{"id":391562,"date":"2026-04-22T04:45:16","date_gmt":"2026-04-22T04:45:16","guid":{"rendered":"https:\/\/www.newsbeep.com\/nz\/391562\/"},"modified":"2026-04-22T04:45:16","modified_gmt":"2026-04-22T04:45:16","slug":"laser-induced-nucleation-of-magnetic-hopfions","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/nz\/391562\/","title":{"rendered":"Laser-induced nucleation of magnetic hopfions"},"content":{"rendered":"<p>Specimen preparation<\/p>\n<p>FeGe TEM specimens were prepared from a single crystal of B20-type FeGe using a standard lift-out method from the bulk using a FIB instrument (Helios 5 CX DualBeam, Thermo Fisher Scientific). The FeGe lamellae used in this work had lateral dimensions of the order of a few micrometres in each direction.<\/p>\n<p>In situ optical Lorentz TEM experiments<\/p>\n<p>Fresnel defocus images were recorded in a four-dimensional electron microscopy system based on a Thermo Fisher Talos F200i<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 26\" title=\"Li, Z. et al. Room-temperature sub-100 nm N&#xE9;el-type skyrmions in non-stoichiometric van der Waals ferromagnet Fe3&#x2212;xGaTe2 with ultrafast laser writability. Nat. Commun. 15, 1017 (2024).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR26\" id=\"ref-link-section-d74646172e3113\" rel=\"nofollow noopener\" target=\"_blank\">26<\/a>. Experiments were performed at 95\u2009K using a double-tilt cooling holder (Gatan model 915). Lorentz images were recorded in Fresnel mode with an out-of-plane magnetic field applied to the sample via the objective lens by tuning its current. The defocus distance for all Fresnel defocus Lorentz images was 700\u2009\u03bcm.<\/p>\n<p>The experimental set-up allows single-shot femtosecond laser excitation under Lorentz phase imaging conditions. Femtosecond laser pulses (515\u2009nm for 240\u2009fs) were triggered by a digital delay generator, which delivered single shots with fluences ranging from 0\u2009mJ\u2009cm\u22122 to 40\u2009mJ\u2009cm\u22122. The laser spot was adjusted to ~50\u2009\u03bcm to ensure uniform illumination of the sample. According to a previous report<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 48\" title=\"Guritanu, V. et al. Optical evidence for heavy charge carriers in FeGe. Phys. Rev. B 75, 155114 (2007).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR48\" id=\"ref-link-section-d74646172e3125\" rel=\"nofollow noopener\" target=\"_blank\">48<\/a>, under the present experimental conditions, the optical reflectivity of FeGe was ~47\u2009% and the penetration depth was estimated to be 15.8\u2009nm.<\/p>\n<p>Micromagnetic simulations<\/p>\n<p>In our theoretical examination of magnetic states, we employed a canonical model designed for isotropic chiral magnets. This model encompasses Heisenberg exchange, DMI and demagnetizing fields. The corresponding energy density functional can be expressed as follows<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 13\" title=\"Zheng, F. et al. Hopfion rings in a cubic chiral magnet. Nature 623, 718&#x2013;723 (2023).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR13\" id=\"ref-link-section-d74646172e3137\" rel=\"nofollow noopener\" target=\"_blank\">13<\/a>,<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 49\" title=\"Savchenko, A. S. et al. Diversity of states in a chiral magnet nanocylinder. APL Mater. 10, 061110 (2022).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR49\" id=\"ref-link-section-d74646172e3140\" rel=\"nofollow noopener\" target=\"_blank\">49<\/a>:<\/p>\n<p>$$\\begin{array}{rcl}{\\mathcal{E}} &amp; = &amp; {\\displaystyle \\int }_{{V}_{{\\rm{m}}}}{\\rm{d}}{\\bf{r}}{\\mathcal{A}}\\mathop{\\sum }\\limits_{i=x,y,z}|{{\\nabla }}{m}_{i}{|}^{2}+{\\mathcal{D}}{\\bf{m}}\\cdot ({{\\nabla }}\\times {\\bf{m}})\\\\ &amp; &amp; -{M}_{{\\rm{s}}}{\\bf{m}}\\cdot {\\bf{B}}+\\frac{1}{2{\\mu }_{0}}{\\displaystyle \\int }_{{{\\mathbb{R}}}^{3}}{\\rm{d}}{\\bf{r}}\\mathop{\\sum }\\limits_{i=x,y,z}|{{\\nabla }}{A}_{{\\rm{d}},i}{|}^{2},\\end{array}$$<\/p>\n<p>\n                    (3)\n                <\/p>\n<p>where Vm is the volume of the magnetic sample, m(r) = M(r)\/Ms is the magnetization vector field, Ms is the saturation magnetization, \u03bc0 is the vacuum permeability, Ad(r) is the component of the magnetic vector potential induced by the magnetization, and \\({\\mathcal{A}}\\) and \\({\\mathcal{D}}\\) are the Heisenberg exchange constant and DMI constant, respectively. It is assumed that the magnetic field B(r) is a sum of the external magnetic field Bext and the demagnetizing field produced by the sample: B = Bext + \u2207 \u00d7 Ad. Here we use material parameters for FeGe: \\({\\mathcal{A}}\\) =\u20094.75\u2009pJ\u2009m\u22121, \\({\\mathcal{D}}\\)\u2009=\u20090.853\u2009mJ\u2009m\u22122 and Ms\u2009=\u2009384\u2009kA\u2009m\u22121. We define the critical field above which the conical phase transitions to the ferromagnetic state as \\(B_{\\rm{c}}=B_{\\rm{D}}+\\mu_0 M_{\\rm{s}}\\), where \\(B_{\\rm{D}}=\\mathcal{D}^2\/(2M_{\\rm{s}}{\\mathcal{A}})\\); the magnetic field is applied perpendicular to the plate. Periodic boundary conditions were applied in the x\u2013y plane. The damaged layer was ~7\u2009nm. All the micromagnetic simulations were performed using Excalibur<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 50\" title=\"Rybakov, F. N. &amp; Babaev, E. Excalibur Software &#010;                http:\/\/quantumandclassical.com\/excalibur\/&#010;                &#010;               (Quantum and Classical Solutions).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR50\" id=\"ref-link-section-d74646172e3730\" rel=\"nofollow noopener\" target=\"_blank\">50<\/a>.<\/p>\n<p>Initial state for hopfions<\/p>\n<p>In this section, we outline the procedure for generating an initial hopfion configuration, which serves as a starting point for micromagnetic simulations. We consider a magnetization vector field m on an \\({{\\mathbb{S}}}^{2}\\) sphere, parameterized by an azimuthal angle \u03a6 and a polar angle \u03b8, that is, \\({\\bf{m}}\\,=\\,(\\sin \\,\\varTheta \\,\\cos \\,\\varPhi ,\\,\\sin \\,\\varTheta \\,\\sin \\,\\varPhi ,\\,\\cos \\,\\varTheta )\\). Making use of equations (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ4\" rel=\"nofollow noopener\" target=\"_blank\">4<\/a>) and (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ5\" rel=\"nofollow noopener\" target=\"_blank\">5<\/a>), we embedded a hopfion into a ferromagnetic background<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 13\" title=\"Zheng, F. et al. Hopfion rings in a cubic chiral magnet. Nature 623, 718&#x2013;723 (2023).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR13\" id=\"ref-link-section-d74646172e3859\" rel=\"nofollow noopener\" target=\"_blank\">13<\/a>:<\/p>\n<p>$$\\varTheta ={\\rm{\\pi }}\\left(1-\\frac{\\eta }{{R}_{1}}\\right),\\,\\,\\,\\,0\\le \\eta \\le {R}_{1},$$<\/p>\n<p>\n                    (4)\n                <\/p>\n<p>$$\\varPhi =\\arctan \\left(\\frac{y}{x}\\right)-\\arctan \\left(\\frac{z}{{R}_{2}-\\rho }\\right)-\\frac{{\\rm{\\pi }}}{2},$$<\/p>\n<p>\n                    (5)\n                <\/p>\n<p>where R1 and R2 are the minor and major radii of the hopfion, respectively. \\(\\rho \\,=\\,\\sqrt{{x}^{2}\\,+\\,{y}^{2}}\\) and \\(\\eta \\,=\\,\\sqrt{{(R-\\rho )}^{2}\\,+\\,{z}^{2}}\\). However, a hopfion created by equations (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ4\" rel=\"nofollow noopener\" target=\"_blank\">4<\/a>) and (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ5\" rel=\"nofollow noopener\" target=\"_blank\">5<\/a>) was found to be unstable in these calculations. To stabilize it, other transformations were performed on this texture.<\/p>\n<p>First, the hopfion was rotated by 90\u00b0 about the y axis, as shown in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">1a<\/a>. This rotation can be expressed in the form:<\/p>\n<p>$${\\bf{m}}^{\\prime} =\\left[\\begin{array}{rcl}\\cos \\frac{{\\rm{\\pi }}}{2} &amp; 0 &amp; \\sin \\frac{{\\rm{\\pi }}}{2}\\\\ 0 &amp; 1 &amp; 0\\\\ -\\sin \\frac{{\\rm{\\pi }}}{2} &amp; 0 &amp; \\cos \\frac{{\\rm{\\pi }}}{2}\\end{array}\\right]{\\bf{m}}.