Enhanced hyperfine magnetic fields for face-centered tetragonal Fe(110) ultrathin films on vicinal Pd(110)

Abstract

The structure and hyperfine magnetic properties of epitaxial Fe ultrathin films on a vicinal Pd͑110͒ surface have been investigated by means of low-energy electron diffraction ͑LEED͒, reflection high-energy electron diffraction ͑RHEED͒ and 57 Fe conversion electron Mössbauer spectroscopy ͑CEMS͒. LEED and RHEED provide evidence for initial pseudomorphic film growth. The RHEED determination of the in-plane atomic distance versus Fe film thickness demonstrates the stabilization of the metastable fcc-like Fe structure on Pd͑110͒. This interpretation is supported by in situ 57 Fe CEMS measurements which indicate an enhanced saturation hyperfine field of ϳ39 T for a 3-monolayers-thick Fe film at 25 K. This is the highest value ever measured for Fe on a metallic substrate. Our results suggest that ultrathin fcc-like ͑face-centered tetragonal͒ Fe films on Pd͑110͒ are in a ferromagnetic high-moment state with an enhanced hyperfine field due to electronic 3d-4d hybridization at the Fe/Pd interface.

Application of Mössbauer spectroscopy in magnetism

“…Therefore, the observed (negative) quadrupole level shift 2ε of −0.09 mm/s for the 3-ML Fe/Pd (110) film provides a model-independent proof for a locally non-cubic structure (lattice distortion) of the 3 ML Fe film, in qualitative agreement with our RHEED result [35]. 2ε obtained for the 8 ML Fe film is negligible within error bars, pointing to a much less distorted lattice of this thicker Fe film.…”

“…This oscillating behaviour was explained by a superposition of two effects: first, an exponential short-range exchange interaction (mainly due to 3 d-4 d hybridization) within the first three Fe monolayers from the interface, and second an oscillating RKKY-type long-range interaction via conduction electrons in deeper Fe layers. A similar hybridization effect at the fct-Fe/Pd (110) interface may contribute to the strongly enhanced B hf value observed in our work [35].…”

“…The relative in-plane Fe atomic distance (relative to the Pd(110)-substrate atomic distance) perpendicular to the scattering plane has been determined as a function of the 57 Fe film thickness from the distance of the RHEED streaks in reciprocal space. This demonstrated the stabilization of the metastable face-centered tetragonal (fct) Fe structure on Pd (110) at small Fe film thickness ( ≤∼3 ML Fe) due to the in-plane Fe lattice expansion by the Pd substrate [35]. This lattice expansion was found to be reduced with increasing Fe film thickness.…”

“…The structural and hyperfine magnetic properties of epitaxial Fe ultrathin films on the vicinal Pd(110) surface were investigated by means of LEED (low-energy electron diffraction), RHEED (reflection high-energy electron diffraction) and 57 Fe CEMS [35]. Earlier experimental work demonstrated that Pd becomes ferromagnetic when in proximity to Fe [36,37] and carries a magnetic moment [38].…”

“…Further details on the sample preparation are given in Ref. [35]. After 57 Fe film preparation and LEED/RHEED studies the sample was transferred (110) measured in situ in UHV at T = 25 K. The corresponding distribution of hyperfine magnetic fields, P(B hf ), is shown on the right-hand side.…”