direct coupling between skyrmion motion and the correspondingly excited magnons, we identify a dynamical mass for the skyrmion in motion. The direct coupling between skyrmion motion and magnons can be employed to engineer skyrmion dynamics with magnons through ingenious material and geometry design.

Hybrid Bloch-Néel spiral states in Mn1.4PtSn probed by resonant soft x-ray scattering

Multiple intriguing phenomena have recently been discovered in tetragonal Heusler compounds, where $D_{2d}$ symmetry sets a unique interplay between Dzyaloshinskii-Moriya (DM) and magnetic dipolar interactions. In the prototype $D_{2d}$ compound Mn$_{1.4}$PtSn, this has allowed the stabilization of exotic spin textures such as first-reported anti-skyrmions or elliptic Bloch-type skyrmions. While less attention has so far been given to the low-field spiral state, this remains extremely interesting as a simplest phase scenario on which to investigate the complex hierarchy of magnetic interactions in this materials family. Here, via resonant small-angle soft x-ray scattering experiments on high-quality single crystals of Mn$_{1.4}$PtSn at low temperatures, we evidence how the underlying $D_{2d}$ symmetry of the DMI in this material is reflected in its magnetic texture. Our studies reveal the existence of a novel and complex metastable phase, which possibly has a mixed character of both the N\'{e}el-type cycloid and the Bloch-type helix, that forms at low temperature in zero fields upon the in-plane field training. This hybrid spin-spiral structure has a remarkable tunability, allowing to tilt its orientation beyond high-symmetry crystallographic directions and control its spiral period. These results broaden the reachness of Heusler $D_{2d}$ materials exotic magnetic phase diagram and extend its tunability, thus enhancing a relevant playground for further fundamental explorations and potential applications in energy saving technologies.

Chirality flips of skyrmion bubbles

The investigation of three-dimensional magnetic textures and chirality switching has attracted enormous interest from the perspective of fundamental research. Here, the three-dimensional magnetic structures of skyrmion bubbles in the centrosymmetric magnet MnNiGa were reconstructed with the vector field tomography approach via Lorentz transmission electron microscopy. The magnetic configuration of the bubbles was determined based on the reconstructed magnetic induction (B-field) at their surfaces and centers. We found that the bubbles easily switched their chirality but preserved their polarity to retain their singularity in the matrix of the material. Our results offer valuable insights into the chirality behavior of skyrmion bubbles.

Size-dependent mobility of skyrmions beyond pinning in ferrimagnetic GdCo thin films

Magnetic skyrmions are swirling magnetic textures that can be efficiently driven with spin-orbit torques with a deflected trajectory. However, pinning slows skyrmions down and alters their trajectory, which prevents a quantitative comparison to analytical models. Here, we study skyrmions driven by spin-orbit torques at room temperature in ferrimagnetic GdCo thin films, an amorphous material with low pinning. Above a sharp current depinning threshold, we observe a clearly linear velocity increase with current that extrapolates to zero and a constant deflection angle, reaching high velocities up to 200 m/s. The mobility increases and the depinning threshold current decreases with the skyrmion diameter, which we vary using an external magnetic field. An analytical model based on the Thiele equation quantitatively reproduces these findings with a single fitting parameter. This validates the linear flow regime description and shows, in particular, the important role of skyrmion size in its dynamics.

Quartic asymmetric exchange for two-dimensional ferromagnets with trigonal prismatic symmetry

We suggest a possible origin of noncollinear magnetic textures in ferromagnets (FMs) with the $D_{3h}$ point group symmetry. The suggested mechanism is different from the Dzyaloshinskii-Moriya interaction (DMI) and its straightforward generalizations. The considered symmetry class is important because a large fraction of all single-layer intrinsic FMs should belong to it. In particular, so does a monolayer Fe$_3$GeTe$_2$. At the same time, DMI vanishes identically in materials described by this point group, in the continuous limit. We use symmetry analysis to identify the only possible contribution to the free energy density in two dimensions that is of the fourth order with respect to the local magnetization direction and linear with respect to its spatial derivatives. This contribution predicts long-range conical magnetic spirals with both the average magnetization and the average chirality dependent on the spiral propagation direction. We relate the predicted spirals to a recent experiment on Fe$_3$GeTe$_2$. Finally, we demonstrate that, for easy-plane materials, the same mechanism may stabilize bimerons.

