Light-induced bimerons in a chiral magnet

Domain: biology
Published: 2026-04-01
Editorial outcome: major_revision

Abstract
Controlled generation of topological spin textures, such as merons and their bound state, the bimerons, is essential for advancing spintronic technologies and elucidating soliton physics in condensed matter. Using in situ Lorentz transmission electron microscopy coupled with femtosecond laser pulse, we demonstrate the creation of two distinct Bloch-type bimeron states in chiral magnet Co8Zn8Mn4 thin plates at room temperature. Magnetic imaging and micromagnetic simulations reveal that bimeron density varies with applied magnetic field strength, enabling dynamic topological control. We further establish that the topological classification of laser-generated bimerons is invariant with specimen thickness. Field-driven reversible transformations between elongated and circular bimeron morphologies are observed, governed by the competition of Zeeman energy and magnetic shape anisotropy. Micromagnetic simulations quantitatively reproduce these metastable states, validating a unified meron-skyrmion topological framework. This work establishes a single-pulse protocol for optical manipulation of topological spin textures.

Introduction
Topological spin textures exhibit remarkable stability even at nanoscale dimensions, making them attractive for various non-volatile applications1–4. Their rapid response to external stimuli, such as electric currents, further positions them as promising candidates for information transfer elements in racetrack memory and quantum computing devices5–8. The prototypical two-dimensional topological magnetic solitons are skyrmions, which can be stabilized in magnetic systems with uniaxial magnetic anisotropy. In contrast, magnetic systems that exhibit easy-plane anisotropy host topological textures in the form of merons and related configurations. The stabilization of skyrmions generally results from a competition between Heisenberg exchange and the Dzyaloshinskii-Moriya interaction (DMI) originating from a broken inversion symmetry of the crystal structure9–11. Besides that, skyrmions can also be stabilized by dipolar interactions12, competing high-order interactions13, and other factors14. Recent experimental studies have revealed a vast diversity of topological magnetic solitons, including high-order dipolar skyrmions12,15, bimerons16,17, skyrmion bags18,19, and even hopfions20–22 in the case of three-dimensional (3D) bulk samples.

Skyrmions in isotropic systems and systems with uniaxial anisotropy typically emerge within a magnetic background with collinear out-of-plane magnetization. The topology of such skyrmions is described in terms of the homotopy group \pi2(S2)=Z. The corresponding topological index, Q2Z can be computed using the Kronecker integral23:

Q(m)= 1/4\pi \int \Omega m \cdot [\partial r1 m \times \partial r2 m] dr1 dr2 , (1)

where m \equiv m(r) is the unit magnetization vector field, and \Omega denotes the skyrmion localization region. This topological index is also widely used to describe merons and their related configurations. However, it is important that Eq. (1) requires a very strict boundary condition – collinearity of spins at the perimeter of the integration domain, m(r \in \partial \Omega)=m0, for many real systems, such alignment around the textures of interest does not occur even approximately. Moreover, Eq. (1) is not invariant to the choice of coordinate system and can give values opposite in sign20,24. A theory of topological invariants for skyrmion-meron systems25–28, which is free from the above-mentioned shortcomings, was presented in a recent work29. According to Ref. 29, the topological invariants of skyrmions and(or) merons must be described by a pair of integers (qt, qb), where qt = Q(mt) and qb = Q(mb) can be calculated with using Eq. (1). Here mt and mb denote the spin textures obtained via an auxiliary mapping of the magnetization and are referred to as the top and bottom projections, respectively. The principle of that auxiliary mapping and details of the topological charge calculation are provided in Supplementary Note 1. The corresponding homotopy group is isomorphic to the direct product of integers, which can be illustrated by the following relation29: \pi2(S2, S2 \setminus {P1, P2}) = Z \times Z.

In idealized 2D chiral magnets with easy-plane anisotropy, merons can appear either as isolated objects (in a sense, that their cores can be isolated29) or in various bound configurations25–28. Due to topological and energetic constraints, they most often occur as coupled pairs, commonly referred to as vortex-antivortex pairs or bimerons16,17. Such bimerons have been theoretically predicted in a range of magnetic systems30–33, and have been experimentally observed in epitaxial antiferromagnetic films34, ferromagnetic multilayers17, and thin plates of chiral magnets16. As particle-like spin textures, bimerons substantially have a promising potential for applications in electronics and spintronics30–33,35,36. Realizing controllable manipulation of bimerons is essential for advancing these applications37,38.

Achieving controllable nucleation of topological spin textures is a crucial requirement for the practical application in spintronic devices. Light irradiation offers an ultrafast and versatile method for controlling the structural and magnetic properties of materials39,40. A femtosecond laser pulse can quench the long-range magnetic order on a picosecond timescale, producing a transient high-temperature state in which the magnetization undergoes abrupt local reversals, enabling skyrmion nucleation via localized spin dynamics41,42. On timescales from nanoseconds to microseconds, the dominant effect of the laser pulse is lattice heating. Even in this regime, relatively slow, thermally activated magnetization dynamics can still facilitate skyrmion nucleation43. The nucleation of vortex and antivortex in thin film with in-plane magnetization by means of femtosecond laser irradiation has also been reported44–46. However, the absence of particle-like behavior in isolated vortices and their random chirality make them less suitable for electronic and spintronic applications. The generation of bimerons in chiral magnets via femtosecond laser excitation has not yet been confirmed experimentally and is highly desirable for future applications.

