Topological Hall Effect Research Articles

Verification of topological magnetic properties of patterned ferromagnetic films

Topological order of magnetic films shows promise due to its unconventional electromagnetic transport effects, but the discontinuity and the miniaturization of patterned magnetic films impose a limit on research of topological properties. In this article, a technique is proposed, the application of which improves the ease of electrical measurement in sub-micrometer-sized magnetic thin film elements. To obtain the topological Hall resistivity of the discretized magnetic textures, a ferromagnetic bilayer film was fabricated into specific geometries. Micromagnetic simulations are presented that demonstrate the topological spin distribution in an exchange coupled patterned ferromagnetic film. Experiments are performed on FePt/FeNi bilayered films that are formed with square arrays of FeNi disks on top of a continuous FePt film in a topological spin distribution. The observation of topological Hall effect was made in the FePt/FeNi film without a nanoscale Hall-bar device. The technique presented facilitates the electrical property measurement even in nanometer elements and offers a pathway for promoting spintronics applications.

Spontaneous magnetic bubbles and large topological Hall effect in Mn3-xFexSn compound

Magnetic and transport properties as well as magnetic domain structures have been investigated in a hexagonal polycrystalline Mn2.1Fe0.9Sn compound. The in-situ observation of Lorentz transmission electron microscopy shows that spontaneous magnetic bubbles owing a pair of Bloch lines and biskyrmions consisting of two skyrmions of opposite spin helicities coexist in this compound at low temperature. A large topological Hall resistivity up to about 9 µΩ·cm at 50 K has been found owing to the formation of noncoplanar spin textures deriving from the competition among magnetocrystalline anisotropy, antiferromagnetic coupling, and ferromagnetic interaction.

Unusual Anomalous Hall Effect in a Co2MnSi/MnGa/Pt TrilayerSupported by the National Program on Key Basic Research Project under Grant No. 2018YFB0407601, the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant Nos. XDB44000000 and QYZDY-SSW-JSC015, and the National Natural Science Foundation of China under Grant Nos. 11874349 and 11774339.

An ultra-thin Co2MnSi(0.5 nm)/MnGa(1.5 nm) bilayer capped with Pt (5 nm) has been successfully grown by molecular-beam epitaxy. It is a potential candidate of synthetic antiferromagnets due to antiferromagnetic coupling between Co2MnSi and MnGa, which is a promising skyrmion-racetrack-memory medium without skyrmion Hall effect after capping with a Pt layer. Unusual humps in transverse Hall resistance loops are clearly observed in the temperature range from 260 to 400 K. This anomaly is generally attributed to topological Hall effect, but other than that, we prove that non-uniform rotation of magnetic moments in the bilayer with magnetic field sweeping is also a possible mechanism contributed to the unusual hump.

Motion tracking of 80-nm-size skyrmions upon directional current injections.

Nanometer-scale skyrmions are prospective candidates for information bits in low-power consumption devices owing to their topological nature and controllability with low current density. Studies on skyrmion dynamics in different classes of materials have exploited the topological Hall effect and current-driven fast motion of skyrmionic bubbles. However, the small current track motion of a single skyrmion and few-skyrmion aggregates remains elusive. Here, we report the tracking of creation and extinction and motion of 80-nm-size skyrmions upon directional one-current pulse excitations at low current density of the order of 109 A m-2 in designed devices with the notched hole. The Hall motion of a single skyrmion and the torque motions of few-skyrmion aggregates have been directly revealed. The results exemplify low-current density controls of skyrmions, which will pave the way for the application of skyrmions.

Magnetic field direction dependence of topological Hall effect like features in synthetic ferromagnetic and antiferromagnetic multilayers

The anomalies in the transverse resistivity are usually thought to be Topological Hall Effects (THEs), which have been considered as the trait of topologically nontrivial spin textures, such as the skyrmion phase. However, the origin of the THE-like features is still under debate. Here, we present the observation of THE-like features in synthetic antiferromagnetic [Pd/Co]/Ru/[Co/Pd] and synthetic ferromagnetic [Pd/Co]/NiO/[Co/Pd] structures. The Pd-rich alloys, which are formed due to the heterogeneous component and the gradual intermixing at the Co/Pd interface, result in a negative anomalous Hall effect coefficient, causing the peak and dip features in transport measurements. By changing the external magnetic field from out-of-plane to in-plane, the magnitude and width of the bump feature in THE curves can be modified, which is caused by the different anisotropy energy of the components in the heterogeneous ferromagnets. The present work broadens the perception of THE-like features and may add a different dimension to understand the magnetization reversal in magnetic multilayers.