Real-space determination of the isolated magnetic skyrmion deformation under electric current flow

The manipulation and control of electron spins, the fundamental building blocks of magnetic domains and spin textures, are at the core of spintronics. Of particular interest is the effect of the electric current on topological magnetic skyrmions, such as the current-induced deformation of isolated skyrmions. The deformation has consequences ranging from perturbed dynamics to modified packing configurations. In this study, we measured the current-driven real-space deformation of isolated, pinned skyrmions within Co10Zn10 at room temperature. We observed that the skyrmions are surprisingly soft, readily deforming during electric current application into an elliptical shape with a well-defined deformation axis (semimajor axis). We found that this axis rotates unidirectionally toward the current direction irrespective of electric current polarity and that the elliptical deformation reverses back upon current termination. We quantified the average distortion δ, which increased by ∼90% during the largest applied current density |j| = 8.46 ×109 A/m2 when compared with the skyrmion's intrinsic shape ([Formula: see text]). Additionally, we demonstrated an approximately 120% average skyrmion core size expansion during current application, highlighting the skyrmions' inherent topological protection. This evaluation of in situ electric current-induced skyrmion deformation paints a clearer picture of spin-polarized electron-skyrmion interactions and may prove essential in designing spintronic devices.

Breathing mode dynamics of coupled three-dimensional chiral bobbers

Recently, three-dimensional (3D) magnetic textures have moved into the focus of spintronics as both technologically relevant and physically intriguing on a fundamental level. A rich variety of 3D textures is currently being investigated; however, their unambiguous experimental detection and detailed study remains challenging. In this work, a new type of chiral 3D spin-texture, consisting of two antiferromagnetically coupled Néel bobbers, is explored. The static properties of this structure depend on the chirality of the individual bobbers. Different chirality combinations are studied with regard to their phase stability regions by micromagnetic simulations and compared to antiferromagnetically coupled skyrmion tubes. Furthermore, the coupled internal breathing modes are investigated by application of a periodically alternating external magnetic field. The breathing modes of each studied system possess a unique fingerprint, which might allow for the identification of the resonating spin textures via their dispersion curves.

Quantized topological charges of ferroelectric skyrmions in two-dimensional multiferroic materials

In multiferroic materials, the microscopic magnetic and electric textures are strongly correlated with each other by magnetoelectric (ME) coupling. Therefore, topological electric dipole textures, such as ferroelectric (FE) dipole skyrmions and periodic FE dipole crystals, are expected to be induced if ferromagnetic (FM) skyrmions and FM skyrmionic crystals (or lattices so as to be abbreviated as SLs for convenience) can be stabilized. In the present work, a quantum computational approach is utilized to simulate the topological textures of FE SLs in a two-dimensional (2D) multiferroic system. Consequently, we find that, FE SLs can indeed be induced once the FM SLs are formed; each FE skyrmion is an ferroelectric dipolar complex formed around an FM skyrmion; the topological charges of these FE skyrmions are usually quantized to be integers, half integers and multiples of certain fractional values, so that the FE SL coincides precisely with the corresponding FM SL; the topological charge density of each FE SL also forms periodic pattern; and a normally applied electric field is able to change the sizes of FM skyrmions, elevate their formation temperatures, and destroy FM SLs below the critical temperatures.

Laser focus on magnetic materials

Exciting a magnetic material with ultrashort laser pulses can strongly modify the macroscopic properties of the medium. Waves of the magnetic texture, called spin waves, can be induced in a broad frequency range, from a few GHz up to tens of THz. How do these spin waves behave? Can their propagation be controlled and manipulated on extreme time- and length-scales? These questions lie at the heart of Dr Davide Bossini’s research.

Nano-scale collinear multi-Q states driven by higher-order interactions

Complex magnetic order arises due to the competition of different interactions between the magnetic moments. Recently, there has been an increased interest in such states not only to unravel the fundamental physics involved, but also with regards to applications exploiting their unique interplay with moving electrons. Whereas it is the Dzyaloshinskii-Moriya interaction (DMI) that has attracted much attention because of its nature to induce non-collinear magnetic order including magnetic-field stabilized skyrmions, it is the frustration of exchange interactions that can drive magnetic order down to the nano-scale. On top of that, interactions between multiple spins can stabilize two-dimensional magnetic textures as zero-field ground states, known as multi-Q states. Here, we introduce a two-dimensional itinerant magnet with various competing atomic-scale magnetic phases. Using spin-polarized scanning tunneling microscopy we observe several zero-field uniaxial or hexagonal nano-scale magnetic states. First-principles calculations together with an atomistic spin model reveal that these states are stabilized by the interplay of frustrated exchange and higher-order interactions while the DMI is weak. Unexpectedly, it is found that not only non-collinear magnetic states arise, but that higher-order interactions can also lead to collinear nano-scale multi-Q states.

