Laser induced ultrafast Gd 4f spin dynamics in Co 100-x Gd x alloys by means of time-resolved XMCD

We have studied the laser induced ultrafast quenching of Gd 4f magnetic order in ferrimagnetic Co 100-x Gd x alloys to highlight the role of the Co 3d–Gd 5d inter-atomic exchange coupling. We have taken advantage of the ultrashort soft X-ray pulses deliver by the femtoslicing beamline at the BESSY II synchrotron radiation source at the Helmholtz–Zentrum Berlin to perform element- and time-resolved X-ray Magnetic Circular Dichroism spectroscopy. Our results show that the laser induced quenching of Gd 4f magnetic order occurs on very different time-scales for the Co 72 Gd 28 , the Co 77 Gd 23 and the Co 79 Gd 21 alloys. Most of the magnetic moment losses occur within the first picosecond (ps) while the electron distribution is strongly out of equilibrium. After the equilibration of the electrons and lattice temperatures ( t > 1 ps), the magnetic losses occur on slower rates that depend on the alloy composition: increasing the Co composition speeds up the demagnetization of Gd 4f sublattice. The strength of the Co 3d–Gd 5d inter-atomic exchange coupling which depends on composition, determines the efficiency of the angular momentum flow from the Gd 4f spin towards the lattice. Our results are in qualitative agreements with the predictions of the microscopic three temperatures model for ferrimagnetic alloys.