D.J. Higley, K. Hirsch, E. Jal, T. Liu, E. Yuan, G. L. Dakovski, A. Lutman, J. MacArthur, A.H. Reid, H.A. Durr, W.F. Schlotter

SLAC National Accelerator Laboratory

X-Ray Magnetic Circular Dichroism Spectroscopy (XMCD) in the soft x-ray range is a powerful technique to element-specifically probe the spin and orbital moments of matter through their absorption of circularly polarized x-rays at elemental resonances. When performed with femtosecond-scale x-ray pulses, XMCD can be used to probe the element-specific time evolution of spin and orbital moments in ultrafast magnetization dynamics. One example is all-optical switching, where a magnetic material controllably switches its magnetization when excited with a femtosecond pump laser pulse. The recent addition of circular polarization capability at LCLS enables ultrafast XMCD in the soft x-ray range with an unprecedented average flux of circularly polarized x-rays provided in femtosecond pulses. Using this new capability, we record static XMCD spectra which agree with those recorded at synchrotrons as well as XMCD spectra recorded at specific time delays after an optical pump on thin film GdFeCo samples. For all recorded spectra after an optical pump (a minimum of 500 fs after the pump), we find that the electronic contribution to the change in XMCD spectra is very small compared to the large magnetic changes. We also systematically monitor the ultrafast dynamics of the magnetization of the Gd and Fe sub-lattices in 20 nm GdFeCo thin films which are above their magnetic compensation temperature when excited with fluences below, at, and above their all-optical switching switching fluence range. We find that the switching of the two sub-lattices is much smaller and slower than previous studies on GdFeCo thin films below their compensation temperature.