arXiv subjects

Santanu Pakhira

Publications and source records attributed to Santanu Pakhira.

At least 19 recordsLinked to original sources

arXiv2026

Topological spin-texture transitions in van der Waals magnets revealed by X-ray Fourier transform holography

Nontrivial topological spin-textures, such as skyrmions, merons, bimerons, and skyrmioniums, are envisioned as robust building blocks for future memory and logic devices. Controllable transformations between these states require a quantum-mechanical description of electronic degrees of freedom and atomic-scale insight beyond existing phenomenological models. Here, we report an atomic-scale investigation of topological phase transitions and their protection in the two-dimensional van der Waals ferromagnet Fe$_3$GeTe$_2$ (FGT) using a combined experimental-theoretical approach. Synchrotron-based Fourier transform holography directly images labyrinth domains, isolated skyrmions, mixed labyrinth-skyrmion phases, and skyrmion bags with high spatial resolution. We compare these observations to simulations based on an electronic lattice Hamiltonian that captures both metallicity and relativistic spin-orbit coupling in FGT. By systematically exploring a broad range of temperatures and magnetic fields, we map the mechanisms governing topological transitions and their stability. This sequential-integrated experimental-theoretical framework advances understanding of spin-texture interactions and enables precise control of external tuning parameters. Our results establish a platform for creating, stabilizing, and manipulating topological states, paving the way for engineered spin-texture transitions in next-generation spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2026

Disproportionate influence of site disorder on the evolution of magnetic phases in anti-Heusler alloy Al$_2$MnFe

Anti-Heusler alloys, being a new addition to the Heusler alloys family, exhibit atomic disorders, and almost all of them are reported as a re-entrant spin-glass system. Although such spin-glass feature is generally attributed to the inherent atomic disorder, a comprehensive and extensive investigation on the individual roles of different types of disorders in magnetic interactions remains lacking for any of the reported anti-Heusler systems. As an illustrative case, we have carried out an in-depth experimental as well as theoretical investigation of structural, magnetic, and transport properties of a polycrystalline anti-Heusler alloy, Al$_2$MnFe. While the major atomic disorder is found to be among Fe and Mn atoms, which are randomly distributed among the two octahedral sites, 4$a$ and 4$b$ (B2-type disorder), a relatively small fraction ($\sim$12\%) of Mn atoms also replace Al atoms at the tetrahedral 8$c$ site. Magnetically, the system undergoes two transitions: a paramagnetic to a ferromagnetic transition at $T_{\rm C}\sim$113~K, followed by a spin-glass phase transition below $T_{\rm f}\sim$20~K. Here, the magnetic moment is primarily confined to Mn atoms. Very interestingly, our theoretical analysis reveals that the ferromagnetic spin arrangement remains rather robust in spite of the 50\% disorder of moment-carrying Mn atoms between the two octahedral sites, but a much smaller ($\sim$12\%) cross-distribution of Mn atoms between octahedral and tetrahedral sites are sufficient to impose a reentrant spin-glass state at low temperature. Our analysis brings forth the importance of understanding the role of individual types of swap-disorder on magnetic properties in the anti-Heusler family of materials.

cond-mat.mtrl-sci↗

arXiv2025

Unveiling Excitonic Insulator Signatures in Ta$_\mathrm{2}$NiSe$_\mathrm{5}$

The high-temperature phase of Ta$_\mathrm{2}$NiSe$_\mathrm{5}$, a near-zero-gap semiconductor ($E_G$ = 0), is a promising candidate for an excitonic insulator. Given the dome-like evolution expected for an excitonic insulator around $E_G$, we investigated Ta$_\mathrm{2}$NiSe$_\mathrm{5}$, the more semi-metallic Ta$_\mathrm{2}$(Ni,Co)Se$_\mathrm{5}$, and semiconducting Ta$_\mathrm{2}$NiS$_\mathrm{5}$ using high-resolution single-crystal x-ray diffraction and near-edge x-ray absorption fine structure (NEXAFS). Our findings reveal a second-order structural phase transition from orthorhombic (space group: $Cmcm$) to monoclinic (space group: $C2/c$) in Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ and Ta$_\mathrm{2}$(Ni,Co)Se$_\mathrm{5}$, but no transition in Ta$_\mathrm{2}$NiS$_\mathrm{5}$ down to 2 K. This transition breaks two mirror symmetries, enabling and enhancing the hybridization of Ta, Ni, and Se atoms, shortening bond lengths, and strengthening orbital interactions. NEXAFS data confirm stronger hybridization, significant changes in excitonic binding energies, and a key alteration in orbital character, suggesting an excitonic insulating state in Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ and emphasizing the crucial electronic role of orbitals in the formation of the excitonic insulator state.