$$<\/p>\n<p>\n                    (6)\n                <\/p>\n<p>Next, spiralization was applied to the texture in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">1a<\/a> in the +z direction, resulting in the configuration shown in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">1b<\/a>.<\/p>\n<p>By relaxing the state in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">1b<\/a>, a hopfion embedded in a cone state was obtained, as shown in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">1c<\/a>. This configuration agrees well with our experimental observations. The spiralization along the z direction can be expressed in the form:<\/p>\n<p>$${{\\bf{m}}}^{{\\prime\\prime} }=\\left[\\begin{array}{rcl}\\cos \\varphi &amp; -\\sin \\varphi &amp; 0\\\\ \\sin \\varphi &amp; \\cos \\varphi &amp; 0\\\\ 0 &amp; 0 &amp; 1\\end{array}\\right]{{\\bf{m}}}^{{\\prime} },\\,\\,\\,\\varphi =\\frac{2{\\rm{\\pi }}}{{L}_{{\\rm{D}}}}z.$$<\/p>\n<p>\n                    (7)\n                <\/p>\n<p>After energy minimization (relaxation) of the initial configuration, the texture shown in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig1\" rel=\"nofollow noopener\" target=\"_blank\">1a<\/a> and Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">1c<\/a> was obtained. In Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">1c<\/a>, the hopfion is visualized via the isosurface at mx = 0, whereas in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig1\" rel=\"nofollow noopener\" target=\"_blank\">1a<\/a>, the isosurface for mz &lt; 0 is displayed. An alternative way to visualize a hopfion embedded in a helical background is through despiralization\u2014the opposite procedure to spiralization\u2014defined by \\(\\varphi ^{\\prime} \\,=\\,-\\varphi\\). When it is applied to the texture shown in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">1c<\/a>, this transformation yields a hopfion embedded in a ferromagnetic background, as illustrated in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">1d<\/a>.<\/p>\n<p>Hopfion stability diagram<\/p>\n<p>The stability diagram shown in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4a<\/a> was calculated using Excalibur with the following parameters. The cuboid size in the finite difference scheme was fixed at 1.75\u2009nm \u00d7\u20091.75\u2009nm \u00d7\u20091.75\u2009nm. To suppress interactions between periodic images of the hopfion resulting from the boundary conditions in the x\u2013y plane, a relatively large simulation domain of 560\u2009nm \u00d7\u2009560\u2009nm (320 cuboids in each lateral direction) was used. The sample thickness was defined by the number of cuboid layers along the z axis.<\/p>\n<p>For each sample thickness, the stability of the hopfion was tested by starting in zero field and gradually increasing the applied magnetic field Bext\u2009\u2225\u2009ez, where ez is the unit vector along the z direction, until the hopfion collapsed. Calculations were performed both with and without the presence of a FIB-damaged layer, which was modelled as four cuboid layers (~7\u2009nm) at each sample surface.<\/p>\n<p>MEP calculations<\/p>\n<p>To compute the MEPs shown in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig5\" rel=\"nofollow noopener\" target=\"_blank\">5<\/a>, the regularized geodesic nudged elastic band (RGNEB) method<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 51\" title=\"Kuchkin, V. M. et al. Stability and nucleation of dipole strings in uniaxial chiral magnets. Phys. Rev. B 111, 174410 (2025).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR51\" id=\"ref-link-section-d74646172e4663\" rel=\"nofollow noopener\" target=\"_blank\">51<\/a> was used. RGNEB extends the standard GNEB approach<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 45\" title=\"Bessarab, P. F. et al. Method for finding mechanism and activation energy of magnetic transitions, applied to skyrmion and antivortex annihilation. Comput. Phys. Commun. 