Extended Skyrmion Phase in EpitaxialFeGe(111)Thin Films

B20 phase magnetic materials have been of significant interest because they enable magnetic Skyrmions. One major effort in this emerging field is the stabilization of Skyrmions at room temperature and zero magnetic field. We grow phase-pure, high crystalline quality FeGe epitaxial films on Si(111). Hall effect measurements reveal a strong topological Hall effect after subtracting the ordinary and anomalous Hall effects, demonstrating the formation of high density Skyrmions in FeGe films between 5 and 275K. In particular, a substantial topological Hall effect was observed at a zero magnetic field, showing a robust Skyrmion phase without the need of an external magnetic field.

Thin films of skyrmionic materials offer the stage of stable skyrmions in a wide temperature range below the magnetic transition temperature, which is important for applications to non-volatile memory devices. In this section, we investigate the stability of skyrmions in thin films by means of transport measurements. With the use of planar Hall effect (PHE), we have revealed the formation of the in-plane skyrmions in the MnSi epitaxial thin films, which can hardly be detected by the conventional detection methods such as Lorentz TEM and topological Hall effect. We also investigate the stability of quasi-two-dimensional skyrmion by mapping the magnitude of topological Hall resistivity.

In this chapter, we focus on the topological Hall effect in itinerant helimagnets. As described in Chap. 1, there have been few helimagnets which show topological Hall effect except for Skyrmion lattice phases of chiral helimagnets (e.g. MnSi and MnGe). This is because the total scalar spin chirality summed over the whole lattice sites often becomes zero due to structural symmetry, although the noncoplanar spin configuration is generally realized in helical magnets under applied \(H\). In the first and second sections of this chapter, we report two new examples of helimagnets in which the scalar spin chirality does not cancel out owing to the modulation of spin structure by DM interaction and thus topological Hall effect occurs. In the third section, we investigate the topological Hall effects by heat current, i.e. topological thermal Hall effect and topological Nernst effect, for MnGe. In contrast to the cases of MnSi and (Fe,Co)Si, the Skyrmion lattice phase of MnGe has been considered to prevail in wide temperature and magnetic-field window. This enables us to examine thoroughly the topological Hall effects by heat current in the Skyrmion lattice phase.KeywordsTopological Hall effectItinerant helimagnetScalar spin chiralityDzyaloshinsky-Moriya interactionBerry phase of electrons

The topological Hall effect is a hallmark of topologically non-trivial magnetic textures such as magnetic skyrmions. It quantifies the transverse electric current that is generated once an electric field is applied and occurs as a consequence of the emergent magnetic field of the skyrmion. Likewise, an orbital magnetization is generated. Here we show that the charge currents are orbital polarized even though the conduction electrons couple to the skyrmion texture via their spin. The topological Hall effect is accompanied by a topological orbital Hall effect even for s electrons without spin-orbit coupling. As we show, antiferromagnetic skyrmions and antiferromagnetic bimerons that have a compensated emergent field, exhibit a topological orbital Hall conductivity that is not accompanied by charge transport and can be orders of magnitude larger than the topological spin Hall conductivity. Skyrmionic textures serve as generators of orbital currents that can transport information and give rise to considerable orbital torques.

Fe3GaTe2 is a van der Waals (vdW) ferromagnet with a Curie temperature TC ranging from 350 to 380 K, followed upon cooling by a ferrimagnetic transition near room temperature. Substituting Fe with Co was previously reported to induce antiferromagnetism (AFM) at a Co fraction-dependent Néel temperature TN. In this work, we confirm the overall phase diagram of the Fe3-xCoxGaTe2 series as a function of x and temperature via magnetization and electrical transport measurements. For x ⩾ 0.6, the Hall effect is observed to mimic the magnetization as the AF ground state is suppressed by the external magnetic field via a metamagnetic transition, thus displaying an anomalous Hall response. At low temperatures, we also observe a pronounced topological Hall signal peaking at μ0H = 4 T, or within the metamagnetic transition region of fields. This observation points to the presence of magnetic field-induced chiral spin textures, such as skyrmions, upon approaching magnetization saturation. Magnetic force microscopy (MFM) reveals the emergence of nearly circular magnetic domains, with diameters on the order of 100-200 nm, within the antiferromagnetic phase. A detailed analysis of the MFM images indicates that the topological Hall effect is closely linked to the field-induced stabilization of magnetic domain structures, likely exhibiting chiral textures. This observation suggests the possible formation of skyrmions already in the AFM phase, i.e., AFM skyrmions, that evolve into ferromagnetic (FM) ones upon increasing the magnetic field. Consequently, Co-doped Fe3GaTe2 might provide a platform to investigate the transformation of skyrmions, initially coupled antiferromagnetically into ferromagnetic ones, and to explore its impact on the topological and skyrmion Hall effects.