cond-mat.mtrl-sci↗

arXiv2025

Al$_2$MnCu: A magnetically ordered member of the Heusler alloy family despite having a valence electron count of 24

The magnetic property of the Heusler alloys can be predicted by the famous Slater-Pauling (S-P) rule, which states the total magnetic moment ($m_t$) of such materials can be expressed as $m _t\,=\,(N_V-24)\,μ_B/f.u.$, where $N_V$ is the total valence electron count (VEC). Consequently, no Heusler alloys having VEC = 24 are theoretically expected as well as experimentally reported to have any magnetic ordering. Recently, a special class of Heusler alloys with 50\% concentration of $p$-block elements (anti-Heusler) have been identified, although none of such reported compounds belong to the VEC 24 category. Here, we report a new anti-Heusler alloy, Al$_2$MnCu, that undergoes long-range ferromagnetic (FM) ordering with $T_{\rm C}\sim$315 K and a large magnetic moment of $\sim$1.8 $μ_B$/f.u. despite having VEC 24. A phenomenological model based on molecular orbital hybridization is also proposed to understand the magnetism and unusual deviation from the standard S-P rule.

cond-mat.mtrl-sci↗

arXiv2025

Spiral Spin Liquid State in the Corrugated Honeycomb Lattice of CaMn$_2$P$_2$

CaMn$_2$P$_2$ exemplifies the realization of a frustrated $J_1$-$J_2$-$J_3$ Heisenberg model of a corrugated honeycomb magnetic lattice. Previous studies show that below the Néel temperature ($T_{\rm N}$), the system forms a cycloidal $6\times 6$ $ab$-plane magnetic unit cell that conforms with various magnetic space groups. Here, we present single-crystal neutron-diffraction studies across expansive reciprocal-space volumes, confirming the cycloidal magnetic structure while uncovering further distinctive features. We find evidence for three magnetic domains, the analysis of which narrows the possible magnetic model structures. At $T_{\rm N}$, the insulator exhibits a sharp phase transition, above which the spin structure transforms into a spiral spin liquid state, evident via a continuous ring of scattering with degenerate wavevectors corresponding to a collection of short-range spiral spin configurations. These degenerate states emerge as thermal fluctuations effectively reduce the $J_3$ interaction. The integration of experimental, theoretical, and real-space simulation results reveals the intricate balance of exchange interactions ($J_1$-$J_2$-$J_3$) that stabilizes the ground-state magnetic structure and drives the emergence of a sought-after $U$(1)-symmetric spiral spin-liquid state with easy-plane anisotropy above the transition temperature.

cond-mat.str-el↗

arXiv2024

Low-Energy Electronic Structure in the Unconventional Charge-Ordered State of ScV$_6$Sn$_6$

Kagome vanadates {\it A}V$_3$Sb$_5$ display unusual low-temperature electronic properties including charge density waves (CDW), whose microscopic origin remains unsettled. Recently, CDW order has been discovered in a new material ScV$_6$Sn$_6$, providing an opportunity to explore whether the onset of CDW leads to unusual electronic properties. Here, we study this question using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The ARPES measurements show minimal changes to the electronic structure after the onset of CDW. However, STM quasiparticle interference (QPI) measurements show strong dispersing features related to the CDW ordering vectors. A plausible explanation is the presence of a strong momentum-dependent scattering potential peaked at the CDW wavevector, associated with the existence of competing CDW instabilities. Our STM results further indicate that the bands most affected by the CDW are near vHS, analogous to the case of {\it A}V$_3$Sb$_5$ despite very different CDW wavevectors.

