arXiv subjects

Hari Bhandari

Publications and source records attributed to Hari Bhandari.

12 recordsLinked to original sources

arXiv2026

Anisotropy, frustration and saddle point in the twisted Kagome antiferromagnet ErPdPb

The kagome lattice, with its inherent geometric frustration, provides a rich platform for exploring intriguing magnetic phenomena and topological electronic structures. In reduced-symmetry structures, such as twisted kagome systems involving rare earth elements, additional anisotropy can arise, enabling intriguing properties including spin-ice states, magnetocaloric effects, noncollinear magnetic ordering, and anomalous Hall effect. Here, we report the synthesis of single crystals of ErPdPb, which features a twisted kagome lattice net of Er atoms within the hexagonal ZrNiAl-type structure, and we investigate its magnetic, electronic, and thermal properties. The material exhibits antiferromagnetic ordering below 2.2 K, consistently observed in magnetic, transport, and heat capacity measurements. Magnetization measurements reveal 1/3 metamagnetic steps along the c-axis below the Néel temperature, suggesting an Ising-spin-like state on the twisted kagome lattice. A pronounced anisotropy between in-plane and out-of-plane resistivity is observed throughout the temperature range of 1.8-300 K, and the compound exhibits a significant frustration index of 13.6 (12.7) along the c-axis (ab-plane). Heat capacity measurements show a broad hump at 2.2 K, with an additional increase below 0.5 K. The anisotropic magnetic properties are further explored through density functional theory (DFT) calculations, which suggest strong easy-axis anisotropy, consistent with experimental magnetic measurements and crystal-field model expectations, and quasi-one-dimensional bands and a spin-split saddle point at the zone center.

cond-mat.str-el↗

arXiv2025

Domain wall induced topological Hall effect in the chiral-lattice ferromagnet Fe$_x$TaS$_2$

Magnetic topology and its associated emergent phenomena are central to realizing intriguing quantum states and spintronics functionalities. Designing spin textures to achieve strong and distinct electrical responses remains a significant challenge. Layered transition metal dichalcogenides offer a versatile platform for tailoring structural and magnetic properties, enabling access to a wide spectrum of topological magnetic states. Here, we report a domain-wall-driven, large, and tunable topological Hall effect (THE) in a non-centrosymmetric intercalated transition metal dichalcogenides series Fe$_x$TaS$_2$. By systematically varying the Fe intercalation level, we exert precise control over the magnetic ground states, allowing manipulation of the topological Hall effect. Real-space magnetic force microscopy (MFM) provides direct evidence of periodic magnetic stripe domain formation, confirming the microscopic origin of the observed topological transport phenomena. Our findings establish a promising way for tuning the topology of domains to generate substantial electromagnetic responses in layered magnetic materials.

cond-mat.mtrl-sci↗

arXiv2025

Doping-induced Spin Reorientation in Kagome Magnet TmMn6Sn6

The kagome-lattice compounds RMn6Sn6 (R is a rare earth element), where the Mn atoms form a kagome net in the basal plane, are currently attracting a great deal of attention as they have been shown to host complex magnetic textures and electronic topological states strongly sensitive to the choice of the R atom. Among the magnetic R atoms, TmMn6Sn6 orders with the easy-plane magnetization forming a complex magnetic spiral along the c-axis. Previous neutron studies, carried on polycrystalline, samples found that Ga doping changes the magnetic anisotropy from easy-plane to easy-axis. Here we present magnetic and magnetotransport measurements on a single crystal and first principles calculations in the doping series of TmMn6Sn6-xGax. We find that the magnetic properties are highly sensitive even to a small concentration of Ga. With minimal Ga substitution, the easy-plane anisotropy is maintained, which gradually changes to the easy-axis anisotropy with increasing Ga. We discuss these observations with respect to the effect of Ga doping on magnetocrystalline anisotropy and Tm crystal field

cond-mat.mtrl-sci↗

arXiv2025

Skyrmion Bubbles by Design in a Centrosymmetric Kagome Magnet

Topologically protected nanoscale spin textures, such as magnetic skyrmions, have attracted significant interest for spintronics applications. While skyrmions in noncentrosymmetric materials are known to be stabilized by Dzyaloshinskii$-$Moriya interaction (DMI), their deliberate design in centrosymmetric materials remains a challenge. This difficulty largely stems from the complexity of controlling magnetocrystalline anisotropy $-$ a critical factor in the absence of DMI. Here, we demonstrate the chemical tuning of magnetocrystalline anisotropy in the centrosymmetric Kagome magnet TmMn$_6$Sn$_6$. The resulting compound exhibits a spin reorientation transition accompanied by an emergent skyrmion bubble lattice, confirmed by Lorentz transmission electron microscopy. Our findings overcome a key materials design challenge and open possibilities for deliberate design of skyrmionic textures in centrosymmetric systems.