196, 335&#x2013;347 (2015).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR45\" id=\"ref-link-section-d74646172e4667\" rel=\"nofollow noopener\" target=\"_blank\">45<\/a>,<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 46\" title=\"Bessarab, P. F. Comment on &#x2018;Path to collapse for an isolated N&#xE9;el skyrmion&#x2019;. Phys. Rev. B 95, 136401 (2017).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR46\" id=\"ref-link-section-d74646172e4670\" rel=\"nofollow noopener\" target=\"_blank\">46<\/a> to the regularized micromagnetic model introduced in ref. <a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 52\" title=\"Kuchkin, V. M. et al. Quantum and classical magnetic Bloch points. Phys. Rev. Res. 7, 013195 (2025).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR52\" id=\"ref-link-section-d74646172e4674\" rel=\"nofollow noopener\" target=\"_blank\">52<\/a>. In the regularized micromagnetic model, the order parameter is a four-dimensional vector \u03bd = (\u03bd1, \u03bd2, \u03bd3, \u03bd4) constrained to the \\({{\\mathbb{S}}}^{3}\\) sphere, so that \u2223\u03bd\u2223\u2009=\u20091. The first three components correspond to the magnetization m = (mx, my, mz), whereas the fourth, auxiliary component satisfies \\({\\nu }_{4}^{2}\\,=\\,1-| {\\bf{m}}{| }^{2}\\). This representation imposes the natural constraint \u2223m\u2223\u2009\u2264\u20091 across the entire sample. Within this model, the Heisenberg exchange term in equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ3\" rel=\"nofollow noopener\" target=\"_blank\">3<\/a>) is replaced by<\/p>\n<p>$${\\mathcal{A}}\\mathop{\\sum }\\limits_{i=x,y,z}| \\nabla {m}_{i}{| }^{2}\\mapsto {\\mathcal{A}}\\mathop{\\sum }\\limits_{i=1}^{4}| \\nabla {\\nu }_{i}{| }^{2}+\\kappa {\\nu }_{4}^{2},$$<\/p>\n<p>\n                    (8)\n                <\/p>\n<p>where \u03ba is a phenomenological constant (J\u2009m\u22123) that determines the Bloch point localization<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 52\" title=\"Kuchkin, V. M. et al. Quantum and classical magnetic Bloch points. Phys. Rev. Res. 7, 013195 (2025).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR52\" id=\"ref-link-section-d74646172e4978\" rel=\"nofollow noopener\" target=\"_blank\">52<\/a>. All the other terms in the micromagnetic Hamiltonian follow from the substitution (mx, my, mz) \u21a6 (\u03bd1, \u03bd2, \u03bd3) and are independent of \u03bd4.<\/p>\n<p>RGNEB calculations were performed using an open-source micromagnetic simulation package<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 53\" title=\"Kuchkin, V. M. Regularized LLG simulations MuMax3 (extension). GitHub &#010;                http:\/\/github.com\/kuchkin\/mumax3-gneb&#010;                &#010;               (2023).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR53\" id=\"ref-link-section-d74646172e5021\" rel=\"nofollow noopener\" target=\"_blank\">53<\/a> that is based on a fork of the MuMax3 code<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 54\" title=\"Vansteenkiste, A. et al. The design and verification of MuMax3. AIP Adv. 4, 107133 (2014).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR54\" id=\"ref-link-section-d74646172e5025\" rel=\"nofollow noopener\" target=\"_blank\">54<\/a>. The material parameters were identical to those used in micromagnetic simulations, with \u03ba = 10\u22123MsBD (\u2248 76.54\u2009J\u2009m\u22123). As shown earlier in ref. <a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 51\" title=\"Kuchkin, V. M. et al. Stability and nucleation of dipole strings in uniaxial chiral magnets. Phys. Rev. B 111, 174410 (2025).