Near room-temperature formation of a skyrmion crystal in thin-films of the helimagnet FeGe

The topological Hall effect (THE) of electrons in skyrmion crystals (SkXs) is strongly related to the quantum Hall effect (QHE) on lattices. This relation suggests to revisit the QHE because its Hall conductivity can be unconventionally quantized. It exhibits a jump and changes sign abruptly if the Fermi level crosses a van Hove singularity. In this Paper, we investigate the unconventional QHE features by discussing band structures, Hall conductivities, and topological edge states for square and triangular lattices; their origin are Chern numbers of bands in the SkX (THE) or of the corresponding Landau levels (QHE). Striking features in the energy dependence of the Hall conductivities are traced back to the band structure without magnetic field whose properties are dictated by the lattice geometry. Based on these findings, we derive an approximation that allows us to determine the energy dependence of the topological Hall conductivity on any two-dimensional lattice. The validity of this approximation is proven for the honeycomb lattice. We conclude that SkXs lend themselves for experiments to validate our findings for the THE and—indirectly—the QHE.

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.

Magnetic anti-skyrmions are one of several chiral spin textures that are of great current interest both for their topological characteristics and potential spintronic applications. Anti-skyrmions were recently observed in the inverse tetragonal Heusler material Mn1.4Pt0.9Pd0.1Sn. Here we show, using Lorentz transmission electron microscopy, that anti-skyrmions are found over a wide range of temperature and magnetic fields in wedged lamellae formed from single crystals of Mn1.4Pt0.9Pd0.1Sn for thicknesses ranging up to ~250 nm. The temperature-field stability window of the anti-skyrmions varies little with thickness. Using micromagnetic simulations we show that this intrinsic stability of anti-skyrmions can be accounted for by the symmetry of the crystal lattice which is imposed on that of the Dzyaloshinskii-Moriya exchange interaction. These distinctive behaviors of anti-skyrmions makes them particularly attractive for spintronic applications.

We have carried out detailed measurements on the electric field vs. current density (E-J) characteristics of GdBa2Cu3O7−δ (GdBCO) coated conductors (CC) in a wide range of temperature and external magnetic fields. Four probe measurements were performed in a 20 T superconducting magnet system. Applying a constant electric field criterion, we have evaluated critical currents and n-values as a function of the temperature, the magnetic field and the field angle. Those results are relevant for the understanding of the practical performance of the tapes, and therefore to improve the process conditions effectively. The GdBCO CC was fabricated by the reel-to-reel PLD methods on the IBAD-MgO based substrate. The Self-field critical current per cm-width of the tape was above 400 A at 77 K. The results were also compared with our previous results on YBCO CC obtained from the similar deposition process. It is noted that the critical current of GdBCO CC is superior to those of previous YBCO CC over a wide range of practical external field and temperature.

