J.
Phys.
France 49
(1988)
7-11
JANVIER
7
1988,
Classification
Physics Abstracts
76.80 - 74.70
A Mössbauer
study
YBa2Cu3O7201403B4
of
a
superconducting sample
of 57Fe
doped
P. Imbert and G. Jéhanno
Service de Physique du Solide et de Résonance
91191 Gif-sur-Yvette Cedex, France
(Regu
le QO aoict
1987, accepti
sous
Magnétique,
forme d£finitive
CEN -
le 4 novembre
Saclay,
1987)
Résumé. 2014 A partir de l’étude des spectres Mössbauer d’impuretés de 57Fe substituées au cuivre
dans le composé supraconducteur YBa2Cu3O7201403B4, nous concluons à l’absence d’ordre magnétique
statique dans ce composé au-dessus de 4,2K.
study of the Mössbauer absorption of 57Fe impurities substituted for Cu in
superconducting YBa2Cu3O7_03B4 we conclude that there are no static ordered magnetic moments
within the Cu sublattices in this compound down to 4.2K.
Abstract. - From
a
"Magnetism and superconductivity are usually mutually exclusive, but they seem to be intimately related in the new high-temperature superconducting compounds", so writes A.L. Robinson [1]. He adds : "Superconductivity and
antiferromagnetism are the Jekyll and Hyde of
these systems" .
A question of much current interest in these
compounds is whether or not there is coexistence
of superconductivity and antiferromagnetic ordering of the copper atoms. According to P.G. de
Gennes, the high critical temperatures (T, ) could
arise from an attractive interaction between carriers mediated by spin waves within the framework of a canted antiferromagnetic structure [2].
According to P.W. Anderson, the copper valence
electrons could, in contrast, be associated as
nearest neighbour singlet pairs [3]. In addition,
it seems to be experimentally established that in
the La2-,,Sr.,,CU04 system, the long-range antiferromagnetic order in non superconducting La2
Cu04 disappears in the superconducting compounds of the series [4]. Here we present an
experimental contribution to the current discussion, this time concerning Y Ba2Cu307 _ h: a
Mossbauer study of this compound doped with
57 Fe shows that ordered magnetic
moments
are
absent at temperatures much lower than Tc.
1.
Sample preparation
and control.
samples of YBa2(CU1-,,Fe,,)307-h were
prepared under identical conditions with x 0,
0.8 and 5 % respectively. The Mossbauer study
was carried out mainly on the sample with x
0.8 % which was prepared using 57 Fe enriched
iron. A mixture of appropriate amounts of Y203,
BaC03, CuO and Fe203 were cold pressed and
sintered in air for 10h at 900C. The heating rate
was near 300C/h and the cooling rate down to
room temperature was near 150C/h. The samples were then finely crushed, cold pressed and
again heat treated. We have verified that a third
heat treatment at 900° C for the sample with
x
0.8 % followed by an anneal at 500°C for 5h
as suggested by P. Strobel et al. [5] so as to possibly increase the oxygen content, did not appreciably modify either the crystallographic properThree
=
=
=
ties
or
the Mossbauer data.
X-ray study of the three samples showed
the presence of a single phase and that the orAn
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019880049010700
8
Tab. I) strongly decreases
increases and becomes essentially equal to
1.0 for x = 5.0 %. This shows that the Fe is
well incorporated into the YBa2Cu307_b lattice.
0 and 0.8 % are slightly
Our b/a values for x
smaller than the value (b/a
1.01821(4)) obtained by Cava et al. [6] for YBaZCu306,9. As
the amount of orthorhombicity is a function of
the oxygen content (following Bordet et al. [7]
the lattice becomes tetragonal for the composition YBa2Cu306) we can estimate that the mean
oxygen content in our samples is slightly lower
than 6.9.
thorhombicity ( b/a,
as x
=
=
Orthorhombicity (b/a ratios) measured
samples YBa2(CU1-.Fe.).307-6
Table Lour
as well as that at 77K and 4.2K clearly shows
the presence of electric quadrupole interactions.
Only the spectrum at 1.4K (Fig. 2) shows the
presence of a magnetic hyperfine structure. From
a more detailed analysis of the
hyperfine interwe
discuss below the number and the
actions,
nature of the sites occupied by the 57Fe probe as
well as the electronic nature of these iron atoms.
We conclude that there is no static magnetic ordering within the Cu sublattices at least down to
4.2K.
in
Fig.l.-
M6ssbauer spectrum of YBa2 (CUO.992
at 295K (see Tab. IIa for the fit-
,57Feo.008 )3 07-6
ted
parameters).
