Berry curvature origin of the thickness-dependent anomalous Hall effect in a ferromagnetic Weyl semimetal

This thesis explores the magnetic properties of Heusler alloy thin films for spintronics applications. The skyrmion generation under different externalstimulation was focused in MgO/Mn2CoAl/Pd ultrathin films and the anomalous Hall effect of Co2MnGa thin films. The perpendicularly magnetized MgO/Mn2CoAl/Pd ultrathin films were firstly optimised by changing the thickness of Pd layer so that the effective magnetic anisotropy can be tuned and various magnetic textures can be obtained for different purposes. Based on the understanding of Pd dependence of magnetic properties, The skyrmion generation by applying an in-plane magnetic field was investigated to fine tune the effective magnetic anisotropy. Further the fractal analysis was used to describe the evolution of the magnetic states and categorise the formation of skyrmions. Then skyrmion generation by ionic liquid gating has been investigated in this trilayer. Both non-volatile and volatile skyrmions can be generated by applying a range of voltage sequences. The potential mechanisms, magneto-ionic and electrostatic charge effects, have been discussed as well. Finally, the thickness dependence of Co2MnGa thin films was studied. This material can be used as a spin-orbit generator for manipulating skyrmions. A large anomalous Hall angle (AHA) was demonstrated in Co2MnGa thin films (20 - 50 nm) showing a AHA ~11.4% at low temperature and ~9.7% at room temperature, which can be ascribed to the nontrivial topology of the band structure with large intrinsic Berry curvature. However, the anomalous Hall angle decreases significantly with thicknesses below 20 nm, which band structure calculations confirm is due to the reduction of the majority spin contribution to the Berry curvature.

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Skyrmion generation and perpendicular magnetic anisotropy modification in Heusler alloy ultrathin films

10.26686/wgtn.16655335.v1 ◽

2021 ◽

Author(s):

◽

Yao Zhang

Keyword(s):

Thin Films ◽

Magnetic Properties ◽

Band Structure ◽

Magnetic Anisotropy ◽

Ultrathin Films ◽

Heusler Alloy ◽

Anomalous Hall Effect ◽

Berry Curvature ◽

Effective Magnetic Anisotropy ◽

Hall Angle

This thesis explores the magnetic properties of Heusler alloy thin films for spintronics applications. The skyrmion generation under different externalstimulation was focused in MgO/Mn2CoAl/Pd ultrathin films and the anomalous Hall effect of Co2MnGa thin films. The perpendicularly magnetized MgO/Mn2CoAl/Pd ultrathin films were firstly optimised by changing the thickness of Pd layer so that the effective magnetic anisotropy can be tuned and various magnetic textures can be obtained for different purposes. Based on the understanding of Pd dependence of magnetic properties, The skyrmion generation by applying an in-plane magnetic field was investigated to fine tune the effective magnetic anisotropy. Further the fractal analysis was used to describe the evolution of the magnetic states and categorise the formation of skyrmions. Then skyrmion generation by ionic liquid gating has been investigated in this trilayer. Both non-volatile and volatile skyrmions can be generated by applying a range of voltage sequences. The potential mechanisms, magneto-ionic and electrostatic charge effects, have been discussed as well. Finally, the thickness dependence of Co2MnGa thin films was studied. This material can be used as a spin-orbit generator for manipulating skyrmions. A large anomalous Hall angle (AHA) was demonstrated in Co2MnGa thin films (20 - 50 nm) showing a AHA ~11.4% at low temperature and ~9.7% at room temperature, which can be ascribed to the nontrivial topology of the band structure with large intrinsic Berry curvature. However, the anomalous Hall angle decreases significantly with thicknesses below 20 nm, which band structure calculations confirm is due to the reduction of the majority spin contribution to the Berry curvature.

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Giant room temperature anomalous Hall effect and tunable topology in a ferromagnetic topological semimetal Co2MnAl

Nature Communications ◽

10.1038/s41467-020-17174-9 ◽

2020 ◽

Vol 11 (1) ◽

Cited By ~ 5

Author(s):

Peigang Li ◽

Jahyun Koo ◽

Wei Ning ◽

Jinguo Li ◽

Leixin Miao ◽

...