Anomalous electrical magnetochiral effect by chiral spin-cluster scattering

The non-collinear spin configurations give rise to many nontrivial phenomena related to the Berry phase. They are often related to the vector and scalar spin chiralities. The scalar spin chirality leads to the topological Hall effect in metals, while the vector spin chirality to the ferroelectricity of spin origin, i.e., multiferroics in insulators. However, the role of the vector spin chirality in conducting systems has not yet been elucidated. Here we show theoretically that the spin correlation with vector spin chirality in chiral magnets scatters electrons asymmetrically, resulting in nonreciprocal transport phenomena, i.e., electrical magnetochiral effect (eMCE). This asymmetric scattering appears in the leading-order scattering term, implying a large nonreciprocity in the charge and spin currents. We find that the temperature and magnetic field dependence of the eMCE reproduces that observed in MnSi. Our results reveal the microscopic mechanism of eMCE and its potential in producing a large nonreciprocal response.

Room-temperature skyrmions in strain-engineered FeGe thin films

Skyrmions hold great promise for low-energy consumption and stable high-density information storage, and stabilization of the skyrmion lattice (SkX) phase at or above room temperature is greatly desired for practical use. The topological Hall effect can be used to identify candidate systems above room temperature, a challenging regime for direct observation by Lorentz electron microscopy. Atomically ordered FeGe thin films are grown epitaxially on Ge(111) substrates with $\ensuremath{\sim}4%$ tensile strain. Magnetic characterization reveals enhancement of the Curie temperature to 350 K due to strain, well above the bulk value of 278 K. A strong topological Hall effect was observed between 10 and 330 K, with a significant increase in magnitude observed at 330 K. The increase in magnitude occurs just below the Curie temperature, similar to the onset of the SkX phase in bulk FeGe. The results suggest that strained FeGe films may host a SkX phase above room temperature when significant tensile strain is applied.

Crystal Hall and crystal magneto-optical effect in thin films of SrRuO3

Motivated by the recently observed topological Hall effect in ultra-thin films of SrRuO3 (SRO) grown on the SrTiO3 [001] substrate, we investigate the magnetic ground state and anomalous Hall response of the SRO ultra-thin films by virtue of spin density functional theory (DFT). Our findings reveal that in the monolayer limit of an SRO film, a large energy splitting of Ru-t2g states stabilizes an anti-ferromagnetic (AFM) insulating magnetic ground state. For the AFM ground state, our Berry curvature calculations predict a large anomalous Hall response upon doping. From the systematic symmetry analysis, we uncover that the large anomalous Hall effect arises due to a combination of broken time-reversal and crystal symmetries caused by the arrangement of non-magnetic atoms (Sr and O) in the SRO monolayer. We identify the emergent Hall effect as a clear manifestation of the so-called crystal Hall effect in terminology of Šmejkal et al., Crystal Hall effect in collinear antiferromagnets (2019), and demonstrate that it persists at finite frequencies, which is the manifestation of the crystal magneto-optical effect. Moreover, we find a colossal dependence of the anomalous Hall effect on the degree of crystal symmetry breaking also in ferromagnetic SRO films, which all together points to an alternative explanation of the emergence of the topological Hall effect observed in this type of systems.

