arXiv subjects

Suchanda Mondal

Publications and source records attributed to Suchanda Mondal.

8 recordsLinked to original sources

arXiv2026

Nanoscale Electronic Phase Separation Driven by Fe-site Ordering in Fe\textsubscript{5-x}GeTe\textsubscript{2}

Understanding how local structural order governs electronic correlations is essential for revealing the microscopic mechanism underlying emergent behavior in two-dimensional magnets. In the layered van der Waals ferromagnet Fe\textsubscript{5-x}GeTe\textsubscript{2}, intrinsic Fe-site disorder provides a natural platform to probe this interplay. Here, we establish a direct atomic scale correlation between Fe-site ordering and local electronic structure by combining high-resolution scanning tunneling microscopy with density functional theory calculations. Scanning tunneling microscopy resolves two coexisting surface phases, a $\sqrt{3} \times \sqrt{3}$ superstructure associated with ordered Fe(1) configurations and an undistorted $1 \times 1$ hexagonal Te lattice in Fe(1)-deficient regions. Spatially resolved spectroscopy shows that the $\sqrt{3}$-ordered domains exhibit metallic behavior, whereas Fe(1) vacant areas display a suppressed density of states(DOS) near the Fermi level, indicative of pseudogapped electronic states. The nanoscale coexistence of these distinct electronic responses provides direct evidence of electronic phase separation driven by Fe-site ordering. First-principles calculations reveal that symmetry allowed hybridization between Fe 3d and Te 5p orbitals reconstructs the low-energy electronic structure, giving rise to the contrasting tunneling signatures of ordered and disordered phases. Bias-dependent local DOS simulations reproduce the experimentally observed contrast evolution and reveal that hybridization induced out of plane orbital character governs the spatial modulation of tunneling conductance. These results provide a microscopic framework linking atomic-scale structural order to nanoscale electronic inhomogeneity in van der Waals magnets.

cond-mat.mes-hall↗

arXiv2026

Topological and Planar Hall Effect in Monoclinic van der Waals Ferromagnet NbFeTe$_2$

Two-dimensional (2D) van der Waals (vdW) ferromagnets have emerged as a critical class of quantum materials for next-generation, low-dimensional spintronic devices. In this study, we report a comprehensive study of the transport properties of the layered soft ferromagnet $\text{NbFeTe}_2$. We report the first observation of the topological Hall effect (THE) and the planar Hall effect (PHE) in metallic $\text{NbFeTe}_2$. THE signatures persist up to 45 K, while PHE remains evident well above Curie temperature ($T_C$). The observed negative longitudinal magnetoresistance, along with the PHE, provides strong evidence for a nontrivial electronic band structure. The coexistence of perpendicular magnetic anisotropy and a substantial THE: two key properties that are highly desirable for future spintronics applications, makes monoclinic vdW ferromagnetic $\text{NbFeTe}_2$ a promising platform to advance spintronics applications.

cond-mat.supr-con↗

arXiv2025

Spin Reorientation Driven Renormalization of Spin-Phonon Coupling in Fe$_4$GeTe$_2$

Quasi-2D van der Waals ferromagnet Fe$_4$GeTe$_2$, featuring the simultaneous presence of high Curie temperature ($T_\mathrm{C}$ $\sim 270$ K) and a spin-reorientation transition at $T_\mathrm{SR}$ $\sim 110$ K, is a rare system where strong interplay of spin dynamics, lattice vibrations, and electronic structure leads to a wide range of interesting phenomena. Here, we investigate the lattice response of exfoliated Fe$_4$GeTe$_2$ nanoflakes using temperature-dependent Raman spectroscopy. Polarization-resolved measurements reveal that, while one Raman mode exhibits a purely out-of-plane character, the rest display mixed symmetry, reflecting interlayer vibrational nonuniformity and symmetry-driven mode degeneracies. Below $T_\mathrm{C}$, phonons harden, and the linewidth narrows, consistent with reduced anharmonicity, while across the spin reorientation transition at $T_\mathrm{SR}$ they display anomalous softening, linewidth broadening, and a peak in lifetime, which are signatures of strengthened spin-phonon coupling. Complementary DFT+DMFT calculations and atomistic spin dynamical simulations reveal temperature-dependent spin excitations whose energies overlap with the Raman-active phonons, providing a natural route for the observed magnon-phonon interaction. Together, these insights establish Fe$_4$GeTe$_2$ as a versatile platform for exploring intertwined spin, lattice, and electronic degrees of freedom, with relevance for dynamic spintronic and magneto-optic functionalities near technologically meaningful temperatures.

