Browsing by Author "Gaurav K. Shukla"

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Anomalous Hall effect from gapped nodal line in the Co2FeGe Heusler compound
(American Physical Society, 2021) Gaurav K. Shukla; Jyotirmoy Sau; Nisha Shahi; Anupam K. Singh; Manoranjan Kumar; Sanjay Singh
Full Heusler compounds with cobalt as a primary element show anomalous transport properties owing to the Weyl fermions and broken time-reversal symmetry. We present here the study of anomalous Hall effect (AHE) in the Co2FeGe Heusler compound. The experiment reveals anomalous Hall conductivity (AHC) ∼100 S/cm at room temperature with an intrinsic contribution of ∼78 S/cm. The analysis of anomalous Hall resistivity suggests the scattering independent intrinsic mechanism dominates the overall behavior of anomalous Hall resistivity. The first principles calculation reveals that the Berry curvature originated by a gapped nodal line near EF is the main source of AHE in the Co2FeGe Heusler compound. The theoretically calculated AHC is in agreement with the experiment. © 2021 American Physical Society.
Atomic disorder and Berry phase driven anomalous Hall effect in a Co2FeAl Heusler compound
(American Physical Society, 2022) Gaurav K. Shukla; Ajit K. Jena; Nisha Shahi; K.K. Dubey; Indu Rajput; Sonali Baral; Kavita Yadav; K. Mukherjee; Archana Lakhani; Karel Carva; Seung-Cheol Lee; Satadeep Bhattacharjee; Sanjay Singh
Co2-based Heusler compounds are promising materials for spintronics applications due to their high Curie temperature, large spin polarization, large magnetization density, and exotic transport properties. In the present paper, we report the anomalous Hall effect (AHE) in a polycrystalline Co2FeAl Heusler compound using combined experimental and theoretical studies. The Rietveld analysis of high-resolution synchrotron x-ray diffraction data reveals a large degree (∼50%) of antisite disorder between Fe and Al atoms. The analysis of anomalous transport data provides the experimental anomalous Hall conductivity (AHC) about 227 S/cm at 2 K with an intrinsic contribution of 155 S/cm, which has nearly constant variation with temperature. The detailed scaling analysis of anomalous Hall resistivity suggests that the AHE in Co2FeAl is governed by the Berry phase driven intrinsic mechanism. Our theoretical calculations reveal that the disorder present in the Co2FeAl compound enhances the Berry curvature induced intrinsic AHC. © 2022 American Physical Society.
Enhanced spin Hall conductivity and charge to spin conversion efficiency in strained orthorhombic SnSe through orbital selective hybridization
(American Institute of Physics Inc., 2023) E. Ketkar; Gaurav K. Shukla; Seung-Cheol Lee; Satadeep Bhattacharjee; Sanjay Singh
The realization of the spin Hall effect has opened new frontiers for the design of efficient memory storage devices facilitated by the conversion of charge currents to spin currents. Here, using the Kubo formula, we calculate the intrinsic spin Hall conductivity (SHC) of orthorhombic tin selenide (o-SnSe) under the influence of isotropic compressive strain in the ab-plane. As the strain is gradually increased, we obtain a substantial hybridization between the pz orbitals of Sn and Se atoms of an electron pocket from the lowest conduction band and the topmost valence band, respectively. This hybridization process greatly enhances the SHC at the Fermi level and charge-to-spin conversion efficiency, the latter of which is superior to that of popular transition metals such as Ta and Pt. This makes strained o-SnSe an attractive candidate for use in spintronic devices. © 2023 Author(s).
Intrinsic anomalous Hall conductivity and real space Berry curvature induced topological Hall effect in Ni2MnGa magnetic shape memory alloy
(Institute of Physics, 2023) Anupam K. Singh; Gaurav K. Shukla; Sanjay Singh
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. © 2022 IOP Publishing Ltd.
Nodal-line symmetry breaking induced colossal anomalous Hall and Nernst effects in Cu2CoSn Heusler compound
(American Institute of Physics Inc., 2023) Gaurav K. Shukla; Ujjawal Modanwal; Sanjay Singh
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: Fm 3 ¯ 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. © 2023 Author(s).