cond-mat.mtrl-sci↗

arXiv2025

Tunable topological transitions in the frustrated magnet HoAgGe

The kagome lattice, known for its strong frustration in two dimensions, hosts a variety of exotic magnetic and electronic states. A variation of this geometry, where the triangular motifs are twisted to further reduce symmetry, has recently revealed even more complex physics. HoAgGe exemplifies such a structure, with magnetic and electronic properties believed to be driven by strong in-plane anisotropy of the Ho spins, effectively acting as a two-dimensional spin ice. In this study, using a combination of magnetization, Hall conductivity measurements, and density functional theory calculations, we demonstrate how various spin-ice states, stabilized by external magnetic fields, influence the Fermi surface topology. More interestingly, we observe sharp transitions in Hall conductivity without concurrent changes in magnetization when an external magnetic field is applied along a particular crystallographic direction, underscoring the role of strong magnetic frustration and providing a new platform for exploring the interplay between magnetic frustration, electronic topology, and crystalline symmetry. These results also highlight the limitations of a simple spin-ice model, suggesting that a more sophisticated framework is necessary to capture the subtle experimental nuances observed.

cond-mat.str-el↗

arXiv2024

Three-dimensional nature of anomalous Hall conductivity in YMn6Sn6-xGax, x ~ 0.55

The unique connectivity of kagome lattices gives rise to topological properties, such as flat bands and Dirac cones. When combined with ferromagnetism and a chemical potential near the 2D Dirac points, this structure offers the potential to realize the highly sought-after topological Chern magnetotransport. Recently, there was considerable excitement surrounding this possibility in the ferrimagnetic kagome metal TbMn$_\mathbf{6}$Sn$_\mathbf{6}$. However, density functional theory (DFT) calculations reveal that the 2D Chern gap lies well above the Fermi energy, challenging its relevance in the observed anomalous Hall conductivity. Here, we investigate YMn$_\mathbf{6}$Sn$_\mathbf{5.45}$Ga$_\mathbf{0.55}$, a compound with similar crystallographic, magnetic, and electronic properties to TbMn$_\mathbf{6}$Sn$_\mathbf{6}$. Our findings show that the intrinsic anomalous Hall conductivity in this material, while comparable in magnitude to that in TbMn$_\mathbf{6}$Sn$_\mathbf{6}$, is fully three-dimensional, thus providing experimental evidence that Hall conductivity in this class of materials does not originate from 2D Chern gaps. Additionally, we confirm that the newly proposed empirical scaling relation for extrinsic Hall conductivity is universally governed by spin fluctuations.

cond-mat.mtrl-sci↗

arXiv2024

Origin of spin reorientation and intrinsic anomalous Hall effect in the kagome ferrimagnet TbMn6Sn6

TbMn$_6$Sn$_6$ has attracted a lot of recent interest for a variety of reasons, most importantly, because of the hypothesis that it may support quantum-limit Chern topological magnetism, derived from the kagome geometry. Besides, TbMn$_6$Sn$_6$ features a highly unusual magnetic reorientation transition about 100 K below the Curie point, whereby all spins in the system, remaining collinear, rotate by 90$^\circ$. In this work, we address both issues combining experiment, mean-field theory and first-principle calculations. Both magnetic reorientation and the unusual temperature dependence of the anomalous Hall conductivity (AHC) find quantitative explanation in the fact that Mn and Tb, by virtue of the Mermin-Wagner theorem, have very different spin dynamics, with Tb spins experiencing much more rapid fluctuation. We were able to cleanly extract the intrinsic AHC from our experiment, and calculated the same microscopically, with good semiquantitative agreement. We have identified the points in the band structure responsible for the AHC and showed that they are not the kagome-derived Dirac points at the K-corner of the Brillouin zone, as conjectured previously.

cond-mat.str-el↗

arXiv2024

Altermagnetism in the layered intercalated transition metal dichalcogenide CoNb$_4$Se$_8$

