arXiv subjects

Tim Drevelow

Publications and source records attributed to Tim Drevelow.

9 recordsLinked to original sources

arXiv2025

Topological spin textures in an antiferromagnetic monolayer

Topological spin structures such as magnetic skyrmions are of fundamental interest and promising for various types of applications in spintronics. Skyrmions have been predicted to emerge also in antiferromagnetic materials where they exhibit superior transport properties. They were experimentally revealed in synthetic antiferromagnets, however, still remain elusive in intrinsic antiferromagnets. Here, we demonstrate the stabilization of topological spin structures in an antiferromagnetic monolayer. Using spin-polarized scanning tunneling microscopy, we observe an antiferromagnetic spin spiral in the Mn monolayer and a collinear antiferromagnetic state in the Mn double-layer on Ta(110). Near the boundary to the double-layer half-skyrmions form in the monolayer as revealed in combination with first-principles calculations and micromagnetic simulations. Our work shows how the topological state in antiferromagnetic material systems can be controlled by the configuration within a lateral heterostructure, resulting in trivial non-coplanar states or antiferromagnetic skyrmions.

cond-mat.mtrl-sci↗

arXiv2025

Network of localized magnetic textures revealed using a saddle-point search framework

A computational framework is presented for the sampling of the energy surface of magnetic systems via the systematic identification of first-order saddle points that determine connectivity of metastable states and define the mechanisms of transitions between them. The framework combines four stages: first, the symmetry of a given minimum-energy configuration is identified and used to define subsystems whose eigenmodes provide relevant deformation directions; the subsystem eigenmodes are then used to guide the system toward the vicinity of different saddle points surrounding the energy minimum; next, the geodesic minimum mode following method is employed to efficiently converge onto the saddle points; and finally, the identified saddle points are embedded into the state network. Applied to metastable textures in two-dimensional chiral magnets described by a lattice Hamiltonian, the method reveals a hierarchy of transition mechanisms governing the nucleation, annihilation, and rearrangement of the fundamental components of localized magnetic textures. The identified saddle points enable the construction of the network of metastable states, where saddle points define the connectivity between them, providing a comprehensive map of accessible transitions and their associated energy barriers. Transitions corresponding to both homotopies that preserve the topological charge and transformations that change it are identified. By scaling the interaction parameters, the distinct behavior of these two classes is obtained as the continuum limit is approached. Finally, it is shown that textures with the same topological charge are not always connected by a homotopy corresponding to a minimum-energy path: in specific parameter regimes, the total topological charge necessarily increases and then decreases (or vice versa) during the transition, returning to its initial value at the final state.

cond-mat.mtrl-sci↗

arXiv2025

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↗

arXiv2025

Spin-polarized edge modes between different magnet-superconductor-hybrids

The interplay of magnetism and superconductivity can lead to intriguing emergent phenomena. Here we combine two different two-dimensional antiferromagnetic magnet-superconductor hybrids (MSH) and study their properties using spin-polarized scanning tunneling microscopy. Both MSHs show the characteristics of a topological nodal point superconducting phase with edge modes to the trivial substrate superconductor. At the boundary between the two MSHs we find low-energy modes which are spin-polarized. Based on a tight-binding model we can explain the experimental observations by considering two different topological nodal point superconductors. This gives rise to spin-polarized chiral edge modes that connect topological nodal points of the two different MSH. We demonstrate via the complex band structure that due to an asymmetric lateral decay these edge modes are spin-polarized, regardless of the details of the spin structure at the boundary. The presence of spin-polarized edge states between different topological superconductors enables advanced functional design for the exploitation of MSHs as a platform for topology-based applications.

cond-mat.supr-con↗

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 N\'eel 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

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. N\'eel-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↗

arXiv2023

Evidence for a conical spin spiral state in the Mn triple-layer on W(001): spin-polarized scanning tunneling microscopy and first-principles calculations

The spin structure of a Mn triple layer grown pseudomorphically on surfaces is studied using spin-polarized scanning tunneling microscopy (SP-STM) and density functional theory (DFT). In SP-STM images a c$(4 \times 2)$ super structure is found. The magnetic origin of this contrast is verified by contrast reversal and using the c$(2 \times 2)$ AFM state of the Mn double layer as a reference. SP-STM simulations show that this contrast can be explained by a spin spiral propagating along the [110] direction with an angle close to $90^\circ$ between magnetic moments of adjacent Mn rows. To understand the origin of this spin structure, DFT calculations have been performed for a large number of competing collinear and non-collinear magnetic states including the effect of spin-orbit oupling (SOC). Surprisingly, a collinear state in which the magnetic moments of top and central Mn layer are aligned antiparallel and those of the bottom Mn layer are aligned parallel to the central layer is the energetically lowest state. We show that in this so-called "up-down-down" ($\uparrow \downarrow \downarrow$) state the magnetic moments in the Mn bottom layer are only induced by those of the central Mn layer. Flat spin spirals propagating either in one, two, or all Mn layers are shown to be energetically unfavorable to the collinear $\uparrow \downarrow \downarrow$ state even upon including the Dzyaloshinskii-Moriya interaction (DMI). However, conical spin spirals with a small opening angle of about $10^\circ$ are only slightly energetically unfavorable within DFT and could explain the experimental observations. Surprisingly, the DFT energy dispersion of conical spin spirals including SOC cannot be explained if only the DMI is taken into account. Therefore, higher-order interactions such as chiral biquadratic terms need to be considered which could explain the stabilization of a conical spin spiral 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↗

arXiv2022

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/{\AA} 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↗