arXiv subjects

Soumyajyoti Haldar

Publications and source records attributed to Soumyajyoti Haldar.

At least 19 recordsLinked to original sources

arXiv2026

Topological and spin-orbit effects on orbital moments in ultra-thin magnetic films

Topological orbital moments (TOMs) are a direct hallmark of a magnetic texture with a non-trivial spin topology. In addition to giving insight into the topology of the magnetic texture, TOMs could also be used to manipulate magnetic structures with a compensated total spin moment. Experimental evidence of TOMs has been provided via transport measurements of the Hall effect in intercalated van-der-Waals materials. However, a direct observation of TOMs is still missing. Another complication arises for the unambiguous proof of the topological origin since orbital moments can also occur due to spin-orbit coupling. Here, we use first-principles electronic structure theory to investigate the origin of orbital moments in different compensated spin structures with a non-trivial topology. We focus on ultrathin magnetic films at surfaces such as Pd/Mn bilayers on Re(0001) which represent ideal model systems for the detection of TOMs since it is possible to apply experimental techniques with local resolution of magnetic properties such as spin-polarized scanning tunneling microscopy. Due to its trivial topology we use the row-wise antiferromagnetic (RW-AFM) state in a hexagonal monolayer to analyze the spin-orbit induced contributions. The triple-Q (3Q) state is a superposition state of three RW-AFM (1Q) states and thus electronically similar, however, due to its non-trivial spin topology it exhibits TOMs. The comparison between these two spin states allows us to disentangle the topological and spin-orbit contributions to the orbital moments. We find that TOMs have an important contribution in spin-compensated systems, since the spin-orbit coupling induced orbital moments are nearly compensated. First-principles calculations for atomic-scale skyrmion lattices in Fe monolayers on different surfaces exhibit the same general trend found for the 3Q state.

cond-mat.mtrl-sci↗

arXiv2026

Lifetime of bimerons and antibimerons in two-dimensional magnets

Soliton-based computing architectures have recently emerged as a promising avenue to overcome fundamental limitations of conventional information technologies, the von Neumann bottleneck. In this context, magnetic skyrmions have been widely considered for in-situ processing devices due to their mobility and enhanced lifetime in materials with broken inversion symmetry. However, modern applications in non-volatile reservoir or neuromorphic computing raise the additional demand for non-linear inter-soliton interactions. Here, we report that solitons in easy-plane magnets, such as bimerons and antibimerons, show greater versatility and potential for non-linear interactions than skyrmions and antiskyrmions, making them superior candidates for this class of applications. Using first-principles and transition state theory, we predict the coexistence of degenerate bimerons and antibimerons at zero field in a van der Waals heterostructure Fe$_3$GeTe$_2$/Cr$_2$Ge$_2$Te$_6$ -- an experimentally feasible system. We demonstrate that, owing to their distinct structural symmetry, bimerons exhibit fundamentally different behavior from skyrmions and cannot be regarded as their in-plane counterparts, as is often assumed. This distinction leads to unique properties of bimerons and antibimerons, which arise from the unbroken rotational symmetry in easy-plane magnets. These range from anisotropic soliton-soliton interactions to strong entropic effects on their lifetime, driven by the non-local nature of thermal excitations. Our findings reveal a broader richness of solitons in easy-plane magnets and underline their unique potential for spintronic devices.

cond-mat.mes-hall↗

arXiv2026

Emergence of multiple topological spin textures in an all-magnetic van der Waals heterostructure

Magnetic solitons such as skyrmions and bimerons show great promise for both fundamental research and spintronic applications. Stabilizing and controlling topological spin textures in atomically thin van der Waals (vdW) materials has gained tremendous attention due to high tunability, enhanced functionality, and miniaturization. Here, we present an efficient spin-spiral approach based on first-principles, a method for mapping magnetic interactions from collective models onto arbitrary lattice symmetries, such as hexagonal and honeycomb lattices. Using atomistic spin models parametrized from first-principles, we predict the emergence of multiple topological spin textures in an all-magnetic vdW heterostructure Fe$_3$GeTe$_2$/Cr$_2$Ge$_2$Te$_6$ (FGT/CGT) -- an experimentally feasible system. Interestingly, the FGT layer favors out-of-plane magnetization, whereas the CGT layer prefers in-plane magnetocrystalline anisotropy. Néel-type nanoscale skyrmions are formed at zero field in the FGT layer due to interfacial Dzyaloshinskii-Moriya interaction (DMI), while nanoscale bimerons and antibimerons can co-exist in the CGT layer by the interplay between exchange frustration and DMI. Using the collective approach we apply, we reveal significant discretization effects in hexagonal and honeycomb geometries. In particular, we demonstrate that the lifting of geometric exchange frustration on the honeycomb significantly affects soliton barriers and pinning energetics. These fundamental results not only highlight the importance of spin simulations in discrete models for topological magnetism, especially in 2D materials, but may also help to pave the way for solitonic devices based on atomically thin vdW heterostructures.

