Mechanism Of Anomalous Hall Effect Research Articles

Colossal anomalous Hall effect in the layered antiferromagnetic EuAl2Si2 compound.

The anomalous Hall effect (AHE), significantly enhanced by the extrinsic mechanism, has attracted attention for its almost unlimited Hall response, which exceeds the upper limit of the Berry curvature mechanism. However, due to the high conductivity in the clean regime and weak skew scattering, it is a great challenge to obtain large anomalous Hall conductivities and large anomalous Hall angles at the same time. Here, we unveil a new magnetic metal system, EuAl2Si2, which hosts both colossal anomalous Hall conductivity (σAxy ≥ 104 Ω-1 cm-1) and large anomalous Hall angle (AHA >10%). The scaling relation suggests that the skew scattering mechanism is dominant in the colossal anomalous Hall response and gives rise to a large skew scattering constant. The large effective SOC and large magnetic moment may account for this anomaly. Our results indicate that EuAl2Si2 is a good platform to study the extrinsic AHE mechanism.

Topological Hall effect and the transition of the anomalous Hall effect mechanism in hexagonal alloys Mn3.1− xFexGe0.9 (x = 1.6, 1.8, 2.0)

Hexagonal Mn3Ge, with both kagome lattice and triangular antiferromagnetism, has gained significant attention due to its large anomalous Hall effect (AHE), resulting from the non-vanishing Berry phase. In this study, we present the magnetic and anomalous transport properties of a series hexagonal D019 type Fe-doped Mn3Ge alloys with the composition of Mn3.1−xFexGe0.9 (x = 1.6, 1.8, 2.0). The ferromagnetic interactions gradually increase with increasing Fe content. The longitudinal resistivity of all alloys exhibits a typical metallic behavior, increasing with temperature from 5 to 390 K. The residual resistivity decreases from 120.4 to 67.8 μΩ·cm as x increases from 1.6 to 2.0. A temperature-driven Lifshitz transition and a spin reorientation have been observed in the x = 1.6 alloy. Topological Hall effect accompanied by the spin reorientation is demonstrated. The maximum value of the topological Hall resistivity ρxyT is approximately 0.16 μΩ·cm. The relationship of ρxyA∝ ρxx in x = 1.6 alloy suggests that the extrinsic skew scattering predominantly contributes to the AHE mechanism. In the case of x = 1.8 and 2.0, both intrinsic and extrinsic factors contribute to the AHE. The anomalous Hall conductivity of our polycrystalline samples at room temperature is comparable to that of single crystal Mn3Ge, which is advantageous for practical applications. This study reveals the effectiveness of chemical engineering in tailoring nontrivial spin textures and the AHE.

Novel anomalous Hall effect mechanism in ferrimagnetic GdCo alloy

In this work, we shed light on the mechanism behind the complexity of the anomalous Hall effect (AHE) in the rare earth-transition metal (RE-TM) compound. To achieve a comprehensive picture, we first theoretically investigate the pure elemental metals and observe that the AHE of RE Gd is negative while that of TM Co is positive. We then investigate magnetic properties and AHE behavior for two typical cases of TM-rich and RE-rich compositions that correspond to GdCo5 and GdCo3 alloys, respectively. The observation that net magnetization is not a decisive factor in assessing the AHE value is consistent with recent claims. Besides, our investigations reveal a significant role of the Gd sublattice to the AHE. The reduction AHE signal under a high-applied field is attributed to the counteraction between the positive and negative AHC contributions when the TM and RE sub-moments are aligned parallel. On a bright note, our new findings can be successfully employed to unravel two types of aberrant humps of AHE signals near compensation temperature.

Sign change of competing contributions to the side jump by impurity, phonon, and magnon scattering

The side jump is one important mechanism of anomalous Hall effect, which is generally believed that it does not depend on the scattering strength and disorder density. Recent theoretical calculations indicate that different types of disordered scattering contribute to side jump. The competition between them will make the magnitude of side jump sensitive to their relative disorder strength. However, the evolution of competing scattering contribution in side jump has not been experimentally observed. In this paper, a series of high-qulity FeCo films with different thickness have been grown on MgO(001) substrate. Based on multivariable scaling, the sign change of the competing contribution between different scatterings to side jump is clearly observed as increasing sample thickness. Meanwhile, we demonstrate that the contribution of magnon side jump may play an irreplaceable role in the competition. Our work provides evidence for the shift of dominated scattering in competition, and shows that competing scattering still exists and is not negligible even in conductivity region with ${\ensuremath{\sigma}}_{xx}\ensuremath{\sim}{10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, which is beneficial to further experimental and theoretical studies of side jump.

