Intrinsic Anomalous Hall Conductivity Research Articles

Weyl orbits as probe of chiral separation effect in magnetic Weyl semimetals

We consider magnetic Weyl semimetals. First of all we review relation of intrinsic anomalous Hall conductivity, band contribution to intrinsic magnetic moment, and the conductivity of chiral separation effect (CSE) to the topological invariants written in terms of the Wigner transformed Green functions (with effects of interaction and disorder taken into account). Next, we concentrate on the CSE. The corresponding bulk axial current is accompanied by the flow of the states in momentum space along the Fermi arcs. Together with the bulk CSE current this flow forms closed Weyl orbits. Their detection can be considered as experimental discovery of chiral separation effect. Previously it was proposed to detect Weyl orbits through the observation of quantum oscillations (Potter et al 2014 Nat. Commun. 5 5161). We propose the alternative way to detect existence of Weyl orbits through the observation of their contributions to Hall conductance.

Extended Berry Curvature Tail in Ferromagnetic Weyl Semimetals NiMnSb and PtMnSb.

Heusler compounds belong to a large family of materials and exhibit numerous physical phenomena with promising applications, particularly ferromagnetic Weyl semimetals for their use in spintronics and memory devices. Here, anomalous Hall transport is reported in the room-temperature ferromagnets NiMnSb (half-metal with a Curie temperature (TC) of 660 K) and PtMnSb (pseudo half-metal with a TC of 560 K). They exhibit 4 µB/f.u. magnetic moments and non-trivial topological states. Moreover, NiMnSb and PtMnSb are the first half-Heusler ferromagnets to be reported as Weyl semimetals, and they exhibit anomalous Hall conductivity (AHC) due to the extended tail of the Berry curvature in these systems. The experimentally measured AHC values at 2 K are 1.8×102Ω-1cm-1 for NiMnSb and 2.2×103Ω-1 cm-1 for PtMnSb. The comparatively large value between them can be explained in terms of the spin-orbit coupling strength. The combined approach of using ab initio calculations and a simple model shows that the Weyl nodes located far from the Fermi energy act as the driving mechanism for the intrinsic AHC. This contribution of topological features at higher energies can be generalized.

Intrinsic Anomalous Hall Conductivity of Strained Fe: A Combination Approach

In this study, the effect of elastic biaxial strain on electronic structure and the anomalous Hall conductivity (AHC) of α‐Fe has been systematically studied by using the combination of a first principles calculation with the tight binding method through the Wannier function. It is found that applying strain in cubic structure of α‐Fe results in the reduction of crystal symmetry and gives the modification on the bands near Fermi level, which causes a significant change in the AHC of strained systems. The AHC of strained α‐Fe is found to be sensitive with applying strain, which shows the large enhancement of AHC at +2% tensile strain with AHC value is up to 929 Ω−1 cm−1 (increases more than 30%) compared with the unstrained one, while slightly reduces when applying compressive strain. The modification of AHC is found to be related to the change of Berry curvature contributed from the spin orbit coupling (SOC) induced energy gap near Fermi level. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Engineering the topological states of Weyl ferromagnetic CoxMnGay films grown by molecular beam epitaxy

Due to its nontrivial topological state, a magnetic Weyl semimetal often exhibits exotic transport properties that are important for both fundamental physics and potential spintronics applications. In this Letter, we investigate the composition dependences of the structural order, magnetism, and transport properties for the epitaxial Heusler alloy CoxMnGay (CMG) topological Weyl semimetal films grown via molecular beam epitaxy. Our results show that the saturated magnetization, anomalous Hall conductivity, and anomalous Hall angle of CMG are influenced by its composition and structural order. Specifically, we observed that the optimized L21–Co2MnGa alloy exhibits a high intrinsic anomalous Hall conductivity of approximately 913 Ω−1 cm−1 at its maximum, which is attributed to the substantial Berry curvature within its electronic band structures. This study provides valuable insights into how to engineer the topological ferromagnetic state of the Weyl semimetals for future applications.

