Mössbauer study of 57Fe-doped La2-xSrxCuO4 : local magnetic properties in the semiconducting, superconducting and normal metallic regions of the phase diagram

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Mössbauer study of 57Fe-doped La2-xSrxCuO4 : local magnetic properties in the semiconducting,

superconducting and normal metallic regions of the phase diagram

P. Imbert, G. Jéhanno, P. Debray, C. Garcin, J. Hodges

To cite this version:

P. Imbert, G. Jéhanno, P. Debray, C. Garcin, J. Hodges. Mössbauer study of 57Fe-doped La2- xSrxCuO4 : local magnetic properties in the semiconducting, superconducting and normal metallic regions of the phase diagram. Journal de Physique I, EDP Sciences, 1992, 2 (7), pp.1405-1430.

�10.1051/jp1:1992219�. �jpa-00246630�

J. Phys. Ifrance 2

(1992)

1405-1430 JuLY1992, PAGE 1405

Classification Physics Abstracts

74.70V-75.25-76.80

M6ssbauer study of ~~Fe-doped La2_g~Srg~Cu04

_:

local magnetic properties in the semiconducting, superconductblg and normal

metallic regions of the phase diagram

P.

Imbert,

G.

Jdhanno,

P.

Debray,

C. Garcin and J-A-

Hodges

Service de Physique de l'Etat Condens6, Departement de Recherche _sur l'Etat Condens6, les Atomes et les Mo16cules, Centre d'Etudes de Saclay 91191 Gif-sur-Yvette Cedex, France

(Received

28 February 1992, accepted 24 March

1992)

Rdsumd. A partir de _mesures de spectrom4trie M6ssbauer _et de r4sistivit4, _{nous avons} explor4 le diagramme de phase de la s4rie de compos4s La2-xSrzCu04 dop4s _en ~~Fe _et

nous

avons 4tud14 les propr14t4s magn4tiques locales dans les phases semiconductrice, supraconduc-

trice _et m4taflique normale. Nous _{avons notamment} observ6 la coexistence de deux fractions pr6sentant _un comportement local di16rent, dans _une partie importante

(0,06

_{£ z £}

o,16)

de la _zone supraconductrice totale (0, 06 £ z j~

0,25)

dans la premibre fraction, dont le volume

relatif d6croit quand _z augmente, _un

gel

de spins intrinsbque apparait au-dessous de 8 K dans le _sous r6seau du cuivre; _nous attribuons cette fraction h des domaines semiconducteurs. Par contre, il n'apparait _pas de moment magn6tique intrinsbque du cuivre dans la seconde fraction, pr6sum4e supraconductrice, mars _{on y} observe _un ordre

magn6tique

local induit _par le fer. Dans les r6gions semiconductrices, les _moments magn6tiques de Cu restent essentiellement parallbles

au plan de base, alors _que dans la fraction supraconductrice l'aimantation locale induite _par le fer _se rapproche de l'axe _c lorsque _z augmente. Nous _avons r6alis6 des _mesures hyperfines pr6cises _sur ~~Fe _dans

un 6chantillon quasi-stoechiom6trique de La2Cu04 recuit _{sous argon} et observ6 _une d6croissance abrupte de l'aimantation £ la temp6rature de N6el. Dans

un 6chan- tilion de

La2Cu04+y

recuit dans l'air, _{nous avons} mis

en 6vidence la pr6sence d'intersticiels

d'oxyg+ne

et montr6 _que le couplage magn6tique eiectif _entre les couches 6tait fortement r6duit par rapport ^h l'6chantillon oh _{y ci} 0.

Abstract. From M6ssbauer and resistivity measurements _on

~~Fe-doped

La2-rSrzCu04,

we have explored the phase diagram of the series for 0 < _z < 0.30 and 1.5 K < T < 295 K and _we have studied the local magnetic properties in the semiconducting, superconducting and normal metalfic phases. The most outstanding result _concerns the coexistence _over part

(o.06

_{j~ z £}

o.16)

of the total

(o,06

_~

z £

0.25)

superconducting _range of two different fractions showing different local behaviours. The first fraction corresponds to intrinsic spin-freezing, below