cond-mat.mtrl-sci↗

arXiv2025

Impact of higher-order exchange on the lifetime of skyrmions and antiskyrmions

Reliable control of skyrmion lifetime is essential for realizing spintronic devices, yet the role of higher-order exchange - which can lead to skyrmion stabilization - remains largely unexplored. Here we calculate lifetimes of isolated skyrmions and antiskyrmions at transition-metal interfaces based on an atomistic spin model that includes all fourth-order exchange terms. Within harmonic transition-state theory, we evaluate both energetic and entropic contributions and find substantially enhanced lifetimes when higher-order exchange is included. The four-spin four-site interaction raises the energy barrier and lowers the curvature of the energy landscape at the collapse saddle point, increasing the pre-exponential factor. We show that skyrmions and antiskyrmions can remain thermally stable even without Dzyaloshinskii-Moriya interaction (DMI), and that tuning the four-spin term by a small amount modulates the prefactor over orders of magnitude. Our results identify higher-order exchange as a promising route to stabilize topological magnetic textures - in particular in systems lacking DMI - and to engineer their thermally activated decay.

physics.comp-ph↗

arXiv2025

A new skyrmion topological transition driven by higher-order exchange interactions in Janus MnSeTe

Two-dimensional (2D) van der Waals magnets offer a promising platform for pushing skyrmion technology to the single-layer limit with high tunability. While Dzyaloshinskii-Moriya interaction (DMI) is often recognized as central to skyrmion formation, their stability, collapse, and topological transition in 2D materials remain largely unexplored. In particular, the effect of higher-order exchange interactions (HOI) on these phenomena is unknown. Here, using first-principles calculations and atomistic spin simulations, we report a new topological transition generated by HOI, which we term 'ferric transition', in single-layer MnSeTe. Surprisingly, skyrmion stability and collapse remain largely unaffected by HOI due to the dominant role of DMI near the saddle point, whereas the Bloch point is strongly modified, giving rise to this novel transition. This mechanism is fundamentally distinct from the well-known radial and chimera transitions. Moreover, we predict that Janus MnSeTe exhibits remarkably high skyrmion energy barriers due to its strong DMI, among the highest reported for intrinsic 2D magnets. Our findings unveil an unexpected role of HOI in skyrmion topological transitions and establish Janus MnSeTe as a robust platform for 2D skyrmionics.

cond-mat.mes-hall↗

arXiv2025

Long lifetimes of nanoscale skyrmions in lithium-decorated van der Waals ferromagnet Fe$_3$GeTe$_2$

The Dzyaloshinskii-Moriya interaction (DMI), which originates from spin-orbit coupling and relies on broken inversion symmetry, is recognized as a key ingredient in forming magnetic skyrmions. However, most 2D magnets exhibit inversion symmetry; therefore, the DMI is suppressed. Here, we propose a strategy to induce large DMI via lithium absorption on the surface of 2D magnets -- an experimentally feasible approach. Using first-principles and atomistic spin simulations, we predict the formation of nanoscale skyrmions in lithium-decorated monolayer Fe$_3$GeTe$_2$ by imposing small out-of-plane magnetic fields ($B_z$). Notably, we find very large skyrmion energy barriers of more than 300 meV at $B_z = 0.4$ T, comparable to those observed in ferromagnet/heavy-metal interfaces. The origin of these unique skyrmions is attributed to the competition between strong DMI, exchange frustration, and small magnetocrystalline anisotropy energy. We further show that the lifetimes of metastable skyrmions exceed one hour for temperatures up to 75 K.