Pressure-Tuned Intrinsic Anomalous Hall Conductivity in Kagome Magnets RV6Sn6 (R = Gd, Tb)

Exploration of exotic phenomena in magnetic topological systems is at the frontier of condensed matter physics, holding a significant promise for applications in topological spintronics. However, complex magnetic structures carrying nontrivial topological properties hinder its progresses. Here, we investigate the pressure effect on the novel topological kagome magnets GdV6Sn6 and TbV6Sn6 to dig out the interplay between magnetic Gd/Tb layers and nonmagnetic V-based kagome sublattice. The pressure-tuned magnetic transition temperature T m in both the compounds exhibit a turning point at the critical pressure P c, accompanied with a sign reversal in anomalous Hall effect (AHE). The separation of intrinsic and extrinsic contributions using the Tian–Ye–Jin scaling model suggests that the intrinsic mechanism originating from the electronic Berry curvature holds the priority in the competition with extrinsic mechanism in AHE. The above-mentioned findings can be attributed to the combined effect of pressure-tuned band topology and magnetic interaction in segregated layers. Our results provide a practical route to design and manipulate the intrinsic AHE in magnetic topological materials.

Sign-flipping intrinsic anomalous Hall conductivity with Berry curvature tunability in a half-metallic ferromagnet NbSe2-VSe2 lateral heterostructure.

Single-layer half-metal magnets offer exciting scope in spin electronic quantum applications owing to improved spin transport, reduced interfacial resistance and streamlined device fabrication. Herein, we report the emergence of sign-flipping intrinsic anomalous Hall conductivity (AHC) as a result of changes in Berry curvature under an external electric field and half metallicity in a lateral heterostructure composed of centrosymmetric metallic monolayers 1T-NbSe2 and 1T-VSe2. The metallic monolayers 1T-NbSe2 and 1T-VSe2 laterally interfaced along the zigzag orientation break inversion symmetry at the interface and result in distinctive Berry curvature features. Furthermore, the half-metallic character was prominent with gapped states in the spin-up channel, while the spin-down state remained conductive; we observed the unique manifestation of sign-flipping intrinsic AHC at the Fermi level in addition to the electron- and hole-doped regions. This sign-flipping aspect of AHC at the Fermi level is of fundamental importance from the prospect of real-time device applications as it eliminates the necessity of supplementary actions, such as doping and strain engineering, which are traditionally employed to achieve AHC sign reversal. Additionally, a phase transition from half metal to metal occurs at a field of 0.5 V Å-1 and beyond. Half metallicity with sign switching AHC via external electric field makes the lateral NbSe2-VSe2 heterostructure a potential candidate for real-time energy-efficient low-power spintronic devices.

Topological properties and anomalous transport in van der Waals ferromagnets Fe n GeTe2: a comparative study

Recently synthesized topological van der Waals (vdW) Fe n GeTe2 (n=3,4, and 5) series of compounds have drawn a lot of research interest due to their interesting magnetic properties and anomalous transport behavior. However, understanding the topological aspect of these materials’ electronic band structure is either scant or absent in the literature. In this work, we perform a comparative study of the electronic structure of all three materials to understand their topological properties and their effect on transport properties using the first-principles calculations and to derive the effective tight-binding model for these systems. All three systems exhibit ferromagnetic behavior, and the contribution of various Fe d-orbitals are also studied. Spin-orbit coupling (SOC) causes the degeneracy to be lifted and results in a finite Berry curvature (BC). Finite BC contributes to the intrinsic anomalous Hall conductivity (AHC) and Nernst conductivity (ANC) of these systems. We show that Fe3GeTe2 and Fe5GeTe2 have similar nodal line formations, while Fe4GeTe2 exhibits nodal points close to Fermi energy E F . We calculate AHC (σ xy ) at zero temperature and ANC (α xy ) at room temperature for n = 3, 4 and 5 and find that our values are consistent with transport and magnetic measurement.

