arXiv subjects

Sang-Eon Lee

Publications and source records attributed to Sang-Eon Lee.

10 recordsLinked to original sources

arXiv2026

Giant Exfoliation Induced Magnetic Coercivity in Fe$_3$GaTe$_2$

Permanent magnets with strong anisotropy and high coercivity underpin modern information and energy technologies, yet rare-earth-free alternatives remain limited. Here, we show that thickness engineering via mechanical exfoliation induces hard magnetic behavior in the van der Waals ferromagnet Fe$_3$GaTe$_2$. Bulk crystals exhibit Curie temperatures above 350 K but negligible room-temperature coercivity. When thinned below 100 nm, the coercive field is dramatically enhanced, reaching nearly 1 T at room temperature for in-plane fields which is comparable to values of conventional hard magnets. Micromagnetic analysis reveals a crossover in magnetization reversal from domain-mediated processes in bulk samples to quasi-coherent rotation in thin flakes, driven by increased effective anisotropy and suppressed domain formation. This thickness-dependent transition enables tuning of magnetic hardness without chemical modification. Combined with high saturation magnetization and robust room-temperature performance, Fe$_3$GaTe$_2$ emerges as a promising rare-earth-free material for spintronic applications. Its layered structure further allows integration into van der Waals heterostructures, where large in-plane coercivity can stabilize magnetic states against perturbations and interlayer coupling, offering potential for high-density nonvolatile memory and domain-wall-based devices.

cond-mat.mtrl-sci↗

arXiv2026

Topological Hall Effect in Antiferromagnetic Co doped Fe$_3$GaTe$_2$

Fe$_3$GaTe$_2$ is van der Waals (vdW) ferromagnet with a Curie temperature $T_C$ ranging from 350 K to 380 K, followed upon cooling by a ferrimagnetic transition near room temperature. Substituting Fe with Co was previously reported to induce antiferromagnetism (AFM) at a Co fraction dependent Neel temperature $T_N$. In this work, we confirm the overall phase diagram of the Fe$_{3-x}$Co$_x$GaTe$_2$ series as a function of $x$ and temperature via magnetization and electrical transport measurements. For $x \simeq 0.6$ the Hall effect is observed to mimic the magnetization as the AF ground state is suppressed by the external magnetic field via a metamagnetic transition, thus displaying an anomalous Hall response. At low temperatures, we also observe a pronounced topological Hall signal peaking at $μ_0H$ = 4 T, or within the metamagnetic transition region of fields. This observation points to the presence of magnetic field-induced chiral spin textures, such as skyrmions upon approaching magnetization saturation. Magnetic force microscopy (MFM) reveals the emergence of nearly circular magnetic domains, with diameters on the order of 100 to 200 nm, within the antiferromagnetic phase. A detailed analysis of the MFM images indicates that the topological Hall effect is closely linked to the field-induced stabilization of magnetic domain structures, likely exhibiting chiral textures. This observation suggests the possible formation of skyrmions already in the AFM phase, i.e., AFM skyrmions, that evolve into ferromagnetic (FM) ones upon increasing the magnetic field. Consequently, Co-doped Fe$_3$GaTe$_2$ might provide a platform to investigate the transformation of skyrmions, initially coupled antiferromagnetically into ferromagnetic skyrmions, and to explore its impact on the topological and skyrmion Hall effects.

cond-mat.mes-hall↗

arXiv2026

High magnetic field response of superconductivity dome in quantum artificial High Tc superlattices with variable geometry

It is known that cuprate artificial high Tc superlattices (AHTS) with period d, composed of quantum wells confining interface space charge in stoichiometric Mott insulator layers (S), with thickness L, at the interface with overdoped normal metallic cuprate layers (N) show a superconducting dome by tuning the geometric L over d ratio of the SNSN superlattice with the top predicted by quantum material design engineering quantum size effects. Here we report high-field magneto transport measurements up to 41 Tesla of AHTS across the entire superconducting dome. The results show the universal upward-concave behavior of the temperature dependent upper critical magnetic field in low Tc samples at rising edge and drop edge of the dome providing strong evidence consistent with two-band superconductivity for two-band superconductivity in agreement with multigap theory used for quantum design of the SNSN superlattices. The measured superconducting coherence length demonstrates that atomic-scale engineering controls not only the critical temperature but also the intrinsic pair size at Fano-Feshbach resonances physics paving the way toward next generation quantum devices and shedding light on unconventional superconductivity.

