Strong Hall Effect Research Articles

Inducing a tunable skyrmion-antiskyrmion system through ion beam modification of FeGe films

Skyrmions and antiskyrmions are nanoscale swirling textures of magnetic moments formed by chiral interactions between atomic spins in magnetic noncentrosymmetric materials and multilayer films with broken inversion symmetry. These quasiparticles are of interest for use as information carriers in next-generation, low-energy spintronic applications. To develop skyrmion-based memory and logic, we must understand skyrmion-defect interactions with two main goals—determining how skyrmions navigate intrinsic material defects and determining how to engineer disorder for optimal device operation. Here, we introduce a tunable means of creating a skyrmion-antiskyrmion system by engineering the disorder landscape in FeGe using ion irradiation. Specifically, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au4+ ions at varying fluences, inducing amorphous regions within the crystalline matrix. Using low-temperature electrical transport and magnetization measurements, we observe a strong topological Hall effect with a double-peak feature that serves as a signature of skyrmions and antiskyrmions. These results are a step towards the development of information storage devices that use skyrmions and antiskyrmions as storage bits, and our system may serve as a testbed for theoretically predicted phenomena in skyrmion-antiskyrmion crystals.

Proximity‐Induced Fully Ferromagnetic Order with Eightfold Magnetic Anisotropy in Heavy Transition Metal Oxide CaRuO3

AbstractFerromagnetic materials with a strong spin‐orbit coupling (SOC) have attracted much attention in recent years because of their exotic properties and potential applications in energy‐efficient spintronics. However, such materials are scarce in nature. Here, a proximity‐induced paramagnetic to ferromagnetic transition for the heavy transition metal oxide CaRuO3 in (001)‐(LaMnO3/CaRuO3) superlattices is reported. Anomalous Hall effect is observed in the temperature range up to 180 K. Maximal anomalous Hall conductivity and anomalous Hall angle are as large as ∼15 Ω−1 cm−1 and ∼0.93%, respectively, by one to two orders of magnitude larger than those of the typical 3d ferromagnetic oxides such as La0.67Sr0.33MnO3. Density functional theory calculations indicate the existence of avoid band crossings in the electronic band structure of the ferromagnetic CRO layer, which enhances Berry curvature thus strong anomalous Hall effects. Further evidences from polarized neutron reflectometry show that the CaRuO3 layers are in a fully ferromagnetic state (∼0.8 μB/Ru), in sharp contrast to the proximity‐induced canted antiferromagnetic state in 5d oxides SrIrO3 and CaIrO3 (∼0.1 μB/Ir). More than that, the magnetic anisotropy of the (001)‐(LaMnO3/CaRuO3) superlattices is eightfold symmetric, showing potential applications in the technology of multistate data storage.

Some Peculiarities of Plasma Flow and Acceleration in End-Face and Magnetoplasmodynamic Accelerators under Strong Hall Effect

Some Peculiarities of Plasma Flow and Acceleration in End-Face and Magnetoplasmodynamic Accelerators under Strong Hall Effect

The giant orbital Hall effect in Cr/Au/Co/Ti multilayers

The spin–orbit torques originating from the spin Hall effect of heavy metals are of vital importance for applications in spintronics due to its low consumption of energy. Theoretical calculations have predicted that 3d and 4d light metals can produce a similar amount of torques to heavy metals via the strong orbital Hall effect (OHE). However, few experiments have been conducted since it is technically challenging to directly detect the orbital current from the OHE. Here, we report an effective approach to demonstrate the strong orbital torques in the light metal Cr with the aid of a conversion process from the orbital current to the spin current by introducing an Au interfacial layer in the Cr/ferromagnet structures. A rather large orbital torque efficiency and an increase with the increasing thickness of the Cr-layer are attained in the perpendicularly magnetized Cr/Au/Co/Ti multilayers. Moreover, an energy efficient magnetization switching and the domain wall motion in Cr/Au/Co/Ti multilayers induced by the OHE have also been observed. Our findings confirm the existence of the orbital Hall torques in Cr and provide an effective way to investigate the OHE.