In the following, we experimentally demonstrate that two distinct types of bimerons can be efficiently nucleated by laser pulses in a thin plate of Co8Zn8Mn4 – a room temperature chiral magnet with isotropic (bulk type) DMI47. We further show that a relatively weak applied magnetic field can significantly shift the energy balance between them, favoring one type of bimeron over the other. We systematically investigated laser-generated bimeron clusters under various conditions, demonstrating that their density can be efficiently controlled by the strength of external out-of-plane magnetic field, and verifying that the topological classification of bimerons remains invariant across thin plates of differing thicknesses. Furthermore, we demonstrate the structural in-field evolution of bimerons, driven by the competition between the applied field and the shape anisotropy. These experimental findings are corroborated by micromagnetic simulations showing very good agreement with the experimental observations.

Results
Laser pulse-generated bimerons
In this work, we use a thin plate of Co8Zn8Mn4 fabricated by the focused ion beam milling technique from a bulk crystal (see “Methods”). This compound belongs to a \beta-Mn-type Co-Zn-Mn alloys representing a unique type of chiral magnet with a Curie temperature above room temperature47. For instance, in our sample, we estimated the Curie temperature, Tc \sim 350 K, and the saturation magnetization at room temperature is Ms \sim 240 kAm-1 (Supplementary Fig. S2), which agrees with earlier studies48,49. In this study, all measurements were performed at room temperature.

We prepared a series of rectangular samples with thicknesses ranging from 90 to 200 nm. The characterization of the samples and a wide-field view of the samples are provided in Supplementary Figs. S3 and S5. In a thick Co8Zn8Mn4 plate with thickness >200 nm, the ground state is a helix with the k-vector lying in the sample plane. The equilibrium helix period was measured to be LD = 140 nm. This value allows us to estimate the ratio between the Heisenberg exchange stiffness A and the DMI constant D using the well-established relation LD = 4\pi A/D. In case of a plate with thickness much less than the smallest lateral dimensions of the sample t \ll L (L = min(Lx, Ly)), the effective easy-plane anisotropy can be approximated by the leading term of the demagnetizing field interaction, Ks = 1/2 \mu0 Ms2, as follows from the \Gamma-convergence limit50–52. In our case, the ratio t/L \sim 0.02 and such an approximation is meaningful. For the experimentally measured value of Ms, we estimated the constant of shape anisotropy for our sample as Ks \approx 36 kJm-3.

In agreement with experimental observations, the presence of easy-plane anisotropy changes the typical energy balance and favors the ground state of helix with k-vector pointing perpendicular to the plate (Supplementary Fig. S3). Due to the chiral surface twist effect53–55, the system exhibits additional surface modulations which, in the case of a thin film, produce extra contrast in Lorentz transmission electron microscopy (TEM) images, most pronounced at low magnetic fields. These modulations are clearly visible in the experimental images in Supplementary Fig. S3, which show the ground state of the system for samples with thicknesses of 90, 140, and 200 nm.

Consistent with earlier studies, the micromagnetic model employed here accounts for the so-called FIB-damaged layer at the surfaces of our plates18,20. We approximate this layer as an 8 nm-thick region at both the top and bottom surfaces, where amorphization (loss of crystal symmetry) leaves the exchange stiffness and saturation magnetization nearly unchanged, but reduces the DMI to zero. This approximation proved essential for achieving quantitative agreement between theory and experiment. Supplementary Fig. S4 illustrates different behavior of the system in our simulations with and without a damaged layer.

To investigate the creation of room-temperature bimerons, we performed Lorentz TEM measurements on a Co8Zn8Mn4 thin plate while exciting it with femtosecond laser pulses. The experimental setup is schematically illustrated in Fig. 1a. In a sample with a thickness of t = 90 nm (~0.65 LD), the initial magnetic configuration evolves into randomly distributed bimerons through an intermediate state that can be regarded as a pulsed laser induced disordered spin configuration (Fig. 1b). Four elementary merons that form bimerons are schematically depicted in Fig. 1c–f. Figure 1g, h shows representative spin textures of two types of bimerons along with the corresponding simulated Lorentz TEM images. The bimeron in Fig. 1g consists of a pair of merons topologically equivalent to those in Fig. 1c, d, whereas the bimeron in Fig. 1h consists of merons equivalent to those in Fig. 1e, f. These two spin textures correspond to topologically stable configurations. By contrast, bimerons composed of the merons in Fig. 1c, f, or in Fig. 1d, e, form topologically trivial configurations, since their indices satisfy (1, 0) + (-1, 0) = (0, 0) and (0, -1) + (0, 1) = (0, 0), respectively. Such bimerons collapse immediately. Other combinations, such as merons from Fig. 1c, e, or from Fig. 1d, f, are topologically stable but energetically unfavorable due to the isotropic DMI. Consequently, among the whole variety of possible bimerons, only the two bimerons shown in Fig. 1g, h remain both topologically and energetically stable.

With increasing perpendicular applied magnetic field or decreasing easy-plane anisotropy, the bimerons may undergo a transition into the pure skyrmions, as depicted in Fig. 1i, j. These two skyrmions can be superimposed onto each other and therefore differ only in polarity for the viewer. In terms of homotopy group analysis29, the transition from (bi)merons to pure skyrmions can be described by the group homomorphisms Z \times Z \to Z. Under such a transition, the topological indices of the merons shown in Fig. 1g and h transform according to the rules: (qt, qb) \mapsto qt and (qt, qb) \mapsto qb, respectively. Contrary to the skyrmion configurations in Fig. 1i and j, which are characterized by axial symmetry, bimerons are asymmetric spin textures that emerge under the action of isotropic DMI. The (1, -1) bimeron consists of a crescent-shaped (1, 0) meron and a more circular (0, -1) meron. Similarly, the (-1, 1) bimeron is composed of a crescent-shaped (0, 1) meron and a circular (-1, 0) meron. As with the skyrmions in Fig. 1i, j, bimerons exhibit mutually inverted contrast in Lorentz TEM images, which allows one to distinguish them in experiment.