cond-mat.supr-con↗

arXiv2025

Inhomogeneity identification by measuring magnetic quantum oscillations

This study explores the identification of sample inhomogeneity via magnetic quantum oscillations analysis in semimetal NbSb$_2$. By doping Bi and Cr, we obtained a homogeneous Bi-doped sample and an inhomogeneous Cr-doped sample, whose homogeneity was confirmed by comparing the magnetic quantum oscillation before and after grinding the samples. The magnetic quantum oscillations in the inhomogeneous sample exhibited a distinct phase shift and unusual field-dependent amplitude, believed to result from a non-uniform Fermi energy. The analysis of the magnetic quantum oscillations demonstrated that the homogeneous Bi-doped sample can be interpreted by the symmetric and Lorentzian effective Fermi energy distribution, while the inhomogeneous Cr-doped sample exhibited an asymmetric distribution, illustrating an unconventional violation of the Lifshitz-Kosevich formula. This research provides a novel method for identifying material inhomogeneity and mitigating potential misinterpretations of magnetic quantum oscillations' unusual phase, commonly seen as a nontrivial Berry phase indicator in topological materials studies.

cond-mat.mtrl-sci↗

arXiv2025

Disorder driven crossover between anomalous Hall regimes in Fe$_3$GaTe$_2$

The large anomalous Hall conductivity (AHC) of the Fe$_3$(Ge,Ga)Te$_2$ compounds has attracted considerable attention. Here, we expose the intrinsic nature of AHC in Fe$_3$GaTe$_2$ crystals characterized by high conductivities, which show disorder-independent AHC with a pronounced value $σ_{xy}^{\text{c}}\approx$ 420 $Ω^{-1}$cm$^{-1}$. In the low conductivity regime, we observe the scaling relation $σ_{xy}\proptoσ_{xx}^{1.6}$, which crosses over to $σ_{xy} \simeq σ_{xy}^{\text{c}}$ as $σ_{xx}$ increases. Disorder in low-conductivity crystals is confirmed by the broadening of a first-order transition between ferromagnetism and the ferrimagnetic ground state. Through density functional theory (DFT) calculations, we reveal that the dominant sources of Berry curvature are located a few hundred meV below the Fermi energy around the $Γ$-point. Therefore, Fe$_3$GaTe$_2$ clearly exposes the disorder-induced crossover among distinct AHC regimes, previously inferred from measurements on different ferromagnets located in either side of the crossover region.

cond-mat.mtrl-sci↗

arXiv2025

Local Inversion Symmetry Breaking and Thermodynamic Evidence for Ferrimagnetism in Fe3GaTe2

The layered compound Fe3GaTe2 is attracting attention due to its high Curie temperature, low dimensionality, and the presence of topological spin textures above room temperature, making Fe$_3$GaTe$_2$ a good candidate for applications in spintronics. Here, we show, through transmission electron microscopy (TEM) techniques, that Fe$_3$GaTe$_2$ single crystals break local inversion symmetry while maintaining global inversion symmetry according to X-ray diffraction. Coupled to the observation of Néel skyrmions via Lorentz-TEM, our structural analysis provides a convincing explanation for their presence in centrosymmetric materials. Magnetization measurements as a function of the temperature displays a sharp first-order thermodynamic phase-transition leading to a reduction in the magnetic moment. This implies that the ground state of Fe$_3$GaTe$_2$ is globally ferrimagnetic and not a glassy magnetic state composed of ferrimagnetic, and ferromagnetic domains as previously claimed. Neutron diffraction studies indicate that the ferromagnetic to ferrimagnetic transition upon reducing the external magnetic field is associated with a change in the magnetic configuration/coupling between Fe1 and Fe2 moments. We observe a clear correlation between the hysteresis observed in both the skyrmion density and the magnetization of Fe$_3$GaTe$_2$. This indicates that its topological spin textures are affected by the development of ferrimagnetism upon cooling. Observation, via magnetic force microscopy, of magnetic bubbles at the magnetic phase boundary suggests skyrmions stabilized by the competition among magnetic phases and distinct exchange interactions. Our study provides an explanation for the observation of Néel skyrmions in centrosymmetric systems, while exposing a correlation between the distinct magnetic phases of Fe$_3$GaTe$_2$ and topological spin textures.

cond-mat.mtrl-sci↗

arXiv2025

Type-II Weyl nodes, flat bands, and evidence for a topological Hall-effect in the new ferromagnet FeCr$_3$Te$_6$

The interplay between linearly dispersing or Dirac-like, and flat electronic bands, for instance, in the kagome ferromagnets, has attracted attention due to a possible interplay between topology and electronic correlations. Here, we report the synthesis, structural, electrical, and magnetic properties of a single-crystalline ferromagnetic compound, namely Fe$_{1/3}$CrTe$_2$ or FeCr$_3$Te$_6$, which crystallizes in the $P\bar{3}m1$ space group instead of the $I2/m$ previously reported for FeCr$_2$Te$_4$. Electronic band structure calculations reveal type-II Dirac nodes and relatively flat bands near the Fermi level ($\varepsilon_F$). This compound shows onset Curie temperature $T_{\text{c}}\simeq 120$ K, followed by an additional ferromagnetic transition near $T_{\text{c2}} \sim 92.5 $ K. Below $T_{\text{c}}$, FeCr$_3$Te$_6$ displays a pronounced anomalous Hall effect, as well as sizable coercive fields that exceed $μ_0H = 1$~T at low $T$s. However, a scaling analysis indicates that the anomalous Hall effect results from a significant intrinsic contribution, as expected from the calculations, but also from the extrinsic mechanism, i.e., scattering. The extrinsic contribution probably results from occupational disorder at the 1b Fe-site within the van der Waals gap of the CrTe$_2$ host. We also observe evidence for a topological Hall component superimposed onto the overall Hall response, suggesting the presence of chiral spin textures akin to skyrmions in this centrosymmetric system. Their possible presence will require experimental confirmation.

