Friday, October 17, 2008, Condensed Matter Seminar, Dr. Martin Weinelt, Max-Born-Institut und freie Universitaet, Berlin, "Hot Spots and Spin Waves," 4:30 PM, 319 Allen Hall, PITT

Abstract

Recent experiments demonstrate that significant demagnetization of thin ferromagnetic films can be achieved within a picosecond upon optical excitation. Within this timescale, the excited electronic system and the underlying lattice are not in equilibrium and it seems that the transient hot electron population is responsible for the change of the magnetization. It remains controversial to date, which microscopic processes are fast enough to provoke femtomagnetism. To approach these problems we combined time-, angle- and energy-resolved two-photon photoemission with spin-resolved electron detection and investigated ultrathin iron and cobalt films on Cu(100).

In purely non-relativistic electron-electron or electron-phonon scattering a spin-flip is not possible because the Coulomb operator does not act on the spin part of the electron wave-function. Electrons only undergo a spin flip in the presence of spin-orbit coupling significantly enhanced at hybridization points in the band structure. We have identified these so-called spin hot-spots by linear magnetic dichroism. Initial bulk and surface states with minority spin-character are the source for the dichroic intensities and the apparent dichroic lifetimes of the image-potential states. Excellent agreement with ab initio fully relativistic calculations of the cobalt fcc band-structure allows us to precisely determine spin-orbit hybridization points close to the Fermi level.

Experimental access to the spin-dependent relaxation processes of low-energy electrons in d-band ferromagnets proves a challenge, as the dynamics occur on the femtosecond timescale and it is difficult to distinguish between refilling and the various scattering processes. For these purposes the well-defined dispersing image-potential-state electron can be employed as test charge or primary electron. While at the minimum of the image- potential band the decay rate is determined by the strongly spin-dependent density of d-states, the increase of the decay rate with energy above the band bottom is governed by spin-independent states only. We observe twice the increase in decay rate for minority-spin electrons than for majority-spin electrons on thin iron films. The factor of two is explained if electron magnon scattering is included. Then minority-spin electrons may also scatter into the majority-spin bands via magnon emission and thus gain twice the phase space of their majority-spin counterparts. These magnon-enhanced electron scattering processes allow for transfer of angular momentum of hot electrons on a femtosecond timescale.