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR51\" id=\"ref-link-section-d74646172e5044\" rel=\"nofollow noopener\" target=\"_blank\">51<\/a>, for 10\u22124 &lt; \u03ba\/MsBD &lt; 10\u22122, the energy barrier varies negligibly. The calculations were performed in a domain with sizes in the x and y directions of 210\u2009nm and a thickness of 105\u2009nm. The mesh density was 128 \u00d7 128 \u00d7 64 nodes. For consistency with the micromagnetic simulations, in the MEP calculations, periodic boundary conditions were applied in the x\u2013y plane and a FIB-damaged layer of thickness ~7\u2009nm was assumed on each sample surface. The reaction coordinates in the MEP calculations shown in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig5\" rel=\"nofollow noopener\" target=\"_blank\">5a<\/a> and Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig15\" rel=\"nofollow noopener\" target=\"_blank\">9a\u2013c<\/a> are given in reduced units.<\/p>\n<p>The MEP shown in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig5\" rel=\"nofollow noopener\" target=\"_blank\">5<\/a> comprises three segments that connect local energy minima representing a skyrmion\u2013antiskyrmion pair, a hopfion and a conical state with weak surface modulations. Snapshots of the skyrmion\u2013antiskyrmion pair and hopfion are shown in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig5\" rel=\"nofollow noopener\" target=\"_blank\">5b,e<\/a>, respectively. More snapshots of intermediate states along each MEP are provided in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Fig15\" rel=\"nofollow noopener\" target=\"_blank\">9<\/a>. Initial paths were constructed using snapshots of the system obtained during micromagnetic simulations of corresponding processes<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 55\" title=\"Kuchkin, V. M. &amp; Kiselev, N. S. Homotopy transitions and 3D magnetic solitons. APL Mater. 10, 071102 (2022).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR55\" id=\"ref-link-section-d74646172e5093\" rel=\"nofollow noopener\" target=\"_blank\">55<\/a>. For example, to generate an initial path describing a skyrmion\u2013antiskyrmion merger into a hopfion, a skyrmion and an antiskyrmion were placed at a distance at which they begin to merge under energy minimization. Intermediate snapshots of this evolution were then used as images forming the initial guess for the MEP calculation. These pseudo-dynamical simulations were performed using the Excalibur code<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 50\" title=\"Rybakov, F. N. &amp; Babaev, E. Excalibur Software &#010;                http:\/\/quantumandclassical.com\/excalibur\/&#010;                &#010;               (Quantum and Classical Solutions).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR50\" id=\"ref-link-section-d74646172e5097\" rel=\"nofollow noopener\" target=\"_blank\">50<\/a>. The final MEPs were obtained using an iterative RGNEB algorithm.<\/p>\n<p>Homotopy group derivation<\/p>\n<p>In this section, we use algebraic topology, focusing on the long exact sequence of homotopy groups, \u03c0i(A) and \u03c0j(B), and relative homotopy groups, \u03c0k(A, B), for the pair of spaces A \u2287 B. The sequence is a chain of group homomorphisms, such that the image of one homomorphism equals the kernel of the next<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 35\" title=\"Hatcher, A. Algebraic Topology (Cambridge Univ. Press, 2002).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR35\" id=\"ref-link-section-d74646172e5147\" rel=\"nofollow noopener\" target=\"_blank\">35<\/a>. We derive group G \u2261 \u03c03(A, B) based on other groups surrounding it in the chain. The third relative homotopy group \u03c03 is a homotopy group classifying maps introduced in equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ1\" rel=\"nofollow noopener\" target=\"_blank\">1<\/a>), so that the domain for the function f is the cube I3 and the overall codomain is the set A, f(r \u2208 I3) \u2208 A. On the boundary of the cube, the function takes values in B, f(r \u2208 \u2202I3) \u2208 B. For our order parameter, the