Resistivity measurements show that for the
sample with x = 0, Tc is slightly lower than 90K
and that for x = 0.8 %, Tc is reduced by about
4K with the transition now spread out over several degrees. A test of the Meissner effect for
5 % shows that it is not
the sample with x
at
77K.
The Meissner effect is
superconducting
on
each
of
the
other two samples
clearly visible
at this temperature. The observed rapid vari=
ation of Tc with Fe content shows that the Fe
probably substitute for the Cu atoms
whose sublattices are thought to be responsible
for the superconducting properties. This substitution would be expected for two 3d elements
having comparable atomic volumes.
atoms very
Finally,
a
part of the
x
=
0.8
Mossbauer spectrum of YBa2 (Cuo.992
"FeO.008)3 07-6 at 1.4K (Note the change in the velocity scale compared with Fig. 1). The solid line was
fitted using a Zeeman sextuplet with He ff
230kOe,
on
a
residual
quadrupole
paramagnetic
superimposed
doublet.
Fig.2.-
=
% sample
annealed at 650 C for 10h in an argon atmosphere. From the observed weight loss, the
oxygen content 7 - 8 of the resulting tetragonal
non superconducting phase was estimated to be
about 6.2.
was
2.
57Fe M6ssbauer results.
hyperfine structure observed on the x
% superconducting sample at 295K (Fig. 1)
The
0.8
=
At 295K and 77K, that is either side of
the
Mossbauer spectra are practically identiT,
cal. Satisfactory line fits are obtained in terms
of three quadrupole doublets (two strong and
one weak).
However the four prominent experimental lines associated with the two dominant quadrupole doublets can be paired equally
well in two different ways. This gives for the
first choice an outer and an inner doublet hav-
9
the same isomer shift value (a "symmetric lines" fit, leading to the doublets D1 and
D2 in Tab. IIa), or for the second choice two
doublet$ with quite different isomer shift values
(a "crossed lines" fit, leading to the doublets
Di and D2 in Tab. IIb). In both types of fit
the components of each doublet were allowed to
have different linewidths. Although both procedures give fits of equally good quality, the "symmetric lines" fit is favoured for the following
For x = 5 %, the "symmetric lines"
reasons.
fit leads to the pairing of lines with the same
widths, whereas very different linewidths are required for two lines linked by the "crossed lines"
fit. Besides, preliminary results obtained on the
x = 0.8 % sample treated at 650C in an argon
atmosphere, where the relative areas of the external lines are much enhanced, are consistent with
a "symmetric lines" fit. Finally, when comparing the two fitting procedures, we have to keep
in view that the two dominant quadrupole doublets are very likely related to the two possible
sites where the iron impurities can substitute for
the Cu : the 5-oxygen coordinated pyramidal site
and the 4-oxygen coordinated planar site [7]. In
this respect, it is difficult to understand the hyperfine parameters (almost identical quadrupole
splittings and widely different isomer shifts) provided by the "crossed lines" fit. In contrast, it
is easier to understand the hyperfine parameters
(very different quadrupole splittings and almost
equal isomer shifts) provided by the "symmetric
lines" fit. The very different quadrupole splittings are in accord with nearest neighbours point
charge calculations, which show that the electric
field gradient is much larger (in absolute value)
for the planar configuration than for the pyramidal configuration [8]. In addition, NQR measurements on 63Cu in YBa2Cu307 [4,8] show that
ing nearly
the
planar
ous as
the
sites Cu 1, which are half as numerpyramidal sites Cu 2, give the largest
quadrupole frequency
(v(’) 32MHz, V(2)
For all these reasons we conclude in
f avour of the ’asymmetric lines" fit (Tab. IIa)
and we assign the largest quadrupole splitting,
?S’(Di) - 1.96mm/s, to the 57Fe impurities substituted for Cu in the planar sites and the small-
22MHz).
est
quadrupole splitting QS(D2) = 1.15mm/s
to
the 57Fe impurities substituted an the pyramidal sites. This assignment is further corroborated by the fact that the doublet D2 is approximately twice as intense as the doublet Dl for
the x = 0.8 % sample (however the D21D, area
5 %).
ratio is close to 1 for x
=
The isomer shift values of the doublets Di
and D2 are both close to zero, relative to Fe
metal. Such a low isomer shift value is not
linked to the metallic character of the matrix, because a comparable value is observed in the nonmetallic sample obtained after argon annealing.