Magnetic and anomalous Hall effect investigations of co-sputtered Co2MnGa Heusler alloy thin films

The cobalt-based full Heusler alloy Co2MnGa (CMG) is well known for exhibiting an exotic phenomenon such as magnetic Weyl semimetallic nature with a high Curie temperature of ∼700 K and a giant anomalous Hall effect. Here, we report a detailed study of structural, electrical, and magnetic properties of Co2MnGa thin films (thickness in the 40–10 nm range) grown on Si(100) by the direct-current magnetron co-sputtering technique using Co and MnGa targets. Structural analysis of the samples revealed the polycrystalline nature of these films with B2 type structural ordering. The damping parameter decreases with the increase in film thickness and reaches the minimum value of 6.1 × 10−3 for a 40 nm thin CMG film. These CMG films are magnetically isotropic and soft ferromagnetic in nature. A remarkably high value of anomalous Hall conductivity (AHC) of 1920 S/cm (2 K) is found for the 40 nm thin film, which is comparable to earlier reported values on highly ordered CMG films. Nearly 73% of this AHC value originates from the intrinsic contribution. The AHC and longitudinal conductivity both increase with the film thickness. Different scaling mechanisms are used to compute the intrinsic and extrinsic contributions playing a role in AHC. The analysis of advanced scaling [by Tian et al., Phys. Rev. Lett. 103, 1–4 (2009)] performed on these CMG films suggests the consistency in the enhanced intrinsic AHC value irrespective of the thickness and a decrease in skew scattering contribution with thickness. These results will enhance the understanding about the magnetic and transport properties of Co2MnGa thin films of different thicknesses and suggest it to be a promising material for topospintronic applications.

CoFeSn, a possible contender for spintronics: A first-principles study

Anomalous carrier transport in magnetic Heusler compounds has evolved as a lively field of research owing to their unusual band structure and broken time-reversal symmetry. They have attractive properties for spintronics due to their high Curie temperature (Tc), high spin polarization, and extravagant transport properties. Here, we scrutinize CoFeSn. Based on the inferences from structural stability, lattice dynamics, and magnetic analysis, we propose a cubic polymorph of hexagonal CoFeSn and explain why we must consider cubic CoFeSn. Through density-functional-theory calculations, we predict a robust 3D half-metallic ferromagnetic compound, CoFeSn (P4¯3m) with a Tc ∼ 693 K, calculated via the Heisenberg magnetic exchange interactions under mean-field approximation, and a magnetic moment of 3 μB. In addition, Wannier interpolation suggests anomalous Hall conductivity (AHC) and spin Hall conductivity (SHC) in cubic CoFeSn, the largest SHC at the Fermi level being ≈ 47 (h/2πe) S/cm. Our theoretical results show that spin–orbit interaction at the Fermi level brings on finite Berry flux that gives an intrinsic AHC ∼ 122 S/cm at room temperature. We note that adjusting the Fermi level can be a sensible way to achieve high values of AHC or SHC. Our findings pave the way for the realization of the quantum anomalous and spin Hall effect in half-Heusler compounds.

Nodal-line symmetry breaking induced colossal anomalous Hall and Nernst effects in Cu2CoSn Heusler compound

The presence of topological band crossings near the Fermi energy is essential for the realization of large anomalous transport properties in the materials. The topological semimetals host such properties owing to their unique topological band structure, such as Weyl points or nodal lines (NLs), those are protected by certain symmetries of the crystal. When the NLs break out in the system, a large Berry curvature arises in the surrounding area of the gapped NL. In the present work, we studied anomalous transport properties of Cu2CoSn compound, which has a cubic Heusler crystal structure (space group: Fm3¯m). Cu2CoSn full Heusler compound possesses NLs at the Fermi level that is protected by mirror reflection symmetries of the lattice. Upon introducing the spin–orbit coupling in the Hamiltonian and by setting the magnetization axis along the [001] direction, we found that the NLs are gapped out and large Berry curvature arises in the system. The integral of Berry curvature gives the intrinsic anomalous Hall conductivity (AHC) about 1003 S/cm and the anomalous Nernst conductivity (ANC) about 3.98 A/m K at the Fermi level. These values of AHC and ANC are comparable to the largest reported values for the Co2MnGa Heusler compound. Therefore, Cu2CoSn becomes a newborn member of the family of full Heusler compounds, which possesses giant AHC and ANC that can be useful for the spintronics application.