+W 8 K, in the Cu sublattice. The relative sample volume of this fraction which is attributed to

semiconducting domains decreases _{as z} increases. In the second fraction which is presumably superconducting, there is _no evidence of intrinsic Cu moments, ^{however iron induced} local magnetic ^{order is} present. ^{In the}

semiconducting

fraction the Cu magnetic moments remain essentially parallel _to the basal plane, whfle in the

superconducting

fraction the iron-induced

local magnetization _moves towards the _c axis _{as z} increases. We have performed accurate ~~Fe hyperfine measurements in _an argon annealed quasi-stoichiometric La2Cu04 sample and _we have

observed

an abrupt variation of the magnetization _near the N6el temperature. ^In _an ^air annealed

La2Cu04+y

sample, _we have evidenced the presence of _oxygen interstitials and

we have shown that the elective interlayer

magnetic coupling

is strongly reduced with respect to the _y

= 0

sample.

1 Introduction.

The connection between

superconductivity

and

magnetism

is _one of the _most

intriguing

_as- pects ^{of the}

high

Tc

layered

Cu-oxide

superconductors.

As is well

known, adding

holes to pure

La2Cu04 by substituting Sr~+ (or Ba~+)

for

La~+ rapidly

_suppresses the

long

_range antiferrc-

magnetic ordering II,2].

However, _as shown

by microscopic probe techniques

such _as

NQR [3-7],

~SR [8-lS]

or M6ssbauer spectroscopy

[1fi-18],

_some kind of local

magnetic ordering persists

in

La2-rSrzCu04

and in

La2-xBa~Cu04

at low temperatures over a wide _range of

doping

levels

(0.02

_{j~ z j~}

0,16)

within both the

semiconducting

and

superconducting regions

of the

phase diagram.

In

addition,

neutron

scattering experiments

have evidenced unusual

spin dynamics (2D

quantum

spin-fluid state)

in

La2Cu04

above TN ^and

they

^{have also} shown the

persistence

of 2D

spin

correlations with reduced correlation

lengths

in

Sr-doped samples [19-23].

In this _{paper, we} present a

comprehensive

M6ssbauer

study

_on ^5~Fe

impurities

substituted for Cu in the series

(La2-xsrz) (Cui-c ~~fec)04

for 0 < _z < 0.30. In order to

study

the _com-

petition

between

magnetism

and

superconductivity,

_we chose

initially

this series in

preference

to the

YBa2Cu306+x

series because of the presence of _a

single

octahedral Cu site where iron substitutes

easily.

We thus avoided the

complications

induced

by

the _presence of Cu chain sites and the controversial Fe site

assignment problems

encountered in

~~Fe-doped YBa2Cu306+x samples. Using

^~~Fe as a local

probe

in the Cu

sublattice,

_we have scanned the

phase

di- agram

by varying

the temperature ^{and the Sr} content z and _we have studied the relevant

magnetic properties respectively

in the

semiconducting, superconducting

and metallic

regions.

The standard ~~Fe

doping

level _{c was}

0.005,

but in order to

appreciate

the influence of the iron concentration _on the observed

properties,

_{c was} varied between 0.002 and 0.01 for _some

particular

_z values. The M6ssbauer

study

_was

complemented by resistivity

measurements.

Partial accounts of _our results have

already

been

reported

elsewhere

[16-18].

In section

2,

we describe the

preparation

conditions and the

X-ray

characterization of the

samples

and _we

give

an account of the

resistivity

measurements.

In section 3, _we report ^the ~~Fe M6ssbauer measurements

performed

in the parent

compound La2Cu04.

First _we describe the results obtained in _a

sample

with _oxygen concentration close to

stoichiometry

and _we discuss the electronic state of the Fe

probe (Sect. 3.I); then, using

a

sample containing

_{some oxygen excess, we} examine the influence of oxygen interstitials _on both the

quadrupole coupling

and the effective

interlayer magnetic coupling (Sect. 3.2);

_we

also discuss the local symmetry at the Fe site in

La2Cu04

and in Sr substituted

compounds (Sect. 3.3).

In section

4,

_we describe the _room temperature ^M6ssbauer spectra of ~~Fe in the

La2-x

SrzCu04 series and _we comment _on the variation of the isomer shift and the

quadrupole

splitting

for

increasing

_z values.

N°7

M6SSBAUER

_{STUDY OF ~~Fe DOPED} La2-rSrzCu04 1407

In section S, _we describe the

magnetic hyperfine

structures observed at low temperatures

on the ~~Fe

probe.