Electrical Detection of Magnetic Skyrmions in a Magnetic Tunnel Junction

AbstractMagnetic skyrmions are promising information carriers for dense and energy‐efficient information storage owing to their small size, low driving‐current density, and topological stability. Electrical detection of skyrmions is a crucial requirement to drive skyrmion devices toward applications. The use of a magnetic tunnel junction (MTJ) is commonly suggested for this purpose as MTJs are key spintronic devices for large‐scale commercialization that can convert magnetic textures into electrical signals. To date, however, it has been challenging to realize skyrmions in MTJs due to incompatibility between standard skyrmion materials and highly efficient MTJ electrodes. Here, a material stack combining magnetic multilayers, which host 100 nm scale skyrmions, with a perpendicularly magnetized MTJ, is reported. The devices are designed so that the skyrmions in the multilayer are imprinted into the MTJ's free layer via magnetostatic interactions. The electrical response of a single skyrmion is successfully identified by employing simultaneous imaging of the magnetic texture and the electrical measurement of the MTJ resistance. The results are an important step toward all‐electrical detection of skyrmions.

Tuning the Dynamics of Chiral Domain Walls of Ferrimagnetic Films by Magnetoionic Effects

The manipulation of magnetism with a gate voltage is expected to lead the way towards the realization of energy-efficient spintronics devices and high-performance magnetic memories. Exploiting magneto-ionic effects under micro-patterned electrodes in solid-state devices adds the possibility to modify magnetic properties locally, in a non-volatile and reversible way. Tuning magnetic anisotropy, magnetization and Dzyaloshinskii-Moriya interaction allows modifying at will the dynamics of non trivial magnetic textures such as skyrmions and chiral domain walls in magnetic race tracks. In this work, we illustrate efficient magneto-ionic effects in a ferrimagnetic Pt/Co/Tb stack using a ZrO2 thin layer as a solid state ionic conductor. When a thin layer of terbium is deposited on top of cobalt, it acquires a magnetic moment that aligns antiparallel to that of cobalt, reducing the effective magnetization. Below the micro-patterned electrodes, the voltage-driven migration of oxygen ions in a ZrO2 towards the ferrimagnetic stack partially oxidizes the Tb layer, leading to the local variation not only of the spontaneous magnetization, but also of the effective magnetic anisotropy and of the Dzyaloshinskii-Moriya interaction. This leads to a huge increase of the domain wall velocity, which varies from 10 m/s in the pristine state to 250 m/s after gating. This non-volatile and reversible tuning of the domain wall dynamics may lead to applications to reprogrammable magnetic memories or other spintronic devices.

Magnetized FeSi film with micrometer thickness inserted with Cr layers as [formula omitted]-flippers for neutron spin-echo spectrometry

Magnetized soft ferromagnetic films with micrometer thickness can be used as π-flippers for a spin echo small-angle neutron scattering (SESANS) instrument. Two FeSi monolayers and two FeSi/Cr multilayers were fabricated using a direct-current magnetron sputtering technique. The thicknesses of the two FeSi monolayers were 114 nm and 1140 nm, respectively. The two FeSi/Cr multilayers with 2 and 20 periods, respectively, were composed of FeSi layers having thicknesses of 1000 nm and 100 nm, respectively, and 10 nm-thick Cr layers. Hysteresis loop measurements and X-ray diffraction measurements were used to characterize their magnetic properties and texture, respectively. Polarized neutron flipping efficiency measurements were conducted using the TPNR (Time-of-flight and Polarized Neutron Reflectometer) to determine the flipping efficiency of the FeSi/Cr multilayer with [100 nm FeSi/10 nm Cr]20. The experimental results show that the saturation magnetization of the FeSi-monolayers decreased and the coercivity increased as their thickness increased. The FeSi/Cr multilayer with [100 nm FeSi/10 nm Cr]20 had higher saturation magnetization (1300 emu/cm 3) and lower coercivity than those of [1000 nm FeSi/10 nm Cr]2, although the total thickness of the FeSi layers was almost the same, and its maximum flipping efficiency was up to 98% for polarized neutrons.

Circular stripe domains and cone state vortices in disk-shaped exchange coupled magnetic heterostructures

Vertically stacked exchange coupled magnetic heterostructures of cylindrical geometry can host complex noncolinear magnetization patterns. By tuning the interlayer exchange coupling between a layer accommodating magnetic vortex state and an out-of-plane magnetized layer, one can efficiently realize new topological chiral textures such as cone state vortices and circular stripe domains. We study how the number of circular stripes can be controlled by both the interlayer exchange coupling and the sample geometrical parameters. By varying geometrical parameters, a continuous phase transition between the homogeneous state, cone state vortex, circular stripe domains, and the imprinted vortex takes place, which is analysed by full scale micromagnetic simulations. The analytical description provides an intuitive pictures of the magnetization textures in each of these phases. The possibility to realize switching between different states allows for engineering magnetic textures with possible applications in spintronic devices.