In all the studies presented below, we employ laser pulses with a duration of 300 fs and a fluence of 4.77 mJ cm-2 to irradiate the sample. To additionally examine the effect of laser parameters on bimeron generation, we systematically varied both the laser fluence and pulse width. Single-pulse excitations were applied with pulse durations of 200 fs, 800 fs, 2 ps, and 4 ps at different fluence levels. As shown in Supplementary Fig. S6, the laser fluences required for both bimeron generation and for reaching the maximum bimeron density do not exhibit a significant dependence on the laser pulse duration. From these data, we estimate the threshold laser fluence to be approximately 2 mJ cm-2. Below this value, the magnetic texture does not exhibit noticeable changes after a single laser pulse. Since the beam’s full width at half maximum (FWHM) is much larger than the sample size (see “Methods”), we can reasonably assume that the entire sample is homogeneously irradiated. A femtosecond laser pulse initially raises the sample temperature above the Curie temperature Tc, temporarily melting the long-range magnetic order44,45. The subsequent rapid cooling dissipates the system energy, bringing the sample temperature back below Tc. Simultaneously, the spin system begins to remagnetize, and the fast, non-equilibrium cooling generates numerous nanoscale spin clusters56,57. On a longer timescale, the system continues to lose energy to the environment, allowing the sample temperature to settle well below Tc. At this stage, these nanoscale spin clusters have sufficient time to interact, merge, and reorganize, ultimately coalescing into long-range, topologically non-trivial spin textures56,57. The details of the bimerons formation process are provided in Supplementary Note 3. Supplementary Video 1 shows an in-situ Lorentz TEM observation of a single laser pulse induced bimeron formation in a 30 mT perpendicular field, displaying the domain configuration before and after laser pulse excitation. A laser pulse rapidly excites the system, producing a post-pulse state independent of the initial configuration. As a result, consecutive pulses from low-repetition-rate lasers generate mutually independent states. This is evidenced by the observation that, at a fixed fluence, single-pulse excitations at repetition rates of 1, 10, and 100 Hz yield reproducible yet mutually independent bimeron configurations (Supplementary Fig. S8).

Finally, we emphasize that the threshold laser fluence of approximately 2 mJ cm-2 does not represent a sharply defined value. Instead, it spans a broad range between 1.75 and 2.3 mJ cm-2 (see Supplementary Fig. S6a, d). Therefore, the scenario in which laser excitation raises the sample temperature to just below, but close to, Tc cannot be excluded. As seen in Supplementary Fig. S6d, for laser fluences within this intermediate range, the initial magnetic configuration may already be perturbed, allowing bimeron nucleation to occur. However, the highest bimeron densities are observed at substantially higher laser fluences. In this regime, the sample is most likely heated well above Tc, as indicated by the completely altered domain structure after every laser pulse (Supplementary Fig. S6d).

Control of bimeron density by external magnetic field
By applying current to the electron microscope objective lenses with an external current source, we can freely adjust the magnetic field applied perpendicular to the sample, and achieve free control of positive/zero/negative magnetic field. Figure 2 presents the response of a 90-nm-thick plate of Co8Zn8Mn4 to femtosecond laser pulse excitations under positive/zero/negative magnetic fields. At zero magnetic field, bimeron with topological indices (1, -1) and (-1, 1) are energetically equivalent, and laser excitation generates both types of bimeron. However, even a relatively weak applied magnetic field can significantly alter the energy balance between them and favor bimerons of specific configurations. At small magnetic fields, such as 10 mT, the positive magnetic field reduces the energy of (1, -1) bimeron. This leads to more bimerons of this type to nucleate under laser pulse excitation. At low field, these bimerons are strongly disordered and accompanied by a small number of isolated (-1, 0) merons and (-1, 1) bimerons. Similar results can be obtained with laser excitation at an inverted magnetic field of -10 mT, with disordered (-1, 1) bimerons accompanied by a small number of (0, -1) merons and (1, -1) bimerons. Figure 2 also shows that at a relatively strong magnetic field of 40 mT, the laser pulses produce bimerons of only one type: (1, -1) bimerons in the case of positive field direction and (-1, 1) bimerons in the case of negative field direction.

It is worth noting that at low magnetic fields, where multiple bimerons coexist in close proximity, the distinction between individual bimerons becomes less well-defined. Since each bimeron consists of two merons, neighboring bimerons may share merons located in their vicinity. This overlap can lead to an apparent ambiguity in assigning a specific meron to a given bimeron. Nevertheless, the overall topological charge of the magnetic texture is determined by the total number of merons and antimerons. At higher magnetic fields, as shown below, this ambiguity disappears, and individual bimerons become well separated and easy to identify.

To emphasize the critical role of laser excitation in the nucleation of bimerons in our samples, we performed in-situ field cooling experiments on the samples. The samples were cooled from both the vicinity of the Curie temperature and temperatures well above it to room temperature under varying magnetic fields. In all field-cooling runs, no bimerons or other topological spin textures were observed. Instead, the system consistently relaxed into the global energy minimum state, i.e., the conical state. (Supplementary Fig. S9). This observation suggests that laser excitation is the direct cause of bimeron formation as an excited (metastable) state.