It has been demonstrated that perovskite oxides with the general formula ABO3 (A - lanthanides, alkaline-earth metals, B - transition metal cations) exhibit the colossal magnetoresistance (CMR) phenomenon and can be used for various spintronics applications [1]. Many efforts were made in structural and chemical engineering of such materials in order to tune their functional properties along a wide range of temperatures and magnetic fields. It was found that epitaxial films exhibit high magnetoresistance (MR) values only in the vicinity of phase transition temperature, while polycrystalline films exhibit significant MR in a wide range of temperatures. Engineering of lanthanum manganites with special nanocrystalline (polycrystalline with nanosize grains) structure enabled to develop so-called CMR-B-scalar sensors, which are capable to measure the magnitude of pulsed magnetic fields in very small volumes independently on field direction [2]. Such sensors have been used in advanced scientific equipment to measure magnetic field dynamics [3-4]. It is worth to note, that in the past decades the most extensive research was performed on A-site doped manganites. This doping results in a mixed valance state of the manganese ions (Mn3+ and Mn4+) giving rise to the ferromagnetic (FM) double exchange (DE) interaction, which determines transport and magnetic properties of these materials. On the other hand, it was proposed that the B-site doping with 3d ions (Co, etc.) would destroy the long-range FM ordering of the Mn3+-O2--Mn4+ network resulting in a complex glassy state which arises from the competition of FM and antiferromagnetic (AFM) interactions. Therefore, significant change of the magnetic and electrical properties of manganites is expected. It was demonstrated, that substitution of Co for Mn in La1-xSrxMnO3 results in increased room temperature MR [5]. A variety of competing magnetic interactions in Co-doped manganites [6] still remains unclear, especially in nanostructured films, therefore, their investigations are of great interest. In addition, the long-term stability [7] of magnetoresistive properties of such films is very important for magnetic field sensors applications.In this study, the resistivity ρ and MR of nanostructured Co-doped manganite films deposited at different growth rate was investigated in magnetic fields up to 20 T at room and cryogenic temperatures. The change of these parameters were studied during the long-term storage of the films and accelerated ageing by annealing the films at O2 and Ar atmosphere.The La0.81Sr0.19Mn1.09Co0.06O3 films (LSMCO) with a thickness of 360 nm were deposited by using a Pulsed-Injection Metal-Organic Chemical Vapor Deposition (PI MOCVD ) technique onto a polycrystalline Al2O3 substrate at 600 °C temperature keeping different growth rates (9 nm/min, 13 nm/min, and 27 nm/min). The Ag electrodes of the samples were thermally deposited on a Cr sublayer and post annealed at 450°C for 1 h. The ρ dependence on temperature was measured in the temperature range of (5–310) K. The MR measurements were performed in permanent (0-0.8) T magnetic field at (25–290) K and pulsed fields up to 20 T at (80–290) K temperatures. The ρ and MR of films deposited at different growth rates was investigated after such preparation steps: 1) formation of electrodes and post annealing at 450°C for 1 h in Ar or O2 atmosphere; 2) long-term storage at room temperature for 6 months; 3) accelerated ageing procedure: additional annealing at 100 °C for 8 h in Ar or O2.It was found that electrode formation conditions and the long-term storage significantly changes the properties of the films (see Fig.1). After 6 months the resistivity maximum ρm of all films significantly increased and the metal-insulator transition temperature Tm shifted to lower temperatures. The annealing at 100 °C for 8 h in O2 decreased the ρ due to oxygen saturation of the films. The resistivity of nanostructured films mostly depends on the quality of grain boundary material, therefore, the largest changes were obtained for LSMCO film grown at the highest rate (27 nm/min). The annealing in Ar only slightly affected the ρ values. The low-field (0.7 T) MR (Fig.2) changed insignificantly in all temperature range (25-290) K. This is probably related with the change of a number of charge carriers’ hopping centers Mn3+-O2--Mn4+, which depends on the oxygen saturation level. Therefore, despite the resistivity changes, the MR=100%×[ρ(B)/ρ(0)-1] was only slightly affected. At 20 T the MR also insignificantly depended on preparation conditions and was ~(65-70)% at 80 K and (55-60)% at 290 K.It was concluded, that for magnetic field sensors application the LSMCO films grown at lower rate (9 nm/min) and annealed at Ar atmosphere are preferable due more stable parameters. **

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 have investigated the in-field current transport property in BaHfO3 doped GdBa2Cu3O7 - δ coated conductors in a wide range of temperatures and magnetic fields. Significant improvement of in-field critical current Ic was observed, e.g., Ic@77 K, 3 T = 93 A/cm-w, Ic@20nK, 17nT = 700 A/cm-w, which is a comparable value to that of Nb3Sn wire at 4.2 K. Enhancement of the irreversibility field was also observed. These results suggest that BaHfO3 is one of the most promising materials as effective artificial pinning centers and leads to the enhancement of in-field Ic. Furthermore, we have also shown that our analytical expression of electric field versus current density characteristics based on the percolation transition model [1-3] agrees well with the experimental results over a wide range of magnetic fields and temperatures. This analytical expression is useful for the design of superconducting devices because this allows us to predict the current carrying capability of coated conductors not only Jc but also n-value at arbitrary operating conditions of temperature and magnetic field.

Magnetic skyrmions hold promise for spintronic devices. To explore the dynamical properties of skyrmions in devices, a nanoscale method to image spin textures in response to a stimulus is essential. Here, we apply a technique for operando electrical current pulsing of chiral magnetic devices in a Lorentz transmission electron microscope. In ferromagnetic multilayers with interfacial Dzyaloshinskii–Moriya interaction, we study the creation and annihilation of skyrmions localized by point-like pinning sites due to defects. Using a combination of experimental and micromagnetic techniques, we establish a thermal contribution for the creation and annihilation of skyrmions in our study. Our work reveals a mechanism for controlling skyrmion density, which enables an examination of skyrmion magnetic field stability as a function of density. We find that high-density skyrmion states are more stable than low-density states or isolated skyrmions resisting annihilation over a magnetic field range that increases monotonically with density.