space is the sphere \\(A={{\\mathbb{S}}}^{2}\\), and the subspace is associated with restrictions on this sphere, so we denote it by the set-theoretic difference \\(B={{\\mathbb{S}}}^{2}\\backslash {\\bigcup }_{i=1}^{n}{X}_{i}\\). Each Xi in the above union is either a point on the sphere or a contractible region (such as a spherical cap) with or without a boundary. Also, none of these domains overlap, \\({X}_{i}\\cap {X}_{j\\ne i}=\\varnothing\\). The total number of punctured domains (holes) is any positive integer, \\(n\\in {{\\mathbb{Z}}}^{+}\\). For every such B, there is a deformation retraction of B into some connected graph \u0393. This mapping can be thought of as expanding every hole Xi so that, at the end, all that remains from the sphere is a set of lines connected by points. Thus, we have homotopy equivalence, B \u2243 \u0393. In turn, the higher homotopy groups of a connected graph are trivial, \u03c0i\u22652(\u0393) = 0, which can be revealed, for example, by taking the tree (it is a contractible space) as the universal cover of the graph<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 35\" title=\"Hatcher, A. Algebraic Topology (Cambridge Univ. Press, 2002).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR35\" id=\"ref-link-section-d74646172e5433\" rel=\"nofollow noopener\" target=\"_blank\">35<\/a>. The last important component here is the classic result of homotopy theory established by Hopf et al.: the third homotopy group of the sphere is isomorphic to the integers, \\({\\pi }_{3}({{\\mathbb{S}}}^{2})={\\mathbb{Z}}\\).<\/p>\n<p>Substituting all the above equalities into the long exact sequence, we obtain:<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/www.newsbeep.com\/nz\/wp-content\/uploads\/2026\/04\/41567_2026_3236_Equ1_HTML.png\" class=\"u-display-block\" alt=\"\"\/><\/p>\n<p>\n                    (9)\n                <\/p>\n<p>In the resulting short exact sequence (chain of three homomorphisms bounded by zeros), the central homomorphism can only be an isomorphism. Therefore:<\/p>\n<p>which completes the derivation of equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ2\" rel=\"nofollow noopener\" target=\"_blank\">2<\/a>) presented in the main text.<\/p>\n<p>Below, we provide some important special cases of equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ2\" rel=\"nofollow noopener\" target=\"_blank\">2<\/a>) in full notation:<\/p>\n<p>$${\\pi }_{3}({{\\mathbb{S}}}^{2},{{\\mathbb{S}}}^{2}\\backslash \\{{P}_{1}\\})={\\pi }_{3}({{\\mathbb{S}}}^{2},{P}_{0})\\equiv {\\pi }_{3}({{\\mathbb{S}}}^{2})={\\mathbb{Z}},$$<\/p>\n<p>\n                    (11)\n                <\/p>\n<p>$${\\pi }_{3}({{\\mathbb{S}}}^{2},{{\\mathbb{S}}}^{2}\\backslash \\{{P}_{1},{P}_{2}\\})={\\pi }_{3}({{\\mathbb{S}}}^{2},{{\\mathbb{S}}}^{1})={\\mathbb{Z}},$$<\/p>\n<p>\n                    (12)\n                <\/p>\n<p>$${\\pi }_{3}({{\\mathbb{S}}}^{2},{{\\mathbb{S}}}^{2}\\backslash \\{{P}_{1},{P}_{2},{P}_{3}\\})={\\pi }_{3}({{\\mathbb{S}}}^{2},{{\\mathbb{S}}}^{1}\\vee {{\\mathbb{S}}}^{1})={\\mathbb{Z}},$$<\/p>\n<p>\n                    (13)\n                <\/p>\n<p>where Pi are points on the sphere.<\/p>\n<p>Hopf index calculation<\/p>\n<p>Our method of finding the integer topological invariant H for equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ2\" rel=\"nofollow noopener\" target=\"_blank\">2<\/a>) involves a two-step procedure. First, we apply an auxiliary map \\(p:{\\bf{m}}({\\bf{r}})\\mapsto {\\mathfrak{m}}({\\bf{r}})\\), such that it does not change the topological invariant H but simplifies the boundary conditions: \\({\\mathfrak{m}}({\\bf{r}}\\in \\partial \\varOmega )=\\,\\mathrm{const}\\), where \u03a9 denotes the domain of hopfion localization and \u2202\u03a9 is the boundary of this domain. Second, we use the Whitehead integral formula.