cond-mat.str-el↗

arXiv2024

Europium $c$-axis ferromagnetism in Eu(Co$_{1-x}$Ni$_{x}$)$_{2-y}$As$_{2}$: A single-crystal neutron diffraction study

We report neutron diffraction results for the body-centered-tetragonal series Eu(Co$_{1-x}$Ni$_x$)$_{2-y}$As$_2$, $x=0.10$, $0.20$, $0.42$, and $0.82$, $y\leq0.10$, that detail changes to the magnetic ordering with nominal hole doping. We report the antiferromagnetic (AFM) propagation vectors, magnetic transition temperatures, and the ordered magnetic moments. We find a nonmonotonic change of the AFM propagation vector with $x$, with a minimum occurring at the tetragonal to collapsed-tetragonal phase crossover. For $x=0.10$ and $0.82$ we find $c$-axis helix ordering of the Eu magnetic moments (spins) similar to $x=0$ and $1$, with the spins oriented within the $ab$-plane. For $x=0.20$ and $0.42$ we find higher-temperature $c$-axis FM order and lower-temperature $c$-axis cone order. Using the extinction conditions for the space group, we discovered that the Eu spins are ordered in the higher-temperature $c$-axis FM phase for intermediate values of $x$, contrary to a previous report suggesting only Co/Ni spin ordering. Although we cannot directly confirm that the Co/Ni spins are also ordered, we suggest that $c$-axis itinerant-FM ordering of the Co/Ni spins could provide a molecular field that drives FM ordering of the Eu spins, which in turn provides the anisotropy for the lower-temperature $c$-axis cone order.

cond-mat.str-el↗

arXiv2024

Antiferromagnetic domains in a single crystal of the A-type spin-7/2 trigonal topological insulator EuSn$_2$As$_2$

EuSn$_2$As$_2$ is a trigonal A-type antiferromagnetic topological insulator with the moments aligned in the $ab$ plane and with a Néel temperature $T_{\rm N} = 23.5$ K. Here we report that an EuSn$_2$As$_2$ crystal exhibits a broad peak at $H_{\rm c1} = 1100$ Oe in the field derivative $dM_{ab}/dH$ of the $ab$-plane magnetization $M_{ab}(H)$ at temperature $T=2$ K, demonstrating the presence of trigonal antiferromagnetic domains. We model these $M_{ab}(H,\,T=2\,{\rm K})$ data and obtain the trigonal anisotropy-energy coefficient $K_3$ that is 13.3 and 3.7 times larger than those we previously reported for single crystals of the trigonal compounds EuMg$_2$Sb$_2$ and EuMg$_2$Bi$_2$, respectively.

cond-mat.str-el↗

arXiv2024

Coexistence of Ferromagnetism and Antiferromagnetic Dimers in Topological Insulators

The addition of magnetic impurities in topological insulators can drive ferromagnetic order that leads to novel quantum anomalous Hall transport well below the Curie temperature. The fragility of the quantized regime has been ascribed to the random nature of the magnetic moment distribution. Here, we refine this hypothesis by using inelastic neutron scattering and density-functional theory calculations to show that two antagonistic components define the magnetism in Mn-substituted SnTe, thereby limiting the effectiveness of dilute magnetic TIs. One component is strongly bound antiferromagnetic dimers that compete with ferromagnetic order. The other component consists of undimerized moments where ferromagnetic order develops via long-range interactions.

cond-mat.str-el↗

arXiv2023

Large saturation moment and high ferromagnetic transition temperature in a structurally disordered inverse Heusler alloy Fe2RuGe