To investigate the magnetic structures arising from femtosecond laser pulse excitation, we conducted micromagnetic simulations where the initial state was set to a random spin distribution. These simulations were based on direct energy minimization and thereby represent a pseudo-dynamics. The resulting configurations are of primary importance, whereas the associated pathway may still offer qualitative insight without claiming to capture the true dynamics. The simulated domain has dimensions 2 \mu m \times 2 \mu m \times 90 nm with periodic boundary conditions applied in the xy-plane. Supplementary Fig. S7 illustrates the evolution of the system and the corresponding theoretical Lorentz TEM contrast as the simulation approaches the equilibrium configuration corresponding to a specific local energy minimum. At the very beginning of the simulations, the system undergoes the scattering, nucleation, and annihilation of spin waves. Such a chaotic process gives rise to the nucleation of large, more ordered, and less quickly moving spin textures that relax into stable bimerons with well-defined configurations (Supplementary Fig. S7). Supplementary Video 2 illustrates this dynamic evolution process under zero magnetic field, which can be considered a rough approximation of the actual laser excitation process. Figure 2 displays snapshots of simulated final stable bimerons under varying magnetic fields after complete minimization of the micromagnetic energy, along with corresponding simulated Lorentz TEM images. These simulations demonstrate excellent agreement with experimental Lorentz TEM data.

Thickness effect
Figure 3 compares the laser-induced magnetic textures in samples with thicknesses of 90 and 140 nm under identical laser pulses. An extended series of images showing the nucleation of laser-induced bimerons under various positive and negative magnetic fields is provided in Supplementary Figs. S10–S13. With increasing applied magnetic field, the density of magnetic textures first rises to a maximum and then gradually decreases. Remarkably, the bimeron density in both thick and thin samples follows nearly the same field dependence. In both cases, the maximum density occurs at approximately 55 \pm 5 mT. The threshold field above which laser pulses no longer nucleate new magnetic textures is about 125 \pm 5 mT for both thicknesses.

Despite their nearly identical bimeron densities, samples of different thicknesses exhibit distinctly different Lorentz TEM contrasts for individual bimerons. In the thin film, each dark spot is bordered on one side by a bright, crescent-shaped halo. In the thick sample, the contrast is more symmetric, with the dark spot surrounded by an almost perfect bright ring. This might suggest that the texture in the thick film represents a skyrmion similar to those shown in Fig. 1i, j. Such interpretations are also frequently encountered in the literature10,11. Micromagnetic simulations, however, reveal that in both cases the underlying spin textures should be classified as bimerons.

The bottom row of images in Fig. 3a, d shows the theoretical Lorentz TEM contrast calculated for equilibrium spin configurations obtained by minimizing the micromagnetic energy functional. In these simulations, the bimeron density for each value of the external magnetic field was adjusted to visually match the experimental images. The resulting configurations reproduce the experimental contrasts with high fidelity, confirming the correspondence between the observed magnetic textures and those obtained in simulations.

To examine the magnetic texture in more detail, we select relatively isolated objects marked by red and blue squares in the thin and thick samples, respectively. Figure 3b and c show the magnetization configuration of a bimeron in a thin 90-nm film, while Fig. 3e, f display the corresponding configuration in a thick 140-nm sample. A clear similarity is observed between the mid-plane magnetic textures in Fig. 3c, f: in both cases, the same (1, -1) bimeron appears, with the only difference being the relative positions of the meron cores. The isosurfaces of the bimerons in the thin and thick samples, shown in Fig. 3b, e, also reveal this similarity. The white isosurface corresponds to the core of the (1, 0) meron, while the colored isosurface surrounds the (0, -1) meron. The main difference between Fig. 3b, e is that, in the thick sample, the white isosurface winds around the colored one. As a result, the magnetization averaged over the thickness and projected into the plane perpendicular to the electron beam exhibits partial blurring of the (1, 0) meron around the circular core of the (0, -1) meron. This produces a more axially symmetric, ring-like contrast in Lorentz TEM images for the thick film. In the thicker plate (t > 1 LD), the bimeron contrast appears more symmetric in Lorentz TEM, and is generally interpreted as a skyrmion tube surrounded by the cone phase58–61. However, in a thin plate (t \le 1 LD), the surrounding cone phase is more meron-like, and it is appropriate to describe the topological texture as bimeron. Nevertheless, bimerons in thin and thick plates depicted in Fig. 3 remain topologically equivalent. The complete three-dimensional spin configurations corresponding to Fig. 3a (100 mT) and d (110 mT) are provided in the Source Data and can be used for alternative visualization of the magnetic textures shown in Fig. 3b, c, e, f.

Field-induced bimeron-skyrmion transition
The transitions between bimerons and skyrmions, illustrated in Fig. 1g, i (or Fig. 1h, j), can in principle be induced by increasing the external magnetic field above the critical value Bc = 2(Ks + KD)/Ms, where Ks = 1/2 \mu0 Ms2 is the shape-anisotropy contribution and KD = D2/(4A) is the effective anisotropy induced by bulk DMI61,62. For Co8Zn8Mn4, this critical field is estimated to be Bc \approx 400 mT. Above Bc, the cone phase surrounding the bimeron transforms into a saturated ferromagnetic state, and the bimeron texture undergoes a transition into a skyrmion regardless of sample thickness. To investigate this transition, we performed a systematic study of bimerons in a 90-nm-thick sample under an applied magnetic field. (1, -1) Bimerons were first generated by a laser pulse at a low positive magnetic field. The field was then reduced to zero, and an image was recorded. Subsequently, the applied magnetic field was gradually increased, and images of the magnetic states were captured at various field strengths. The row of images in Fig. 4a presents experimental Lorentz TEM images obtained in a 90-nm-thick sample over the field range 0–320 mT. With increasing field, the asymmetric contrast characteristic of an individual bimeron gradually evolves into a more symmetric shape. In the under-focused Lorentz TEM images, the dark spot becomes smaller, while the surrounding bright, crescent-shaped halo becomes less pronounced. In the range from 0 to 250 mT, the field-driven evolution of the bimerons is fully reversible (see Supplementary Fig. S15). Above 250 mT, the number of bimerons steadily decreases, and above 320 mT, they disappear completely.

