{"id":31452,"date":"2025-07-29T08:56:12","date_gmt":"2025-07-29T08:56:12","guid":{"rendered":"https:\/\/www.newsbeep.com\/ca\/31452\/"},"modified":"2025-07-29T08:56:12","modified_gmt":"2025-07-29T08:56:12","slug":"lets-twist-again-seeing-spin-spirals-in-action","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/ca\/31452\/","title":{"rendered":"Let\u2019s Twist Again: Seeing Spin Spirals in Action"},"content":{"rendered":"

Daniel Schick<\/a>Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Berlin, German<\/p>\n

July 28, 2025&bullet; Physics 18, 139<\/p>\n

Using ultrafast x-ray pulses, researchers have probed the chirality of spin spirals in synthetic antiferromagnets.<\/p>\n

\"Figure<\/a>\"expand<\/p>\n

Figure 1:<\/a> Top: A range of magnetic orders is shown from top to bottom: ferromagnetic, antiferromagnetic, and N\u00e9el-type spin spiral. Bottom: Researchers have created a synthetic antiferromagnet, in which magnetic layers are stacked together with each layer hosting a N\u00e9el-type spin spiral. Since the spins of adjacent layers are anti-aligned, the net magnetization is zero.<\/p>\n

\"Figure<\/p>\n

Figure 1: Top: A range of magnetic orders is shown from top to bottom: ferromagnetic, antiferromagnetic, and N\u00e9el-type spin spiral. Bottom: Researchers have created a synthetic antiferromagnet, in which magnetic layers are stacked together with each layer hosting a N\u00e9el-type spin spiral. Since the spins of adjacent layers are anti-aligned, the net magnetization is zero.\u00d7<\/a><\/p>\n

Magnetism is a constant companion in our daily lives. Data storage, sensors, electric motors\u2014none of these devices would function without it. Yet most technologies exploit only the simplest form of magnetic order: ferromagnetism, in which all magnetic moments within a domain align in the same direction. But magnetic order can be far more intricate. In conventional antiferromagnets (AFMs), the magnetic moments align in opposite directions to produce zero net magnetization, a type of order which has several advantages over ferromagnetism in many next-generation technological applications. In more exotic materials, the magnetic moments can twist into spirals, vortices, and other spin structures that might one day be used to store information. Occurring in both ferromagnets and AFMs, these spin structures are defined by their chirality, the direction in which the spins rotate relative to a fixed axis.<\/p>\n

The chirality is a key fingerprint of the competing interactions at play in complex magnetic systems. However, observing the dynamics of chirality and magnetization in AFMs has been experimentally challenging, as both can evolve over nanometer length scales and on femtosecond timescales. In a new study, Zongxia Guo from the French National Centre for Scientific Research and colleagues have taken a major step forward by probing both quantities with ultrashort and ultrabright pulses from a free-electron laser (FEL) [1<\/a>]. The researchers look specifically at spin spirals in an AFM, and they find that\u2014under laser excitation\u2014the chirality and magnetization evolve together in near unison and on significantly faster time scales than is observed for ferromagnets. Such fast spin dynamics in chiral spin structures offers a promising new route for how we will store, transfer, and compute information in the future.<\/p>\n

Chiral spin structures can emerge from the interplay of various magnetic interactions such as the Dzyaloshinskii-Moriya interaction, which involves an antisymmetric exchange between two spins [2<\/a>]. The structures can take the form of extended cycloids or helices (called spirals) and compact vortices (called skyrmions). Skyrmions are especially appealing because of their topological protection, nanoscale size (typically, 10\u2013100 nm), and mobility under electrical currents [3<\/a>].<\/p>\n

However, skyrmions in ferromagnets come with drawbacks. Their stabilization requires external magnetic fields, and their topological charge (a quantity that describes how often their local magnetization vector can be wrapped around a unit sphere) is nonzero. A nonzero topological charge implies that a magnetic field will deflect the motion of these skyrmions through the so-called skyrmion Hall effect [4<\/a>]\u2014just as a moving electric charge is deflected in the conventional Hall effect.<\/p>\n