The nature and the

origin

of the local

magnetic ordering

_are examined

respectively

in the

semiconducting composition

_range

(Sect. S.I),

in the

superconducting composition

_range

(Sect. S.2)

^{and in the metallic}

composition

_range

(Sect. S.3).

^One of the most remarkable features of _our results _concerns the two

phase

^{behaviour which is evidenced}

in the

superconducting compounds

for _z j~ ^0.16.

Section 6 is devoted to _a

general

discussion and _to

a

comparison

of

our results with those obtained

by

other

techniques. Finally,

the main conclusions _are summarized in section 7.

2.

Sa~nple preparation. X-ray

and

resistivity

measurements.

2,I SAMPLE PREPARATION AND X-RAY CHARACTERIZATION. -Various

samples

of

(La2-rsrz)(Cui-c ~~fec)04

_were

prepared

with _z

ranging

from _zero to

0.30, using

_a stan- dard ~~Fe concentration _c

= 0,005. Additional

samples

_were

prepared

^with _{c =} 0.002

(for

z =

0.03, 0.05, 0.08,

0,13 and

0.20)

and with _c

= 0.01

(for

_{z =} 0.13 and

0.20).

After

mixing appropriate

amounts of

La203, SrC03>

CUO and

~~Fe-enriched Fe203,

the

powder

_was

pressed

into

pellets,

about I _mm

thick,

which _were sintered in air in _a

platinum

vessel _{or a} BeO crucible

at 920 °C for 12h. This _{treatment was} followed

by

three other

cycles

of

regrinding, sifting

to

20 ~,

pressing

and

sintering

in air at l100 °C for 12h. After each

sintering,

the

samples

were

furnace cooled. At this stage, a minor _amount of _an extra

phase

_was ^detected for _z > 0,18

by X-ray

measurements. This

phase,

^which

develops mainly

at the surface of the

pellets,

corre-

sponds unambiguously

to

La2-xSrzCu2Os,

_as described

by

several authors

[24-27].

As shown

by thermogravimetric

measurements, this

impurity phase

_appears at T

= 104S °C when

slowly cooling

^{in air from} l100

°C,

whereas it does _not form when

quenched

from l100 °C. However the

quenching

has the

disadvantage

of

lowering

the oxygen content in the

samples,

which _re- duces the hole concentration and the lattice parameter _c.

Therefore,

_{as a} final treatment, the

samples

used in this

study

_were

quenched

from l100 °C and then annealed at 700 °C in air for at least 3

days

in order _to stabil12e the oxygen content

(only

_one

sample

was annealed under

argon

atmosphere:

_see Sect.

3.I).

Room temperature

X-ray

examination showed that all the

samples prepared

under these conditions _are

single phase.

The

La2Cu04-type

structure with the usual orthorhombic distortion

(space

_group

Abma)

_was observed in the

samples

with low Sr content

(z

_<

0.09)

while the

tetragonal K2NiF4-type

structure _was observed for

higher

_z

values. In table I _are

listed,

for the various

samples,

the cell parameters ^which were obtained

using

^Co K~ radiation and _a

powder

diffractometer. The variations of the cell parameters

versus z are illustrated in

figure

I. One _{can see} that the parameter c increases almost

linearly

for _z values up ^to 0.18 and that it increases _more

slowly

for

higher

_z values. These results

are

roughly

consistent with those

previously reported

for

compounds

elaborated in similar conditions

[28-31].

. Remark _on the

early

measurements

[16-18].

^{We have} to

point

out _a

difficulty

encountered at the

beginning

of this

study,

which relates to the _presence of

large

amounts of

La(OH)3

ⁱⁿ

commercial

La203 powders

and the

propensity

^of

La203

to

rapidly

form

La(OH)3

in the _open air. As

reported by Payzant

et al.

[32],

^if some

La(OH)3

is present, ^{but it is} not accounted for when

weighing

the La203 _Precursor, the

resulting compound

has _a Sr content which is

somewhat

higher

than

expected

and _moreover, the La

deficiency

facilitates the formation of the

extra

La2-rSrzCu2Os Phase.

^Our

preliminary experiments

_were affected

by

this inconvenience.

Later _{on, we}

systematically

^heated the commercial

La203 Powder

at 800 ° C for several hours in

flowing

_{argon gas} in order to transform the

La(OH)3 impurity

into

La203

before each

sample

preparation.