Growth of bilayer MoTe2 single crystals with strong non-linear Hall effect

The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe2 crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T′ MoTe2 single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe2 shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer Td phase MoTe2 under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer Td MoTe2 produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers.

Giant nonlinear Hall effect in strained twisted bilayer graphene

Recent studies have shown that moir\'e flat bands in twisted bilayer graphene (TBG) can acquire nontrivial Berry curvatures when aligned with a hexagonal boron nitride substrate, which can be manifested as a correlated Chern insulator near the 3/4 filling. In this Letter, we show that the large Berry curvatures in the moir\'e bands lead to a strong nonlinear Hall (NLH) effect in strained TBG with general filling factors. Under a weak uniaxial strain $\ensuremath{\sim}0.1%$, the Berry curvature dipole which characterizes the nonlinear Hall response can be as large as $\ensuremath{\sim}200\phantom{\rule{0.28em}{0ex}}\text{\AA{}}$, exceeding the values of previously known nonlinear Hall materials by two orders of magnitude. The dependence of the giant NLH effect as a function of electric gating, strain, and twist angle is further investigated systematically. Importantly, we point out that the giant NLH effect appears generically for a twist angle near the magic angle due to the strong susceptibility of nearly flat moir\'e bands to symmetry breaking induced by strains, which can even induce a topological band inversion. Our results establish TBG as a promising platform for investigating nonlinear effects such as the NLH effect, the nonlinear Nernst effect, and the nonlinear thermal Hall effect due to its giant Berry curvature dipole.

Giant orbital Hall effect and orbital-to-spin conversion in 3d , 5d , and 4f metallic heterostructures

The orbital Hall effect provides an alternative means to the spin Hall effect to convert a charge current into a flow of angular momentum. Recently, compelling signatures of orbital Hall effects have been identified in $3d$ transition metals. Here, we report a systematic study of the generation, transmission, and conversion of orbital currents in heterostructures comprising $3d$, $5d$, and $4f$ metals. We show that the orbital Hall conductivity of Cr reaches giant values of the order of $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ $[\frac{\ensuremath{\hbar}}{2e}]$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ and that Pt presents a strong orbital Hall effect in addition to the spin Hall effect. Measurements performed as a function of thickness of nonmagnetic Cr, Mn, and Pt layers and ferromagnetic Co and Ni layers reveal how the orbital and spin currents compete or assist each other in determining the spin-orbit torques acting on the magnetic layer. We further show how this interplay can be drastically modulated by introducing $4f$ spacers between the nonmagnetic and magnetic layers. Gd and Tb act as very efficient orbital-to-spin current converters, boosting the spin-orbit torques generated by Cr by a factor of 4 and reversing the sign of the torques generated by Pt. To interpret our results, we present a generalized drift-diffusion model that includes both spin and orbital Hall effects and describes their interconversion mediated by spin-orbit coupling.

Current-driven dynamics and ratchet effect of skyrmion bubbles in a ferrimagnetic insulator.

Magnetic skyrmions are compact chiral spin textures that exhibit a rich variety of topological phenomena and hold potential for the development of high-density memory devices and novel computing schemes driven by spin currents. Here, we demonstrate the room-temperature interfacial stabilization and current-driven control of skyrmion bubbles in the ferrimagnetic insulator Tm3Fe5O12 coupled to Pt, showing the current-induced motion of individual skyrmion bubbles. The ferrimagnetic order of the crystal together with the interplay of spin-orbit torques and pinning determine the skyrmion dynamics in Tm3Fe5O12 and result in a strong skyrmion Hall effect characterized by a negative deflection angle and hopping motion. Further, we show that the velocity and depinning threshold of the skyrmion bubbles can be modified by exchange coupling Tm3Fe5O12 to an in-plane magnetized Y3Fe5O12 layer, which distorts the spin texture of the skyrmions and leads to directional-dependent rectification of their dynamics. This effect, which is equivalent to a magnetic ratchet, is exploited to control the skyrmion flow in a racetrack-like device.