The micromagnetic simulations shown in Fig. 4b exhibit good agreement with the experimental images. Similar to the experimental protocol, the simulations were initialized under an applied field of 30 mT starting with a random magnetization distribution, performed energy minimization, and then the field was reduced to zero. The main difference between the experimental and simulated results is that bimeron collapse is not observed at the same field values in the simulations. This discrepancy is typical for micromagnetic simulations that do not include temperature effects. On the other hand, the results of direct energy minimization are in good agreement with the stochastic LLG simulations within the micromagnetic model (Supplementary Fig. S16), even though finite-temperature effects cannot be fully captured in continuum descriptions.

In micromagnetic simulations, the bimerons remain stable up to, and even above, the critical field Bc = 400 mT. Thus, in the simulations, we observe a field-driven transition from bimerons to skyrmions. This transition is evident from the background magnetization color in the bottom row of Fig. 4b. At low field, the background has a reddish hue, indicating that in the mid-plane the surrounding magnetization has an in-plane component oriented along the x-axis. With increasing field, the background between bimerons becomes brighter, and at 400 mT it appears white, indicating that the bimerons have transformed into skyrmions embedded in a ferromagnetic background. In the experiment, however, all bimerons collapse at much lower fields, so strictly speaking, such a transition does not occur. Therefore, even though at high magnetic fields the experimentally observed contrast resembles that of skyrmions (Fig. 4a), we classify these spin textures as bimerons embedded in a conical background. In fact, most of the spin textures reported as skyrmions in isotropic chiral magnets with bulk DMI should more accurately be classified as bimerons58,59,61,63.

Methods
Micromagneticsimulations
In this work, we follow the micromagnetic model approach with the total energy of the system, including four energy terms: the Heisenberg exchange energy, DMI energy, Zeeman energy, and self-energy of the demagnetizing field:

\epsilon = \int V [A \sum i=x,y,z |\nabla mi|2 + D m \cdot (\nabla \times m) - Ms m \cdot B + 1/2 \mu0 Ms2 \sum i=x,y,z |\nabla Ad,i|2] dr,

where m(r) = M(r)/Ms is a unit vector field that defines the direction of the magnetization, Ms = |M(r)| is the saturation magnetization, A is the exchange stiffness constant, D is the constant of isotropic bulk DMI, and \mu0 is the vacuum permeability. The magnetic field in Eq. (2), B = Bext + \nabla \times Ad, is the sum of the external magnetic field and the demagnetizing field, where Ad(r) is the component of the magnetic vector potential induced by the magnetization. In our simulations, we used the following material parameters for Co8Zn8Mn4: A = 5.8 pJ m-1, D = 0.52 mJ m-2, and Ms = 240 kAm-1. The value of Ms corresponds to room temperature, as shown in Supplementary Fig. S2. With these parameters, the equilibrium period of helical modulations at zero field is LD = 4\pi A/D = 140 nm, in agreement with the experiment (see Supplementary Fig. S3). The saturation field of the conical phase59 is given by Bc = BD + \mu0 Ms \approx 400 mT, where BD = D2/(2Ms A).

The simulation domain was set to 2 \times 2 \mu m2 in the xy-plane, with the thickness varied between 90 and 200 nm. The discretization size in the finite-difference scheme was 4 \times 4 \times 4 nm3. To approximate an extended plate, periodic boundary conditions were applied in the xy-plane. The FIB-damaged surface layer, 8 nm thick, was modeled as two cuboid layers at the top and bottom surfaces, where D = 0.

All simulations were performed using the Excalibur code64 and independently reproduced with MuMax65. Energy minimization was carried out using the conjugate gradient method, as implemented in Excalibur59. Lorentz TEM images were calculated in Excalibur following the approach described in Ref. 59. Consistent with the experimental setup, all simulated Lorentz TEM images were computed for a defocus distance of -1 mm.

Bulk sample preparation
Polycrystalline Co8Zn8Mn4 samples were synthesized following the method described in ref. 47. Stoichiometric amounts of pure Co, Zn, and Mn pieces, with a total mass of 5 g, were sealed in evacuated quartz tubes (10-3 Pa) and heated to 1000 °C for 12 h. The samples were subsequently cooled slowly at a rate of 1 °C h-1 to 925 °C and maintained at this temperature for 75–100 h. Finally, the products were subjected to water quenching.

TEM sample preparation
Co8Zn8Mn4 thin lamellae were fabricated from bulk crystals using a scanning electron microscopy-focused ion beam (SEM-FIB) dual-beam system (JIB-4700F MultiBeam System) equipped with a gas injection system and a micromanipulator. A lamella with an initial thickness of ~1 \mu m was extracted from the bulk crystal using the lift-out method, then transferred onto a copper lift-out grid via the deposited Pt layers. Depending on the experimental requirements, the samples were subsequently thinned to a final thickness of 90–200 nm. A low ion beam current was applied for final polishing to minimize the surface amorphous layer.

