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We have studied the laser induced ultrafast quenching of Gd 4f magnetic order in ferrimagnetic Co100-xGdx 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 Co72Gd28, the Co77Gd23 and the Co79Gd21 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. [ABSTRACT FROM AUTHOR]

We have investigated the laser induced ultrafast dynamics of Gd 4f spins at the surface of Co x Gd 100-x alloys by means of surface-sensitive and time-resolved dichroic resonant Auger spectroscopy. We have observed that the laser induced quenching of Gd 4f magnetic order at the surface of the Co x Gd 100-x alloys occur on a much longer time scale than that previously reported in "bulk sensitive" time-resolved experiments. In parallel, we have characterized the static structural and magnetic properties at the surface and in the bulk of these alloys by combining Physical Property Measurement System (PPMS) magnetometry with X-ray Magnetic Circular Dichroism in absorption spectroscopy (XMCD) and X-Ray Photoelectron spectroscopy (XPS). The PPMS and XMCD measurements give information regarding the composition in the bulk of the alloys. The XPS measurements show non-homogeneous composition at the surface of the alloys with a strongly increased Gd content within the first layers compared to the nominal bulk values. Such larger Gd concentration results in a reduced indirect Gd 4f spin-lattice coupling. It explains the "slower" Gd 4f demagnetization we have observed in our surface-sensitive and time-resolved measurements compared to that previously reported by "bulk-sensitive" measurements. • Characterization of electronic, magnetic and structural properties of ferrimagnetic CoGd amorphous alloys. • Gd 4f spin dynamics at the surface of CoGd alloys by means of Time-Resolved Dichroic Resonant Auger Spectroscopy. • Disparate Gd 4f ultrafast spin dynamics in CoGd alloys due to non-homogeneous in-depth composition. [ABSTRACT FROM AUTHOR]

We have investigated ultrafast quenching of the Tb 4f magnetic order in Co74Tb26 alloys, induced by femtosecond hot-electron pulses. The hot-electron pulses were produced in specific nonmagnetic capping layers by infrared femtosecond laser pulses. Our experimental results show that subpicosecond dynamics of Tb 4f magnetic moments can be induced by nonthermal and thermal hot-electron. We further demonstrate that the demagnetization efficiencies of nonthermal and thermal hot-electron are similar. However, the characteristic demagnetization times show values of 0.35 ps for nonthermal hot-electron excitations and 1.2 ps for thermal hot-electron excitations. We explain this temporal elongation by the propagation time of thermal hot-electron through the 15-nm-thick CoTb film. [ABSTRACT FROM AUTHOR]

Femtosecond-laser-induced ultrafast magnetization dynamics have been studied in multisublattice CoxDy1-x alloys. By performing element- and time-resolved x-ray spectroscopy, we distinguish the ultrafast quenching of Co 3d and Dy 4f magnetic order when the initial temperatures are below (T=150K) or above (T=270K), the temperature of magnetic compensation (Tcomp). In accordance with former element-resolved investigations and theoretical calculations, we observe different dynamics for Co 3d and Dy 4f spins. In addition, we observe that for a given laser fluence, the demagnetization amplitudes and demagnetization times are not affected by the existence of a temperature of magnetic compensation. However, our experiment reveals a twofold increase of the ultrafast demagnetization rates for the Dy sublattice at low temperature. In parallel, we measure a constant demagnetization rate of the Co 3d sublattice above and below Tcomp. This intriguing difference between the Dy 4f and Co 3d sublattices calls for further theoretical and experimental investigations. [ABSTRACT FROM AUTHOR]