The topological Hall effect (THE), given by a composite of electric and\ntopologically non-trivial spin texture is commonly observed in magnetic\nskyrmion crystals. Here we present a study of the THE of electrons coupled to\nantiferromagnetic Skyrmion lattices (AF-SkX). We show that, in the strong Hund\ncoupling limit, topologically non-trivial phases emerge at specific fillings.\nInterestingly, at low filling an external field controlling the magnetic\ntexture, drives the system from a conventional insulator phase to a phase\nexhibiting THE. Such behavior suggests the occurrence of a topological\ntransition which is confirmed by a closing of the bulk-gap that is followed by\nits reopening, appearing simultaneously with a single pair of helical edge\nstates. This transition is further verified by the calculation of the the Chern\nnumbers and Berry curvature. We also compute a variety of observables in order\nto quantify the THE, namely: Hall conductivity and the orbital magnetization of\nelectrons moving in the AF-SkX texture.\n

We have investigated the low-energy dynamics of the triangular lattice of Skyrmions in a helimagnetic insulator Cu2OSeO3 in terms of microwave response. We have observed two elementary excitations of the Skyrmion with different polarization characteristics: the counterclockwise circulating mode at 1 GHz with the magnetic field polarization parallel to the Skyrmion plane and the breathing mode at 1.5 GHz with a perpendicular magnetic field polarization. These modes reflect the topological nature of Skyrmions and may play a central role in the Skyrmion dynamics.

The topological Hall effect (THE) is the transport response of chiral spin textures and thus can serve as a powerful probe for detecting and understanding these unconventional magnetic orders. So far, the THE is only observed in either noncentrosymmetric systems where spin chirality is stabilized by Dzyaloshinskii-Moriya interactions, or triangular-lattice magnets with Ruderman-Kittel-Kasuya-Yosida-type interactions. Here, a pronounced THE is observed in a Fe-Co-Ni-Mn chemically complex alloy with a simple face-centered cubic (fcc) structure across a wide range of temperatures and magnetic fields. The alloy is shown to have a strong magnetic frustration owing to the random occupation of magnetic atoms on the close-packed fcc lattice and the direct Heisenberg exchange interaction among atoms, as evidenced by the appearance of a reentrant spin glass state in the low-temperature regime and the first principles calculations. Consequently, THE is attributed to the nonvanishing spin chirality created by strong spin frustration under the external magnetic field, which is distinct from the mechanism responsible for the skyrmion systems, as well as geometrically frustrated magnets.

In the class of cubic B20 transition metal silicides and germanides, the skyrmions and skyrmion lattices have so far been studied most extensively. Certain groups of materials in which skyrmions were identified include the Perovskites and Heusler systems. In this paper, the skyrmions chiral Neel characterization is illustrated via Lorentz transmission electron microscopy. Further utilizing magnetic-imaging and Hall-transport in a functionally viable multilayer sample, the topological-hall resistivity rises over a wide range of temperature and magnetic field with the isolated-skyrmion density is observed, verifying the effect of the skyrmion geometric-phase on electron transport. The observed bulk Neel skyrmions in an exchange coupled cobalt/palladium(Co/Pd) based multi-layers of metallic compounds at high temperatures up to a maximum of 220 K followed by a larger region characterized by spin from in-to out-of-plane. In fact, the skyrmions are extremely resilient to in-plane magnetic fields and can be stable in a zero magnetic field using appropriate cooling methods in field over a very wide ambient temperature of up to 5.5 K. At low temperatures (of < 13 K) the Neel skyrmions have been observed recently in distinct non-metallic compounds, in bulk crystals with broken inversion symmetry with a non-adiabatic footprint, multiband transport, interfacial interactions.