Oxygen Vacancy‐Induced Topological Hall Effect in a Nonmagnetic Band Insulator

AbstractThe discovery of skyrmions has recently sparked tremendous interest in topologically nontrivial spin textures. The signature of the noncoplanar nature of magnetic moments can be observed as topological Hall effect (THE) in electrical measurement. Realization of such nontrivial spin textures in new materials and through new routes is an ongoing endeavor due to their huge potential for future ultra‐dense, low‐power memory applications. In this work, oxygen vacancy (OV)‐induced THE and anomalous Hall effect (AHE) in a 5d0 system KTaO3 are reported. The observation of weak antilocalization behavior and THE in the same temperature range strongly implies the crucial role of spin–orbit coupling (SOC) behind the origin of THE. Ab initio calculations reveal the formation of the magnetic moment on Ta atoms around the OV and Rashba‐type spin texturing of conduction electrons. In the presence of Rashba SOC, the local moments around vacancy can form bound magnetic polarons (BMPs) with noncollinear spin texture, resulting in THE. Scaling analysis between transverse and longitudinal resistance establishes skew scattering‐driven AHE in the present case. This study opens a route to realize topological phenomena through defect engineering.

Investigation of the Role of Rare-Earth Elements in Spin-Hall Topological Hall Effect in Pt/Ferrimagnetic-Garnet Bilayers.

Topological magnetic textures such as skyrmions are being extensively studied for their potential application in spintronic devices. Recently, low-damping ferrimagnetic insulators (FMI) such as Tm3Fe5O12 have attracted significant interest as potential candidates for hosting skyrmions. Here, we report the detection of the spin-Hall topological Hall effect (SH-THE) in Pt/Tm3Fe5O12 and Pt/Y3Fe5O12 bilayers grown on various orientations of Gd3Ga5O12 substrates as well as on epitaxial buffer layers of Y3Sc2Al3O12, which separates the FMI from the substrate without sacrificing the crystal quality. The presence of SH-THE in all of the bilayers and trilayers provides evidence that rare-earth ions in either the FMI or substrate may not be critical for inducing an interfacial Dzyaloshinskii-Moriya interaction that is necessary to stabilize magnetic textures. Additionally, the use of substrates with various crystal orientations alters the magnetic anisotropy, which shifts the temperatures and strength of the SH-THE.

Topological Hall effect in weakly canted antiferromagnets

Motivated by a recent experiment on manganese oxide thin films, we theoretically study the topological Hall effect in a weakly canted antiferromagnet with textured N\'eel ($\mathbit{n}$) and uniform ($\mathbit{l}$) components of magnetization. Treating the N\'eel texture by a spin gauge field and the uniform component perturbatively, we obtain an analytical expression for the topological Hall conductivity. The result is proportional to the emergent magnetic field, $\mathbf{\ensuremath{\nabla}}\ifmmode\times\else\texttimes\fi{}{\mathbit{A}}_{\mathrm{AF}}$, where ${A}_{\mathrm{AF},i}=\mathbit{l}\ifmmode\cdot\else\textperiodcentered\fi{}({\ensuremath{\partial}}_{i}\mathbit{n}\ifmmode\times\else\texttimes\fi{}\mathbit{n})$ is an emergent vector potential in antiferromagnets, which consists of the spin-chirality density, $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbit{l}}\ifmmode\cdot\else\textperiodcentered\fi{}({\ensuremath{\partial}}_{x}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbit{l}}\ifmmode\times\else\texttimes\fi{}{\ensuremath{\partial}}_{y}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbit{l}})$, formed by the normalized uniform moment $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbit{l}}=\mathbit{l}/|\mathbit{l}|$, and one formed by the N\'eel and uniform components in the presence of spatial variation of canting. The result is discussed in comparison with the previous study on ferromagnets in the weak-coupling regime.

Two-channel anomalous Hall effect in SrRuO3

The Hall effect in SrRuO$_3$ thin-films near the thickness limit for ferromagnetism shows an extra peak in addition to the ordinary and anomalous Hall effects. This extra peak has been attributed to a topological Hall effect due to two-dimensional skyrmions in the film around the coercive field; however, the sign of the anomalous Hall effect in SrRuO$_3$ can change as a function of saturation magnetization. Here we report Hall peaks in SrRuO$_3$ in which volumetric magnetometry measurements and magnetic force microscopy indicate that the peaks result from the superposition of two anomalous Hall channels with opposite sign. These channels likely form due to thickness variations in SrRuO$_3$, creating two spatially separated magnetic regions with different saturation magnetizations and coercive fields. The results are central to the development of strongly correlated materials for spintronics.

