Modern techniques for the investigation of correlated materials in the time domain combine selective excitation in the THz frequency range with selective probing of coupled structural, electronic and magnetic degrees of freedom using x-ray scattering techniques. Cryogenic sample temperatures are commonly required to prevent thermal occupation of the low energy modes and to access relevant material ground states. Here, we present a chamber optimized for high-field THz excitation and (resonant) x-ray diffraction at sample temperatures between 5 and 500 K. Directly connected to the beamline vacuum and featuring both a Beryllium window and an in-vacuum detector, the chamber covers the full (2–12.7) keV energy range of the femtosecond x-ray pulses available at the Bernina endstation of the SwissFEL free electron laser. Successful commissioning experiments made use of the energy tunability to selectively track the dynamics of the structural, magnetic and orbital order of Ca2RuO4 and Tb2Ti2O7 at the Ru (2.96 keV) and Tb (7.55 keV) L-edges, respectively. The chamber has been employed extensively in user operation since 2021, during which THz field amplitudes up to 1.12 MV cm−1 peak field were demonstrated and used to excite the samples at temperatures as low as 5 K. New developments, which allow the chamber to be used in optical pump, x-ray probe experiments in grazing incidenc geometry down to 5 K sample temperature have been commissioned recently.
For more details, see https://iopscience.iop.org/article/10.1088/1361-648X/ac08b5
Magnetic skyrmions, vortex-like topological spin textures, have attracted much attention in terms of both fundamental physics and spintronics applications. Thus far, skyrmions have been observed in thin-film multilayers with interfacial Dzyaloshinskii-Moriya interaction (DMI) and structurally-chiral magnets with bulk DMI. Recently, bulk-DMI-induced skyrmions have been observed above room temperature in Co-Zn-Mn alloys with a β-Mn-type chiral structure [1]. In most chiral magnets, however, skyrmions exist as a thermodynamically equilibrium state only in a narrow temperature and magnetic field region just below the magnetic transition temperature Tc. The limited region of the stable skyrmions is unfavorable for applications.
Here, we focused on β-Mn-type Co9Zn9Mn2 (Tc ~ 400 K) and performed small-angle neutron scattering, magnetic susceptibility and Lorentz transmission electron microscope measurements. We demonstrated that skyrmions persist over almost the whole temperature region below 400 K as a long-lived metastable state by performing a conventional field-cooling process. Once created, metastable skyrmions survive above room temperature after removal of magnetic field [2]. These findings exemplify the topological robustness of the once-created skyrmions and provide a significant step toward applications of skyrmions in bulk chiral magnets.
In the presentation, we will discuss the details of the above observations, and also show novel skyrmion states in Co-Zn-Mn alloys with other chemical compositions.
[1] Y. Tokunaga et al., Nat. Commun. 6, 7638 (2015).
[2] K. Karube et al., Phys. Rev. Mater. 1, 074405 (2017).
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