Magnetic imaging and femtosecond laser excitation
Fresnel-defocus Lorentz TEM was employed for magnetic domain analysis using a JEOL JEM-2100 Plus microscope operated at 200 kV. The objective lens current was precisely regulated to apply out-of-plane magnetic fields ranging from -1.0 T to +1.0 T normal to the specimen plane. All images were acquired at room temperature using a Gatan OneView 4k \times 4k detector, with the defocus distance maintained at 1 mm unless otherwise specified. The JEM-2100 Plus microscope was retrofitted with a femtosecond laser system (Spirit 1040-4, Spectra-Physics), enabling direct generation of bimerons in Co8Zn8Mn4 thin plates via pulsed laser excitation. Linearly polarized 520 nm femtosecond pulses (300 fs FWHM duration) were focused to a ~40 \mu m FWHM spot on the specimen. These pulses were generated by frequency-doubling the fundamental 1040 nm output in a \beta-barium borate (BBO) crystal, with pulse energy and repetition rate digitally controlled.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Zhu et al. reported ultrafast fs laser-induced bimeron topological magnetic structures in the chiral magnet Co8Zn8Mn4. They investigated the relationship between the generated topological magnetic structures and external magnetic fields, sample thickness, etc., studied their magnetic structures through micromagnetic simulations, which showed high consistency with experimental results. Topological magnetic structures are considered one of the potential information carriers for next-generation non-volatile information storage. To generate, read, and control topological magnetic structures have practical significance. Although fs lasers have been previously used to induce topological magnetic structures like skyrmions, this work presents the first successful attempt to apply this method to induce bimerons. However, the current manuscript needs to address the following issues before considering publication in Nature Communications:\n1. Laser energy and pulse width of the applied laser pulses are key experimental parameters for inducing topological magnetic structures, but this study did not conduct systematic characterization of them.\n2. The authors argue that the short-range spin correlations induced by fs lasers are crucial for inducing bimerons. This explains why bimerons cannot be observed when cooling the sample from above the Curie temperature to room temperature. But why a random initial state in simulations can still yield bimeron structures? It seems from the simulations that short-range spin correlation states may not be necessary for inducing bimerons.\n3. Can an estimation be provided for the magnitude of electron temperature rise (ΔT_el) after a single laser pulse, and compared with the Curie temperature? This could be crucial for understanding whether the laser induces short-range correlated spin states or a completely random paramagnetic spin state.\n4. During simulations, surface amorphization was considered (i.e., the 8 nm surface thickness kept other bulk material parameters but zero DMI), but there is no relevant characterization in experiment to justify it. Whether this assumption affects the model's results needs to be demonstrated through comparative simulations without the amorphous surface layers.\n5. The text states that the fs laser spot is much larger than the actual sample size, but Figure 1a shows an extremely small laser beam, failing to truly reflect the experimental setup.\n6. This study does not provide the time scale for bimeron generation. Figure 1b only shows a pseudo-time coordinate. It is recommended to provide the time evolution process of the magnetic structure after the laser pulse through micromagnetic simulations.\n7. Both 'bimeron' and 'meron pair' are used in the maintext. Do they refer to the same structure? If so, it is best to unify the concept.", "reviewer_2": "In the present manuscript, Zhu et al. study the laser-induced creation of bimeron states in an in-plane chiral magnet by Lorentz TEM at room-temperature. Their key observation is that dense bimeron patterns emerge after fs laser illumination, while such textures are absent after adiabatic field cooling. This behavior is studied quite systematically as a function of out-of-plane magnetic field and sample thickness. The interpretation of the L-TEM data is supported by micromagnetic simulations, and a detailed theoretical framework is presented to understand the topology of the nucleated textures. Overall, the study is an important and non-trivial extension of light-induced topological switching to chiral materials with weak or vanishing perpendicular magnetic anisotropy. The topic is of significant contemporary interest in the ultrafast and magnetism communities, making the paper in principle suitable for Nature Communications. However, I have a few remarks that should be clarified before I can give a final recommendation.\n1. In the abstract and throughout the paper, the process is referred to as “ultrafast,” “energy-efficient,” and “non-thermal”. I cannot find evidence for any of these claims. The experiment is not time-resolved and even the simulations do not have a time axis. Note that the mere usage of a fs drive does not have any implication on the speed of the subsequent process. For example, Berruto et al. [10.1103/PhysRevLett.120.117201] (should be cited) studied the nucleation of skyrmions in lamella of bulk chiral magnets, similar to the present paper, by time-resolved L-TEM and found microsecond topological switching dynamics after fs laser excitation. Energy-efficiency and the claim of “non-thermal manipulation” are not discussed at all. I think these claims should be removed.\n2. Related, while it makes sense to discuss the literature on how magnetic order emerges in other magnetic systems after ultrafast heating, one should use a more speculative language when applying this interpretation to the present system.\n3. On page 1: “only chiral magnets provide a unique platform that allows the observation of a vast diversity of skyrmions with various morphologies, including the coexistence of ordinary skyrmions with antiskyrmions [15], bimerons[16], skyrmion bags [17], and even hopfions [18]” -> Most, if not all, of these textures were observed in non-chiral materials, too. Or in materials where chiral interactions play a minor role. For example, [10.1002/adma.202501250, 10.1038/s41467-021-22600-7, 10.1038/s41467-021-21846-5, 10.1038/s41467-022-34370-x].\n4. On page 2: “merons can appear either as isolated objects …”. Can a single meron (not bimeron) really be an isolated object?\n5. On page 2: “the vortices […] lack […] particle-like properties”. Vortex cores and vortex domain walls were studied for many years as quasi-particles. Indeed, in continuous films, they cannot exist as isolated objects, but neither can merons to my knowledge (see previous point). In such continuous films, vortices and antivortices are generated in pairs and I do not see why they would not behave as loosely bound quasi-particles in the same manner as bimerons.