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Magnetismus Poster: Do., 13:00–15:30 D-P266
NMR and XAS study of Fe-Mo double perovskites
Dariusz A. Zajac 1,2 , Czeslaw Kapusta 1 , Marta Borowiec 1 , Marcin Sikora 1,3 ,
Vit Prochazka 1,4 , Damian Rybicki 1 , Clara I. Marquina 5 , Jose M. de Teresa 5 ,
Javier Blasco 5 , Manuel R. Ibarra 5
1 AGH University of Science and Technology, Faculty of Physics Applied Computer Science,
av. Mickiewicza 30, 30-059 Cracow, Poland – 2 DESY Hasylab, Notkestrasse 85,
22 607 Hamburg, Germany – 3 European Synchrotron Radiation Facility, BP 220, 38043
Grenoble Cedex, France – 4 Charles University, Faculty of Mathematics and Physics, V
Holesovickach 2, 180 00 Prague 8, Czech Republic – 5 Universidad de Zaragoza, CSIC
c/ Pedro Cerbuna 12, 50009 Zaragoza, Spain
The results of molybdenum NMR and XAS measurements at the Fe and Mo K edges of
the A2FeMoO6 double perovskites (DP) (A2=Sr2,SrBa,Ba2,Ca2) are reported. The information
obtained on the individual site electronic and magnetic properties is analysed
and discussed. The compounds studied belong to the family of materials exhibiting
a huge negative magnetoresistance. Magnetoresistive properties of these compounds
arise from their half-metallicity, i.e. only one spin direction being populated in the
conduction band, which consists of overlapping spin down 3d Fe, 2p O and 4d Mo
electron bands [1]. The molybdenum NMR measurements revealed the existence of a
non-integer magnetic moment at Mo and Fe [2,3,4], which can be attributed to the
DE-like interaction. The 95 Mo 97 Mo NMR measurements on Fe-Mo DP presented in
this work show different values of the Mo hyperfine field and the corresponding Mo
magnetic moment. This is attributed to a dependence of the Mo magnetic moment
on the structural tolerance factor f related to the ionic radius of the alkaline earth.
A similar dependence on f is also observed for TC. This indicates that the strength
of magnetic interaction is related to the magnetic moment at the molybdenum site.
X-ray absorption measurements in the XANES range provide the Fe and Mo K-edge
energies correspond to intermediate valence states of Fe (Mo) between 2.5+ and 3+
(between 5+ and 6+) for all the investigated compounds, with opposite tendencies of
the dependence on f. For the compounds Ba2FeMoO6 and SrBaFeMoO6 a two-step
feature of the absorption edge is observed. The step at higher energies is attributed
to the iron valence states close to Fe 3+ , whereas the lower energy one is attributed
to an intermediate Fe valence, in analogy with the XAS spectrum of Fe3O4. XANES
measurements at the Mo K edge show an intermediate valence state of Mo, similarly to
that reported in [7]. A change of the edge energy with f is observed, with the opposite
tendency to that obtained for Fe. The results of NMR measurements are compared to
those of XANES study and a relation to bulk magnetic and electrical transport properties
is discussed. [1] K.-I. Kobayashi, et al., Nature 395, 677 (1998), [2] Cz. Kapusta,
et al., JMMM, 242-245, 701, (2002), [3] Cz. Kapusta, et al., JMMM, 272-276, e1619,
(2004), [4] D. Zajac, PhD thesis, Cracow-Zaragoza 2006, [5] P.A. Algarabel, et al.,
JMMM, 226-230, 1089, (2001), [6] J. Linden, et al., Phys. Rev. B, 68, 174415, (2003),
[7] J. Herrero-Martin, et al., J. Phys.: Condens. Matter, 16, 6877, (2004).