We carry out a thorough study of the topological Hall and topological spin Hall effects in disordered skyrmionic systems: the dimensionless (spin) Hall angles are evaluated across the energy-band structure in the multiprobe Landauer-Büttiker formalism and their link to the effective magnetic field emerging from the real-space topology of the spin texture is highlighted. We discuss these results for an optimal skyrmion size and for various sizes of the sample and find that the adiabatic approximation still holds for large skyrmions as well as for nanoskyrmions. Finally, we test the robustness of the topological signals against disorder strength and show that the topological Hall effect is highly sensitive to momentum scattering.

Artificial single skyrmion and biskyrmion states achieved in magnetic nanodot pairs by micromagnetic simulation

Skyrmions hold promise for next-generation magnetic storage as their nanoscale dimensions may enable high information storage density and their low threshold for current-driven motion may enable ultra-low energy consumption. Skyrmion-hosting nanowires not only serve as a natural platform for magnetic racetrack memory devices but also stabilize skyrmions. Here we use the topological Hall effect (THE) to study phase stability and current-driven dynamics of skyrmions in MnSi nanowires. THE is observed in an extended magnetic field-temperature window (15–30 K), suggesting stabilization of skyrmions in nanowires compared with the bulk. Furthermore, we show in nanowires that under the high current density of 108–109 A m−2, the THE decreases with increasing current densities, which demonstrates the current-driven motion of skyrmions generating the emergent electric field in the extended skyrmion phase region. These results open up the exploration of skyrmions in nanowires for fundamental physics and magnetic storage technologies.

We study, in the absence of a magnetic field, the stabilization of skyrmions in a single-layered ferromagnet in the presence of next-nearest-neighbor exchange interactions including both the ferromagnetic exchange interaction and Dzyaloshinskii–Moriya exchange interaction. The stabilization of skyrmion depends on not only magnetic anisotropy but also the next-nearest-neighbor ferromagnetic exchange interaction. The latter stabilizes bimeron in the presence of in-plane magnetic anisotropy, while it enhances the stabilization of the ferromagnetic background in the presence of perpendicular magnetic anisotropy. Numerical simulations show that the next-nearest-neighbor ferromagnetic exchange interaction is a viable tool to control the creation and annihilation of skyrmionic states with a small size. This study may open an alternative avenue to the generation, stabilization, and control of magnetic skyrmions in the two-dimensional thin films.

Magnetic skyrmions with a topological particle nature have recently attracted attention as a potential information carrier for novel magnetic storage devices. For single-phase bulk crystals, skyrmions usually appear for a very narrow temperature region just below the magnetic ordering temperature ${T}_{c}$, and the stabilization of skyrmions for a wider temperature range remains an important challenge. Here, by investigating the impact of uniaxial tensile stress for a chiral magnet ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$, we demonstrate that only less than 0.2% of uniaxial elongation can dramatically stabilize skyrmions for an entire temperature range from ${T}_{c}$ to the lowest temperature. The stability of skyrmions essentially depends on the geometrical relationship among the directions of strain, magnetic field, and crystallographic axes, which is consistently explained in terms of the anisotropic modulation of the Dzyaloshinskii-Moriya interaction and magnetocrystalline anisotropy. Our finding may provide a good strategy for materials design to enhance the stability of skyrmions.

Magnetic skyrmions with topologically protected spin textures have recently attracted much attention due to their fascinating properties and potential application in advanced spintronics devices. For ferromagnetic thin films, skyrmions usually appear only in the presence of magnetic field. The stabilization of skyrmions in the absence of magnetic field remains an important challenge. Here, using a real-space phase field model based on Ginzburg-Landau theory, we demonstrate that a nonuniform strain can stabilize skyrmions in the FeGa thin film without a magnetic field. The phase field simulations show that the FeGa thin film exhibits a metastable skyrmion phase in the absence of magnetic field when a nonuniform strain with a cosine profile is applied. It is found that the metastable skyrmions can be transformed into a helical phase if a localized magnetic field or pulse of spin-polarized current is applied, resulting in the coexistence of skyrmion and helical phases in the ferromagnetic thin films. Furthermore, the skyrmion and helical phases can remain dynamically stable during the motion driven by a spin-polarized current. The coexistence of skyrmion and helical phases in the ferromagnetic thin films without magnetic field has potential application in skyrmion-based spintronic devices.

Although current induced motion of magnetic skyrmions have been extensively studied in recent work [1-3], manipulation of magnetic skyrmions that are fixed in space can also be utilized to implement such devices. We have recently shown that, skyrmion core reversal and switching between ferromagnetic states via an intermediate skyrmion state can be achieved in the free layer of an MTJ using Voltage Control of Magnetic Anisotropy (VCMA) which could result in energy efficient memory devices with smaller footprint [4-5]. In this work, we demonstrate experimental evidence of voltage controlled switching of fixed skyrmions in both continuous film [6] and confined geometry. The continuous heterostructure film stack consists of IrMn/CoFeB/MgO layers. The exchange biased structure allows stabilization of skyrmions without any external magnetic field. Upon application of a voltage pulse, the perpendicular magnetic anisotropy (PMA) changes at the ferromagnet/oxide interface. When the PMA is increased by applying a negative voltage pulse, skyrmions are annihilated. On the other hand, skyrmions can be recreated using a positive voltage pulse. We will also present micromagnetic simulations which reveal the detailed magnetization dynamics of this switching. Next, we will show voltage control of skyrmions in patterned nanodots composed of Ta/CoFeB/MgO multilayers. The geometric confinement is expected to influence the switching dynamics as shown in Fig. 1 [4,5]. To systematically modulate the confinement strength, we fabricated nanodots of varying lateral dimensions. The magnetization configurations are imaged using Magnetic Force Microscopy (MFM). Preliminary result of MFM imaging of skyrmion states under 1300 mV applied voltage in a ~5 µm dot is shown in Fig. 2. We will present detailed analysis of the confinement effect on skyrmion switching using further in-situ MFM imaging and rigorous micromagnetic simulations and compare these observations with the switching in thin films. Acknowledgement: NSF CCF collaborative grants: 1909030 and 1909416.

