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impurities in the high TC superconductors La2 -xSrxCuO4
C. Garcin, P. Imbert, P. Bonville, J. Hodges
To cite this version:
C. Garcin, P. Imbert, P. Bonville, J. Hodges. Mössbauer emission measurements on highly dilute 57Fe impurities in the high TC superconductors La2 -xSrxCuO4. Journal de Physique I, EDP Sciences, 1994, 4 (12), pp.1829-1842. �10.1051/jp1:1994224�. �jpa-00247037�
J Phys I France 4 (1994) 1829-1842 DECEMBER 1994, PAGE1829
Classification Pliysics Abstracts
74.70V 76.80 75.30H
Môssbauer emission measurements on highly dilute ~~Fe impurities in the high Tc superconductors La2_~SrJ~Cu04
C. Garcin, P. Imbert, P. Bonville and J-A- Hodges
Service de Physique de l'Etat Condensé, Centre d'Etudes de Saclay, 91191 Gif-sur-Yvette,
France
(Received 5 July1994, accepted in final form 9 September 1994)
Résumé Des expériences d'émission Môssbauer effectuées sur des impuretés quasi-isolées de
~~Fe substituées à Cu dans La2-xSrxCu04 montrent un changement rapide du comportement magnétique local quand la teneur ~ en Sr excède une valeur effective ~t, située approximative-
ment au centre du domaine en ~ associé à la supraconductivité dans le diagramme de phase.
Pour ~ < ~t, l'impureté de Fe3+ s'entoure systématiquement d'une zone verre de spin, où les spins du
cuivre s'orientent au voisinage du plan de base, avec une température de gel de
8 ~1 K vis-à-vis du temps de réponse caractéristique de la sonde Môssbauer. Pour ~ > ~t, la sonde ne détecte plus d'ordre antiferromagnétique local, mais elle révèle l'existence d'une
aiman-
tation locale induite dans le sous-réseau du cuivre, qui présente une orientation préférentielle
suivant l'axe é pour les valeurs de
~ les plus grandes. Les résultats sont discutés et comparés aux
autres mesures disponibles, y compris celles de notre étude précédente par absorption Môssbauer
sur ~~Fe dans la même série.
Abstract Môssbauer emission measurements on quasi-isolated ~~Fe1nlpurities substituted
for Cu in La2-xSrxCu04 show a rapid change in the local magnetic behaviour when the Sr content ~ exceeds an effective value ~t, which is roughly at the centre of the superconducting
~ range in the phase diagram For ~ < ~t, the Fe3+ impurity systematically pins a CU spin-
glass cluster where the spms are oriented near the basai plane, with a freezing temperature of 8 ~1 K with respect to the characteristic time scale of the Môssbauer probe. For ~ > ~t,
the probe no longer detects local antiferromagnetic ordenng, but it reveals the presence of an induced Iocahzed magnetization in the CU sublattice, which is preferentially onented near the é axis for the Iargest ~ values. The results are discussed and compared with other available
measurements, includmg our previous ~~Fe Môssbauer absorption study in the same serres.
l Introduction.
Doping trie Cu site of the cuprate superconductors by varions elements is a method commonly used to investigate the properties of these compounds and to gain information on trie mecha-
nism of the high Tc superconductivity. In particular, magnetic measurements have shown that
@ Les Editions de Physique1994
a localized spin density was induced around 3d impurities (Fe, Co, Ni, Zn) and sp impurities (Ga, Al) substituted for copper in superconducting La2-xSr~Cu04, regardless of the magnetic
or non magnetic character of the dopant [1-3]. Microscopic type measurements, such as EPR,
NMR or Môssbauer spectroscopy, are particularly relevant and provide additional information
on the nature of these local induced magnetic effects and on their dependence on various pa-
rameters, such as the temperature, the hole carrier density and the impurity concentration level.
We describe here Môssbauer emission measurements, which have been carried out on dilute
~~Fe atoms at the Cu site in La2-xSr~Cu04. This study, partially reported in reference [4],
presents two outstanding features.
. First, the Fe3+ Môssbauer ion, which substitutes for copper, can be viewed as a "perturbative probe", since it is both the cause and the investigation tool of the local perturbation induced in the matrix. We recall indeed that iron doping rapidly destroys the superconducting prop-
erties, and that Tc vanishes for a substitution level Fe/Cu
= 0.018 ~ 0.001 in the compound Lai s5Sro.15Cu04 Ill.
