The University of Tokyo demonstrates various topological spin structures in rare-earth alloys under variable external magnetic fields

A research group including Graduate Student Haruto Yoshimochi, Assistant Professor Rina Takagi (at the time of the research), and Associate Professor Shinichiro Seki from the Graduate School of Engineering, in collaboration with Associate Professor Taro Nakajima from the Institute of Solid State Physics at the University of Tokyo and Associate Professor Satoru Hayami from the Faculty of Science, Hokkaido University, has succeeded in observing various spin structures in the rare-earth alloy GdRu₂Ge₂ by changing the magnitude of the external magnetic field, such as elliptical skyrmions, meron−antimeron structures, and circular skyrmions. The results were published in Nature Physics.

Magnetic skyrmions, which are vortex structures of electron spins observed in magnetic materials, are attracting attention as candidates for next-generation information carriers because they behave as stable particles protected by topology. Previously, skyrmions were thought to be produced only in materials with low-symmetry crystal structures. In recent years, extremely small skyrmions with a diameter of several nanometers formed via a new mechanism have been reported even in highly symmetric materials.

The research group focused on the rare-earth alloy GdRu₂Ge₂, which has a square lattice structure with spatial inversion symmetry. The structure of this material consists of alternating layers of a two-dimensional square lattice of trivalent gadolinium ions, which elicit magnetism, and ruthenium and germanium layers, which are responsible for electrical conduction. Magnetization and electrical transport measurements were performed on this alloy to measure the magnetic field dependence of magnetization, longitudinal resistivity, and Hall resistivity.

The group clarified that when an external magnetic field is applied in the stacking direction, the magnetic structure undergoes a multistage phase transition and an increase in the Hall resistivity is observed for two of the magnetic phases, namely phases 2 and 4. It is known that when electrons move on a skyrmion, a topological Hall effect occurs wherein the electron movement direction changes due to a virtual magnetic field, reflecting the unique topology of the skyrmion. The increased hole resistivity observed in this case is thought to be caused by such a topological Hall effect, indicating that multiple skyrmion phases are formed in this alloy. To directly observe the microscopic spin arrangement, the research group performed neutron scattering experiments using the High-Resolution Chopper Spectrometer (HRC) installed at the beamline of the Japan Proton Accelerator Research Complex J-PARC along with resonant X-ray scattering experiments at the KEK Photon Factory beamline.

The results showed that in magnetic phases 2 and 4, skyrmions with a diameter of 2.7 nm were stabilized in a lattice state. Moreover, the research group revealed that this material undergoes a multistep magnetic structural phase transition in response to an external magnetic field. In particular, in magnetic phases 2, 3, and 4, various topological spin structures were manifested, such as elliptical skyrmions, meron−antimeron structures, and circular skyrmions.

These findings present new material design guidelines for extremely small skyrmions. In addition, since skyrmions and meron−antimeron structures are characterized by different topological numbers, these results may lead to the development of new applications, such as multilevel memory operations using external magnetic fields.

Journal Information
Publication: Nature Physics
Title: Multistep topological transitions among meron and skyrmion crystals in a centrosymmetric magnet
DOI: 10.1038/s41567-024-02445-9