Structure and strain tunings of topological anomalous Hall effect in cubic noncollinear antiferromagnet Mn3Pt epitaxial films

Antiferromagnets (AFMs) with chiral noncollinear spin structure have attracted great attention in recent years. However, the existing research has mainly focused on hexagonal chiral AFMs, such as Mn3Sn, Mn3Ga, Mn3Ge with low crystalline symmetry. Here, we present our systematical study for the face-centered cubic noncollinear antiferromagnetic Mn3Pt. By varying the alloy composition ( x ), we have successfully fabricated antiferromagnetic Mn1− x Pt x epitaxial films on MgO substrates and have observed a crystalline structure transition from L 10 MnPt to L 12 Mn3Pt. The Mn3Pt exhibits a large anomalous Hall effect, which is in the same order of magnitude as those of ferromagnetic materials. Moreover, a large thickness-evolved strain effect is revealed in Mn3Pt films by X-ray diffraction (XRD) analysis based on the Scherrer method. Our work explores Mn3Pt as a promising candidate for topological antiferromagnetic spintronics.

Chiral Magnonics: Reprogrammable Nanoscale Spin Wave Networks Based on Chiral Domain Walls.

SummarySpin waves offer promising perspectives as information carriers for future computational architectures beyond conventional complementary metal-oxide-semiconductor (CMOS) technology, owing to their benefits for device minimizations and low-ohmic losses. Although plenty of magnonic devices have been proposed previously, scalable nanoscale networks based on spin waves are still missing. Here, we demonstrate a reprogrammable two-dimensional spin wave network by combining the chiral exchange spin waves and chiral domain walls. The spin-wave network can be extended to two dimensions and offers unprecedented control of exchange spin waves. Each cell in the network can excite, transmit, and detect spin waves independently in the chiral domain wall, and spin-wave logics are also demonstrated. Our results open up perspectives for integrating spin waves into future logic and computing circuits and networks.

Giant Topological Hall Effect and Superstable Spontaneous Skyrmions below 330 K in a Centrosymmetric Complex Noncollinear Ferromagnet NdMn2Ge2.

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.

Validity of magnetotransport detection of skyrmions in epitaxial SrRuO3 heterostructures

A technically simple way of probing the formation of skyrmions is to measure the topological Hall resistivity that should occur in the presence of skyrmions as an additional contribution to the ordinary and anomalous Hall effect. This type of probing, lately intensively used for thin film samples, relies on the assumption that the topological Hall effect contribution can be extracted unambiguously from the measured total Hall resistivity. Ultrathin films and heterostructures of the $4d$ ferromagnet ${\mathrm{SrRuO}}_{3}$ have stirred up a lot of attention after the observation of anomalies in the Hall resistivity, which resembled a topological Hall effect contribution. These anomalies, first reported for bilayers in which the ${\mathrm{SrRuO}}_{3}$ was interfaced with the strong spin-orbit coupled oxide ${\mathrm{SrIrO}}_{3}$, were attributed to the formation of tiny N\'eel-type skyrmions. Here we present the investigation of heterostructures with two magnetically decoupled and electrically parallel connected ${\mathrm{SrRuO}}_{3}$ layers. The two ${\mathrm{SrRuO}}_{3}$ layers deliberately have different thicknesses, which affects the coercive field and ferromagnetic transition temperature of the two layers, and the magnitude and temperature dependence of their anomalous Hall constants. The ${\mathrm{SrRuO}}_{3}$ layers were separated by ultrathin layers of either the strong spin-orbit coupling oxide ${\mathrm{SrIrO}}_{3}$ or of the large band-gap insulator ${\mathrm{SrZrO}}_{3}$. Our magnetic and magnetotransport studies confirm the additivity of the anomalous Hall transverse voltages for the parallel conducting channels originating from the two ferromagnetic ${\mathrm{SrRuO}}_{3}$ layers as well as the possibility to tune the global anomalous Hall resistivity by magnetic field, temperature, or structural modifications at the epitaxial all-oxide interfaces. The Hall voltage loops of these two-layer heterostructures demonstrate the possibility to generate humplike structures in the Hall voltage loops of ${\mathrm{SrRuO}}_{3}$ heterostructures without the formation of skyrmions and emphasize that the detection of skyrmions only by Hall measurements can be misleading.

