STUDIES OF MAGNETITE SURFACES
BY MEANS OF LOW-ENERGY ION SCATTERING TECHNIQUE
N.-T.H. Kim-Ngan*, W. Soszka
Institute of Physics, Pedagogical University, 30-084 Kraków, Poland.
(*Corresponding author: Email address: tarnawsk@ap.krakow.pl)
Magnetite (Fe3O4), a promising material for the development of spintronic devices due
to its half-metallic behaviour, has recently attracted much interest. Its well-know feature is the
metal-insulator phase transition around 120 K. We have used the low-energy ion scattering
(LEIS) technique to investigate several MBE thin-film and single-crystalline surfaces of
magnetite. The experiments have been carried out in the temperature range of 85 K – 300 K
using 4 - 8 keV He+, Ne+ and Ar+ ion beams at a small- and a large-scattering-angle geometry
favorable respectively for the multiple- and single-scattering process. Distinct anomalies in
the temperature dependence of the scattered ion yield, R+(T), were observed in the metalinsulator phase transition region (Fig. 1). For the multiple ion scattering (from the MBE
magnetite surfaces) a strong dependence of the characteristic of the R+(T) curves on the ion
type, primary energy and geometrical structures of the surface semi-channels was found. For
the single ion scattering (from the single-crystalline magnetite surfaces), despite of such a
strong influence, the main trend of the R+(T) curves was maintained at different energies.
Figure 1. Temperature dependence of the normalized ion yield of magnetite surfaces revealed
anomalies in the metal-insulator phase transition region. Left: ions multiply scattered off MBE (100) Fe 3O4
surface under 5.5 keV (shown by markers) and 6.5 keV He +, Ne+ and Ar+ ion bombardments (thick solid lines).
Right: ions singly scattered off the single-crystalline Fe3O4 (001) surface under 5.0 keV He+, 6.0 keV Ne+ and
Ar+ ion bombardments.
Complicated behaviors of the R+(T) curves indicated a complex interplay between the
dominant Auger neutralization and re-ionization. The Auger neutralization was related to a
large increase of the electron localization degree at the transition point, while the re-ionization
concerned to the ionizing trajectories from the zigzag collisions from the topmost atomic rows
and in the collision cascade inside the surface channels (in the case of multiple scattering),
and/or in the isolated binary collision of ions with atoms lying in the deeper layers than the
topmost surface layer (in the case of single scattering). The later one was found to be largely
affected by a change of the crystal transparency due to a crystal distortion.
Analyzing the LEIS data of singly-scattered ions within the framework of the
shadowing effect related to the screening action of the Fe ions have given an evidence of a
dominant contribution from Fe-octahedral ions in the Fe-O planes on the (001) surface and
from Fe ions in the mix-trigonal layer consisting of both tetrahedrally and octahedrally
coordinated sites on the (111) surface.