Sign change of anomalous Hall effect with temperature in Cr2.63V0.25Te4 single crystal

Anomalous Hall effect, an interesting transport behavior, is of importance topic for fundamental physics and device application. Recently, the sign change of anomalous Hall effect (AHE) in some materials has being concerned. Here, we study the electrical transport, magnetic properties, and AHE of Cr2.63V0.25Te4 single crystals. Cr2.63V0.25Te4 shows a bad metal behavior with a resistivity kink at 178 K, corresponding to a peak on a heat capacity–temperature curve, and the strong electron correlation should be a dominant transport mechanism below 178 K. At this temperature, the magnetization shows a sharp magnetic transition. The linear Hall resistivity at a high field with a positive slope suggests a p-type conductivity for bulk Cr2.63V0.25Te4. The clear AHE is observed below 180 K with a larger anomalous Hall conductivity σxy ∼ 2000 Ω−1 cm−1 at 3 K. For 3–60 K, an AHE coefficient Rs is negative, and Rs is positive between 100 and 180 K. The sign change of AHE may be due to Fermi level crossing the overlap of 3d band in ferromagnetic transition-metal materials. Based on our analysis, the AHE mechanism should be the skew scattering. Our results reveal the interesting physical properties in Cr2.63V0.25Te4 single crystals and give another system to study AHE for future Hall device design.

Anomalous Hall Effect and Magneto-Optic Kerr Effect in Pt/Co/Pt Heterostructure

Magnetic multilayer with large perpendicular magnetic anisotropy (PMA) has attracted sustained interest owing to its importance to fundamental physics and applications. In this work, the high quality of Pt/Co/Pt heterostructures with large PMA was successfully achieved to exhibit a large anomalous Hall effect (AHE) with squared Hall loops. By calculating the proportional relationship between the longitudinal resistivity (ρxx) and the abnormal Hall coefficient (Rs), it is confirmed that the basic mechanism of AHE comes from the external skew scattering (SS) and side jump (SJ), while SS contribution, related to asymmetric scattering from impurities, is dominant in the AHE. Furthermore, the obvious magneto-optical Kerr effect (MOKE) was also observed using the polar MOKE microscopy. The obviously circular magnetic domain can form and propagate in response to the applied out-of-plane magnetic field, resulting in the magnetization reversal of the entire film. This work offers important information in terms of both AHE and MOKE in the ultrathin ferromagnetic films with perpendicular anisotropy, establishing the application foundation for the nonvolatile memories and spintronics.

Dominant Mechanism of Anomalous Hall Effect in Thin Film of Manganites

Dominant Mechanism of Anomalous Hall Effect in Thin Film of Manganites

Anomalous Hall effect of facing-target sputtered ferrimagnetic Mn4N epitaxial films with perpendicular magnetic anisotropy

Epitaxial Mn4N films with different thicknesses were fabricated by facing-target reactive sputtering and their anomalous Hall effect (AHE) is investigated systematically. The Hall resistivity shows a reversed magnetic hysteresis loop with the magnetic field. The magnitude of the anomalous Hall resistivity sharply decreases with decreasing temperature from 300 K to 150 K. The AHE scaling law in Mn4N films is influenced by the temperature-dependent magnetization, carrier concentration and interfacial scattering. Different scaling laws are used to distinguish the various contributions of AHE mechanisms. The scaling exponent γ > 2 for the conventional scaling in Mn4N films could be attributed to the residual resistivity ρ xx0. The longitudinal conductivity σxx falls into the dirty regime. The scaling of is used to separate out the temperature-independent ρ xx0 from extrinsic contribution. Moreover, the relationship between ρ AH and ρxx is fitted by the proper scaling to clarify the contributions from extrinsic and intrinsic mechanisms of AHE, which demonstrates that the dominant mechanism of AHE in the Mn4N films can be ascribed to the competition between skew scattering, side jump and the intrinsic mechanisms.