. Secondly, Môssbauer emission spectroscopy (MES) enables trie ~~Fe probes to be studied at
highly dilute concentration levels. In MES, trie matrix is doped by trie radioactive parent ~~Co using substitution levels Co/Cu < 10~~, and trie ~~Fe atoms generated by trie decay of ~~Co
can be considered as quasi-isolated impunties. The present MES study extends our previous Môssbauer absorption spectroscopy (MAS) measurements on ~~Fe
in La2-xSr~Cu04 là, 6j,
where the substitution level Fe/Cu rangea from o.01 to 0.002. The comparison of the MAS and MES results provides information on the influence of the interaction between impurities.
In section 2, we describe the preparation and the ~~Co doping process of the La2-xSr~Cu04 samples (0.13 < x < 0.35), paying particular attention to the precautions taken to reduce the
chemical attack of the matrix by the ~~Co/HO solution and to ensure a convenient diffusion of the ~~Co in the lattice.
In section 3, we report the room temperature MES measurements and we compare the
isomer shift and quadrupole splitting data to our previous MAS measurements, and aise to
the MES measurements of Seregin et ai. I?i.
In section 4, we describe the hyperfine magnetic structures observed at low temperature
on the ~~Fe probe. The main result is the evidencing of two very different local magnetic behaviours for x < xt and for x > xt, where xt is near trie centre of the x range associated with superconductivity. Trie absence of any visible coexistence of trie two behaviours near xt,
in contrast with the MAS measurements, leads us to revise our previous interpretation in terms
of spontaneous phase separation in these compounds.
In section 5, we present a general discussion of the results. In the superconducting com- pounds for x < xt, where ail trie Môssbauer probes experience a local Cu spin-glass ordering,
we conclude that trie ~~Fe~+ ares net act
as a passive probe with respect to the separation into magnetic (spin-glass) regions and non magnetic (metallic) regions. We compare our measure-
ments with those performed by other techniques, and we aise discuss the results in connection
with the model of frustrated phase separation developped by Emery and Kivelson [8]. For
x > xt, where the ~~Fe probe no longer experiences Cu spin-glass behaviour, we explain why
the observed magnetic hyperfine structure actually imphes a couphng of the probe with a localized induced Cu magnetization.
The section 6 is devoted ta the conclusions.
N°12 ~~Fe MOSSBAUER EMISSION STUDY IN La2-xSrxCu04 1831
2. Sample preparation and experimental conditions.
2.1 La2-xSr~Cu04 MATRIX PREPARATION AND X-RAY CHARACTERIZATION. Varions
samples of La2-xSr~Cu04 were prepared with x ranging from 0.13 ta 0.35, using the same process as described in reference [Si. We recall in particular that the lanthanum oxyde com-
mercial powder was systematically heated at 800 °C in flowing argon ta transform the La(OH)3 phase usually present into La203 before each sample preparation. Moreover, as a final treat-
ment, the matrix samples were quenched from l100 °C in order to avoid the formation of the La2-xSr~Cu205 impurity which occurs near 1045 °C, and they were annealed at 700 °C in air for 3 days in order ta eliminate the oxygen vacancies. The X-ray examinations showed that
each sample was single phase. The oeil parameters a and c of the K2NiF4-type structure were
consistent with those previously reported for the ~~Fe substituted samples in reference [Si and
they showed a tendency to a saturation when x reaches 0.35.
2. 2 ~~Co DOPING PROCESS. Preliminary preparation attempts showed that the solution
of ~~CoC12 in HO used for the doping, induced a chemical attack of the matrix and that the level of the attack increased with the strength of the HO solution and with the level of the
Sr content. Through a probable formation of SrC12, this attack led to a noticeable superficial
lowering of the Sr content of the La2-xSr~Cu04 Phase at the place of the solution deposit
on the matrix pellet. In order to reduce this unwanted effect, we used dilute HO solutions
(0.1 M) with high ~~Co volumic activity (50mCilcm3). Besides, when depositing the ~~Co/HO solution, we added a similar amount of NH40H alcoolic solution with the same concentration, in order to neutralize the acidity and to facilitate the wetting of the pellet. After drying,
the diffusion of the ~~Co was performed at l100 °C for 12h under weak air flow and then the
sample was quenched and annealed at 700 °C for several days, as previously done for the matrix
(parent sample). After these treatments, the Môssbauer tests revealed the persistence of local fluctuations m the Sr content and of vicinity effects between ~~Co impurities. So, in order to improve the homogeneity of the material, we reground the radioactive samples as finely as possible, we pressed the powder into pellets and we repeated the heat treatment. Finally, in order to optimize the source thickness and to avoid radioactive contamination during handling,
we again ground the pellet and incorporated the powder into a resin disk (stycast). The activity of the samples was in the range 1 ta 3 mci.