Semiclassical kinetic theory for systems with non-trivial quantum geometry and the expectation value of physical quantities

Starting from the Keldysh theory, for a general low-energy N-band Hamiltonian in the clean limit, we perform a manifestly gauge-invariant semiclassical expansion. A generalized Berry curvature tensor is shown to control a redistribution of spectral weights. New expressions for certain physical quantities ensue, establishing the limits of a previously proposed correction to the density of states. In the two-band case, we derive a completely general semiclassical kinetic theory including all quantum corrections. As an application, we show how one can recover, out of a single simple calculation, the chiral anomaly, intrinsic anomalous Hall conductivity and chiral magnetic effect, in all generality. The demonstrated flexibility and efficiency of our formalism derives from the insulation it provides from the underlying complexity of the quantum kinetics, notwithstanding its rigorous connection to this deeper level.

Intrinsic Berry curvature driven anomalous Nernst thermopower in the semimetallic Heusler alloy CoFeVSb

Understanding of spin-heat coupling mechanisms and magnetothermoelectric phenomena, including the anomalous Nernst effect (ANE), in emergent quaternary Heusler alloys is of practical importance for applications in thermal management and energy harvesting. Here, we demonstrate an intrinsic Berry curvature mediated anomalous Nernst thermopower in CoFeVSb, which orders magnetically at high temperature (${T}_{C}\ensuremath{\approx}850\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) with a large saturation magnetization of $\ensuremath{\approx}2.2\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.$ at room temperature. We show that the electron-electron elastic and electron-magnon inelastic scattering dominate longitudinal electrical transport at low temperatures ($T\ensuremath{\le}50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$), whereas the electron-phonon and electron-magnon scatterings govern it at higher $\phantom{\rule{0.28em}{0ex}}T$. The longitudinal thermopower is resulted mainly from the diffusive contribution with a very large longitudinal Seebeck coefficient ($42\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 395 K). The value of the anomalous Nernst coefficient (${S}_{\mathrm{ANE}}$) for CoFeVSb at room temperature is $0.039\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ which is higher than the compressively strained $\mathrm{SrRu}{\mathrm{O}}_{3}$ film ($0.03\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$) as well as the spin gapless semiconductor CoFeCrGa ($0.018\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$). On lowering $T$, both the ordinary Nernst coefficient and carrier mobility increase but an opposite trend is found for ${S}_{\mathrm{ANE}}$. Our ab initio simulations reveal the topological semimetallic nature of CoFeVSb with a pair of Weyl points. These Weyl crossings result in a significant contribution to the Berry curvature, leading to an intrinsic anomalous Hall conductivity (${\ensuremath{\sigma}}_{xy}^{\mathrm{AHE}}$) of $\ensuremath{\approx}85$ S/cm, which matches well with experiment (77 S/cm at 2 K). Our experimental findings and ab initio calculations support the dominance of the intrinsic Berry curvature in the observed ANE. The ratio of ${\ensuremath{\sigma}}_{xy}^{\mathrm{AHE}}$ to the transverse anomalous thermoelectric conductivity (${\ensuremath{\alpha}}_{xy}^{\mathrm{ANE}}$) shows an increasing trend with $T$ attaining a sizable fraction of $\phantom{\rule{0.28em}{0ex}}\frac{{k}_{B}}{e}\phantom{\rule{0.28em}{0ex}}(\ensuremath{\approx}0.35\frac{{k}_{B}}{e})$ at room temperature.

