Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho $L_3$ absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Tuning the x-ray energy to the electric dipole ($E1$, $2p\to 5d$) or quadrupole ($E2$, $2p\to 4f$) transition allows us to selectively and independently study the spin dynamics of the itinerant $5d$ and localized $4f$ electronic subsystems via the suppression of the magnetic (2 1 $3\text{−}\tau$) satellite peak. We find demagnetization time scales very similar to ferromagnetic $4f$ systems, suggesting that the loss of magnetic order occurs via a similar spin-flip process in both cases. The simultaneous demagnetization of both subsystems demonstrates strong intra-atomic $4f\text{−}5d$ exchange coupling. In addition, an ultrafast lattice contraction due to the release of magneto-striction leads to a transient shift of the magnetic satellite peak.

• Received 9 November 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.257202