Keyword(s):

Hall Effect ◽

Room Temperature ◽

Transport Phenomena ◽

Surface States ◽

Anomalous Hall Effect ◽

Spintronic Devices ◽

Structure Topology ◽

Weyl Semimetal ◽

Weyl Semimetals ◽

Record Value

Abstract Weyl semimetals exhibit unusual surface states and anomalous transport phenomena. It is hard to manipulate the band structure topology of specific Weyl materials. Topological transport phenomena usually appear at very low temperatures, which sets challenges for applications. In this work, we demonstrate the band topology modification via a weak magnetic field in a ferromagnetic Weyl semimetal candidate, Co2MnAl, at room temperature. We observe a tunable, giant anomalous Hall effect (AHE) induced by the transition involving Weyl points and nodal rings. The AHE conductivity is as large as that of a 3D quantum AHE, with the Hall angle (ΘH) reaching a record value ($$\tan {\Theta }^{H}=0.21$$ tan Θ H = 0.21 ) at the room temperature among magnetic conductors. Furthermore, we propose a material recipe to generate large AHE by gaping nodal rings without requiring Weyl points. Our work reveals an intrinsically magnetic platform to explore the interplay between magnetic dynamics and topological physics for developing spintronic devices.

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Large anomalous Hall effect driven by a nonvanishing Berry curvature in the noncolinear antiferromagnet Mn3Ge

Science Advances ◽

10.1126/sciadv.1501870 ◽

2016 ◽

Vol 2 (4) ◽

pp. e1501870 ◽

Cited By ~ 240

Author(s):

Ajaya K. Nayak ◽

Julia Erika Fischer ◽

Yan Sun ◽

Binghai Yan ◽

Julie Karel ◽

...

Keyword(s):

Hall Effect ◽

Data Storage ◽

Room Temperature ◽

Theoretical Calculations ◽

Anomalous Hall Effect ◽

Spin Hall Effect ◽

Storage Devices ◽

Berry Curvature ◽

Ferromagnetic Metals ◽

Hall Effect Measurements

It is well established that the anomalous Hall effect displayed by a ferromagnet scales with its magnetization. Therefore, an antiferromagnet that has no net magnetization should exhibit no anomalous Hall effect. We show that the noncolinear triangular antiferromagnet Mn3Ge exhibits a large anomalous Hall effect comparable to that of ferromagnetic metals; the magnitude of the anomalous conductivity is ~500 (ohm·cm)−1 at 2 K and ~50 (ohm·cm)−1 at room temperature. The angular dependence of the anomalous Hall effect measurements confirms that the small residual in-plane magnetic moment has no role in the observed effect except to control the chirality of the spin triangular structure. Our theoretical calculations demonstrate that the large anomalous Hall effect in Mn3Ge originates from a nonvanishing Berry curvature that arises from the chiral spin structure, and that also results in a large spin Hall effect of 1100 (ħ/e) (ohm·cm)−1, comparable to that of platinum. The present results pave the way toward the realization of room temperature antiferromagnetic spintronics and spin Hall effect–based data storage devices.

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Interface-induced sign reversal of the anomalous Hall effect in magnetic topological insulator heterostructures

Nature Communications ◽

10.1038/s41467-020-20349-z ◽

2021 ◽

Vol 12 (1) ◽

Cited By ~ 1

Author(s):

Fei Wang ◽

Xuepeng Wang ◽

Yi-Fan Zhao ◽

Di Xiao ◽

Ling-Jie Zhou ◽

...

Keyword(s):

Hall Effect ◽

Electric Fields ◽

First Principles ◽

Berry Phase ◽

Condensed Matter ◽

Anomalous Hall Effect ◽

First Principles Calculations ◽

Sign Reversal ◽

Berry Curvature ◽

Topological Materials

AbstractThe Berry phase picture provides important insights into the electronic properties of condensed matter systems. The intrinsic anomalous Hall (AH) effect can be understood as the consequence of non-zero Berry curvature in momentum space. Here, we fabricate TI/magnetic TI heterostructures and find that the sign of the AH effect in the magnetic TI layer can be changed from being positive to negative with increasing the thickness of the top TI layer. Our first-principles calculations show that the built-in electric fields at the TI/magnetic TI interface influence the band structure of the magnetic TI layer, and thus lead to a reconstruction of the Berry curvature in the heterostructure samples. Based on the interface-induced AH effect with a negative sign in TI/V-doped TI bilayer structures, we create an artificial “topological Hall effect”-like feature in the Hall trace of the V-doped TI/TI/Cr-doped TI sandwich heterostructures. Our study provides a new route to create the Berry curvature change in magnetic topological materials that may lead to potential technological applications.

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