cond-mat.mtrl-sci↗

arXiv2025

Bilayer triple-Q state driven by interlayer higher-order exchange interactions

Using first-principles calculations and an atomistic spin model we predict the stabilization of a bilayer triple-Q state in an atomic Mn bilayer on Ir(111) due to interlayer higher-order exchange interactions. Based on density functional theory (DFT) we study the magnetic interactions and ground state in a Mn monolayer and bilayer on the Ir(111) surface. We calculate the energy dispersion of spin spirals (single-Q states) to scan a large part of the magnetic phase space and to obtain constants of pair-wise exchange interactions. By including spin-orbit coupling we determine the strength of the Dzyaloshinskii-Moriya interaction. To reveal the role of higher-order exchange interactions in these films, we consider multi-Q states obtained by a superposition of spin spirals. For the Mn monolayer in fcc stacking on Ir(111), the triple-Q state exhibits the lowest total energy in DFT, while the Néel state is most favorable for hcp stacking. For the Mn bilayer on Ir(111), two types of the triple-Q state are possible. In both magnetic configurations, a triple-Q state occurs within each of the Mn layers. However, only in one of them the spin alignment between the layers is such that nearest-neighbor spins of different layers also exhibit the tetrahedron angles which characterize the triple-Q state. We denote this state -- which has the lowest total energy in our DFT calculations -- as the ideal bilayer triple-Q state. This state exhibits significant topological orbital moments within each of the two Mn layers which are aligned in parallel resulting in a large topological orbital magnetization. We interpret the DFT results within an atomistic spin model which includes pair-wise Heisenberg exchange, the Dzyaloshinskii-Moriya interaction, as well as higher-order exchange interactions....

cond-mat.mtrl-sci↗

arXiv2024

Strain-driven domain wall network with chiral junctions in an antiferromagnet

Materials with antiferromagnetic order have recently emerged as promising candidates in spintronics based on their beneficial characteristics such as vanishing stray fields and ultra-fast dynamics. At the same time more complex localized non-coplanar magnetic states as for instance skyrmions are in the focus of applications due to their intriguing properties such as the topological Hall effect. Recently a conceptual shift has occurred to envision the use of such magnetic defects not only in one-dimensional race track devices but to exploit their unique properties in two-dimensional networks. Here we use local strain in a collinear antiferromagnet to induce non-coplanar domain wall junctions, which connect in a very specific way to form extended networks. We combine spin-polarized scanning tunneling microscopy with density functional theory to characterize the different building blocks of the network, and unravel the origin of the handedness of triple-junctions and the size of the arising topological orbital moments.

cond-mat.mes-hall↗

arXiv2024

First-principles investigation of magnetic exchange force microscopy on adatoms adsorbed on an antiferromagnetic surface

Using density functional theory (DFT), we calculate the magnetic short-ranged exchange forces between a magnetic tip and an adatom adsorbed on the antiferromagnetic Mn monolayer on the W(110) surface [Mn/W(110)]. These exchange forces can be measured in magnetic exchange force microscopy allowing atomic-scale imaging of spin structures on insulating and conducting surfaces. We consider two types of $3d$ transition-metal atoms with intrinsic magnetic moments: Co and Mn and Ir as an example of a $5d$ transition-metal atom exhibiting an induced magnetic moment on Mn/W(110). The tips are modeled by Fe pyramids and terminated either with an Fe or a Mn apex atom. From our total energy DFT calculations for a parallel and antiparallel alignment between tip and adatom magnetic moments we obtain the exchange energy $E_{\rm ex}(d)$ as a function of tip-adatom distance $d$. The exchange forces, $F_{\rm ex}(d)$, are calculated based on the Hellmann-Feynman theorem. We show that structural relaxations of tip and sample due to their interaction need to be taken into account. Due to the exchange interaction the relaxations depend on the alignment between tip and adatom magnetization -- an effect which will affect the tunneling magnetoresistance that can be measured by a scanning tunneling microscope. A maximum in the exchange energy and force curves is obtained for magnetic adatoms at tip-adatom separations of about 3 to 4~Å. The exchange forces with an Fe terminated tip reach a maximum value of up to 0.2~nN and 0.6~nN for Co and Mn adatoms, respectively, and prefer an antiferromagnetic coupling. Surprisingly, we also find an exchange force of up to 0.2~nN for Ir adatoms. We analyze the exchange interaction between tip and adatom based on the spin-polarized electronic structure of the coupled system......

cond-mat.mtrl-sci↗

arXiv2024

Prediction of stable nanoscale skyrmions in monolayer Fe$_5$GeTe$_2$

Using first-principles calculations and atomistic spin simulations, we predict stable isolated skyrmions with a diameter below 10 nm in a monolayer of the two-dimensional van der Waals ferromagnet Fe$_5$GeTe$_2$, a material of significant experimental interest. A very large Dzyaloshinskii-Moriya interaction (DMI) is observed due to the intrinsic broken inversion symmetry and strong spin-orbit coupling for monolayer Fe$_5$GeTe$_2$. We show that the nearest-neighbor approximation, often used in literature, fails to describe the DMI. The strong DMI together with moderate in-plane magnetocrystalline anisotropy energy allows to stabilize nanoscale skyrmions in out-of-plane magnetic fields above $\approx 2$~T. The energy barriers of skyrmions in monolayer Fe$_5$GeTe$_2$ are comparable to those of state-of-the-art transition-metal ultra-thin films. We further predict that these nanoscale skyrmions can be stable for hours at temperatures up to 20 K.