Epitaxial growth of Ho<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub> films with perpendicular magnetic anisotropy and spin transport properties in Ho<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>/Pt heterostructures

Rare-earth iron garnet films with perpendicular magnetic anisotropy could open new perspectives for spintronics. Holmium iron garnet (Ho<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>, HoIG) films with thickness ranging from 2 to 100 nm are epitaxially grown on (111) orientated gadolinium gallium garnet single crystal substrate doped with yttrium and scandium (Gd<sub>0.63</sub>Y<sub>2.37</sub>Sc<sub>2</sub>Ga<sub>3</sub>O<sub>12</sub>, GYSGG) by ultra-high vacuum magnetron sputtering. A 3-nm Pt film is further deposited on each of the HoIG films. The magnetic anisotropy and magneto-transport properties of heterostructures at room temperature are investigated. It is shown that the HoIG film as thin as 2 nm (less than two unit cells in thickness) exhibits the ferromagnetic properties at room temperature, and perpendicular magnetic anisotropy is achieved in the 2-60 nm thick films, and a maximum effective perpendicular anisotropy field reaches 350 mT due to the strain induced magnetoelastic anisotropy. The HoIG/Pt heterostructure shows significant anomalous Hall effect (AHE) and appreciable spin-Hall magnetoresistance (SMR) and/or anisotropic magnetoresistance (AMR). Remarkably, the AHE starts to decline gradually when the HoIG thickness is less than 4 nm, but the magnetoresistance decreases rapidly with the HoIG layer becoming less than 7 nm in thickness. The fact that the AHE in the heterostructure is less sensitive to the HoIG thickness suggests that the interface effect is more dominant in the AHE mechanism, whereas the bulk magnetic properties of the HoIG plays a more important role for the observed magnetoresistance. In addition, the spin Seebeck effect decreases exponentially with the decrease of HoIG thickness till the ultrathin limit, which was previously validated in the micrometer-thick YIG/Pt stacks in the frame of thermally excited magnon accumulation and propagation. The present results show that the nanometer HoIG/Pt heterostructure with tunable perpendicular magnetic anisotropy and efficient interfacial spin exchange interaction could be a promising candidate for insulating magnet based spintronic devices.

Anomalous Nernst effect in Cox(MgO)1-x granular thin films

The anomalous Nernst effect (ANE) is investigated for 100 nm Cox(MgO)1-x granular films as a function of the Co volume fraction (x) changes. The results show that by slightly adding MgO into the Co matrix, the anomalous Nernst angle (θAN) can be largely enhanced more than three times compared with that of pure Co, which also results in the obvious change in ANE voltage. The transverse conductivity σXYAH vs the longitudinal conductivity σXX shows distinct linear behavior in three regimes, which indicates different anomalous Hall effect (AHE) mechanisms. By qualitative Mott-relation analysis, we found that the non-monotonic dependence of ANE on x is associated with the change in AHE mechanisms.

Piezostrain control of anomalous Hall resistivity of [Co/Pt]3–Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructure

The anomalous Hall effect (AHE) in [Co/Pt]3 thin films with perpendicular magnetic anisotropy grown on Pb(Mg1/3Nb2/3)O3–Pb0.7Ti0.3O3 substrates is investigated under application of different electric fields at various temperatures. As the sweeping mode of application of the electric field changes from symmetric bipolar to asymmetric bipolar, the butterfly-like behavior of the resistivity vs electric field curves is converted into loop-like behavior, which is attributed to the piezostrain effect. The anomalous Hall conductivity σAH vs longitudinal conductivity σxx characteristics at the corresponding temperatures indicate two AHE mechanisms, which include the side jump and/or intrinsic mechanisms. In addition, the σAH–σxx curve shows obvious changes under different electric fields, which means that the AHE can be mediated. These results are highly important for further understanding of the mechanism by which the electric field can control the AHE.