Skyrmions with topologically nontrivial spin textures are promising information carriers in next-generation ultralow power consumption and high-density spintronic devices. To promote their further development and utilization, the search for new room temperature skyrmion-hosting materials is crucial. Considering that most of the previous skyrmion-hosting materials are noncollinear magnets, here, the detection of the topological Hall effect (THE) and the discovery of skyrmions at room temperature are first reported in a centrosymmetric complex noncollinear ferromagnet NdMn2Ge2. Below 330 K, the compound can host stable Bloch-type skyrmions with about 75 nm diameter in a wide window of magnetic field and temperature, including zero magnetic field and room temperature. Moreover, the skyrmions can induce a giant topological Hall effect in a wide temperature range with a maximum value of -2.05 μΩ cm. These features make the compound attractive for both fundamental research and potential application in novel spintronic devices.

Magnetic skyrmions offer promising prospects for constructing future energy-efficient and high-density information technology, leading to extensive explorations of new skyrmionic materials recently. The topological Hall effect has been widely adopted as a distinctive marker of skyrmion emergence. Alternately, here we propose a novel signature of skyrmion state by quantitatively investigating the magnetoresistance (MR) induced by skyrmionic bubbles in CeMn2Ge2. An intriguing finding was revealed: the anomalous MR measured at different temperatures can be normalized into a single curve, regardless of sample thickness. This behavior can be accurately reproduced by the recent chiral spin textures MR model. Further analysis of the MR anomaly allowed us to quantitatively examine the effective magnetic fields of various scattering channels. Remarkably, the analyses, combined with the Lorentz transmission electron microscopy results, indicate that the in-plane scattering channel with triplet exchange interactions predominantly governs the magnetotransport in the Bloch-type skyrmionic bubble state. Our results not only provide insights into the quantum correction on MR induced by skyrmionic bubble phase, but also present an electrical probing method for studying chiral spin texture formation, evolution, and their topological properties, which opens up exciting possibilities for identifying new skyrmionic materials and advancing the methodology for studying chiral spin textures.

To the Editor.—The article by Thomson et al,1 “External Quality Assessment in the Examination of Blood Films for Malarial Parasites Within Ontario, Canada,” correctly identified a significant and potentially serious problem in malaria speciation. However, I am bewildered by their statement “These and other studies support the need for prompt examination of thick and thin blood films … ,” when their data provide a strong argument for abandoning thick films. Whereas there is no significant difference in the rate of false positives when thick and thin films are compared, the false-negative rate for thick films is 17 times higher than that for thin films. Unfortunately, the authors do not provide data that allow one to determine whether speciation was facilitated by the examination of thick films. However, it would be my experience and educated guess that speciation, as well, was better performed in thin films than thick films.I found it encouraging that a significant number of laboratories had shown the good sense to abandon the thick film preparation and examination despite obsolete recommendations and even accreditation requirements to the contrary. Virtually all modern laboratories have individuals skilled at preparing and interpreting thin blood smears. Very few have people with the skills to make good quality thick films or to interpret them. We should capitalize on our areas of strength. It makes no sense to continue to encourage shortcutting, the original intent in the preparation of thick films.Sam Thomson BA, ARTMDS Laboratory Services Kitchener, Ontario, Canada N2M 5N4Reinhard C. Lohmann MD, FRCPCLondon Health Sciences Centre London, Ontario, Canada N6A 4G5Linda Crawford MT(ASCP); Ruby Dubash; Harold Richardson MD, FRCPCLaboratory Proficiency Testing Program Toronto, Ontario, Canada M4W 1E6In Reply. Dr Sheehan questions the value of preparing and interpreting thick films for malarial parasites largely on the basis of lack of technical expertise. We are fully aware of these difficulties. However, it was not the purpose of our review to examine the value of the thick film versus the thin film.We do not believe that our results infer that “the false-negative rate for thick films is 17 times higher than that for thin films.” Nor did we discuss the value, if any, of thick films in malaria speciation. As stated in our article, scanty parasites and Plasmodium falciparum gametocytes may be easier to detect in thick films and therefore might be of value in low infections and in P falciparum speciation.It would be of interest to test scientifically the accuracy of the thick film in low-level parasitemia compared to the thin film. Until such time, we believe that prompt examination of both thick and thin films is advisable.