Advanced neutron scattering techniques for complex magnetic structures and their dynamics: case studies on magnetic skyrmion lattices

by Prof. Taro Nakajima (The Institute for Solid State Physics, the University of Tokyo RIKEN Center for Emergent Matter Science)

Monday 22 Nov 2021, 14:30 → 15:30 Europe/Berlin

Zoom Webinar

Neutron scattering is one of the most useful tools to study magnetic structures and their dynamics. For instance, magnetic structures are determined by comparing intensities of magnetic Bragg reflections, which are obtained in neutron diffraction measurements, with calculations. Dispersion relations of magnetic excitations are observed by neutron inelastic scattering experiments. These methodologies have been well established, and successfully applied to numerous number of magnetic systems. But we still need to develop new neutron scattering techniques to study complex magnetic orders, such as magnetic skyrmions, which are topologically-nontrivial vortex-like spin objects attracting considerable attention in recent years[1]. In this talk, I will introduce the advanced neutron scattering techniques which we employed to investigate the structural and dynamical aspects of magnetic skyrmion lattices (SkLs).

Polarized neutron scattering is quite useful to determine orientations of magnetic moments from a limited number of magnetic Bragg reflections. We performed polarized neutron small-angle scattering on VOSe2O5, which has a polar crystal structure, at the KWS-1 beamline in FRM-II. By measuring spin-flip and non-spin-flip scatterings from magnetic Bragg reflections appearing in a small-Q region, we revealed that VOSe2O5 exhibits a Neel-type SkL consisting of cycloidal magnetic modulations[2].

 Magnetic skyrmions are expected to have robust stability when the thermal fluctuation is negligible, because transitions from the SkL state to other topologically-trivial magnetic orders have large energy barriers. Indeed, it was demonstrated that a metastable skyrmion state with an extremely long lifetime can be realized in MnSi by quenching the thermoequilibrium SkL phase to low temperatures[3]. To observe the quenching process, we performed stroboscopic small-angle neutron scattering, in which time-resolved neutron scattering patterns are obtained by synchronizing rapid temperature sweepings and pulsed neutron beam, in the TAIKAN instrument in the materials and lifescience experimental facility MLF in J-PARC[4].  

Finally, I will introduce the MIEZE-type neutron spin echo (NSE) technique, which is suited for observing slow magnetic fluctuations in a magnetic field[5]. We performed the MIEZE-type NSE measurements on MnSi at the VIN ROSE instrument in MLF of J-PARC, observing coexistence of static SkL and diffusive spin fluctuations at the boundary between the SkL phase and the paramagnetic phase[6].

 

[1] S. Mühlbauer et al., Science 323, 915 (2009).

[2] T. Kurumaji et al., Phys. Rev. Lett. 119, 237201 (2017), J. Phys. Soc. Jpn. 90, 024705 (2021).

[3]  H. Oike et al., Nat. Phys. 12, 62 (2016).

[4] T. Nakajima et al., Phys. Rev. B 98, 014424 (2018),

[5] J. Kindervater et al. Phys. Rev. X 9, 041059 (2019).

[6] T. Nakajima et al., Phys. Rev. Research 2, 043393 (2020).