Anomalous Hall effect mechanisms in the quasi-two-dimensional van der Waals ferromagnet Fe0.29TaS2

The recent emergence of two-dimensional (2D) van der Waals ferromagnets has provided a new platform for exploring magnetism in the flatland and for designing 2D ferromagnet-based spintronics devices. Despite intensive studies, the anomalous Hall effect (AHE) mechanisms in 2D van der Waals ferromagnets have not been investigated yet. In this paper, we report the AHE mechanisms in quasi-2D van der Waals ferromagnet Fe0.29TaS2 via systematically measuring Fe0.29TaS2 devices with thickness from 14 nm to bulk single crystal. The AHE mechanisms are investigated via the scaling relationship between the anomalous Hall and channel conductivities. As the Fe0.29TaS2 thickness decreases, the major AHE mechanism changes from extrinsic scattering to intrinsic contribution. The crossover of the AHE mechanisms is found to be highly associated with the channel conductivities as the Fe0.29TaS2 thickness varies.

Localization Correction to Anomalous Hall Effect in the Perpendicular CoFeB Thin Films

In this paper, we reported an obvious weak localization (WL) effect in perpendicular CoFeB sandwiched by Ta and MgO layers. The WL correction to the anomalous Hall effect (AHE) arises when the sheet resistance is larger than 1.5[Formula: see text]k[Formula: see text]. Furthermore, it is found that the mechanism of AHE is strongly related to the characteristic of the granularity in the MgO/CoFeB/Ta thin films. Both skew scattering and side jump mechanisms will give comparable contribution in the high disorder regime.

First-principles investigations on the Berry phase effect in spin–orbit coupling materials

In recent years, the Berry phase as a fascinating concept has made a great success for interpreting many physical phenomena. In this paper, we review our past works on the Berry phase effect in solid materials with spin–orbit coupling. Firstly, we developed an exact method for directly evaluating the Berry curvature and anomalous Hall conductivity, then the intrinsic mechanism of anomalous Hall effect was quantitatively confirmed in bcc Fe and CuCr2Se4−xBrx. An effective method was proposed to decompose the anomalous Hall effect into the intrinsic and extrinsic contributions. We also developed computational methods for the spin Hall conductivity and anomalous Nernst conductivity. Secondly, we developed a powerful method for computing the Z2 topological invariant without consideration of special symmetry. We predicted many topological insulators in three-dimensions, including half-Heusler, chalcopyrite, strained InSb, core-holed Ge and InSb, Bi2Te3/BiTeI heterostructure, and in two-dimensions, including silicene, germanene, stanene, X-hydride/halide (X=N–Bi) monolayers and Bi4Br4. We also predicted the quantum anomalous Hall effect in magnetic atoms adsorbed graphene and half-passivated BiH, the quantum valley Hall effect and topological superconducting in silicene and n-dopped BiH. Finally, the summary of our past works and the further outlooks are discussed.

Anomalous Hall Effect in 2D DMS

Transport and magnetic properties of δ-Mn doped GaAs/InGaAs/GaAs quantum wells (QW) with various In content were studied at temperatures 4.2K≤T≤300K. Fluctuation potential (FP) appeared to be crucial for transport characteristics of structures under investigation. The magnetic percolation transition was observed at temperatureTpin the range 20 - 40K. TheTpdependence on the In content is nonmonotonic due to the peculiarities of free-carrier mediated exchange interaction mechanisms. The change of the anomalous Hall effect (AHE) sign with decreasing temperature was detected at temperatures close to theTp. The main reason of the AHE sign change is the variation of contributions of different AHE mechanisms (intrinsic and side-jump) caused by the reduction of spin-dependent scattering intensity with temperature decrease. We believe that our results are the experimental observation of the AHE intrinsic mechanism in 2D.

Anomalous Hall effect in epitaxial permalloy thin films

Anomalous Hall effect (AHE) of epitaxial permalloy thin films grown on MgO (001) substrates is investigated. The longitudinal conductivity independent term (i.e., the sum of intrinsic and side-jump contributions) of the anomalous Hall conductivity (AHC) is found to be much smaller than those of Fe and Ni films. Band theoretical calculations of the intrinsic AHC as a function of the number of valence electrons (band filling) indicate that the AHC of the permalloy is in the vicinity of sign change, thus resulting in the smallness of the intrinsic AHC. The contribution of the phonon scattering is found to be comparable to that of the impurity scattering. This work suggests that the permalloy films are ideal systems to understand the AHE mechanisms induced by impurity scattering.

