Direct evidence for interface-induced perpendicular spin orientation and spin reorientation in terbium-coated thin iron films.

Abstract

Conversion-electron M\"ossbauer spectroscopy in ultrahigh vacuum has been employed to study in situ the average Fe-spin orientation and hyperfine fields in zero external field in uncoated and Tb-coated $^{57}\mathrm{Fe}$ films deposited either onto a clean Tb(0001) single-crystal surface or onto (bulklike) polycrystalline Tb or Ag films. At 30 K, the spectra of 15-\AA{}-thick uncoated Fe films on Tb(0001) indicate structurally disordered (amorphous) Fe layers, whereas spectra of 30- or 35-\AA{}-thick uncoated and coated Fe films are typical for bcc Fe. We show that the orientation of Fe spins in 30--35-\AA{}-thick uncoated Fe films is preferentially in the film plane for temperatures ranging from 30--300 K for all substrates used. In striking contrast to uncoated films Tb-coated 30--35-\AA{}-thick Fe films exhibit a preferential Fe-spin orientation perpendicular to the film plane at \ensuremath{\sim}30 K irrespective of the substrates used. This demonstrates (i) that the perpendicular spin orientation in these films originates from an interface magnetic anisotropy and (ii) that the dominant effect in inducing perpendicular spin direction originates from the interface formed by the Tb-coating layer and the Fe film and to a smaller exent to the Fe/Tb-substrate interface. A temperature-dependent reversible Fe-spin reorientation was observed near 150--200 K which is qualitatively compatible with recent models including spin fluctuations.