Universität Osnabrück, Graduiertenkolleg Mikrostruktur oxidischer

18 UNIVERSITÄT OSNABRÜCK, FACHBEREICH PHYSIK
this system is a non-saturated spin-fluctuation system. The exchange-enhancement factors are not high enough
to lead to the saturation of the amplitude of the local spin fluctuations.
In the case of GdNi5-xAlx system, the aim of our study was to investigate the influence of partially replacing Ni
by Al on the magnetic state of the Ni 3d-electrons in GdNi5-xAlx by using magnetic measurements and XPS
technique.
The valence bands of GdNi5, GdNi4Al (Fig 4) arise mainly from the Ni 3d electrons which are close to the onset
of ferromagnetism. The 3d states are hybridized with 5d states. The occupied part of the valence bands are over
4eV wide. One can see also a marked feature near the maximum of
the valence bands of GdNi5 and GdNi4Al. This may be explained
Intensity (arb. units)
GdNi 5
GdNi 4 Al
GdNiAl 4
pure Ni
-2 0 2 4 6 8 10 12 14 16
Binding energy (eV)
as a partial quenching of the spin fluctuations by the strong magnetic
moments of Gd atoms. Furthermore, the substitution of Ni by
Al leads to an increase of the localization of the Ni 3d electrons,
reflected by an energy shift of the valence band maximum to
higher binding energies. This is also supported by the different
values of the saturation temperature of the spin fluctuations T *
obtained for GdNi5 and GdNi4Al.
Fig 4. XPS valence spectra of the GdNi5-xAlx system.
One can observe marked differences both in the shape and in the
position of the valence bands in GdNiAl4 and Ni. The density of states at the Fermi level in GdNiAl4 is drastically
reduced in comparison with pure Ni and the maximum of the valence band is shifted to higher binding
energy, namely from 0.59 eV in Ni to 2.3eV in GdNiAl4. The states at the Fermi level have 3d and s-p character
in Ni and GdNiAl4, respectively. In d-band metals and alloys the 3d states are shifted gradually to higher binding
energy by increasing the d-state occupancy and consequently a decreasing density of states at EF occurs. These
results reveal the complete filling of the Ni 3d-band by charge transfer of Gd 5d electrons and the s-p character
of the states at the Fermi level in GdNiAl4. This conclusion is also confirmed by comparing the Ni 2p core level
spectra for pure Ni and GdNiAl4. In the Ni 2p core level spectrum of pure Ni are present also satellites as an
evidence for the d character in unoccupied states. The absence of these satellites in the Ni 2p spectrum of
GdNiAl4, shows that the 3d-band in this compound is filled.
Summarising the results, one may conclude that the Ni 3d-band in GdNi5-xAlx compounds is sensitive to the
local environment, showing a large variety of magnetic behaviours by partially replacing Ni by Al atoms.
Outlook
We have very recently investigated the presented ferric-wheel by X-ray emission spectroscopy using synchrotron
radiation. The results will be further analysed. Other magnetic molecules will be investigated by several
spectroscopic methods in the next future. Other rare-earth compounds under investigation are the LaNi5-xAlx and
YCo2-xSix series.
References
[1] See http://zeus.physik.uni-osnabrueck.de/neumann/veroeff_1.html
[2] St. Uhlenbrock, PhD Thesis, Osnabrück 1994
[3] S. Lütkehoff, PhD Thesis, Osnabrück 1997
[4] S. Plogmann, PhD Thesis, Osnabrück, 2000
[5] A review on http://cmpweb.ameslab.gov/magnetic_molecules/
[6] C. Schröder, PhD Thesis, Osnabrück 1999
[7] E. Burzo, S.G. Chiuzbăian, L. Chioncel, M. Neumann, Journal of Physics: Condensed Matter, 12 (2000)
5897
[8] D. Ginoux, D. Givord, and A del Moral, Solid State Communications 19 (1978) 891
[9] J. Palleau and G. Chouteau, Physical Review Letters 41 (1980) L227
[10] M. Coldea, S. Chiuzbaian, M. Neumann, D. Todoran, M. Demeter, R. Tetean, V. Pop, accepted for publication
in Acta Physica Polonica
[11] T. Moriya, Journal of Magnetism and Magnetic Materials 100 (1991) 261 and the references therein
[12] A. Caneschi, D. Gatteschi, C. Sangregorio, R. Sessoli, L. Sorace, A. Cornia, M.A. Novak, C. Paulsen, W.
Wernsdorfer, J. Magn. Magn. Mater. 200 (1999) 182