Separating Exchange Splitting from Spin Mixing in Gadolinium by Femtosecond Laser Excitation

Abstract

Employing spin-, time-, and energy-resolved photoemission spectroscopy, we present the first study on the spin polarization of a single electronic state after ultrafast optical excitation. Our investigation concentrates on the majority-spin component of the d-band-derived Gd(0001) surface state d(z(2))(↑). While its binding energy shows a rapid Stoner-like shift by 90 meV with an exponential time constant of τ(E)=0.6±0.1 ps, the d(z(2))(↑) spin polarization remains nearly constant within the first picoseconds and decays with τ(S)=15±8 ps. This behavior is in clear contrast to the equilibrium phase transition, where the spin polarization vanishes at the Curie temperature.

Perspective: Ultrafast magnetism and THz spintronics

“…A projection of the proper electron state onto the spin quantum number is therefore questionable; 21 the electron state is rather a mixture of spin-up and spin-down states. [22][23][24][25] Similar effects are found in the presence of spinfluctuations. However, this spin-mixing has important implications.…”

Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt

“…In the case of a Stoner-like behavior of itinerant electrons, the magnetic moments are quenched via single-particle excitations that induce a collapse of the exchange splitting and therefore a shift of the spin-polarized bands. In the case of a localized spin or Heisenberg picture, magnetic moments change their orientation via collective excitations, that is, due to spin fluctuations and/or magnon generation ( 15 , 19 – 21 ). This spatially and time-dependent modification of the orientation of the magnetic moments leads to rapidly fluctuating spin-split electronic states in space and time.…”

Stoner versus Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

“…Some authors observe a change of exchange splitting during ultrafast demagnetization 38,[44][45][46] , while others do not 30 . One source of confusion has been that it had not been realized that the behavior of magnetic films on metallic substrates is different, since spin currents can modify the occupations of the spin-polarized bands.…”

“…Also, different probes measure different parts of bands in the Brillouin zone and are thus sensitive to different contributions 10 . In addition, the behavior of 4f ferromagnets is more involved than in pure 3d systems due to coupled reactions of 5d and 4f spin-polarized electrons [44][45][46] . Recently Carpene et al used femtosecond magneto-optical and reflectivity spectroscopy to study demagnetization of an Fe film, and concluded that the process is dominated by transversal spin excitations and not changes in exchange splitting 30 .…”