Anomalous Hall effect in kagome ferromagnet YbMn6Sn6 single crystal

The rare-earth based magnetic kagome materials RMn6Sn6 have attracted tremendous interest for their potential to host exotic topological quantum states. Here, we report the observation of the anomalous Hall effect in the kagome ferromagnet YbMn6Sn6 single crystal. YbMn6Sn6 has an easy-plane ferromagnetic structure till 2 K without magnetic ordering of the R ions. The low-temperature magnetoresistance is positive and does not exhibit a tendency to saturate up to 5 T. The magnetoresistance becomes negative with the increasing temperature. An intrinsic anomalous Hall conductivity with the value of 40.2 Ω−1·cm−1 is obtained in YbMn6Sn6, which is associated with the Berry curvature of electronic band structure in momentum space. Our results indicate that YbMn6Sn6 may be an excellent platform to study the relationship between the magnetic and electronic structure and to explore novel quantum phenomenon.

Berry curvature induced unconventional electronic transport behaviors in magnetic topological semimetals

In recent years, more and more magnetic topological materials, especially magnetic Weyl semimetals, have been discovered, providing a platform for studying the electronic transport behavior. The strong Berry curvature of magnetic topological materials can significantly enhance the conventional transverse transport behaviors, and can also make the transport phenomena that have been overlooked or unobserved appear gradually. In this review, the semi-classical equation is used to understand the anomalous transport behaviors in magnetic topological materials. The intrinsic anomalous Hall conductivity is obtained by integrating the Berry curvature of the occupied states, which is determined by the electronic band structure. The topological electronic state can be modulated by magnetic field and doping, and the anomalous Hall conductivity was changed with the evolution of the Berry curvature. A linear positive magnetoresistance behavior associated with the Berry curvature and magnetic field is introduced, which establishes the relation between the Berry curvature and the longitudinal transport. Due to the presence of tilted Weyl cone, the conductivity terms related to the first power of magnetic field are observed in magnetic Weyl systems. These behaviors under the interaction of topology and magnetic provide a new understanding and insight for the electric transport behaviors. At last, this review also provides a viewpoint on the field of magnetic topological physics.

Intrinsically dominated anomalous Hall effect in pulsed laser deposited epitaxial Co2MnGe ferromagnetic full Heusler alloy thin films.

Large size epitaxial thin films of ferromagnetic Co2MnGe full Heusler alloy are grown over MgO(100) substrate by using pulsed laser deposition technique under optimized growth conditions. Metallic behavior is confirmed from the longitudinal resistivity-temperature data, while a minimum in the resistivity at ∼25 K is attributed to the disorder-induced weak localization effect. Importantly, a dominating intrinsic anomalous Hall conductivity value of ∼21 S cm-1 against an overall anomalous Hall conductivity value of ∼36 S cm-1 at the room temperature has been estimated for the epitaxial Co2MnGe film. The dominating intrinsic mechanism is also evident from the near temperature-independent behavior of the overall anomalous Hall conductivity.

Antisite disorder and Berry curvature driven anomalous Hall effect in the spin gapless semiconducting Mn2CoAl Heusler compound

Spin gapless semiconductors exhibit a finite band gap for one spin channel and a closed gap for another spin channel, and they have emerged as a new state of magnetic materials with a great potential for spintronic applications. The first experimental evidence for spin gapless semiconducting behavior was observed in an inverse Heusler compound ${\mathrm{Mn}}_{2}\mathrm{CoAl}$. Here, we report a detailed investigation of the crystal structure and anomalous Hall effect in ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ using experimental and theoretical studies. The analysis of the high-resolution synchrotron x-ray diffraction data shows antisite disorder between Mn and Al atoms within the inverse Heusler structure. The temperature-dependent resistivity shows semiconducting behavior and follows Mooij's criteria for disordered metal. The scaling behavior of the anomalous Hall resistivity suggests that the anomalous Hall effect in ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ is primarily governed by an intrinsic mechanism due to the Berry curvature in momentum space. The experimental intrinsic anomalous Hall conductivity (AHC) is found to be $\ensuremath{\sim}35$ S/cm, which is considerably larger than the theoretically predicted value for ordered ${\mathrm{Mn}}_{2}\mathrm{CoAl}$. Our first-principles calculations conclude that the antisite disorder between Mn and Al atoms enhances the Berry curvature and hence the value of intrinsic AHC, which is in very good agreement with the experiment.