cond-mat.other↗

arXiv2023

Fermi surface magnetization of Fe-doped NbSb$_2$ investigated by magnetic quantum oscillations

Magnetic quantum oscillations (MQOs) have been widely used as a tool for probing Fermi surfaces. The shape and topology of the Fermi surface and the related physical parameters, such as the cyclotron mass and relaxation time, can be verified by carefully analyzing the frequency, amplitude, and phase of MQOs. In particular, phase analysis, from which we can obtain the Berry phase, has received much attention with the growing interest in the topology of condensed matter physics. Here, beyond the conventional uses of MQOs, we show that MQOs can be used to determine the Fermi surface magnetization. We doped dilute magnetic element Fe into the Dirac semimetal NbSb$_2$ to only introduce magnetism without changing the shape of the Fermi surface. We observed a phase shift in magnetically doped Fe-NbSb$_2$, which is in contrast to the lack of a phase shift in pristine NbSb$_2$ and nonmagnetic Bi-doped NbSb$_2$, indicating the strong exchange interaction between doped magnetic impurities and Fermi surface electrons. We estimated the Fermi surface magnetization introduced by magnetic Fe doping from the phase shift. This work demonstrates not only how tiny magnetic impurities can significantly change the Fermi surface magnetization but also how the Fermi surface magnetization can be investigated by phase analysis of MQOs.

cond-mat.mes-hall↗

arXiv2021

Degenerate magnetic ground state and metastable state on trihexagonal Co-sublattices in Co3Sn2(S,Se)2 single crystals

A trihexagonal lattice has been predicted to retain a degenerate magnetic state which enriches physical properties. Especially, Co3Sn2S2, possessing trihexagonal Co-sublattices, has been observed to own topological quantum properties. Experimentally, Co3Sn2S2 has been reported to have hidden magnetic phases due to magnetic anomalies. To clarify the hidden magnetic phase, we fabricated high-quality single crystals of Co3Sn2S2-xSex (x = 0, 0.26, & 0.86). The Se-substitution is intended to broaden the distance between Co atoms. For each Se-composition, magnetizations of single-crystalline Co3Sn2S2-xSex implicate that the magnetic ground state consists of the out-of-plane ferromagnetism and the in-plane antiferromagnetism: Being degenerate. Meanwhile, along out-of-plane, remanent magnetizations of Co3Sn2S2-xSex have a first-order phase transition, so Co3Sn2S2-xSex owns an excited magnetic state, denoted as the metastable state. Consequently, we have provided semiclassical magnetic structures of the degenerate ground state and the metastable state via both magnetic symmetries and experimental constraints. Moreover, we have discovered mechanisms raising degeneracy and metastability.

cond-mat.mtrl-sci↗

arXiv2021

Orbit topology analysed from $π$ phase shift of magnetic quantum oscillations in three-dimensional Dirac semimetal

With the emergence of Dirac fermion physics in the field of condensed matter, magnetic quantum oscillations (MQOs) have been used to discern the topology of orbits in Dirac materials. However, many previous researchers have relied on the single-orbit Lifshiftz-Kosevich formula, which overlooks the significant effect of degenerate orbits on MQOs. Since the single-orbit LK formula is valid for massless Dirac semimetals with small cyclotron masses, it is imperative to generalize the method applicable to a wide range of Dirac semimetals, whether massless or massive. This report demonstrates how spin-degenerate orbits affect the phases in MQOs of three-dimensional massive Dirac semimetal, NbSb$_2$. With varying the direction of the magnetic field, an abrupt $π$ phase shift is observed due to the interference between the spin-degenerate orbits. We investigate the effect of cyclotron mass on the $π$ phase shift and verify its close relation to the phase from the Zeeman coupling. We find that the $π$ phase shift occurs when the cyclotron mass is 1/2 of the electron mass, indicating the effective spin gyromagnetic ratio is $g_s$ = 2. Our approach is not only useful for analysing MQOs of massless Dirac semimetals with a small cyclotron mass, but also can be used for MQOs in massive Dirac materials with degenerate orbits, especially in topological materials with a sufficiently large cyclotron mass. Furthermore, this method provides a useful way to estimate the precise $g_s$ value of the material.

cond-mat.mes-hall↗