Shear-driven Hall-magnetohydrodynamic dynamos

Nonlinear Hall-magnetohydrodynamic dynamos associated with coherent structures in subcritical shear flows are investigated by using unstable invariant solutions. The dynamo solution found has a relatively simple structure, but it captures the features of the typical nonlinear structures seen in simulations, such as current sheets. As is well known, the Hall effect destroys the symmetry of the magnetohydrodynamic equations and thus modifies the structure of the current sheet and mean field of the solution. Depending on the strength of the Hall effect, the generation of the magnetic field changes in a complex manner. However, a too strong Hall effect always acts to suppress the magnetic field generation. The hydrodynamic/magnetic Reynolds number dependence of the critical ion skin depth at which the dynamos start to feel the Hall effect is of interest from an astrophysical point of view. An important consequence of the matched asymptotic expansion analysis of the solution is that the higher the Reynolds number, the smaller the Hall current affects the flow. We also briefly discuss how the above results for a relatively simple shear flow can be extended to more general flows such as infinite homogeneous shear flows and boundary layer flows. The analysis of the latter flows suggests that interestingly a strong induction of the generated magnetic field might occur when there is a background shear layer.

Magnetism and topological Hall effect in antiferromagnetic Ru2MnSn-based Heusler compounds

Heusler compounds and alloys based on them are of great recent interest because they exhibit a wide variety of spin structures, magnetic properties, and electron-transport phenomena. Their properties are tunable by alloying and we have investigated L21-orderd compound Ru2MnSn and its alloys by varying the atomic Mn:Sn composition. While antiferromagnetic ordering with a Néel temperature of 361 K was observed in Ru2MnSn, the Mn-poor Ru2Mn0.8Sn1.2 alloy exhibits properties of a diluted antiferromagnet in which there are localized regions of uncompensated Mn spins. Furthermore, a noncoplanar spin structure, evident from a topological Hall-effect contribution to the room-temperature Hall resistivity, is realized in Ru2Mn0.8Sn1.2. Our combined experimental and theoretical analysis shows that in the Ru2Mn0.8Sn1.2 alloy, the magnetic properties can be explained in terms of a noncoplanar antiferromagnetic scissor mode, which creates a small net magnetization in a magnetic field and subsequently yields a Berry curvature with a strong topological Hall effect.

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.

Probing the Source of the Interfacial Dzyaloshinskii-Moriya Interaction Responsible for the Topological Hall Effect in Metal/Tm3Fe5O12 Systems

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is responsible for the emergence of topological spin textures such as skyrmions in layered structures based on metallic and insulating ferromagnetic films. However, there is active debate on where the interfacial DMI resides in magnetic insulator systems. We investigate the topological Hall effect, which is an indication of spin textures, in Tm_{3}Fe_{5}O_{12} films capped with various metals. The results reveal that Pt, W, and Au induce strong interfacial DMI and topological Hall effect, while Ta and Ti cannot. This study also provides insights into the mechanism of electrical detection of spin textures in magnetic insulator heterostructures.

Highly Tunable Nonlinear Hall Effects Induced by Spin-Orbit Couplings in Strained Polar Transition-Metal Dichalcogenides

Recently, signatures of nonlinear Hall effects induced by Berry-curvature dipoles have been found in atomically thin 1T'/Td-WTe$_2$. In this work, we show that in strained polar transition-metal dichalcogenides(TMDs) with 2H-structures, Berry-curvature dipoles created by spin degrees of freedom lead to strong nonlinear Hall effects. Under an easily accessible uniaxial strain of order 0.2%, strong nonlinear Hall signals, characterized by a Berry-curvature dipole on the order of 1{\AA}, arise in electron-doped polar TMDs such as MoSSe, and this is easily detectable experimentally. Moreover, the magnitude and sign of the nonlinear Hall current can be easily tuned by electric gating and strain. These properties can be used to distinguish nonlinear Hall effects from classical mechanisms such as ratchet effects. Importantly, our system provides a potential scheme for building electrically switchable energy-harvesting rectifiers.

