HAL Id: jpa-00210677
https://hal.archives-ouvertes.fr/jpa-00210677
Submitted on 1 Jan 1988
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
A Mössbauer study of a superconducting sample of 57Fe doped YBa2Cu3O7-δ
P. Imbert, G. Jéhanno
To cite this version:
P. Imbert, G. Jéhanno. A Mössbauer study of a superconducting sample of 57Fe doped YBa2Cu3O7-δ.
Journal de Physique, 1988, 49 (1), pp.7-11. �10.1051/jphys:019880049010700�. �jpa-00210677�
A Mössbauer study of a superconducting sample of 57Fe doped YBa2Cu3O7201403B4
P. Imbert and G. Jéhanno
Service de Physique du Solide et de Résonance Magnétique, CEN - Saclay,
91191 Gif-sur-Yvette Cedex, France
(Regu le QO aoict 1987, accepti sous forme d£finitive le 4 novembre 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.
Abstract. - From a 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.
Classification
Physics Abstracts
76.80 - 74.70
"Magnetism and superconductivity are usu- ally mutually exclusive, but they seem to be inti- mately related in the new high-temperature su- perconducting compounds", so writes A.L. Ro- binson [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 or- dering of the copper atoms. According to P.G. de Gennes, the high critical temperatures (T, ) could
arise from an attractive interaction between car-
riers mediated by spin waves within the frame-
work 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 anti- ferromagnetic order in non superconducting La2 Cu04 disappears in the superconducting com- pounds of the series [4]. Here we present an
experimental contribution to the current discus-
sion, 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.
Three 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 sam- ples 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 pos-
sibly increase the oxygen content, did not appre-
ciably modify either the crystallographic proper- ties or the Mossbauer data.
An X-ray study of the three samples showed
the presence of a single phase and that the or-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019880049010700
8
thorhombicity ( b/a, Tab. I) strongly decreases
as x 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.
Our b/a values for x = 0 and 0.8 % are slightly
smaller than the value (b/a = 1.01821(4)) ob-
tained 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 composi-
tion YBa2Cu306) we can estimate that the mean
oxygen content in our samples is slightly lower
than 6.9.
Table L- Orthorhombicity (b/a ratios) measured in
our samples YBa2(CU1-.Fe.).307-6
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 sev-
eral degrees. A test of the Meissner effect for the sample with x = 5 % shows that it is not
superconducting at 77K. The Meissner effect is
clearly visible on each of the other two samples
at this temperature. The observed rapid vari-
ation of Tc with Fe content shows that the Fe atoms very probably substitute for the Cu atoms whose sublattices are thought to be responsible
for the superconducting properties. This substi-
tution would be expected for two 3d elements
having comparable atomic volumes.
Finally, a part of the x = 0.8 % sample
was annealed at 650 C for 10h in an argon at-
mosphere. From the observed weight loss, the
oxygen content 7 - 8 of the resulting tetragonal
non superconducting phase was estimated to be
about 6.2.
2. 57Fe M6ssbauer results.
The hyperfine structure observed on the x =
0.8 % superconducting sample at 295K (Fig. 1)
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 inter- actions, we discuss below the number and the 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 or- dering within the Cu sublattices at least down to 4.2K.
Fig.l.- M6ssbauer spectrum of YBa2 (CUO.992 ,57Feo.008 )3 07-6 at 295K (see Tab. IIa for the fit- ted parameters).
Fig.2.- Mossbauer spectrum of YBa2 (Cuo.992
"FeO.008)3 07-6 at 1.4K (Note the change in the ve- locity scale compared with Fig. 1). The solid line was
fitted using a Zeeman sextuplet with He ff = 230kOe, superimposed on a residual paramagnetic quadrupole
doublet.
