Seminars Archive

Novel Insights into Magnetic Vortex Dynamics by Time-Resolved Scanning Transmission X-Ray Microscopy

Abstract
Time-resolved scanning transmission X-ray microscopy was implemented by our group at the Advanced Light Source (ALS), Berkeley, at the Canadian Light Source (CLS), Saskatoon and at BESSY II, Berlin. A sophisticated data acquisition technique allows us to take advantage of the full photon flux of the multi-bunch mode for time-resolved measurements with pump-and-probe or RF excitation at almost any frequency, up to 22 GHz so far at the BESSY II ‘low alpha’ multi bunch mode. By applying the above measurement technique on micron sized magnetic vortex structures we discovered low-field switching of the vortex core when exciting the gyrotropic eigenmode of the vortex structure with an RF magnetic field burst [1]. A model was suggested [1] and later confirmed experimentally [2], which is based on the creation and annihilation of a vortex – anti-vortex (VA) pair. This VA mechanism for vortex core reversal is now generally accepted in the community. By excitation with rotating RF magnetic fields the vortex core can be switched in a ‘unidirectional’ way, either to its up or its down position only, depending on the sense of rotation of this external RF field [3]. Vortex structures possess azimuthal spin wave modes with much higher eigenfrequencies compared to the vortex gyromode. We demonstrated by experiments and micromagnetic simulations that unidirectional low-field vortex core reversal is also possible by exciting these spin wave eigenmodes with rotating multi-GHz magnetic fields [4] or with orthogonal field pulse sequences in x and y direction [5]. In that way we achieved vortex core switching times below 100 ps [5] (from the start of the orthogonal pulse sequence to the end of the vortex core reversal). Outlook: The implementation of the diffraction-based ptychography removed the lateral resolution limit of soft X-ray microscopy imposed by the characteristics of the X-ray optics. For instance, 5 nm structures could be imaged by using a Fresnel zone plate with 60 nm outer zone width [6]. Concerning the time resolution to be achieved, this could be significantly improved from about 100 ps (at BEESY II multi bunch mode) or about 10 ps (at BESSY II ‘low alpha’ mode) to the 100 fs range for Free Electron Lasers like the Fermi FEL. [1] B. Van Waeyenberge et al., Nature 444, 461, (2006) [2] A. Vansteenkiste et al., Nature Physics 5, 332 (2009) [3] M. Curcic et al., Phys. Rev. Lett. 108, 197204 (2008) [4] M. Kammerer et al., Nature Communications 2, 279 (2011) [5] M. Noske et al., Phys. Rev. B. 90, 104415 (2014) [6] D. Shapiro et al., Nature Photonics 8, 765 (2014)