<\/p>\n<p>We focus on the case isolated in equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ11\" rel=\"nofollow noopener\" target=\"_blank\">11<\/a>) and then prove that the same routine is universal for all other cases of equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ2\" rel=\"nofollow noopener\" target=\"_blank\">2<\/a>). For the auxiliary map, we use deformation retraction resembling the half of the \u2018dumbbell\u2019 introduced in ref. <a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 41\" title=\"Rybakov, F. N. et al. Topological invariants of vortices, merons, skyrmions, and their combinations in continuous and discrete systems. Phys. Rev. B 111, 134417 (2025).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR41\" id=\"ref-link-section-d74646172e6237\" rel=\"nofollow noopener\" target=\"_blank\">41<\/a>. For convenience and without loss of generality, we assume that the base point is the north pole (PN), and the punctured point is the south pole (PS):<\/p>\n<p>$$p:\\,({{\\mathbb{S}}}^{2},{{\\mathbb{S}}}^{2}\\backslash \\{{P}_{{\\rm{S}}}\\})\\to ({{\\mathbb{S}}}^{2},{P}_{{\\rm{N}}}),$$<\/p>\n<p>\n                    (14)\n                <\/p>\n<p>$$\\left(\\begin{array}{l}{m}_{x}\\\\ {m}_{y}\\\\ {m}_{z}\\end{array}\\right)\\mapsto \\left(\\begin{array}{l}2\\gamma {m}_{x}\\\\ 2\\gamma {m}_{y}\\\\ 1-2{\\gamma }^{2}(1-{m}_{z})(1+\\mu )\\end{array}\\right),$$<\/p>\n<p>\n                    (15)\n                <\/p>\n<p>where<\/p>\n<p>$$\\gamma =\\left\\{\\begin{array}{l}\\sqrt{\\frac{\\mu -{m}_{z}}{(1-{m}_{z}){(1+\\mu )}^{2}}},\\,\\,\\mathrm{if}\\,{m}_{z} &lt; \\mu ,\\\\ 0,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\,\\mathrm{otherwise},\\end{array}\\right.$$<\/p>\n<p>and where the parameter \u03bc = \u03bc(r) is a monotone function that changes in value from +1 to \u22121 from inside \u03a9 to the boundary \u2202\u03a9. For \u03a9 in the form of a cuboid described in dimensionless coordinates as \\({r}_{1,2,3}\\in [-1\/2,\\,1\/2]\\), we use<\/p>\n<p>$$\\mu ({\\bf{r}})=128\\left(\\frac{1}{4}-{r}_{1}^{2}\\right)\\,\\left(\\frac{1}{4}-{r}_{2}^{2}\\right)\\left(\\frac{1}{4}-{r}_{3}^{2}\\right)-1.$$<\/p>\n<p>\n                    (16)\n                <\/p>\n<p>For the unit vector field \\({\\mathfrak{m}}\\) obtained from equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ15\" rel=\"nofollow noopener\" target=\"_blank\">15<\/a>), we calculate the Whithead integral:<\/p>\n<p>$$H=-\\frac{1}{16{{\\rm{\\pi }}}^{2}}{\\int }_{\\varOmega }{\\rm{d}}{\\bf{r}}\\,{\\bf{F}}\\cdot [{(\\nabla \\times )}^{-1}{\\bf{F}}],$$<\/p>\n<p>\n                    (17)\n                <\/p>\n<p>where F represents the vector of curvature, defined as<\/p>\n<p>$${\\bf{F}}\\equiv \\left(\\begin{array}{c}{\\mathfrak{m}}\\cdot [{\\partial }_{{r}_{2}}{\\mathfrak{m}}\\times {\\partial }_{{r}_{3}}{\\mathfrak{m}}]\\\\ {\\mathfrak{m}}\\cdot [{\\partial }_{{r}_{3}}{\\mathfrak{m}}\\times {\\partial }_{{r}_{1}}{\\mathfrak{m}}]\\\\ {\\mathfrak{m}}\\cdot [{\\partial }_{{r}_{1}}{\\mathfrak{m}}\\times {\\partial }_{{r}_{2}}{\\mathfrak{m}}]\\end{array}\\right).$$<\/p>\n<p>\n                    (18)\n                <\/p>\n<p>As in ref. <a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 13\" title=\"Zheng, F. et al. Hopfion rings in a cubic chiral magnet. Nature 623, 718&#x2013;723 (2023).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR13\" id=\"ref-link-section-d74646172e7333\" rel=\"nofollow noopener\" target=\"_blank\">13<\/a>, the vector field F was calculated using the Berg\u2013L\u00fcscher method<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 56\" title=\"Berg, B. &amp; L&#xFC;scher, M. Definition and statistical distributions of a topological number in the lattice O(3)&#x3C3;-model. Nucl. Phys. B 190, 412&#x2013;424 (1981).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR56\" id=\"ref-link-section-d74646172e7341\" rel=\"nofollow noopener\" target=\"_blank\">56<\/a>, and the following integral was calculated numerically to give the vector potential:<\/p>\n<p>$${(\\nabla \\times )}^{-1}{\\bf{F}}={\\int }_{-1\/2}^{{r}_{1}}{\\rm{d}}{r}_{1}\\,{\\bf{F}}\\times {\\widehat{{\\bf{e}}}}_{{r}_{1}}.