Pressure driven iso-structural phase transition and its implication on the Néel skyrmion host hexagonal PtMnGa
(American Physical Society, 2024) K.K. Dubey; S. Rastogi; Ajit K. Jena; Gaurav K. Shukla; Parul Devi; Seung-Cheol Lee; Satadeep Bhattacharjee; R. Rawat; Boby Joseph; Sanjay Singh
Magnetic skyrmions are nanometer-sized whirling spin textures in the magnetic material, which have the potential to revolutionize the field of spintronics. This study explores the influence of pressure on the structural properties of the PtMnGa hexagonal system, recognized for hosting Néel skyrmions. By employing pressure-dependent synchrotron x-ray powder diffraction (SXRPD), we reveal an isostructural phase transition in this system at approximately 6 GPa. The isostructural transition is evidenced by a deviation of the lattice parameter from the linear dependence, change of trend in the in-plane to out-of-plane lattice parameter ratio, and a description of the pressure-unit cell volume data by two distinct second-order Birch-Muraghan equation of states. The PtMnGa system, however, exhibits reversible structural behavior when pressure is released. Analysis of combined pressure and temperature-dependent SXRPD data provides indirect evidence that the application of moderate pressure (0.8-1.09 GPa) shifts the thermodynamically stable skyrmion regime near to room temperature in the Néel skyrmion-host PtMnGa system. Theoretical calculations on band structure, magnetic moment, and density of states (DOS) under pressure further corroborate the experimental findings, offering a comprehensive understanding of the material's response to pressure changes. The combination of experimental findings and theoretical calculations demonstrates the potential for engineering materials supporting stable skyrmions even at elevated temperatures and nominal pressures, which can be attained in the materials using chemical substitution or epitaxial thin films by strain controlling of the substrate-film lattice parameter mismatch. © 2024 American Physical Society.
Revealing the origin of the topological Hall effect in the centrosymmetric shape memory Heusler alloy Mn2NiGa: A combined experimental and theoretical investigation
(American Physical Society, 2023) Shivani Rastogi; Nisha Shahi; Vishal Kumar; Gaurav K. Shukla; Satadeep Bhattacharjee; Sanjay Singh
Skyrmions are localized swirling noncoplanar spin textures offering a promising revolution in future spintronic applications. These topologically nontrivial spin textures lead to an additional contribution to the Hall effect, called the topological Hall effect. Here, we investigate the origin of the topological Hall effect - a trademark of skyrmions - in a centrosymmetric shape memory Heusler alloy (SMHA) Mn2NiGa. The magnetization measurement unveils the presence of austenite to martensite transition in the studied system. The topological Hall effect (THE) in the present system is examined experimentally and theoretically. The presence of a large THE in the austenite (cubic) phase of the system strongly suggests that the observed THE in Mn2NiGa cannot be attributed to the antiskyrmions stabilized by D2d symmetry as reported earlier. To comprehend the underlying mechanism behind the origin of THE, we have performed micromagnetic simulations for a range of magnetic field with a small value of DMI (local DMI) to consider the possible impact of earlier reported atomic disorder in the centrosymmetric SMHA Mn2NiGa. The results showed the stabilization of Néel-type skyrmions, which can be assigned to the expected local symmetry breaking at the interface of disorder originated ferromagnetic nanoclusters and ferrimagnetic lattice of the system. A theoretical calculation of topological Hall resistivity by utilizing micromagnetic simulations is performed, which is of the same order as the experimentally obtained values in the both martensite and austenite phases. © 2023 American Physical Society.
Role of chemical disorder in tuning the Weyl points in vanadium doped Co2TiSn
(American Physical Society, 2021) Payal Chaudhary; Krishna Kant Dubey; Gaurav K. Shukla; Sanjay Singh; Surasree Sadhukhan; Sudipta Kanungo; Ajit K. Jena; S.-C. Lee; S. Bhattacharjee; Jan Minár; Sunil Wilfred D'Souza
The lack of time-reversal symmetry and Weyl fermions give exotic transport properties to Co-based Heusler alloys. In the present study, we have investigated the role of chemical disorder on the variation of Weyl points in Co2Ti1-xVxSn magnetic Weyl semimetal candidate. We employ the first principle approach to track the evolution of the nodal lines responsible for the appearance of Weyl node in Co2TiSn as a function of V substitution in place of Ti. By increasing the V concentration in place of Ti, the nodal line moves toward Fermi level and remains at Fermi level around the middle composition. Further increase of the V content, leads shifting of nodal line away from Fermi level. Density of state calculation shows half-metallic behavior for the entire range of composition. The magnetic moment on each Co atom as a function of V concentration increases linearly up to x=0.4, and after that, it starts decreasing. We also investigated the evolution of the Weyl nodes and Fermi arcs with chemical doping. The first-principles calculations reveal that via replacing almost half of the Ti with V, the intrinsic anomalous Hall conductivity increased twice as compared to the undoped composition. Our results indicate that the composition close to the 50% V doped Co2TiSn will be an ideal composition for the experimental investigation of Weyl physics. © 2021 American Physical Society.