We report the successful synthesis of a new 4$d$ based polycrystalline inverse Heusler alloy Fe$_2$RuGe by an arc melting process and have studied in detail its structural, magnetic and transport properties complemented with first principle calculations. X-ray and neutron diffraction, Extended X-ray Absorption Fine Structure and $^{57}$Fe Mössbauer spectroscopic studies confirm the single phase nature of the system where the Fe and Ru atoms are randomly distributed in the 4$c$ and 4$d$ Wyckoff positions in a ratio close to 50:50. The formation of the disordered structure is also confirmed by the theoretical energy minimization calculation. Despite the random cross-site disorder of Fe and Ru atoms, magnetic measurements suggest not only a high Curie temperature of $\sim$860\,K, but also a large saturation magnetic moment $\sim$4.9\,$μ_B$ per formula unit at 5\,K, considerably exceeding the theoretical limit (4\,$μ_B$ per formula unit) predicted by the Slater-Pauling rule. Only a few Fe-based inverse Heusler alloys are known to exhibit such high Curie temperatures. Neutron diffraction analysis coupled with the isothermal magnetization value indicates that the magnetic moments in Fe$_2$RuGe are associated with Fe-atoms only, which is also confirmed by Mössbauer spectrometry. Interestingly, in comparison to the cubic or hexagonal phase of the parent compound, Fe$_3$Ge, the Curie temperature of Fe$_2$RuGe has increased significantly despite the substitution of the nonmagnetic, yet isoelectronic element Ru in this structurally disordered compound. Our theoretical calculation reveals that the large Fe moment ($\sim2.8μ_B$/Fe) on the 4$b$ site can be attributed to a charge transfer from this Fe site towards its Ru neighbours. Such a substantial increase in magnetic moment due to electron charge transfer has not previously been reported in a Heusler alloy system.

cond-mat.mtrl-sci↗

arXiv2023

Origin of magnetic ordering in half-Heusler RuMnGa

The half-Heusler alloy RuMnGa having valence electron count (VEC) 18 has recently been theoretically proposed to exhibit compensated ferrimagnetic (CFiM) character instead of the expected nonmagnetic ground state. On the other hand, a preliminary experimental study proposed ferromagnetic (FM) ordering. As no half-Heusler system with VEC 18 is known to exhibit magnetic ordering, we have investigated the details of crystal structure and magnetic properties of RuMnGa using a combination of experimental tools, viz., x-ray and neutron diffraction techniques, dc and ac susceptibility, isothermal magnetisation, heat capacity, resistivity and neutron depolarisation measurements. Rietveld refinements of x-ray and neutron diffraction data suggest single phase nature of the compound with elemental composition RuMn$_{0.86}$Ga$_{1.14}$. We have shown that the system exhibits FM-type ordering owing to the inherent presence of this minor off-stoichiometry, showing very low magnetic moment. The system also exhibits reentrant canonical spin-glass behaviour, which is rarely observed in half-Heusler alloys. The temperature coefficient of resistivity changes its sign from negative to positive and further to negative as the temperature decreases.

cond-mat.mtrl-sci↗

arXiv2023

Experimental observation of spin glass state in highly disordered quaternary Heusler alloy FeRuMnGa

The realization of spin-glass (S-G) state in Heusler alloys is very rare despite the presence of inherent structural and elemental disorder in those compounds. Although a few half and full Heusler alloys are known to exhibit S-G state, there is hardly any manifestation of the same in cases of quaternary Heusler compounds. Here we report the observation of S-G state in a highly disordered equiatomic quaternary Heusler compound: FeRuMnGa, where the S-G state is in between of canonical S-G and cluster glass. Different intricate features of S-G state including non-equilibrium magnetic dynamics at low temperature in the compound are unveiled through our comprehensive magnetic, heat capacity and neutron diffraction studies. The structural disorder in the sample is neither conventional \textit{A2}- nor \textit{B2}-type while those two types are commonly observed for Heusler compounds. The presence of disorder also plays a significant role in electron transport properties of the alloy, which is reflected in its exhibition of semi-metallic behavior and anomalous Hall effect at low temperature.

cond-mat.mtrl-sci↗

arXiv2023

4d-element induced improvement of structural disorder and development of weakly re-entrant spin-glass behaviour in NiRuMnSn