$$<\/p>\n<p>\n                    (19)\n                <\/p>\n<p>Next, we prove that the procedure starting with equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ15\" rel=\"nofollow noopener\" target=\"_blank\">15<\/a>) is universal for all other cases. Because every holey sphere can be considered as an inclusion into the single-punctured sphere,<\/p>\n<p>$${{\\mathbb{S}}}^{2}\\backslash \\underset{i=1}{\\overset{n}{\\cup }}{X}_{i}\\hookrightarrow {{\\mathbb{S}}}^{2}\\backslash \\{{P}_{j}\\}$$<\/p>\n<p>\n                    (20)\n                <\/p>\n<p>there is an induced homomorphism of the corresponding relative homotopy groups:<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/www.newsbeep.com\/nz\/wp-content\/uploads\/2026\/04\/41567_2026_3236_Equ2_HTML.png\" class=\"u-display-block\" alt=\"\"\/><\/p>\n<p>\n                    (21)\n                <\/p>\n<p>A simple analysis reveals that homomorphism equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ22\" rel=\"nofollow noopener\" target=\"_blank\">21<\/a>) is an isomorphism. Indeed, an arbitrary element \\({{\\mathfrak{m}}}_{k}\\) of the group \\({\\pi }_{3}({{\\mathbb{S}}}^{2})\\) is an element of both groups in equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ22\" rel=\"nofollow noopener\" target=\"_blank\">21<\/a>) because one can always choose a base point outside the holes. Let us consider one of these elements that has unit charge, \\(H({{\\mathfrak{m}}}_{k})=1\\). It turns out that a homomorphism from integers to integers (endomorphism<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 57\" title=\"Rotman, J. J. An Introduction to the Theory of Groups (Springer, 1995).\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#ref-CR57\" id=\"ref-link-section-d74646172e7676\" rel=\"nofollow noopener\" target=\"_blank\">57<\/a>), which is always multiplication by an integer, can result in an integer equal to 1. This is possible only if the homomorphism is a multiplication by \u00b11. As such multiplications are in the group of automorphisms \\(\\,{\\rm{Aut}}\\,({\\mathbb{Z}})\\), we obtain that the homomorphism equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ22\" rel=\"nofollow noopener\" target=\"_blank\">21<\/a>) is an isomorphism. Accordingly, to calculate H, the procedure starting with equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41567-026-03236-0#Equ15\" rel=\"nofollow noopener\" target=\"_blank\">15<\/a>) is always applied when choosing a base point outside the holes.<\/p>\n","protected":false},"excerpt":{"rendered":"Specimen preparation FeGe TEM specimens were prepared from a single crystal of B20-type FeGe using a standard lift-out&hellip;\n","protected":false},"author":2,"featured_media":391563,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[24],"tags":[8005,8004,8008,2298,2294,7940,8003,8006,111,139,69,8007,393,147,3304,8002],"class_list":{"0":"post-391562","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-physics","8":"tag-atomic","9":"tag-classical-and-continuum-physics","10":"tag-complex-systems","11":"tag-condensed-matter-physics","12":"tag-general","13":"tag-magnetic-properties-and-materials","14":"tag-mathematical-and-computational-physics","15":"tag-molecular","16":"tag-new-zealand","17":"tag-newzealand","18":"tag-nz","19":"tag-optical-and-plasma-physics","20":"tag-physics","21":"tag-science","22":"tag-spintronics","23":"tag-theoretical"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/391562","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/comments?post=391562"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/391562\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media\/391563"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media?parent=391562"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/categories?post=391562"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/tags?post=391562"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}