cond-mat.mtrl-sci↗

arXiv2024

Kicking Co and Rh atoms on a row-wise antiferromagnet

Diffusion on surfaces is a fundamental process in surface science, governing nanostructure and film growth, molecular self-assembly, and chemical reactions. Atom motion on non-magnetic surfaces has been studied extensively both theoretically and by real-space imaging techniques. For magnetic surfaces density functional theory (DFT) calculations have predicted strong effects of the magnetic state onto adatom diffusion, but to date no corresponding experimental data exists. Here, we investigate Co and Rh atoms on a hexagonal magnetic layer, using scanning tunneling microscopy (STM) and DFT calculations. Experimentally, we "kick" atoms by local voltage pulses and thereby initiate strictly one-dimensional motion which is dictated by the row-wise antiferromagnetic (AFM) state. Our calculations show that the one-dimensional motion of Co and Rh atoms results from conserving the Co spin direction during movement and avoiding high induced Rh spin moments, respectively. These findings demonstrate that magnetism can be a means to control adatom mobility.

cond-mat.mtrl-sci↗

arXiv2024

Proposal for all-electrical skyrmion detection in van der Waals tunnel junctions

A major challenge for magnetic skyrmions in atomically thin van der Waals (vdW) materials is reliable skyrmion detection. Here, based on rigorous first-principles calculations, we show that all-electrical skyrmion detection is feasible in 2D vdW magnets via scanning tunneling microscopy (STM) and in planar tunnel junctions. We use the nonequilibrium Green's function method for quantum transport in planar junctions, including self-energy due to electrodes and working conditions, going beyond the standard Tersoff-Hamann approximation. We obtain a very large tunneling anisotropic magnetoresistance (TAMR) around the Fermi energy for a vdW tunnel junction based on graphite/Fe$_3$GeTe$_2$/germanene/graphite. For atomic-scale skyrmions the noncollinear magnetoresistance (NCMR) reaches giant values. We trace the origin of the NCMR to spin-mixing between spin-up and -down states of $p_z$ and $d_{z^2}$ character at the surface atoms. Both TAMR and NCMR are drastically enhanced in tunnel junctions with respect to STM geometry due to orbital symmetry matching at the interface.

cond-mat.mtrl-sci↗

arXiv2023

Coupling of the triple-Q state to the atomic lattice by anisotropic symmetric exchange

We identify the triple-Q (3Q) state as magnetic ground state in Pd/Mn and Rh/Mn bilayers on Re(0001) using spin-polarized scanning tunneling microscopy and density functional theory. An atomistic model reveals that in general the 3Q state with tetrahedral magnetic order and zero net spin moment is coupled to a hexagonal atomic lattice in a highly symmetric orientation via the anisotropic symmetric exchange interaction, whereas other spin-orbit coupling terms cancel due to symmetry. Our experiments are in agreement with the predicted orientation of the 3Q state. A distortion from the ideal tetrahedral angles leads to other orientations of the 3Q state which, however, results in a reduced topological orbital magnetization compared to the ideal 3Q state.

cond-mat.mtrl-sci↗

arXiv2023

Spontaneous square vs. hexagonal nanoscale skyrmion lattices in Fe/Ir(111)

We study the emergence of spontaneous skyrmion lattices in an Fe monolayer in fcc and hcp stacking on the Ir(111) surface using density functional theory (DFT). For fcc-Fe/Ir(111) we find the well-known square nanoskyrmion lattice. However, for hcp-Fe/Ir(111) the hexagonal skyrmion lattice previously proposed based on experiments is energetically unfavorable with respect to a hexagonal multi-Q state with nearly collinear magnetic moments. By mapping our DFT calculations to an atomistic spin model we demonstrate that the interplay of pair-wise exchange, higher-order exchange, and Dzyaloshinskii-Moriya interaction is decisive for the symmetry and collinearity of the obtained spin lattice.

cond-mat.mtrl-sci↗

arXiv2023

Tuning the magnetic interactions in van der Waals Fe$_3$GeTe$_2$ heterostructures: A comparative study of \textit{ab initio} methods