Intrinsic anomalous Hall conductivity and real space Berry curvature induced topological Hall effect in Ni2MnGa magnetic shape memory alloy

Anomalous and topological Hall effect (THE) are the fascinating electronic transport properties in condensed matter physics and received tremendous interest in the field of spintronics. Here, we report the intrinsic anomalous Hall conductivity (AHC) and THE in the bulk Ni2MnGa magnetic shape memory alloy. The magnetization measurement reveals the premartensite, martensite and magnetic phase transitions. A detailed analysis of AHC reveals that the intrinsic Berry phase mechanism dominates over skew scattering and side jump in all the structural phases of Ni2MnGa. Further, an additional contribution in the Hall resistivity is observed as THE. The magnitude of the THE and its temperature independent behavior indicates that the THE arises due to the real space Berry curvature induced by topologically protected magnetic skyrmion textures in the martensite and premartensite phases of Ni2MnGa. The larger magnetic field is required to vanish the topological Hall resistivity in the martensite phase in comparison to the premartensite phase, which manifests the more stable skyrmion textures in the martensite phase. The present findings open a new direction in the field of functional materials, which hosts skyrmion, exhibits anomalous transport and magnetic shape memory effect.

Surface Oxygen Passivation-Driven Large Anomalous Hall Conductivity in Early Transition Metal-Based Nitride MXenes: Can AHC Be a Tool to Determine Functional Groups in 2D Ferro(i)magnets?

Identifying the existence of specific functional groups in MXenes is a difficult topic that has perplexed researchers for a long time. At the same time, the realization of intrinsic ferromagnetism, which is important for two-dimensional (2D) materials’ families, is one of the fascinating properties that MXenes offer. Here, using first-principle calculations, we have examined the actual magneto-transport phenomena in transition metals and nitride-based-functionalized MXenes. We demonstrate that intrinsic anomalous Hall conductivity (AHC) can be used to identify the presence of more probable functional groups. The maximum AHC at Fermi energy, EF, is found in the case of Mn2NO2 (470 S/cm), which is attributed to the presence of avoided band crossing and a larger density of states. Together, when considering all the studied systems, the AHC can be above 2500 S/cm within EF± 0.25 eV. Our findings could be useful not only in guiding the experimentalists by considering AHC as a possible tool in determining the most probable functional groups in 2D ferro(i)magnets but also in designing memory devices with negligible stray fields.

Intrinsically enhanced anomalous Hall conductivity and Hall angle in Sb-doped magnetic Weyl semimetal Co3Sn2S2

The emerging magnetic topological materials bring a new opportunity to obtain giant transverse transport effects. In this work, a greatly enhanced anomalous Hall effect (AHE) is obtained in electron-doped magnetic Weyl semimetal Co3Sn2−xSbxS2, showing a maximum anomalous Hall conductivity (AHC) of ∼1600 Ω−1 cm−1 and an anomalous Hall angle of ∼26%. Based on the qualitative and quantitative analysis of scaling models, the enhanced AHC comes from the intrinsic mechanism related to the Berry curvature of the topological band structures. A small amount of electron doping still makes the EF around the gapped nodal rings. At the same time, disorder doping leads to the splitting and broadening of bands, which enhance the Berry curvature and intrinsic AHC. Our work provides an important guidance for the design and development of large AHE in magnetic topological materials.

A large anomalous Hall conductivity induced by Weyl nodal lines in Fe70Al30

Materials with one-dimensional Weyl nodal lines are attracting much attention because of rich exotic properties. In this work, based on the first-principles calculations, we predict the existence of Weyl nodal lines in Fe70Al30. A large intrinsic anomalous Hall conductivity is calculated to be −374 S/cm, which stems from the net Berry curvature induced by Weyl nodal lines. To confirm our calculated results, high quality Fe70Al30 has been prepared and did possess a large anomalous Hall conductivity, where the intrinsic Berry curvature plays a role.