Altermagnets (AMs) are a new class of magnetic materials that combine the beneficial spintronics properties of ferromagnets and antiferromagnets, garnering significant attention recently. Here, we have identified altermagnetism in a layered intercalated transition metal diselenide, CoNb$_4$Se$_8$, which crystallizes with an ordered sublattice of intercalated Co atoms between NbSe$_2$ layers. Single crystals are synthesized, and the structural characterizations are performed using single crystal diffraction and scanning tunneling microscopy. Magnetic measurements reveal easy-axis antiferromagnetism below 168 K. Density functional theory (DFT) calculations indicate that A-type antiferromagnetic ordering with easy-axis spin direction is the ground state, which is verified through single crystal neutron diffraction experiments. Electronic band structure calculations in this magnetic state display spin-split bands, confirming altermagnetism in this compound. The layered structure of CoNb$_4$Se$_8$ presents a promising platform for testing various predicted properties associated with altermagnetism.

cond-mat.str-el↗

arXiv2024

Uncovering the Timescales of Spin Reorientation in $TbMn_{6}Sn_{6}$

Kagome ferrimagnets are of fundamental interest because of their tunable magnetic, electronic and topological properties, with potential applications for quantum-enabled technologies and spintronics. Here we present the first direct measurement of the intrinsic timescale of the spin reorientation transition in the Kagome ferrimagnet $TbMn_{6}Sn_{6}$. This material exhibits a magnetic phase transition near room temperature, where spins remain collinear while the total magnetic moment rotates from out-of-plane to in-plane. This reorientation has been attributed to the competing anisotropies of Tb and Mn, whose magnetic moments have very different temperature dependencies. By probing at the Mn M-edge using high harmonic extreme ultraviolet pulses, we measure reorientation timescales between 6 and 18 ps, depending on the laser excitation fluence. Our model of the magnetization dynamics shows that this timescale is consistent with a spin reorientation driven by very large anisotropy energies, on meV energy scales. Our model also predicts a possibility of a 180$^{\circ}$ reorientation of the out-of-plane moment, that could facilitate optically controlled magnetization switching between very stable ground states, for applications in spintronics or data storage.

cond-mat.mtrl-sci↗

arXiv2023

Magnetism and Fermiology of Kagome Magnet YMn6Sn4Ge2

Kagome lattice magnets are an interesting class of materials as they can host topological properties in their magnetic and electronic structures. YMn6Sn6 is one such compound in which a series of competing magnetic phases is stabilized by an applied magnetic field, and both an enigmatic topological Hall effect and a Dirac crossing close to the Fermi energy have been realized. This material also shows a magnetization-induced Lifshitz transition and evidence of a unique charge spin coupling in one of the magnetic phases, namely the fan-like phase. Tuning the magnetism, and thus the interplay with the electronic states, opens new avenues for precise control of these novel properties. Here, we demonstrate the extreme sensitivity of the magnetic phases in YMn6Sn4Ge2 through the investigation of structural, magnetic, and transport properties. The high sensitivity to small doping provides great potential for engineering the magnetic phases and associated electronic properties in this family of rare-earth kagome magnets.

cond-mat.str-el↗

arXiv2022

CoTe2: A quantum critical Dirac metal with strong spin fluctuations

Quantum critical points separating weak ferromagnetic and paramagnetic phases trigger many novel phenomena. Dynamical spin fluctuations not only suppress the long-range order, but can also lead to unusual transport and even superconductivity. Combining quantum criticality with topological electronic properties presents a rare and unique opportunity. Here, by means of ab initio calculations and magnetic, thermal, and transport measurements, we show that the orthorhombic CoTe$_2$ is close to ferromagnetism, which appears suppressed by spin fluctuations. Calculations and transport measurements reveal nodal Dirac lines, making it a rare combination of proximity to quantum criticality and Dirac topology.

cond-mat.str-el↗

arXiv2022

Visualizing the out-of-plane electronic dispersions in an intercalated transition metal dichalcogenide

Layered transition metal dichalcogenides have rich phase diagram and they feature two dimensionality on numerous physical properties. Co1/3NbS2 is one of the newest members of this family where Co atoms are intercalated into the Van der Waals gaps between NbS2 layers. We study the three-dimensional electronic band structure of Co1/3NbS2 using both surface and bulk sensitive angle-resolved photoemission spectroscopy. We show that the electronic bands do not fit into the rigid-band-shift picture after the Co intercalation. Instead, Co1/3NbS2 displays a different orbital character near the Fermi level compared to the pristine NbS2 compound and has a clear band dispersion in kz direction despite its layered structure. Our photoemission study demonstrates the out-of-plane electronic correlations introduced by the Co intercalation, thus offering a new perspective on this compound. Finally, we propose how Fermi level tuning could lead to exotic phases such as spin density wave instability.

cond-mat.mtrl-sci↗