Direct observation of Stoner excitations in itinerant ferromagnets by spin-polarized inelastic electron scattering (invited; abstract)

Abstract

Stoner excitations in the ferromagnetic 3d metals, although fundamental in the theory of itinerant magnetism, have never been observed directly, e.g., by inelastic neutron scattering because the energies involved are too high for present-day neutron sources. We show that these excitations are readily measure by spin-polarized electron energy loss spectroscopy (SPEELS). We have measured the spin polarization spectrum of electrons scattered from a Fe-based ferromagnetic metallic glass (Fe82B12Si6) using unpolarized primary electrons in the energy range Ep=20–240 eV. While the spin polarization P for elastically scattered electrons is always small (*P*≤3%) a maximum of P is found around 2.2 eV energy loss. The value of the maximum Pmax decreases with increasing primary energy (Pmax=12% for Ep=20 eV, Pmax=3% for Ep=240 eV). This maximum is caused by spin-flip (Stoner) excitations (via exchange). In this process an incoming spin-down electron falls into the empty part of the spin-down density of states above EF and excites a spin-up electron from the occupied part of the majority density of states. The energy loss of 2.2 eV then corresponds to the ferromagnetic exchange splitting. This interpretation is corroborated by recent model calculations by Glazer and Tosatti which show good agreement with the experimental data. The spin polarization as a function of energy loss and the dependence on proimary energy are well reproduced by the calculatioons. Kirschner et al. showed that the Stoner excitatioons can also be observed by a slightly different experimental approach, i.e., by using spin-polarized primary electrons and measuring the asymmetry of the scattered intensity upon reversal of the primary spin polarization. For a Ni(110) surface they found an asymmetry maximum around 300 meV energy loss. This value again corresponds to the ferromagnetic exchange splitting. The novel technique of spin-polarized electron energy loss spectroscopy, which is complementary to spin-polarized photoemission and inverse photoemission, promises to yield new insights into the electronic structure and the electronic excitation spectrum of itinerant ferromagnets and their temperature dependence, e.g., for ferromagnetic alloys.