The pursuit of efficient spin-polarization in quaternary Heusler alloys with the general formula $XX'YZ$ (where X, $X'$, and Y are transition metals and Z is a p-block element), has been a subject of significant scientific interest. While previous studies shows that isoelectronic substitution of 4d element in place of 3d element in quaternary Heusler alloy, improves the half-metallic ferromagnetic characteristics, our research on the quaternary Heusler alloy NiRuMnSn reveals a strikingly different scenario. In this study, we present a detailed structural analysis of the material using X-ray absorption fine structure (EXAFS) and neutron diffraction (ND) techniques, which confirms the formation of a single-phase compound with 50:50 site disorder between Ni/Ru atoms at 4c/4d sites. Contrary to expectations, our DFT calculations suggests a considerable decrease in spin-polarization even in the ordered structure. Additionally, we report on the compound's exceptional behavior, displaying a rare re-entrant spin glass property below $\sim$60 K, a unique and intriguing feature for quaternary Heusler-type compounds.

cond-mat.mtrl-sci↗

arXiv2023

Temperature-dependent Eu spin reorientations in the tetragonal A-type antiferromagnet EuGa$_4$ induced by small ab-plane magnetic fields

The body-centered-tetragonal antiferromagnet EuGa$_4$ exhibits A-type antiferromagnetic order below its Néel temperature $T_{\rm N} = 16.4$ K in magnetic field $H = 0$ where the moments are ferromagnetically aligned in the $ab$-plane with the Eu moments in adjacent Eu planes aligned antiferromagnetically. Previous magnetization versus field $M_{ab}(H)$ measurements revealed that the moments exhibit a spin-reorientation transition at a critical field $H_{c1}$ where the Eu moments become perpendicular to an in-plane magnetic field while still remaining in the $ab$ plane. A theory for $T=0$ K was previously presented that successfully explained the observed low-field moment-reorientation behavior at $T = 2$ K. Here we present a theory explaining the observed $T$ dependence of $M_{ab}(H,T<T_{\rm N})$ in the [1,0,0] direction for $H\leq H_{\rm c1}(T)$ from 2 to 14 K arising from a $T$-dependent anisotropy energy.

cond-mat.str-el↗

arXiv2023

High spin-polarization in a disordered novel quaternary Heusler alloy FeMnVGa

In this work, we report the successful synthesis of a Fe-based novel half-metallic quaternary Heusler alloy FeMnVGa and its structural, magnetic and transport properties probed through different experimental methods and theoretical technique. Density functional theory (DFT) calculations performed on different types of structure reveal that Type-2 ordered structure (space group: F-43m, Ga at 4a, V at 4b, Mn at 4c and Fe at 4d) possess minimum energy among all the ordered variants. Ab-initio simulations in Type 2 ordered structure further reveal that the compound is half-metallic ferromagnet (HMF) having a large spin-polarization (89.9 %). Neutron diffraction reveal that the compound crystalizes in disordered Type-2 structure (space group: Fm-3m) in which Ga occupy at 4a, V at 4b and Fe/Mn occupy 4c/4d sites with 50:50 proportions. The structural disorder is further confirmed by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS),57Fe Mossbauer spectrometry results and DFT calculations. Magnetisation studies suggest that the compound orders ferromagnetically below TC ~ 293 K and the saturation magnetization follows Slater-Pauling rule. Mossbauer spectrometry, along with neutron diffraction suggest that Mn is the major contributor to the total magnetism in the compound consistent with the theoretical calculations. First principle calculations indicate that spin-polarization remain high (81.3 %) even in the presence of such large atomic disorder. The robustness of the HMF property in presence of disorder is a quite unique characteristic over other reported HMF in literature and make this compound quiet promising for spintronics applications.