2.3 EXPERIMENTAL CONDITIONS. The Môssbauer emission spectra were recorded using
a moving single hne absorber made of potassium ferrocyanide containing o-1 mg of ~~Fe per cm~. The absorber was displaced with a triangular velocity sweep.
3. Room temperature Môssbauer emission spectra.
At room temperature, the ~~Fe Môssbauer emission spectra in the various samples essentially
appear as single quadrupole doublets whose isomer shift values are compatible with the high
spin (S
= 5/2) Fe3+ charge state (Fig. l). The importance of the quadrupole splitting (for
instance zl
= 1.70 ~ 0.02 mm/s for x
= 0.13) is linked ta the highly elongated nature of the octahedral site: a nearest neighbour point charge calculation performed at the Cu site in
La~Cu04 predicts the correct order of magnitude of the quadrupole splitting (zl m 1.8 mm /s).
Figure 2 shows the variations of the fitted values of the isomer shift e and of the quadrupole splitting zl as a function of x. These variations (fuit fine curves) follow the same trend of
quasi-hnear decay as in the previous MAS measurements in the same x range using a ratio
x 013
xfi
Q~
~É
~d
bÙ
C$
'ÎÎ
~ É
CJx
-z
v
Fig. 1. -
Fe /Cu
bsorption urves the emission curves. This shift probably esults from a lowering of
theeffective local Sr
concentration level during the ~Codoping rocess of the amples, despite
the precautions taken to minimize the attack of the matrix by
the acid solution Co/HCI
to rehomogenize the arnple after doping (see Sect. 2). We noticed ndeed that the shift
was larger in the preliminary easurements where
theserecautions were nottaken.
Dur results are definitely different from those measured by MES on ~~Fe in a2-xSr~Cu04
by Seregin et ai. [7j. The e and zl values reported by
x (for
instance zl m 1.81 mm /s for x= 0.1 and zl ci
that the ffective local Sr level x in the source samples of Seregin et ai. after ~~Co doping
uch lower than the ominal value x itially
present before doping (these uthors ctually do
net specify the conditions of their
~~Co oping
process).
Figure 3 shows the variation of
the linewidths Gi and G2 in the two mponents
doublet, uerstts x. As x is increased, the higher
energy ine (G2 > Gi ). This effect was already bserved in MAS for x > o.2
due to stributions
of the isomer shift and quadrupole splitting parameters, due to vicinity effects etween the probend Sr atoms or oxygen defects.
As a conclusion, the Fe site seems to have essentially the saine haracteristics in
MAS and MES xperiments, contrary to the supposition of Seregin et
ssumed that
a charge was present near Fe3+ in the absorption experiments
but not in the emission because the
~~Co precursor was ivalent. However it seems that
the Co is actually trivalent in a2-xSr~Cu04 il, 3j. Moreover, ~~Fe
MES easurements erformed on oriented crystallites of
La2Cu04
have evidenced similar
N°12 ~~Fe MÔSSBAUER EMISSION STUDY IN La2-xSrxCu04 1833
0 35 0
1 ~ ~~ ~"'
~ , Q
0 25 ',, E
~ ', E
o ~o
~
035 *
lD 2 0
fl fl ~ ~~
Î __
6 "'~ ~ ~~
0 10 0 20 0 30 0 40
Î~ 4 "',, ~
~
0 0 Î~
0 4 ~~~' ~
Jt c
Fig. 2
Fig. 2 ~~Fe hyperfine parameters versus ~ at 295 K in La2-xSr~Cu04. e. isomer shift relative
to iron metal A: quadrupole sphtting Experimental values and fuII hue: from Môssbauer emission
measurements Dashed Iine from absorption expenments [5].
Fig. 3. FUIT hnewidth at half maximum versus ~, m the 2-Iine Môssbauer emission spectra at 295 K.