We investigate the impact of mechanical strain, stacking order, and external electric fields on the magnetic interactions of a two-dimensional (2D) van der Waals (vdW) heterostructure in which a 2D ferromagnetic metallic Fe$_3$GeTe$_2$ monolayer is deposited on germanene. Three distinct computational approaches based on \textit{ab initio} methods are used, and a careful comparison is given: (i) The Green's function method, (ii) the generalized Bloch theorem, and (iii) the supercell approach. First, the shell-resolved exchange constants are calculated for the three Fe atoms within the unit cell of the freestanding Fe$_3$GeTe$_2$ monolayer. We find that the results between methods (i) and (ii) are in good qualitative agreement and also with previously reported values. An electric field of ${\cal E}= \pm 0.5$~V/Å applied perpendicular to the Fe$_3$GeTe$_2$/germanene heterostructure leads to significant changes of the exchange constants. We show that the Dzyaloshinskii-Moriya interaction (DMI) in Fe$_3$GeTe$_2$/germanene is mainly dominated by the nearest neighbors, resulting in a good quantitative agreement between methods (i) and (ii). Furthermore, we demonstrate that the DMI is highly tunable by strain, stacking, and electric field, leading to a large DMI comparable to that of ferromagnetic/heavy metal (FM/HM) interfaces. The geometrical change and hybridization effect explain the origin of the high tunability of the DMI at the interface. The electric-field driven DMI obtained by method (iii) is in qualitative agreement with the more accurate \textit{ab initio} method used in approach (ii). However, the field-effect on the DMI is overestimated by method (iii) by about 50\%. The magnetocrystalline anisotropy energy can also be drastically changed by the application of compressive or tensile strain in the Fe$_3$GeTe$_2$/germanene heterostructure.

cond-mat.mtrl-sci↗

arXiv2022

Strain-Driven Zero-Field Near-10 nm Skyrmions in Two-Dimensional van der Waals Heterostructures

Magnetic skyrmions $-$ localized chiral spin structures $-$ show great promise for spintronic applications. The recent discovery of two-dimensional (2D) magnetic materials opened new opportunities for exploring such topological spin structures in atomically thin van der Waals (vdW) materials. Despite recent progress in stabilizing metastable skyrmions in 2D magnets, their diameters are still beyond 100~nm and their lifetime, which is essential for applications, has not been explored yet. Here, using first-principles calculations and atomistic spin simulations, we predict that compressive mechanical strain leads to stabilizing zero-field skyrmions with diameters close to 10 nm in a Fe$_3$GeTe$_2$/germanene vdW heterostructure. The origin of these unique skyrmions is attributed to the high tunability of Dzyaloshinskii-Moriya interaction and magnetocrystalline anisotropy energy by strain, an effect which is shown to be general for Fe$_3$GeTe$_2$ heterostructures with buckled substrates. Based on our first-principles parameters for the magnetic interactions, we calculate the energy barriers protecting skyrmions against annihilation and their lifetimes using transition-state theory. We show that nanoscale skyrmions in strained Fe$_3$GeTe$_2$/germanene can be stable for hours at temperatures up to 20 K.

cond-mat.mtrl-sci↗

arXiv2022

Nano-scale collinear multi-Q states driven by higher-order interactions

Complex magnetic order arises due to the competition of different interactions between the magnetic moments. Recently, there has been an increased interest in such states not only to unravel the fundamental physics involved, but also with regards to applications exploiting their unique interplay with moving electrons. Whereas it is the Dzyaloshinskii-Moriya interaction (DMI) that has attracted much attention because of its nature to induce non-collinear magnetic order including magnetic-field stabilized skyrmions, it is the frustration of exchange interactions that can drive magnetic order down to the nano-scale. On top of that, interactions between multiple spins can stabilize two-dimensional magnetic textures as zero-field ground states, known as multi-Q states. Here, we introduce a two-dimensional itinerant magnet with various competing atomic-scale magnetic phases. Using spin-polarized scanning tunneling microscopy we observe several zero-field uniaxial or hexagonal nano-scale magnetic states. First-principles calculations together with an atomistic spin model reveal that these states are stabilized by the interplay of frustrated exchange and higher-order interactions while the DMI is weak. Unexpectedly, it is found that not only non-collinear magnetic states arise, but that higher-order interactions can also lead to collinear nano-scale multi-Q states.

cond-mat.mtrl-sci↗

arXiv2021

Topological-chiral magnetic interactions in ultrathin films at surfaces

We demonstrate that topological-chiral magnetic interactions can play a key role for magnetic ground states in ultrathin films at surfaces. Based on density functional theory we show that significant chiral-chiral interactions occur in hexagonal Mn monolayers due to large topological orbital moments which interact with the emergent magnetic field. Due to the competition with higher-order exchange interactions superposition states of spin spirals such as the 2Q state or a distorted 3Q state arise. Simulations of spin-polarized scanning tunneling microscopy images suggest that the distorted 3Q state could be the magnetic ground state of a Mn monolayer on Re(0001).

cond-mat.mtrl-sci↗