\n6. Related to the previous point, the quasi-particle entity of bimerons is not established without doubt in my opinion. A quasi-particle should at least fulfill two criteria: compact extent and a (more or less) rigid displacement mode. None of these criteria are demonstrated in the present work, or rigorously supported by literature references. Displacement dynamics are not studied at all, which may be fine. However, at least the compact support should be verified, and this is not exactly evident from the data. While the high-field data in Fig. 3d shows indeed nicely isolated objects, in none of the other images I would be able to draw a box that would exactly include one bimeron state. To address this comment, I suggest including an illustration in Fig 1 that shows how to identify merons and bimerons in the dense network L-TEM images shown in the subsequent figures, and how to define their compact extent.\n7. On page 2: “simulations showing perfect agreement” -> The agreement is very good, but not exact / perfect.\n8. On page 3: The coordinate system (x,y,z) is not defined.\n9. On page 3: The shape anisotropy is 1/2 mu0 Ms^2 for a uniform magnetic state. For a noncollinear state, the energy penalty for out-of-plane alignment is much lower, especially in the thickness range studied by the authors.\n10. On page 3: Any claims regarding the transient state should be accompanied by a clarification that these are speculations.\n11. On page 3: “Four elementary merons comprising bimerons …” -> hard to parse.\n12. On page 4: “With the establishment of the thermal equilibrium between electrons and phonons …” -> Again, this is not studied in the present paper. Even more, in the cases when spin texture formation was studied by time-resolved experiments, no evidence was found that the transiently different temperatures of electron and phonon baths plays any role. Instead, spin fluctuations transform into long-range ordered textures when the bath that couples to the spins (=electron temperature) cools sufficiently below Tc and the magnetic system is given sufficient time to dissipate its energy. The phonon temperature plays no role in this consideration.\n13. On page 4: Supplementary Video 1 should be accompanied by a description. It is unclear to me if the video shows any time-dependent content, or simply a before-and-after sequence which could be represented in a two-panel figure.\n14. On page 4: What is meant by “*consistent* bimeron configurations?” Does “consistent” refer to a strong correlation between subsequent images, which would be indicative of a pinning-dominated process?\n15. On page 4: “This leads to more of this type of bimeron nucleates” -> Check grammar.\n16. On page 4: “accompanied by a small number of isolated (−1,0) merons and (−1,1) bimerons” -> Please indicate these features in the figure. It is hard for the readers to identify them on their own.\n17. On page 4: “The samples were cooled from above the Curie temperature” -> Yes, but very barely so. It might be that the Curie temperature was underestimated (related: please provide more details on how the data in Fig. S2 was obtained; and why the ZFC sample exhibits any net magnetization). If possible, please repeat the adiabatic cooling experiment from T = 400 K.\n18. On page 5: “… thereby represent a pseudo-dynamics” -> Why did the authors choose to not simulate the actual evolution using LLG, which would provide a time axis (at least assuming instantaneous cooling, which is oversimplifying the experiment)? Moreover, if the authors still believe that their process is ultrafast, then they should explain why they do not (have to) use atomistic simulations.\n19. On page 6: “Unlike the experimental protocol, …” -> Why did the authors deviate in their simulations from the experiment?\n20. On page 6: “we classify these spin textures as bimerons embedded in a conical background” -> A conical background is much more complicated than the uniform in-plane background suggested by the 2D vector images throughout the paper. At least in the Supplement, one should include a discussion on how the bimeron textures look like in 3D (providing more details and explanations then the captions of Fig 3 b, e).\n21. Fig 1: “The topological indices qt and qt correspond” -> Typo.\n22. All figures: Color legends are missing.\n23. SI Section 2: “exhibits a [111] oriented hard magnetic axis” -> What does this mean? Is there an intrinsic anisotropy in the material? This is not included in the modeling. Also, what is the orientation of the lamella?\n24. Fig S5: “at successive stages of energy minimization” is not well defined."}, "author_response": "Dear Editor: We are grateful to the reviewer for their careful reading of our manuscript and for their constructive and valuable comments. In the revised manuscript, we have addressed all concerns and questions raised by the reviewers. We believe that these corrections significantly improve the quality of our manuscript. Below, we provide a point-by-point response to the reviewers’ questions and comments. The text of the original Reviewer reports is highlighted in blue, our reply in black, and corrections made in the revised version of the text in red. List of major changes: • According to the reviewer’s suggestion, we performed an additional experiment with varying laser pulse width and laser fluence. • Followingthereviewer’ssuggestion,werewritethetexttoavoidwordsultrafast,energy-efficient, non-thermal etc. • We also revised the Fig. 1a and Fig. 1b, which now present the schematic of the experimental setup with a lager laser spot, and the light-induced bimeron formation process, respectively. • Following the reviewer’s request, we indicate the position of isolated merons in Fig. 2, and modified the caption accordingly. • Inresponsetothereviewercommentregardingtheroleoflaserexcitation,wecorrectedthetext in the Introduction as well as in the section Laser pulse-generated bimerons. • We added X references, see Refs.[16, 20, 22, 23, 44, 51, 52, 58] in the revised version of the manuscript. • In the Supplementary material, we added two extra figures that expand on the role of the FIB- damaged layer (Fig. S4) and the influence of laser fluence and pulse width on light-induced bimeron generation (Fig. S6); Corresponding references to new supplementary figures were added in the main text. • We have updated Fig. S9 by replacing the previous data with new measurements obtained after heating the sample to 400 K instead of 360 K. • We significantly extended the Supplementary Note, Section 3 Femtosecond laser-induced bimeron nucleation, where we discuss the results of calculations performed with the two-temperature model. • We added two OVF files as Supplementary data. These files allow for the visualization of the three-dimensional magnetic textures presented in Figs. 3b, e. Withtheresubmission,wehaveprovidedaseparatePDFfileinwhichallcorrectionsaremarkedinred. Sincerely, The authors [Detailed point-by-point responses follow...