The importance of Hall effect on the magnetized thin accretion disc

ABSTRACT To study the role of Hall effect on the structure of accretion disc, we have considered a toroidal magnetic field in our paper. To study the vertical structure of the disc, we have written a set of magnetohydrodynamic (MHD) equations in the spherical coordinates (r, θ, ϕ) based on the two assumptions of axisymmetric and steady state. Also, we employed the self-similar solutions in the radial direction to obtain the structure of the disc in the θ-direction. We have solved a set of ordinary differential equations in the θ-coordinate with symmetrical boundary conditions in the equatorial plane. In order to describe the behaviour of Hall effect, we introduced the ΛH parameter that was called the dimensionless Hall Elsasser number. The strength of the Hall effect is measured by the inverse of dimensionless Hall Elsasser number. We have shown that the strong Hall effect decreases the accretion rate or infall velocity and size of inflow part. It has also been found the Hall effect is maximum in the equatorial plane and gets the value close to zero near the boundary, and it has the antidiffusive nature. The results display that the strong Hall effect makes the standard accretion sub-Keplerian disc becomes thinner. Our solutions have shown the Hall effect leads to transport magnetic flux outward in the upper layer of the disc and it produces outflows in the surface of the disc.

Intrinsic spin Hall conductivity of the semimetals MoTe2 and WTe2

We report a comprehensive study on the intrinsic spin Hall conductivity (SHC) of semimetals MoTe2 and WTe2 by ab initio calculation. Large SHC and desirable spin Hall angles have been discovered, due to the strong spin orbit coupling effect and low charge conductivity in semimetals. Diverse anisotropic SHC values, attributed to the unusual reduced-symmetry crystalline structure, have been revealed. We report an effective method on SHC optimization by electron doping, and exhibit the mechanism of SHC variation respect to the energy shifting by the spin Berry curvature. Our work provides insights into the realization of strong spin Hall effects in 2D systems.

Andreev reflection spectroscopy of ferromagnetic Fe<sub>0.26</sub>TaS<sub>2</sub> with layered structure

<sec>An elementary mission of spintronics research is to prevent the interface reacting in spin device and extract spin polarization of ferromagnetic material reliably. Layered transition metal sulfide has very strong anisotropic magnetism, magnetoresistance, and unique Hall effect. It provides a good platform for studying the magnetic order related physical phenomena and may lay a foundation for spintronic applications. In this work, the magnetism, electronic transport and Andreev reflection spectrum of a novel ferromagnetic material Fe<sub>0.26</sub>TaS<sub>2</sub> with a layers-stacked structure are measured. Strong magnetic anisotropy, double-peak magnetoresistance and anomalous Hall effect are found. In the magnetic measurement, the strong magnetic anisotropy behavior in Fe<sub>0.26</sub>TaS<sub>2</sub> single crystal is observed. Curie temperature <i>T</i><sub>C</sub> of the Fe<sub>0.26</sub>TaS<sub>2</sub> single crystal is confirmed by zero field cooling, field cooling and Arrot plot. The electronic transport in the Fe<sub>0.26</sub>TaS<sub>2</sub> single crystal also reveals strong anisotropic behaviors, such as butterfly-like magnetoresistance and obvious anomalous hall effect below <i>T</i><sub>C</sub>.</sec><sec>To obtain the spin polarization of Fe<sub><i>x</i></sub>TaS<sub>2</sub>, we fabricate an Fe<sub><i>x</i></sub>TaS<sub>2</sub>/superconductor Andreev junction to measure the spin polarization that is fitted by the modified Blonder-Tinkham-Klapwijk (BTK) theory. Perhaps the diffusion of Pb can form an alloy structure, creating another superconductor behavior. The two-gap BTK theory confirms our hypothesis, and the result spin polarization can reach 26%. To avoid the interference from Pb alloy superconductor, we also fabricate an Fe<sub>0.26</sub>TaS<sub>2</sub>/Al/Pb superconductor junction by evaporating Al and then Pb film on the surface of Fe<sub>0.26</sub>TaS<sub>2</sub> in sequence. The results of BTK fit show that the spin polarization from the first technical route cannot be reliable due to the tunneling layer on the Al interface. In order to obtain a clean interface, Fe<sub>0.26</sub>TaS<sub>2</sub>/NbSe<sub>2</sub> junction is fabricated through mechanical-exfoliation and dry-transfer method. Through the Andreev reflection spectrum of this junction, the spin polarization of Fe<sub>0.26</sub>TaS<sub>2</sub> is extracted to be 47% ± 7%. For various two-dimensional ferromagnetic materials, our work suggests that the dry-transfer method is well applicable in spin polarization extraction. The results of spin polarization indicate that the Fe<sub>0.26</sub>TaS<sub>2</sub> is a promising candidate of next-generation material of spintronics.</sec>