Chemical Composition Tuning of the Anomalous Hall Effect in IsoelectronicL10FePd1−xPtxAlloy Films

The anomalous Hall effect (AHE) in L1(0)FePd(1-x)Pt(x) alloy films is studied both experimentally and theoretically. We find that the intrinsic contribution (σ(AH)(int)) to the AHE can be significantly increased, whereas the extrinsic side-jump contribution (σ(AH)(sj)) can be continuously reduced from being slightly larger than σ(AH)(int) in L1(0) FePd to being much smaller than σ(AH)(int) in L1(0) FePt, by increasing the Pt composition x. We show that this chemical composition tuning of the intrinsic contribution is afforded by the stronger spin-orbit coupling strength on the Pd/Pt site when the lighter Pd atoms are replaced by the heavier Pt atoms. Our results provide a means of manipulating the competing AHE mechanisms in ferromagnetic alloys for fully understanding the AHE and also for technological applications of ferromagnetic alloys.

Room-temperature anomalous Hall effect and magnetroresistance in (Ga, Co)-codoped ZnO diluted magnetic semiconductor films**Project supported by the National Natural Science Foundation of China (Grant No. 50772122) and the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 51002176).

This paper reports that the (Ga, Co)-codoped ZnO thin films have been grown by inductively coupled plasma enhanced physical vapour deposition. Room-temperature ferromagnetism is observed for the as-grown thin films. The x-ray absorption fine structure characterization reveals that Co2+ and Ga3+ ions substitute for Zn2+ ions in the ZnO lattice and exclude the possibility of extrinsic ferromagnetism origin. The ferromagnetic (Ga, Co)-codoped ZnO thin films exhibit carrier concentration dependent anomalous Hall effect and positive magnetoresistance at room temperature. The mechanism of anomalous Hall effect and magneto-transport in ferromagnetic ZnO-based diluted magnetic semiconductors is discussed.

Anomalous Hall effect

We present a review of experimental and theoretical studies of the anomalous Hall effect (AHE), focusing on recent developments that have provided a more complete framework for understanding this subtle phenomenon and have, in many instances, replaced controversy by clarity. Synergy between experimental and theoretical work, both playing a crucial role, has been at the heart of these advances. On the theoretical front, the adoption of Berry-phase concepts has established a link between the AHE and the topological nature of the Hall currents which originate from spin-orbit coupling. On the experimental front, new experimental studies of the AHE in transition metals, transition-metal oxides, spinels, pyrochlores, and metallic dilute magnetic semiconductors, have more clearly established systematic trends. These two developments in concert with first-principles electronic structure calculations, strongly favor the dominance of an intrinsic Berry-phase-related AHE mechanism in metallic ferromagnets with moderate conductivity. The intrinsic AHE can be expressed in terms of Berry-phase curvatures and it is therefore an intrinsic quantum mechanical property of a perfect cyrstal. An extrinsic mechanism, skew scattering from disorder, tends to dominate the AHE in highly conductive ferromagnets. We review the full modern semiclassical treatment of the AHE together with the more rigorous quantum-mechanical treatments based on the Kubo and Keldysh formalisms, taking into account multiband effects, and demonstrate the equivalence of all three linear response theories in the metallic regime. Finally we discuss outstanding issues and avenues for future investigation.

Optical Probe for Anomalous Hall Resonance in Ferromagnets with Spin Chirality

We have investigated the infrared optical Hall conductivity, sigma(xy)(omega) for band-filling-controlled ferromagnetic crystals of Nd2Mo2O7, revealing the dynamical properties of their anomalous Hall effect (AHE). A resonant structure and its systematic filling dependence were observed in the Hall conductivity spectra in the midinfrared region (typically at 0.1 eV), while similar effects were not discerned in the diagonal (longitudinal or ordinary) conductivity spectra. This property of sigma(xy)(omega) provides crucial and essential information to understand the microscopic mechanism of AHE including its dc limit. Specifically, the interband transition at the magnetic-monopole-like band-anticrossing point, which is split by spin chirality, is the dominant source in AHE.