AFM skyrmions offer solutions to these limitations. These objects can be viewed as two interwoven ferromagnetic skyrmions\u2014one on each AFM sublattice\u2014with opposite spin orientations. The absence of stray fields in AFMs enables field-free stability at room temperature and potentially even smaller sizes (below 10\u202fnm) [5<\/a>]. And because their topological charges cancel, the skyrmion Hall effect is strongly suppressed, allowing straight motion.<\/p>\n

Finding materials that naturally host such properties is difficult. Instead, researchers have turned to synthetic antiferromagnets (SAFs)\u2014nanostructures made of ferromagnetic layers antiferromagnetically coupled through nonmagnetic spacers. These engineered materials allow precise control over magnetic interactions, enabling the stabilization of chiral antiferromagnetic structures such as spirals and skyrmions [6<\/a>].<\/p>\n

Guo and colleagues have taken up the challenge to probe the ultrafast evolution of magnetization and chirality in a SAF made up of several layers of a cobalt-iron-boron magnetic alloy separated by nonmagnetic layers. This layered configuration can potentially host AFM skyrmions [6<\/a>], but it would be challenging to directly probe such compact structures with the researchers\u2019 technique. So instead, they designed their material to host N\u00e9el-type spin spirals, which is a cycloid where the spins rotate in a plane forward or backward with respect to the propagation direction. (This contrasts with Bloch-type spin spirals, where the spins rotate around the direction of propagation like a helix.) Each magnetic layer hosts a spin spiral, with adjacent layers having opposite spin directions (Fig. 1<\/a>).<\/p>\n

The researchers conducted their experiments at the FERMI FEL in Italy\u2014one of the world\u2019s brightest and most stable ultrafast light sources. To resolve spatial variations in the SAF\u2019s magnetic structure, the team performed resonant magnetic scattering experiments, which involve tuning the wavelength of the probing light to the magnetically sensitive absorption resonances in the material. In particular, the researchers targeted the Fe L3 edge, which required using soft x rays (707\u202feV photon energy) at the limit of the FEL\u2019s performance.<\/p>\n

The team recorded the x rays scattering from the SAF, revealing a strong specular diffraction signal from the multilayer structure. In addition, the researchers identified a ring-shaped scattering pattern that emerged from the randomly orientated spin spirals. The radius of this ring corresponds to a spiral period of about 190\u202fnm, as expected. But determining the chirality of the spin spiral is far more demanding, as it requires circular (or at least elliptical) polarization of the soft x rays\u2014a capability only recently demonstrated at FERMI [7<\/a>]. By recording scattering patterns with left and right elliptically polarized x rays, the team extracted the circular dichroism signal, which revealed the sense of spin rotation (left- or right-handed) and the chirality type (N\u00e9el or Bloch) [8<\/a>].<\/p>\n

To investigate the spin dynamics in their SAF, Guo and colleagues used a pump\u2013probe scheme, in which femtosecond infrared laser pulses transiently demagnetize the material, while x rays track the response of the AFM order and the chirality. The time it takes the SAF to demagnetize and remagnetize (180 fs and 500 fs, respectively) is significantly faster (by a factor of around 3) than a comparable ferromagnetic sample. Interestingly, in the SAF, the chirality follows exactly the dynamics of the AFM order, which was not the case in a previous experiment on a chiral ferromagnetic material [9<\/a>]. The researchers suggest that this distinction arises from the topological nature of the continuously winding spin spiral in the SAF, which lacks the discrete domain boundaries present in ferromagnets. The result highlights the importance of considering both local and nonlocal contributions when describing ultrafast spin dynamics.<\/p>\n

This method for monitoring chiral spin structure is an achievement built upon years of progress in sample design, FEL source development, and resonant scattering methodology. Extending the technique to the femtosecond dynamics of individual skyrmions will require further improvements, as skyrmions are much smaller than the typical x-ray beam width (around 100 \u00b5m) and their chirality signal (in terms of dichroic contrast) is much weaker than that of spin spirals. But there are several other directions that researchers might explore. Following this demonstration, future efforts might compare the dynamics of spin spirals with different rotational planes (N\u00e9el type vs Bloch type) or study long-range ordered skyrmion lattices under ultrafast excitation. As chiral textures can also exhibit depth-dependent magnetic order\u2014especially after spatially inhomogeneous laser excitation\u2014a heroic next step would be to capture the full three-dimensional, time-resolved evolution of (anti)ferromagnetic order and chirality [10<\/a>].<\/p>\n