cond-mat.mes-hall↗

arXiv2025

Disentangling the unusual magnetic anisotropy of the near-room-temperature ferromagnet Fe$_{4}$GeTe$_{2}$

In the quest for two-dimensional conducting materials with high ferromagnetic ordering temperature the new family of the layered Fe$_{n}$GeTe$_{2}$ compounds, especially the near-room-temperature ferromagnet Fe$_{4}$GeTe$_{2}$, receives a significant attention. Fe$_{4}$GeTe$_{2}$ features a peculiar spin reorientation transition at $T_\mathrm{SR} \sim 110$ K suggesting a non-trivial temperature evolution of the magnetic anisotropy (MA) - one of the main contributors to the stabilization of the magnetic order in the low-D systems. An electron spin resonance (ESR) spectroscopic study reported here provides quantitative insights into the unusual magnetic anisotropy of Fe$_{4}$GeTe$_{2}$. At high temperatures the total MA is mostly given by the demagnetization effect with a small contribution of the counteracting intrinsic magnetic anisotropy of an easy-axis type, whose growth below a characteristic temperature $T_{\rm shape} \sim 150$ K renders the sample seemingly isotropic at $T_\mathrm{SR}$. Below one further temperature $T_{\rm d} \sim 50$ K the intrinsic MA becomes even more complex. Importantly, all the characteristic temperatures found in the ESR experiment match those observed in transport measurements, suggesting an inherent coupling between magnetic and electronic degrees of freedom in Fe$_{4}$GeTe$_{2}$. This finding together with the observed signatures of the intrinsic two-dimensionality should facilitate optimization routes for the use of Fe$_{4}$GeTe$_{2}$ in the magneto-electronic devices, potentially even in the monolayer limit.

cond-mat.str-el↗

arXiv2023

Unusual magnetotransport and anomalous Hall effect in quasi-two-dimensional van der Waals ferromagnet Fe$_4$GeTe$_2$

Fe$_4$GeTe$_2$, an itinerant vdW ferromagnet (FM) having Curie temperature (T$_C$) close to room temperature ($\sim 270$ K), exhibits another transition (T$_{SR}$ $\sim$ 120 K) where the easy axis of magnetization changes from in-plane to the out-of-plane direction in addition to T$_C$. Here, we have studied the magnetotransport in a multilayer Hall bar device fabricated on 300 nm Si/SiO$_2$ substrate. Interestingly, the zero field resistivity shows a negligible change in resistivity near T$_C$ unlike the typical metallic FM, whereas, it exhibits a dramatic fall below T$_{SR}$. Also, the resistivity shows a weak anomaly at T $ \sim $ 38 K (T$_Q$), below which the resistivity shows a quadratic temperature dependence according to the Fermi liquid behavior. Temperature-dependent Hall data exhibits important consequences. The ordinary Hall coefficient changes sign near T$_{SR}$ indicating the change in majority carriers. In a similar manner, the magnetoresistance (MR) data shows significantly large negative MR near T$_{SR}$ and becomes positive below T$_Q$. The observations of anomaly in the resistivity, sign-change of the ordinary Hall coefficient and maximum negative MR near T$_{SR}$, together suggest a possible Fermi surface reconstruction associated with the spin reorientation transition. Furthermore, analysis of the Hall data reveals a significant anomalous Hall conductivity (AHC) from $\sim 123 \Omega^{-1}$ cm$^{-1}$ (at T $\approx$ 5 K) to the maximum value of $\sim 366 \Omega^{-1}$ cm$^{-1}$ near T$_{SR}$. While the low-temperature part may originate due to the intrinsic KL mechanism, our analysis indicates that the temperature-dependent AHC is primarily appearing due to the side-jump mechanism as a result of the spin-flip electron-magnon scattering. Our study demonstrates an interplay between magnetism and band topology and its consequence on electron transport in Fe$_4$GeTe$_2$.

