  Temperature-dependent magnetic experiments like pump-probe measurements
generated by a pulsed laser have become a crucial technique for switching the
magnetization in the picosecond time scale. Apart from having practical
implications on the magnetic storage technology, the research field of
ultrafast magnetization poses also fundamental physical questions. To correctly
describe the time evolution of the atomic magnetic moments under the influence
of a temperature-dependent laser pulse, it remains crucial to know if the
magnetic material under investigation has magnetic excitation spectrum that is
more or less dependent on the magnetic configuration, e.g. as reflected by the
temperature dependence of the exchange interactions. In this article, we
demonstrate from first-principles theory that the magnetic excitation spectra
in Co with fcc, bcc and hcp structures are nearly identical in a wide range of
non-collinear magnetic configurations. This is a curious result of a balance
between the size of the magnetic moments and the strength of the Heisenberg
exchange interactions, that in themselves vary with configuration, but put
together in an effective spin Hamiltonian results in a configuration
independent effective model. We have used such a Hamiltonian, together with
ab-initio calculated damping parameters, to investigate the magnon dispersion
relationship as well as the ultrafast magnetisation dynamics of Co and Co-rich
CoMn alloys.