ReferencesZ. Guo et al., \u201cUltrafast dynamics of chiral spin structures in synthetic antiferromagnets,\u201d Phys. Rev. B 112, L020408 (2025)<\/a>.I. Dzyaloshinsky, \u201cA thermodynamic theory of \u201cweak\u201d ferromagnetism of antiferromagnetics,\u201d J. Phys. Chem. Solids 4, 241 (1958)<\/a>; T. Moriya, \u201cAnisotropic superexchange interaction and weak ferromagnetism,\u201d Phys. Rev. 120, 91 (1960)<\/a>.J. Sampaio et al., \u201cNucleation, stability and current-induced motion of isolated magnetic skyrmions in nanostructures,\u201d Nat. Nanotechnol. 8, 839 (2013)<\/a>.W. Jiang et al., \u201cDirect observation of the skyrmion Hall effect,\u201d Nat. Phys. 13, 162 (2016)<\/a>; K. Litzius et al., \u201cSkyrmion Hall effect revealed by direct time-resolved X-ray microscopy,\u201d 13, 170 (2016)<\/a>.L. Caretta et al., \u201cFast current-driven domain walls and small skyrmions in a compensated ferrimagnet,\u201d Nat. Nanotechnol. 13, 1154 (2018)<\/a>.W. Legrand et al., \u201cRoom-temperature stabilization of antiferromagnetic skyrmions in synthetic antiferromagnets,\u201d Nat. Mater. 19, 34 (2019)<\/a>.C. Spezzani et al., \u201cCircular dichroism experiments at the L edge of magnetic transition metals enabled by elliptically polarized pulses from a seeded free-electron laser,\u201d Phys. Rev. B 110, 174409 (2024)<\/a>.J. Chauleau et al., \u201cChirality in magnetic multilayers probed by the symmetry and the amplitude of dichroism in x-ray resonant magnetic scattering,\u201d Phys. Rev. Lett. 120, 037202 (2018)<\/a>.N. Kerber et al., \u201cFaster chiral versus collinear magnetic order recovery after optical excitation revealed by femtosecond XUV scattering,\u201d Nat. Commun. 11, 6304 (2020)<\/a>.E. Burgos-Parra et al., \u201cProbing of three-dimensional spin textures in multilayers by field dependent X-ray resonant magnetic scattering,\u201d Sci. Rep. 13, 11711 (2023)<\/a>.About the Author\"Image<\/p>\n

Daniel Schick is a Leibniz junior group leader at the Max Born Institute (MBI) for Nonlinear Optics and Short Pulse Spectroscopy in Germany. His research focuses on ultrafast magnetism, employing resonant magnetic scattering techniques across the XUV to soft x-ray spectral range.\u00a0He is an experienced user of accelerator-based photon sources and is also a developer of laboratory experiments with laser-driven sources of ultrashort x rays. He earned his PhD from the University of Potsdam in Germany in 2013 and subsequently worked as a postdoctoral researcher at the femtoslicing facility at BESSY II, also in Germany, from 2014 to 2017 before joining MBI.<\/p>\n

Ultrafast dynamics of chiral spin structures in synthetic antiferromagnets<\/a><\/p>\n

Zongxia Guo, Raphael Gruber, Dmitriy Ksenzov, Cyril L\u00e9veill\u00e9, Matteo Pancaldi, Emanuele Pedersoli, Carlo Spezzani, Giovanni De Ninno, Flavio Capotondi, Christian Gutt, Mathias Kl\u00e4ui, Vincent Cros, Nicolas Reyren, and Nicolas Jaouen<\/p>\n

Phys. Rev. B 112, L020408 (2025)<\/a><\/p>\n

Published July 28, 2025<\/p>\n

Read PDF<\/a>Subject AreasRelated Articles\"Half-Metals<\/a>\"Spin-to-Charge<\/a>\"Mapping<\/a>Condensed Matter PhysicsMapping Spin Waves with a Strobe Light<\/a>December 20, 2024<\/p>\n

A method for imaging spin waves in magnetic materials uses flash-like intensity variations in a laser beam to capture the wave motion at specific moments in time. Read More \u00bb<\/a><\/p>\n

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