Response of the chiral soliton lattice to spin-polarized currents

Spin polarized currents originate a spin-transfer torque that enables the manipulation of magnetic textures. Here we theoretically study the effect of a spin-polarized current on the magnetic texture corresponding to a chiral soliton lattice in a monoaxial helimagnet under a transverse magnetic field. At sufficiently small current density the chiral soliton lattice reaches a steady motion state with a velocity proportional to the intensity of the applied current, the mobility being independent of the density of solitons and the magnetic field. This motion is accompanied with a small conical distortion of the chiral soliton lattice. At large current density the spin-transfer torque destabilizes the chiral soliton lattice, driving the system to a ferromagnetic state parallel to the magnetic field. We analyze how the deformation of the chiral soliton lattice depends on the applied current density. The destruction of the chiral soliton lattice under current could serve as a possible erasure mechanisms for spintronic applications.

Confinement of Skyrmions in Nanoscale FeGe Device-like Structures.

Skyrmion-based devices have been proposed as a promising solution for low-energy data storage. These devices include racetrack or logic structures and require skyrmions to be confined in regions with dimensions comparable to the size of a single skyrmion. Here we examine skyrmions in FeGe device shapes using Lorentz transmission electron microscopy to reveal the consequences of skyrmion confinement in a device-like structure. Dumbbell-shaped elements were created by focused ion beam milling to provide regions where single skyrmions are confined adjacent to areas containing a skyrmion lattice. Simple block shapes of equivalent dimensions were also prepared to allow a direct comparison with skyrmion formation in a less complex, yet still confined, device geometry. The impact of applying a magnetic field and varying the temperature on the formation of skyrmions within the shapes was examined. This revealed that it is not just confinement within a small device structure that controls the position and number of skyrmions but that a complex device geometry changes the skyrmion behavior, including allowing skyrmions to form at lower applied magnetic fields than in simple shapes. The impact of edges in complex shapes is observed to be significant in changing the behavior of the magnetic textures formed. This could allow methods to be developed to control both the position and number of skyrmions within device structures.

Texture fluctuations and emergent dynamics in coupled nanomagnets

We analyse the thermal fluctuations of magnetization textures in two stray field coupled elements, forming mesospins. To this end, the energy landscape associated with the thermal dynamics of the textures is mapped out and asymmetric energy barriers are identified. These barriers are modified by changing the gap that separates the mesospins. Moreover, the coupling between the edges leads to an anisotropy in the curvature of the energy surface at the metastable minima. This yields a dynamic mode splitting of the edge modes and affects the attempt switching frequencies. Thus, we elucidate the mechanism with which the magnons in the thermal bath generate the stochastic fluctuations of the magnetization at the edges.

Manipulation of generation and evolution of topological spin texture in NdCo5 alloys by rapid solidification

Magnetic skyrmions are topologically stable magnetic textures with novel behaviors suitable for applications in future spintronic devices. Tuning the temperature range of topological spin textures is imperative for their potential applications. Here, by means of the rapid solidification, topological magnetic transitions in the NdCo5 bulk alloy are manipulated whereby skyrmions spontaneously generate at a higher temperature in ribbons. The direct observation of magnetic domain evolution via in situ Lorentz transmission electron microscopy reveals that the topological magnetic transition is dependent on the crystal orientations, where magnetic domain evolution is different within the a-b plane between the bulk and ribbons of NdCo5. Our work provides a way to tune the temperature range of topological spin textures and helps to clarify the importance of magnetic anisotropy on stabilizing the topological spin textures.

Universal Hopping Motion Protected by Structural Topology.

A scaling law elucidates the universality in nature, presiding over many physical phenomena which seem unrelated. Thus, exploring the universality class of scaling law in a particular system enlightens its physical nature in relevance to other systems and sometimes unearths an unprecedented new dynamic phase. Here, the dynamics of weakly driven magnetic skyrmions are investigated, and its scaling law is compared with the motion of a magnetic domain wall (DW) creep. This study finds that the skyrmion does not follow the scaling law of the DW creep in 2D space but instead shows a hopping behavior similar to that of the particle-like DW in 1D confinement. In addition, the hopping law satisfies even when a topological charge of the skyrmion is removed. Therefore, the distinct scaling behavior between the magnetic skyrmion and the DW stems from a general principle beyond the topological charge. This study demonstrates that the hopping behavior of skyrmions originates from the bottleneck process induced by DW segments with diverging collective lengths, which is inevitable in any closed-shape spin structure in 2D. This work reveals that the structural topology of magnetic texture determines the universality class of its weakly driven motion, which is distinguished from the universality class of magnetic DW creep.