Browsing by Author "Nisha Shahi"

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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.
Atomic disorder and intrinsic anomalous Hall effect in a half-metallic ferromagnet Co2VAl
(Elsevier Ltd, 2025) Shivani Rastogi; Vishal Kumar; Ajit K. Jena; Nisha Shahi; Gaurav K Shukla; Sunil Wilfred D'Souza; Satadeep Bhattacharjee; Sanjay Singh
Half-metallic ferromagnets, conducting for one spin channel while insulating for the other, are highly desirable for spintronic applications due to 100 % spin polarization around the Fermi level. Cobalt-based half-metallic Heusler compounds have attracted enormous attention due to their large spin polarization and a high magnetic transition temperature. In the present study, we report the experimental and theoretical investigation of crystal structure and anomalous Hall effect (AHE) in half-metallic ferromagnet Co2VAl. The structural investigation of high-resolution synchrotron x-ray diffraction data reveals 10 % antisite disorder between V and Al atoms within the L21 ordered crystal structure. The scaling analysis of anomalous Hall data shows that the AHE in our system is mainly driven by the Berry curvature in the momentum space. The magnitude of experimental intrinsic anomalous Hall conductivity (AHC) due to the momentum space Berry curvature is about 44.67 ± 0.02 S/cm at 5 K, which is less than the theoretically calculated AHC for the ordered structure. Our theoretical calculations suggest that the lower AHC obtained for the present system is due to the reduced Berry curvature in the disordered case with negligible impact on half-metallicity of the system. © 2024 Elsevier B.V.
Existence of exotic magnetic phases along with exchange bias and memory effect in frustrated beta-Mn Heusler alloy
(American Institute of Physics Inc., 2023) Srishti Dixit; Labanya Ghosh; Mohd Alam; Satya Vijay Kumar; Neha Patel; Swayangsiddha Ghosh; Nisha Shahi; Sanjay Singh; Sandip Chatterjee
Generally, Co-based Heusler alloys are the center of interest because of their properties such as high Curie temperature, spin polarization, and high value of exchange bias. Herein, we have used the macroscopic technique to probe the low-temperature exotic properties of M1.5Co0.5FeAl. First, we have analyzed the dc magnetization data, and it unfolds the presence of a glassy phase at 33 K. The cluster spin glass phase is authenticated by measuring ac susceptibility. Furthermore, using empirical models like power law and Vogel-Fulcher fitting, the relaxation time for the spin is of the order of τ ∼10-9 s, confirming the presence of a cluster spin glass in Mn1.5Co0.5FeAl below an irreversible temperature. The H-T phase space diagram ensures that it follows the Ising spin model. Furthermore, the glassy phase of the system is confirmed by magnetic relaxation, memory effect, and the presence of an exchange bias instead of a minor loop below spin-freezing temperature (TTf ~33 K). © 2023 Author(s).
Experimental and computational approaches to study the high temperature thermoelectric properties of novel topological semimetal CoSi
(IOP Publishing Ltd, 2022) Shamim Sk; Nisha Shahi; Sudhir K Pandey
Here, we study the thermoelectric properties of topological semimetal CoSi in the temperature range 300-800 K by using combined experimental and density functional theory (DFT) based methods. CoSi is synthesized using arc melting technique and the Rietveld refinement gives the lattice parameters of a = b = c = 4.445 Å. The measured values of Seebeck coefficient (S) shows the non-monotonic behaviour in the studied temperature range with the value of 1/4-81 μV K-1 at room temperature. The |S| first increases till 560 K (1/4-93 μV K-1) and then decreases up to 800 K (1/4-84 μV K-1) indicating the dominating n-type behaviour in the full temperature range. The electrical conductivity, σ (thermal conductivity, κ) shows the monotonic decreasing (increasing) behaviour with the values of 1/45.2×105 (12.1 W m-1 K-1) and 1/43.6×105 (14.2 W m-1 K-1) ω-1 m-1 at 300 K and 800 K, respectively. The κ exhibits the temperature dependency as, κ T 0.16. The DFT based Boltzmann transport theory is used to understand these behaviour. The multi-band electron and hole pockets appear to be mainly responsible for deciding the temperature dependent transport behaviour. Specifically, the decrease in the |S| above 560 K and change in the slope of σ around 450 K are due to the contribution of thermally generated charge carriers from the hole pockets. The temperature dependent relaxation time (τ) is computed by comparing the experimental σ with calculated σ/τ and it shows temperature dependency of 1/T 0.35. Further this value of τ is used to calculate the temperature dependent electronic part of thermal conductivity (κ e) and it gives a fairly good match with the experiment. Present study suggests that electronic band-structure obtained from DFT provides a reasonably good estimate of the transport coefficients of CoSi in the high temperature region of 300-800 K. © 2022 IOP Publishing Ltd.
Raman effect and unusual transport properties of Co-doped Mn2FeAl Heusler alloy
(Institute of Physics, 2023) Srishti Dixit; Swayangsiddha Ghosh; Neha Patel; Mohd Alam; Krishanu Bandyopadhyay; Nisha Shahi; Yogendra Kumar; M. Sawada; K. Shimada; Satyen Saha; Sanjay Singh; Sandip Chatterjee
Semiconducting materials with a distinctive blend of high electrical and low thermal conductivity are required for efficient thermoelectric devices. In this aspect, Heusler alloys are potential candidates for thermoelectric materials. It has been observed that Co doping in Mn2FeAl enhances the electrical conductivity as well as reduces the thermal conductivity of the system leading to an improvement in figure of merit. The Seebeck coefficient suggested the p-type behavior over the whole temperature range, followed by a maximum at 150 K. Additionally, the electronic properties of the suggest that the observed Raman mode is due to the electronic excitations in the system. Interestingly, this system shows a decoupling between the Seebeck coefficient and electrical conductivity, suggesting the promising potential of as a thermoelectric material and offering valuable insights into its electronic properties. Copyright © 2024 EPLA.
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.
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.