cond-mat.mes-hall↗

arXiv2022

Anomalous magnetic properties of quasi two-dimensional van der Waals ferromagnet Fe4GeTe2

We have studied the magnetic properties of quasi two-dimensional itinerant ferromagnet Fe$_4$GeTe$_2$ as functions of temperature and magnetic field ($B$) with field parallel to the $ab$-plane ($B\|ab$) and $c$-axis ($B\|c$) of the crystal. This van der Waals compound undergoes a continuous paramagnetic to ferromagnetic phase transition below $T_C$ = 270.0 K and a pronounced spin reorientation transition at around $T_{SR}$ $\sim$ 115 K where the easy axis of magnetization changes its direction from in-plane to out-of-plane. The temperature evolution of magnetization in the FM state is highly anomalous and extremely sensitive to the direction and strength of applied magnetic field. Magnetic entropy change ($ΔS_M$) has been estimated in the vicinity of $T_C$ and $T_{SR}$. $ΔS_M$($T$) is found to be almost isotropic around $T_C$ while it shows very unusual behavior and is sensitive to the direction of applied field at low temperature close to $T_{SR}$. For $B\|c$, $ΔS_M$ is negative and decreases continuously with field and -$ΔS_M$($T$) shows a broad maximum around $T_{SR}$ similar to that observe at $T_C$. On the other hand, $ΔS_M$ is positive in the vicinity of $T_{SR}$ below 1.4 T and -$ΔS_M$($T$) exhibits a peak at $T_{SR}$ which becomes very sharp at high field for $B\|ab$. These results suggest complex nature of the magnetic ground state of Fe$_4$GeTe$_2$, which is possibly due to the presence of multiple inequivalent Fe sites, their ordering and complexity of spin configuration.

cond-mat.str-el↗

arXiv2021

Superconductivity coexisting with ferromagnetism in a quasi-one dimensional non-centrosymmetric (TaSe$_4$)$_3$I

Low-dimensional materials with broken inversion symmetry and strong spin-orbit coupling can give rise to fascinating quantum phases and phase transitions. Here we report coexistence of superconductivity and ferromagnetism below 2.5\,K in the quasi-one dimensional crystals of non-centrosymmetric (TaSe$_4$)$_3$I (space group: $P\bar{4}2_1c$). The unique phase is a direct consequence of inversion symmetry breaking as the same material also stabilizes in a centro-symmetric structure (space group: $P4/mnc$) where it behaves like a non-magnetic insulator. The coexistence here upfront contradicts the popular belief that superconductivity and ferromagnetism are two apparently antagonistic phenomena. Notably, here, for the first time, we have clearly detected Meissner effect in the superconducting state despite the coexisting ferromagnetic order. The coexistence of superconductivity and ferromagnetism projects non-centrosymmetric (TaSe$_4$)$_3$I as a host for complex ground states of quantum matter including possible unconventional superconductivity with elusive spin-triplet pairing.

cond-mat.supr-con↗

arXiv2019

Effect of hydrostatic pressure on ferromagnetism in two-dimensional CrI$_3$

We have investigated the magnetic properties of highly anisotropic layered ferromagnetic semiconductor CrI$_3$ in presence of hydrostatic pressure ($P$). At ambient pressure, magnetization exhibits a clear anomaly below 212 K along with a thermal hysteresis over a wide temperature range (212-180 K), where a first-order structural transition is observed. CrI$_3$ undergoes a second-order ferromagnetic-paramagnetic phase transition with Curie temperature $T_C$=60.4 K. With application of pressure, the transition becomes sharper and $T_C$ is found to increase from 60.4 to 64.9 K as $P$ increases from 0 to 1.0 GPa. $T_C$ increases with $P$ in a sublinear fashion. The thermal hysteresis in magnetization and the increase of $T_C$ with pressure suggest that the spin and lattice degrees of freedom are coupled. The observed increase in $T_C$ has been explained on the basis of change in inter-layer coupling and Cr-I-Cr bond angle with pressure.

cond-mat.mtrl-sci↗