Spin reorientation and sign reversal of Berry curvature induced intrinsic anomalous Hall effect in the manganese pnictide MnSb
(American Physical Society, 2024) Nisha Shahi; Gaurav K. Shukla; Vishal Kumar; Sanjay Singh
The manipulation of the anomalous Hall effect (AHE) by controlling magnetization is of great interest in condensed matter physics due to its potential application for the practical design of spintronic devices. In this study, we report a combined experimental and theoretical investigation of the AHE in the MnSb manganese pnictide. Temperature-dependent magnetization measurement indicates spin reorientation transition (SRT) temperature at ∼ 120 K (TSR). Magnetotransport data shows that negative magnetoresistance increases from room temperature up to SRT temperature 120 K, then decreases and becomes positive at very low temperatures. The anomalous Hall conductivity (AHC) shows temperature-independent behavior from room temperature to TSR followed by a drop and sign reversal at low temperatures. Detailed scaling analysis of anomalous Hall data suggests that the AHE above TSR is primarily governed by the intrinsic Berry curvature and the obtained value of intrinsic AHC is about 310 S/cm. In contrast, below TSR, the extrinsic skew scattering becomes the dominant contributor to the AHE compared to the intrinsic Berry curvature and the obtained value of intrinsic AHC is about-28 S/cm. The first-principles calculations reveal that changes in the sign and magnitude of the intrinsic AHC are attributed to modifications in the Berry curvature when the magnetic moment undergoes rotation from the c-axis to the ab-plane. Our study yields a compound exhibiting large AHC and offers an insightful comprehension of the anisotropic behavior of AHE due to the modification of Berry curvature. © 2024 American Physical Society.
Tuning of nodal line states via chemical alloying in Co2 CrX (X=Ga, Ge) Heusler compounds for a large anomalous Hall effect
(American Physical Society, 2024) Ujjawal Modanwal; Gaurav K. Shukla; Ajit K. Jena; Satadeep Bhattacharjee; Sunil Wilfred D'Souza; Jan Minár; Sanjay Singh
Topological materials have attracted significant interest in condensed matter physics for their unique topological properties leading to potential technological applications. Topological nodal line semimetals, a subclass of topological materials, exhibit symmetry-protected nodal lines, where band crossings occur along closed curves in the three-dimensional Brillouin zone. When the nodal lines are gapped out due to perturbation in the Hamiltonian, a large Berry curvature (BC) arises in the surrounding area of the gapped nodal line, leading to exotic anomalous transport responses. In this paper, we studied the Co2CrX (X=Ga, Ge) Heusler compounds that exhibit mirror symmetry-protected nodal line states below the Fermi level. The BC calculation yields anomalous Hall conductivity (AHC) of about 292 and 217 S/cm for Co2CrX (X=Ga, Ge), respectively, at the Fermi level, which increases by up to 400% at the nodal line energy level. We theoretically analyzed that 20% and 60% zinc (Zn) alloying in Co2CrX (X=Ga, Ge) effectively lowers the Fermi level by 50 meV and 330 meV, respectively, aligning with the protected crossings. Consequently, we identified Co2CrGe0.4Zn0.6 and Co2CrGa0.8Zn0.2 as compositions to achieve the significant AHC of 800 and 1300 S/cm, respectively. The explicit AHC calculation for these alloyed compositions is in good agreement with our predictions. Our findings highlight that chemical alloying is an efficient way to enhance AHC in nodal line hosting materials. © 2024 American Physical Society.