Abstract
The magnetic hyperfine fields Bhf
of 77Br/77Se adatoms on the Co(0001) surface and
on the (001) surface of a 40 monolayers Fe film were measured at T = 300
K applying perturbed angular correlation spectroscopy (PAC). The results
are: |Bhf| = 2.2(2) T for Se on Co(0001) and |Bhf|
= 6.1(3) T for Se on Fe(001). The values of the Bhf of the Se
adatom are strongly reduced in comparison to their respective bulk values:
Bhf = 42(4) T in cobalt [1] and Bhf = 69(5) T in
iron [1]. These experimental results can be explained by first-principle
calculations of the Bhf of 4sp-impurities on the (001) surfaces
of Ni and Fe which were performed by Mavropoulos et al. [2]. The authors
show that a splitting in the local density of states of the impurity s-electrons
due to the lowering of the symmetry and of the coordination number at the
surface is responsible for the observed reduction of Bhf of
Se on Ni [3] and Fe. The same mechanism is likely to be attributed to the
observed reduction of Bhf of Se on Co(0001), though computational
results are not reported. The calculated enhancement of magnetic hyperfine
fields at the beginning and the end of the 4sp-series (Zn, Ga,..., Kr)
on the surface of Fe and Ni compared to the bulk values [2] could be demonstrated
in this work experimentally by measuring the |Bhf| of the 5sp-impurity
111In/111Cd
on Ni(111) which is iso-electric to Zn. The result is | Bhf|
= 16 T as compared to
Bhf = ?6.69(3) T in bulk Ni [4]. Such a considerably stronger
value of a Bhf at a surface is experimentally observed for the
first time.
In the second part of this work the temperature
dependence of the electric field gradient EFG of 77Br/77Se
on Pd(111) and Co(0001) was studied. In both cases a linearly increasing
EFG with increasing temperature was observed. This is in contrast to EFGs
at Se on surfaces with (001) orientation and in most metallic bulk systems.
Computations of the EFG using electronic structure calculations based on
the discrete variational method (DVM) were performed for Se on Ni(001),
Ni(111), Pd(001), Pd(111) and Co(0001) by B. Lindgren [5]. The computation
of the EFG is based on the dependence of the distance between the Se adatom
and the substrate surface at T = 0 K. Extrapolation of the experimental
results of the temperature dependence of the EFG at Se on Ni(001) [3],
Ni(111) [3], Pd(001) [3], Pd(111) and Co(0001) to T = 0 K allows for comparison
of the experimental and theoretical findings. A consistent picture emerges
with the assumption of negative EFGs at the Se nuclei on surfaces with
(001) orientation and positive EFGs at the Se nuclei on surfaces with (111)
orientation.
All experiments were performed in the UHV
chamber ASPIC at the UHV beam line of the online mass separator ISOLDE/CERN.
[1] P. T. Callaghan, N. J. Stone, B. G. Turrell,
Phys. Rev. B 10, 1075 (1974)
[2] P. Mavropoulos, N. Stefanou, B. Nonas,
R. Zeller, P. H. Dederichs,
Phys. Rev. Lett. 81,
1505 (1998)
[3]
H. Granzer, H. H. Bertschat, H. Haas, W. ?D. Zeitz, J. Lohmüller,
G. Schatz,
Phys.
Rev. Lett. 77, 4261 (1996)
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