Gi (open circles). Iower energy hne G2 (fuII circles). higher energy Iine.
orientations of the electric field gradient (EiG) tensor in the two types of experiments [9]. The noticeable tilt-angle of the local EFG axis W, which is observed with respect to the é axis, might imply the presence of a charge compensation in the neighbourhood of the Fe3+ probe m
both MAS and MES measurements.
4. Low temperature Môssbauer emission spectra.
At lop temperature, a magnetic hyperfine structure develops in the MES spectrum of the
~~Fe3+ ion (Fig. 4), showing local spin freezing, with respect to the time scale of the probe (that is the nuclear Larmor period, typically 10~~ s). For all x, these spectra cari be fitted using
a hyperfine field combined with an electric quadrupole interaction. We present on the right part offigure 5 the x variation of three parameters which are derived from these MES spectra:
first, the local spin-freezing temperature Tm, defined by the onset of the magnetic hyperfine
structure; second, the value of the quasi-saturated hyperfine field Ho, which is measured at 1.3 K; third, the average value < > of the angle
= (DZ, Ho), which defines the orientation of the hyperfine field $ with respect to the main axis W of the local EFG (the tilt-angle of
Ù with respect to the tetragonal axis é is 26~5° and it remains essentially constant when
x is varied [9j). For comparison, the left part of figure 5 shows the corresponding MAS data for Tm, Ho and < >, which were previously measured in the samples La2-xSr~Cu04 in the
same x range.
From both the MAS and MES measurements, two different local magnetic behavi'ours are
evidenced, depending on whether x is smaller or larger than a value xt which is roughly located in the middle of the superconducting x range. This limiting value xt can be defined from the
< > variation curves, as the Sr level at which a reorientation of the magnetic moment of the probe occurs when x is increased: xt is about o.19 in the MES measurements and about o.16 in the MAS measurements. We note that the shift ôx
= 0.03 between these two values is
x 013 8K
f
"
~"° '3 T" '3K
~ X ° lé T 8K
j
jf _
~ ~Î , x=0 18 T=1 3K
x=020 ~_
tÙ '
bQ
3K ~
Ç$
°$
bÙ
Ç$
£ x=0 20 T=1 3K
~ " °~ 3K
Îà
C~
çj
©
C~ c
x=0 25 T=1 3K
x 035 T 42K
x=0 35 T=1 3K
-10-8 -8 -4 -2 0 2 4 8 8 10 -10-8 -8 -4 -2 0 2 4 8 8 10
v (mm/s)
v (mm/s)
Fig 4 Low temperature ~~Fe emission spectra in La2-xSrxCu04 for different
~ values. Right part:
spectra at 1.3 K. Lejt part spectra near the spin-freezing tenlperature Tm.
comparable to the average shift ôx which is observed at room temperature m the same region of the phase diagram between the MAS and MES variation curves of e and zl uerstts x. For
the reason given in section 3, we think that the effective Sr concentration in the source samples
is a little smaller than the nominal concentration and therefore that the value xt ci 0.16 is
probably more realistic than the value 0.19. We now compare successively trie results obtained
by MES and by MAS for x < xt, for x > xt and for x near xt.
. For 0.06 < x < xt, that is within the first part of the superconducting x range in the
phase diagram, we have already observed in the MAS measurements that the three parameters Tm, Ho and < > essentially keep the same values as in the preceding region of the phase
diagram (0.02 < x < 0.06), where the compounds present semiconducting and Cu spin-glass properties [Si. The MES measurements confirm this observation and support the conclusion that a Cu spin-glass behaviour locally persists around the highly dilute Fe3+ impunty in the
superconducting samples for x < xt. The spin-freezing temperature Tm
= 8 ~ l K is actually
the sonne for the quasi-isolated Fe3+ impurity studied in MES and for the Fe3+ impurities
studied in MAS where it was found ta be independent of the concentration level (the Fe/Cu
ratio in MAS was varied between 0.002 and 0.01 for x
= 0.13). This observation reflects the fact that in this x range the magnetic behaviour of the probe is mainly governed by the properties of the Cu spin-glass environment. The value < > m 80°, which is observed in both the
MAS and MES experiments, shows that the Cu spins around the Fe probe remain essentially
oriented within the basal plane, as in the antiferromagnetic precursor La2Cu04. Finally, we
note that the quasi-saturated hyperfine field Ho is the same in the MAS and MES experiments,
as long as x is smaller than xt.