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "In the revised manuscript, the authors have thoroughly addressed my concerns with additional results. I believe the conclusion of the manuscript is well supported and the work now meets the publication standard of Nature Communications.", "reviewer_2": "The authors properly addressed almost all my comments. The only clarifications I would ask for are related to comments 6 and 9. Comment #6: I agree that “particle-like state” is a better term than “quasi-particle”. However, I do not understand why a compact support would not be needed to qualify as a “particle-like state”. Finite extent is one of the defining properties of a particle, along with position and shape. It should be clear which part of a magnetic texture belongs to a particle, and this assignment should be unambiguous and unique; a region in space that belongs to one particle should not belong to another one. It should be possible to add or remove individual entity from the system without affecting the system at a global scale. I do not think that bimerons, as presented in the paper, meet these requirements. The issue becomes most evident in Fig. 3. The blue box in Fig. 3d clearly contains the entire magnetic texture; I’m happy to consider this object as a “particle-like state”. On the contrary, there red box in Fig. 3c does not. It is not clear to me which criteria one would use to define the “particle” outline in this case. Comment #9: The outer dimensions of a sample determine the values of the demagnetizing tensor for a uniformly magnetized material. Under this assumption of uniform magnetization, the demagnetizing field can indeed be described as stated by the authors. However, once the sample is not in a uniform magnetic state, and is of finite thickness (as compared to the variation length of the magnetic texture, (\\grad m)^-1, and not as compared to the lateral size of the sample), the stray field interactions become non-local (not anisotropy-like) and samples can exhibit out-of-plane domains even if Ku - 0.5 mu0 Ms^2 << 1.", "reviewer_3": "Zhu et al. employed in situ Lorentz transmission electron microscopy (TEM) in combination with femtosecond laser pulses to generate two distinct Bloch-type bimeron states in chiral magnet Co8Zn8Mn4 thin plates at room temperature. The observed variation in bimeron density as a function of applied magnetic field strength, along with the finding that the topological classification remains invariant with respect to specimen thickness, represents significant advancements in the field. Furthermore, the reversible transformations between elongated and circular bimeron morphologies, driven by Zeeman energy and magnetic shape anisotropy, considerably enhance the impact of this study. The authors conducted micromagnetic simulations to elucidate these results. However, several points require clarification to strengthen the rigor of the work. I encourage the authors to address these concerns. 1. The author claims that the femtosecond laser pulse raises the sample temperature above the Curie temperature (Tc) based solely on a two-temperature model. It is recommended that the author conduct experiments to verify this claim, such as performing femtosecond laser experiments in a zero magnetic field. 2. Alternatively, it is possible that the laser fluence does not elevate the temperature above the Curie temperature; instead, it may only raise the temperature to just below Tc, thereby freezing the topological magnetic state below Tc. The author should provide further details to clarify this point. 3. Throughout the manuscript, the generation of merons and bimerons is discussed in relation to time. However, it appears that the simulations were not performed using a time-resolved Landau-Lifshitz-Gilbert (LLG) equation, which raises concerns regarding the rigor of the approach. 4. Please clarify whether the dynamic generation of merons and bimerons presented in the manuscript is indeed based on time-resolved calculations. 5. This question is closely related to the previous one. If the simulation of meron/bimeron induction by the femtosecond laser did not incorporate time, what then is the difference between the results obtained from the cooling experiment and those from the femtosecond laser experiment?"}, "author_response": "Dear Editor: We are grateful to the reviewer for their careful reading of our manuscript and for their constructive and valuable comments. In the revised manuscript, we have addressed all concerns and questions raised by the reviewers. We believe that these corrections significantly improve the quality of our manuscript. Below, we provide a point-by-point response to the reviewers’ questions and comments. [Detailed point-by-point responses follow...]"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have properly addressed my questions. I recommend publication of this article in Nature Communications.", "reviewer_3": "I have carefully reviewed the authors' revised manuscript and their responses to previous comments, but I still have significant concerns about the adequacy of the experimental support and the interpretation of the simulation data. While the additional explanations are helpful, they do not convincingly address these main issues. For the manuscript to be suitable for publication, it is essential to provide a stronger experimental foundation and a more careful analysis of the simulation results. The authors use fluence-dependent magnetic reconfiguration as indirect evidence because direct transient temperature measurements in a TEM are not possible. However, this evidence alone cannot confirm that the critical temperature (Tc) is exceeded. The observed reconfiguration could also be due to thermal fluctuations near Tc or laser-driven spin dynamics that do not lead to complete magnetic disorder. To verify their claim, the authors should conduct a control experiment where they apply a fixed magnetic field while heating the sample just below Tc and then cool it back to room temperature. This experiment should include creating a temperature-magnetic field (T-B) phase map. Furthermore, the authors dismiss time-dependent Landau-Lifshitz-Gilbert (LLG) simulations as “beyond the scope” of this study because of the lack of time-resolved experimental data. This reasoning is circular and weakens the theoretical basis of their conclusions. Even without femtosecond-resolution data, micromagnetic relaxation trajectories could provide insights into whether the final states observed are metastable states that can be reached after a rapid thermal quench. The uncertainty of the damping parameter does not justify completely excluding dynamic analysis; it is common practice in the field to explore a reasonable range of damping values. I strongly encourage the authors to include at least some qualitative dynamic simulations to better connect the laser quenching process with the resulting magnetic structure, distinguishing their findings from those of traditional cooling experiments."}, "author_response": "Dear Editor, We thank you for the communication regarding our manuscript and for the opportunity to submit a revised response. We also appreciate Reviewer #3 for the valuable comments, which have been instrumental in enhancing our paper. We have carefully considered the two points raised by Reviewer #3 and have provided detailed responses below. We hope our clarifications and revisions will address the reviewer’s concerns satisfactorily. [Detailed point-by-point responses follow...]"}, {"round": 4, "reviewer_comments": {"reviewer_3": "The authors have fully answered all the comments. The present version deserves publication."}, "author_response": ""}]

Decision letter