Band Signatures for Strong Nonlinear Hall Effect in Bilayer WTe_{2}.

Unconventional responses upon breaking discrete or crystal symmetries open avenues for exploring emergent physical systems and materials. By breaking inversion symmetry, a nonlinear Hall signal can be observed, even in the presence of time-reversal symmetry, quite different from the conventional Hall effects. Low-symmetry two-dimensional materials are promising candidates for the nonlinear Hall effect, but it is less known when a strong nonlinear Hall signal can be measured, in particular, its connections with the band-structure properties. By using model analysis, we find prominent nonlinear Hall signals near tilted band anticrossings and band inversions. These band signatures can be used to explain the strong nonlinear Hall effect in the recent experiments on two-dimensional WTe_{2}. This Letter will be instructive not only for analyzing the transport signatures of the nonlinear Hall effect but also for exploring unconventional responses in emergent materials.

Modification of magnetohydrodynamic waves by the relativistic Hall effect.

This study shows that a relativistic Hall effect significantly changes the properties of wave propagation by deriving a linear dispersion relation for relativistic Hall magnetohydrodynamics (HMHD). Whereas, in nonrelativistic HMHD, the phase and group velocities of fast magnetosonic wave become anisotropic with an increasing Hall effect, the relativistic Hall effect brings upper bounds to the anisotropies. The Alfvén wave group velocity with strong Hall effect also becomes less anisotropic than the nonrelativistic case. Moreover, the group velocity surfaces of Alfvén and fast waves coalesce into a single surface in the direction other than near perpendicular to the ambient magnetic field. It is also remarkable that a characteristic scale length of the relativistic HMHD depends on ion temperature, magnetic field strength, and density while the nonrelativistic HMHD scale length, i.e., ion skin depth, depends only on density. The modified characteristic scale length increases as the ion temperature increases and decreases as the magnetic field strength increases.

Strong anisotropic anomalous Hall effect and spin Hall effect in the chiral antiferromagnetic compoundsMn3X(X=Ge, Sn, Ga, Ir, Rh, and Pt)

We have carried out a comprehensive study of the intrinsic anomalous Hall effect and spin Hall effect of several chiral antiferromagnetic compounds, Mn$_3X$ ($X$ = Ge, Sn, Ga, Ir, Rh and Pt) by $ab~initio$ band structure and Berry phase calculations. These studies reveal large and anisotropic values of both the intrinsic anomalous Hall effect and spin Hall effect. The Mn$_3X$ materials exhibit a non-collinear antiferromagnetic order which, to avoid geometrical frustration, forms planes of Mn moments that are arranged in a Kagome-type lattice. With respect to these Kagome planes, we find that both the anomalous Hall conductivity (AHC) and the spin Hall conductivity (SHC) are quite anisotropic for any of these materials. Based on our calculations, we propose how to maximize AHC and SHC for different materials. The band structures and corresponding electron filling, that we show are essential to determine the AHC and SHC, are compared for these different compounds. We point out that Mn$_3$Ga shows a large SHC of about 600 $(\hbar/e)(\Omega\cdot cm)^{-1}$. Our work provides insights into the realization of strong anomalous Hall effects and spin Hall effects in chiral antiferromagetic materials.

Robust Zero-Field Skyrmion Formation in FeGe Epitaxial 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.