cond-mat.str-el↗

arXiv2023

Electronic and magnetic properties of the topological semimetal SmMg$_2$Bi$_2$

Dirac semimetals show nontrivial physical properties and can host exotic quantum states like Weyl semimetals and topological insulators under suitable external conditions. Here, by combining angle-resolved photoemission spectroscopy measurements (ARPES) and first-principle calculations, we demonstrate that Zintl-phase compound SmMg$_2$Bi$_2$ belongs to the close proximity to a topological Dirac semimetallic state. ARPES results show a Dirac-like band crossing at the zone-center near the Fermi level ($E_\mathrm {F}$) which is further confirmed by first-principle calculations. Theoretical studies also reveal that SmMg$_2$Bi$_2$ belongs to a $Z_2$ topological class and hosts spin-polarized states around the $E_\mathrm {F}$. Zintl's theory predicts that the valence state of Sm in this material should be Sm$^{2+}$, however we detect many Sm-4$f$ multiplet states (flat-bands) whose energy positions suggest the presence of both Sm$^{2+}$ and Sm$^{3+}$. It is also evident that these flat-bands and other dispersive states are strongly hybridized when they cross each other. Due to the presence of Sm$^{3+}$ ions, the temperature dependence of magnetic susceptibility $χ(T)$ shows Curie-Weiss-like contribution in the low temperature region, in addition to the Van Vleck-like behaviour expected for the Sm$^{2+}$ ions. The present study will help in better understanding of the electronic structure, magnetism and transport properties of related materials.

cond-mat.str-el↗

arXiv2023

Anisotropic magnetism and electronic structure of trigonal EuAl$_2$Ge$_2$ single crystals

The magnetic and electronic properties of the layered Zintl-phase compound EuAl$_2$Ge$_2$ crystallizing in the trigonal CaAl$_2$Si$_2$-type structure are reported. Our neutron-diffraction measurements show that EuAl$_2$Ge$_2$ undergoes A-type antiferromagnetic (AFM) ordering below $T_{\rm N} = 27.5(5)$~K, with the Eu moments (Eu$^{2+}$, $S = 7/2$) aligned ferromagnetically in the $ab$ plane. The $H = 0$ magnetic structure consists of trigonal AFM domains associated with $ab$-plane magnetic anisotropy and a field-induced reorientation of the Eu spins in the domains is evident at $T = 2$~K below the critical field $H_{c1} = 2.5(1)$ kOe. Electrical resistivity and ARPES measurements show that EuAl$_2$Ge$_2$ is metallic both above and below $T_{\rm N}$. In the AFM phase, we directly observe folded bands in ARPES due to the doubling of the magnetic unit cell along the $c$ axis with an enhancement of quasiparticle weight due to the complex change in the coupling between the magnetic moments and itinerant electrons on cooling below $T_{\rm N}$. The observed electronic structure is well reproduced by first-principle calculations, which also predict the presence of nontrivial electronic states near the Fermi level in the AFM phase with $Z_2$ topological numbers 1;(000).

cond-mat.str-el↗

arXiv2023

Magnetic frustration driven by conduction carrier blocking in Nd$_2$Co$_{0.85}$Si$_{2.88}$

The intermetallic compound Nd$_2$Co$_{0.85}$Si$_{2.88}$ having a triangular lattice could be synthesized in single-phase only with defect crystal structure. Investigation through different experimental techniques indicate the presence of two magnetic transitions in the system. As verified experimentally and theoretically, the high-temperature transition T$_H$ ~ 140 K is associated with the development of ferromagnetic interaction between itinerant Co moments, whereas the low-temperature transition at T$_L$ ~ 6.5 K is due to the coupling among Nd-4f and Co-3d moments, which is antiferromagnetic in nature. Detailed studies of temperature-dependent dc magnetic susceptibility, field dependence of isothermal magnetization, non-equilibrium dynamical behavior, viz. magnetic relaxation, aging effect, magnetic-memory effect, and temperature dependence of heat capacity, along with density functional theory (DFT) calculations, suggest that the ground state is magnetically frustrated spin-glass in nature, having competing magnetic interactions of equivalent energies. DFT results further reveal that the 3d/5d-conduction carriers are blocked in the system and act as a barrier for the 4f-4f RKKY interactions, resulting in spin-frustration. Presence of vacancy defects in the crystal are also conducive to the spin-frustration. This is an unique mechanism of magnetic frustration, not emphasized so far in any of the ternary R$_2$TX$_3$ (R=rare-earth, T=transition elements and X=Si, Ge, In) type compounds. Due to the competing character of the itinerant 3d and localized 4f moments, the compound exhibits anomalous field dependence of magnetic coercivity.

cond-mat.str-el↗