In a purely ionic model, this value would suggest
either a Fe4+ charge state or a low spin Fe3+
state [9] (a low spin Fe2+ state is excluded because it is diamagnetic, which is not compatible
with the magnetic spectrum observed at 1.4K).
But as a large hybridization between the Cu or
Fe d-orbitals with the oxygen p-orbitals is likely
to occur in this material, we suggest rather the
presence of a high spin Fe3+ state strongly modified by covalency effects. The large and temperature independent quadrupole splittings of the
doublets DI and D2 would thus reflect directly
the highly anisotropic charge distribution around
the two copper sites substituted by 57Fe.
The low intensity doublet D3, which has
a smaller quadrupole
splitting (QS(D3) isomer shift (about
and
a
larger
0.6mm/s)
Table IIa.- Values obtained from the 295K Mössbauer spectra using the "symmetric lines" fit (see text). IS :
isomer shift relative to SfiFe - metal ; G : full linewidth ; QS : quadrupole splitting ; P : relative area of the
three quadrupole doublets.
10
Table IIb.- Values obtained from the 295K Mossbauer spectra,
Table IIa for definition of symbols.
relative to Fe-metal) is attributed to
high spin ionic Fe3+ state located in a site less
deformed than the normal Cu sites (ex. Fe3+
substituting for y3+) ; but it is not clear whether
this site belongs in fact to the matrix or to some
separate phase. If such an extra phase exists, it
must correspond to an impurity with a relatively
high iron concentration, as it is not detected by
X-ray diffraction.
0.31mm/s
a
At 4.2K the doublets Dl and D2 are still
0.8 % suvisible in the spectrum of the x
perconducting sample, but they have increased
linewidths. At 1.4K (Fig. 2), they give rise to a
magnetic hyperfine structure, which can be fitted
to a first approximation using a mean effective
field Hff -- 230kOe on the 57Fe nucleus. Such a
=
magnetic
ture
structure appears at
(between 4.2K
a
higher tempera-
and
10K) in the sample with
(x 5 %). This concentra-
a larger iron content
tion dependence suggests that it is the coupling
between the iron impurities which is responsible
for their magnetic ordering. However the magnetization of the 57Fe probe could also reflect the
presence of a long range or a short range magnetic order within the Cu sublattices at these
low temperatures. The dynamic nature of the
magnetic hyperfine interaction can also be envisaged : the low temperature magnetic splitting
can be due either to blocked magnetic Fe moments or to slow paramagnetic relaxation. However, whatever the exact origin and nature of the
magnetic hyperfine structure observed at 1.4K,
the existence of this structure clearly shows that
the electronic configuration of the Fe atoms which
substitute for the Cu is not diamagnetic and that
these Fe impurities are not in a spin-compensated
using
the "crossed lines"
fit (see text).
See
Kondo state, as
quently the 57Fe
for example in Cr [10]. Conseprobe should be sensitive to any
magnetic ordering within the Cu sublattices (we
mention that the existence of magnetic ordering
in antiferromagnetic La2Cu04 is clearly visible
on a 0.5 % 57Fe probe [11]).
The fact that the
main paramagnetic quadrupole doublets D1 and
D2 remain visible down to 4.2K thus excludes the
existence of static magnetic ordering
sublattices at 4.2K and above.
of
the Cu
This conclusion agrees with that obtained
from recent NMR and NQR results on Cu in
superconducting YBa2Cu307 [4]. It is also to
be compared to our 170Yb and 166Er Mossbauer
analysis of superconducting YbBa2Cu307- b and
ErBa2CU307-,b which shows low rare-earth
magnetic-ordering temperatures of 0.35 and 0.7K
respectively ( 12) .
=
Further
experiments using Mossbauer
emis-
sion spectroscopy, which can be carried out at
much lower 57Co doping concentrations than are
required for Mossbauer absorption spectroscopy
with 57Fe, could perhaps elucidate the origin of
the low temperature magnetic hyperfine interaction and determine whether the Cu sublattices
are magnetically ordered at 1.4K. Let us mention
too that specific heat measurements between 30
and 200mK strongly suggest the existence of a
hyperfine field at the copper nuclei in supercon-
ducting Lal.g5Sro.1,5CU04-h 1131.
Acknowledgments.
The authors
resistivity
Hodges
indebted to J.M. Delrieu for
and to A. G6rard and
for useful discussions.
are
measurements
J.A.
11
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