Large anisotropic topological Hall effect in a hexagonal non-collinear magnet Fe5Sn3

We report the observation of a large anisotropic topological Hall effect (THE) in the hexagonal non-collinear magnet Fe5Sn3 single crystals. It is found that the sign of the topological Hall resistivity ρTH is negative when a magnetic field H is perpendicular to the bc-plane (H ⊥ bc-plane); however, it changes form negative to positive when H is parallel to the c-axis (H ∥ c-axis). The value of ρTH increased with the increasing temperature and reached approximately −2.12 μΩ cm (H ⊥ bc-plane) and 0.5 μΩ cm (H ∥c-axis) at 350 K, respectively. Quantitative analyses of the measured data suggest that the observed anisotropic THE may originate from the opposite scalar spin chirality induced by the magnetic fields perpendicular and parallel to the c-axis, respectively.

Topological Hall Effect in Traditional Ferromagnet Embedded with Black-Phosphorus-Like Bismuth Nanosheets.

Topological Hall effect is an abnormal Hall response arising from the scalar spin chirality of chiral magnetic textures. Up to now, such an effect is only observed in certain special materials, but rarely in traditional ferromagnets. In this work, we have implemented the molecular beam epitaxy technique to successfully embed black-phosphorus-like bismuth nanosheets with strong spin-orbit coupling into the bulk of chromium telluride Cr2Te3, as evidenced by atomically resolved energy dispersive X-ray spectroscopy mapping. Distinctive from pristine Cr2Te3, these Bi-embedded Cr2Te3 epitaxial films exhibit not only pronounced topological Hall effects, but also magnetoresistivity anomalies and differential magnetic susceptibility plateaus. All these experimental features point to the possible emergence of magnetic skyrmions in Bi-embedded Cr2Te3, which is further supported by our numerical simulations with all input parameters obtained from the first-principle calculations. Therefore, our work demonstrates a new efficient way to induce skyrmions in ferromagnets, as well as the topological Hall effect by embedding nanosheets with strong spin-orbit couplings.

Topological Hall Effect in Single Thick SrRuO3 Layers Induced by Defect Engineering

AbstractThe topological Hall effect (THE) has been discovered in ultrathin SrRuO3 (SRO) films, where the interface between the SRO layer and another oxide layer breaks the inversion symmetry resulting in the appearance of THE. Thus, THE only occurs in ultrathin SRO films of several unit cells. In addition to employing a heterostructure, the inversion symmetry can be broken intrinsically in bulk SRO by introducing defects. In this study, THE is observed in 60‐nm‐thick SRO films, in which defects and lattice distortions are introduced by helium ion irradiation. The irradiated SRO films exhibit a pronounced THE in a wide temperature range from 5 to 80 K. These observations can be attributed to the emergence of Dzyaloshinskii–Moriya interaction as a result of artificial inversion symmetry breaking associated with the lattice defect engineering. The creation and control of the THE in oxide single layers can be realized by ex situ film processing. Therefore, this work provides new insights into the THE and illustrates a promising strategy to design novel spintronic devices.

Topological thermal Hall effect for topological excitations in spin liquid: Emergent Lorentz force on the spinons

We study the origin of Lorentz force on the spinons in a U(1) spin liquid. We are partly inspired by the previous observation of gauge field correlation in the pairwise spin correlation using the neutron scattering measurement by P.A. Lee an N. Nagaosa [PhysRevB 87,064423(2013)] when the Dzyaloshinskii-Moriya interaction intertwines with the lattice geometry. We extend this observation to the Lorentz force that exerts on the (neutral) spinons. The external magnetic field, that polarizes the spins, effectively generates an internal U(1) gauge flux for the spinons and twists the spinon motion through the Dzyaloshinskii-Moriya interaction. Such a mechanism for the emergent Lorentz force differs fundamentally from the induction of the internal U(1) gauge flux in the weak Mott insulating regime from the charge fluctuations. We apply this understanding to the specific case of spinon metals on the kagome lattice. Our suggestion of emergent Lorentz force generation and the resulting topological thermal Hall effect may apply broadly to other non-centrosymmetric spin liquids with Dzyaloshinskii-Moriya interaction. We discuss the relevance with the thermal Hall transport in kagome materials volborthite and kapellasite.