&lt;sec&gt;It has been found that many magnetic materials possess the properties arising from skyrmions at room temperature. In addition to the common interaction energy, chiral interaction is also needed to form the skyrmion in magnetic material. There are four chiral magnetic interactions, namely: 1) Dzyaloshinskii-Moriya (DM) interaction; 2) long-ranged magnetic dipolar interaction; 3) four-spin exchange interaction; 4) frustrated exchanged interaction. Through the competition between exchange interaction and chiral interaction, magnetic skyrmion can be realized in magnetic material subject to a certain magnetic field and temperature. The skyrmion generated by the DM interaction features small size (5–100 nm), which is easy to adjust. The skyrmion can be driven by magnetic field or ultralow current density. The magnetic materials with skyrmion can exhibit the properties related to the skyrmion Hall effect, the topological Hall effect and the emergent electrodynamics, which are closely related to the skyrmion number. The existence of skyrmion in the magnetic material can be indirectly measured by topological Hall effect. The movement of skyrmion can be driven by spin polarized current in the direction either parallel or perpendicular to the current direction. The movement of the skyrmion driven by spin polarized currents will continue when the current is present, and will disappear when the current disappears. &lt;/sec&gt;&lt;sec&gt;In previous studies, magnetic skyrmions were realized in a variety of materials. However magnetic skyrmions were found only in very limited types of single crystal materials at room temperature or near room temperature. In recent years, scientists have discovered a variety of magnetic skyrmion materials at room temperature, including film materials (such as multilayer materials, artificial skyrmion materials) and crystal materialssuch as &lt;i&gt;β&lt;/i&gt;-Mn-type Co&lt;sub&gt;10–&lt;i&gt;x&lt;/i&gt;/2&lt;/sub&gt;Zn&lt;sub&gt;10–&lt;i&gt;x&lt;/i&gt;/2&lt;/sub&gt;Mn&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;, Fe&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;. Among all kinds of room temperature magnetic skyrmion materials, the most valuable one is the multilayer film material. The Skyrmion multilayer film has the advantages of small size, adjustable material type, simple preparation, good temperature stability, good device integration,etc. At the same time, skyrmion multilayer film is very easy to optimize by adjusting and constructing a special structure that has the wanted types of materials each with a certain thickness. Artificial skyrmion material obtains artificial skyrmion by constructing a micro-nano structure, therefore the artificial skyrmion with high-temperature stability can be realized by choosing high Curie temperature materials. There are a variety of materials which can realize the skyrmion above room temperature, such as Co&lt;sub&gt;9&lt;/sub&gt;Zn&lt;sub&gt;9&lt;/sub&gt;Mn&lt;sub&gt;2&lt;/sub&gt; (300–390 K) and Fe&lt;sub&gt;3&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt; (100–400 K). These room temperature materials further widen the temperature application range of skyrmion. The room temperature materials can be prepared or characterized by a variety of techniquesincluding sputtering for fabrication and X-ray magnetic circular dichroism-photoemission electron microscopy (XMCD-PEEM) for characterization. &lt;/sec&gt;&lt;sec&gt;The discovery of the magnetic skyrmion materials at room temperature not only enriches the research content of materials science, but also makes the skyrmion widely applicable in novel electronic devices (such as racetrack memory, microwave detector, oscillators). Because the skyrmion has the advantages of small size, ultra-low driving current density, and topological stability, it is expected to produce racetrack memory based on the skyrmion with low energy consumption, non-volatile and high density. The MTJ microwave detector based on skyrmion can be achieved with no external magnetic field nor bias current but with low power input (&lt; 1.0 μW); the sensitivity of the microwave detector can reach 2000 V·W&lt;sup&gt;–1&lt;/sup&gt;. The frequency of the oscillator based on skyrmion can be tuned by magnetic field or current, and moreover, the oscillato is very easy to integrate with IC. In this paper, first, the basic characteristic of magnetic skyrmion is introduced; and then room temperature magnetic skyrmion is reviewed; finally the advances of the racetrack memory, microwave detectors and oscillators are introduced, highlighting the development trend of room temperature magnetic skyrmion. &lt;/sec&gt;

The quantum Hall effect is generally understood for free electron gases, in which topologically protected edge states between Landau levels (LLs) form conducting channels at the edge of the sample. In periodic crystals, the LLs are imprinted with lattice properties; plateaus in the transverse Hall conductivity are not equidistant in energy anymore. Herein, crystals with a polyatomic basis are considered. For a stripe arrangement of different atoms, the band structure resorts nontrivially and exhibits strong oscillations that form a salient pattern with very small bandgaps. The Hall conductivity strongly decreases for energies within these bands, and only sharp peaks remain for energies in the gap. These effects are traced back to open orbits in the initial band structure; the corresponding LLs are formed from states with positive and negative effective mass. The partial cancellation of transverse charge conductivity also holds for different polyatomic stripe lattices and even when the magnetic field is replaced by a topologically nontrivial spin texture. The topological Hall effect is suppressed in the presence of magnetic skyrmions. The discussion is complemented by calculations of Hofstadter butterflies and orbital magnetization.

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.