At 295K and 77K, that is either side of
T, the Mossbauer spectra are practically identi-
cal. Satisfactory line fits are obtained in terms of three quadrupole doublets (two strong and
one weak). However the four prominent ex- perimental lines associated with the two domi-
nant quadrupole doublets can be paired equally
well in two different ways. This gives for the
first choice an outer and an inner doublet hav-
ing nearly the same isomer shift value (a "sym-
metric 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 proce- dures give fits of equally good quality, the "sym-
metric lines" fit is favoured for the following
reasons. For x = 5 %, the "symmetric lines"
fit leads to the pairing of lines with the same
widths, whereas very different linewidths are re-
quired 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 exter-
nal lines are much enhanced, are consistent with
a "symmetric lines" fit. Finally, when compar-
ing the two fitting procedures, we have to keep
in view that the two dominant quadrupole dou-
blets 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 hy- perfine parameters (almost identical quadrupole splittings and widely different isomer shifts) pro-
vided 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 split- tings 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 pyrami-
dal configuration [8]. In addition, NQR measure-
ments on 63Cu in YBa2Cu307 [4,8] show that
the planar sites Cu 1, which are half as numer-
ous as the pyramidal sites Cu 2, give the largest quadrupole frequency
(v(’) 32MHz, V(2)
22MHz). 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 sub-
stituted for Cu in the planar sites and the small-
est quadrupole splitting QS(D2) = 1.15mm/s to
the 57Fe impurities substituted an the pyrami-
dal sites. This assignment is further corrobo- rated by the fact that the doublet D2 is approx-
imately twice as intense as the doublet Dl for
the x = 0.8 % sample (however the D21D, area
ratio is close to 1 for x = 5 %).
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, be-
cause a comparable value is observed in the non-
metallic 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 be-
cause 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 mod-
ified by covalency effects. The large and temper-
ature 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) - 0.6mm/s) and a larger isomer shift (about
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, using the "crossed lines" fit (see text). See
Table IIa for definition of symbols.
0.31mm/s relative to Fe-metal) is attributed to
a 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.
At 4.2K the doublets Dl and D2 are still
visible in the spectrum of the x = 0.8 % su- 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 structure appears at a higher tempera-
ture (between 4.2K and 10K) in the sample with
a larger iron content (x = 5 %). This concentra-
tion dependence suggests that it is the coupling
between the iron impurities which is responsible
for their magnetic ordering. However the mag- netization of the 57Fe probe could also reflect the presence of a long range or a short range mag- netic order within the Cu sublattices at these low temperatures. The dynamic nature of the magnetic hyperfine interaction can also be envis-
aged : the low temperature magnetic splitting
can be due either to blocked magnetic Fe mo-
ments or to slow paramagnetic relaxation. How- ever, 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
Kondo state, as for example in Cr [10]. Conse-
quently the 57Fe probe 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 of the Cu
sublattices at 4.2K and above.
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 interac-
tion 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 are indebted to J.M. Delrieu for
resistivity measurements and to A. G6rard and J.A. Hodges for useful discussions.
References
[1] ROBINSON, A.L., Science 267 (1987) 780.
[2] DE GENNES, P.G., C.R. Hebd. Scéan.
Acad. Sci., Paris 305, Série II (1987) 345-
348.
[3] ANDERSON, P.W., Science 235 (1987) 1196.
[4] LÜTGEMEIER, L., PIEPER, M.W., Solid
State Commun. 64 (1987) 267.
[5] STROBEL, P., CAPPONI, J.J., CHAILLOUT, C., MAREZIO, M. and THOLENCE, J.L., Na-
ture 327 (1987) 306.
[6] CAVA, R.J., BATLOGG, B., VAN DOVER, R.B., MURPHY, D.W., SUNSHINE, S., SIE-
GRIST, T., REMEIKA, J.P., RIETMAN, E.A., ZAHURAK, S. and ESPINOSA, G.P., Phys.
Rev. Lett. 58 (1987) 1676.
[7] BORDET, P., CHAILLOUT, C., CAPPONI,
J.J., CHENAVAS, J. and MAREZIO, M. Pre- print.
[8] RIESEMEIER, H., CRABOW, Ch., SCHEIDT, E.W., MÜLLER, V., LÜDERS, K. and RIEGEL, D., Solid State Commun. 64
(1987) 309.
[9] GREENWOOD, N.N. and GIBB, T.C., Möss-
bauer Spectroscopy (Chapman and Hall Ltd,
London) 1971, p. 91.
[10] HERBERT, I.R., CLARK, P.E. and WILSON, G.V.H., J. Phys. Chem. Solids 33 (1972)
979.
[11] To be published.
[12] HODGES, J.A., IMBERT, P. and JÉHANNO, G., Solid State Commun., to be published.
[13] GUTSMIEDL, P., WOLFF, G., and ANDRES, K., Phys. Rev. B36 (1987) 4043.