We also mention that _our first

samples

_were not

quenched,

but furnace cooled

Table I, Lattice parameters ⁱⁿ

(La2-xsr~)(Cuo.99s ~~feo.oos)04.

Left hand side: orthorhom- bic

compounds. Right

hand side:

tetragonal compounds.

The

experimental

_error is estimated to be +0.001

1

_{for parameters}

a and and to be +0.002

1

_for the parameter c. For _a

precise comparison

with iron

undoped samples,

_we mention that the Fe

doping

increases _a and b _very

slightly

and decreases _c

by

about twice _as much.

x 10~

bo(I)

_x 10~

0.0 S.406 S.358 13.149 11.0 3.785 13.205

1.4 S.400 S.356 13.lS4 13.0 3.781 13.216

2.0 S.397 S.356 13.lS6 14.0 3.779 13.226

3.0 S.391 S.354 13.162 lS.0 3.778 13.229

S.0 S.382 S.352 13.176 16.0 3.778

7.0 S.371 S.353 13.183 Ii-S 3.775

8.6 S.361 S.357 13.195 20.0 3.772

25.0 3.767 13.258

30.0 3.764 13.263

from l100

°C,

_so that _a noticeable _amount of extra

phase

_was present for

large

_z values. In these

early samples,

_we have observed _a

pronounced tendency

to _a saturation in the values of the

crystal

parameter c for _{z >} 0.20, as well _as in the values of the isomer shift _e and of the

quadrupole splitting

^A on the ~~Fe

probe

_[18]. This saturation does not appear in the present

measurements.

2. 2 RESISTIVITY MEASUREMENTS. Relative thermal variations of the

resistivity (normal-

ized _{at room}

temperature)

_are

given

in

figure

2 for _some

samples

ⁱⁿ the 0.S$io

~~Fe-doped

series.

A

typical semiconducting

behaviour is observed for _z

= 0.05, whereas

superconducting

tran- sitions _are observed for all the other _z values _up to 0.25. For _z

=

0.30,

the

resistivity clearly

shows _a "normal" metallic behaviour down to at least I-S K.

The critical temperature Tc

(defined

_as the temperature where the

resistivity extrapolates

to

zero)

^is

plotted

versus z in

figure

10

(open squares)

and the

corresponding

_curve defines the

superconducting region

in the

phase diagram

of the

(La2-zsr~)(Cuo.99s ~~feo.oos)04 samples.

Due to the ~~Fe

doping,

the threshold for the appearance of

superconductivity

_seems to occur at _a

slightly higher

Sr substitution level

(z

ci

0.06)

than for the iron free

sanaples (z

_m 0.05 [33] ^{and the} Tc values _are lowered.

Figure

II shows the variation ofTc

(full circles)

_versus the

~~Fe

doping

level _c, for _a

given

Sr concentration

(z

=

0.20). Extrapolation

of the

straight

line

dependence

would

give

Tc

= 0 for _{c m}

0.01S,

which _compares

favourably

with the estimation of Kimball _et al.: Tc ₌ ^{0 for} cd 0.02 in

(Lai.s5Sro.is)(Cui-cfec)04

_[34].

3. Mossbauer

study

of ~~Fe in

La2Cu04.

Before

presenting

the results obtained in the series

La2-rSr~Cu04

for various _z

values,

_we de- scribe the ~~Fe M6ssbauer measurements

performed

in the parent

compound La2Cu04, Paying

particular

attention to the influence of oxygen

stoichiometry

and to local symmetry ^considera- tions. From

perturbed angular

correlation measurements in

La2Cu04+y samples

with variable oxygen excess y,

Saylor

et al. have concluded that the _most

homogeneous

^and stoichiometric

N°7

MbSSBAUER

_{STUDY OF} ~~Fe _DOPED La2-zSrzCu04 1409

c(li

13.26

c

13.20

~~.~~

13.16

~~'~~

aalii

3.80

b~/0

tetragonal

3.75

o-o

Fig. i. - _ell

parameters

urements. In to allow ^a

direct comparison

_{with the tetragonal} unit cell, _the

cell ^rameters a and b

are divided by v5.

sample (y

_ci

0) corresponds

to the

highest

TN value

(TN

f~ 317

K) [35].

^Such a

sample

_was ob- tained

by

_vacuum

heating,

whereas

samples

cooled in air contain _{oxygen excess} and have lower TN ^values. We present ^below a

comparative

M6ssbauer

study

in both types of

compounds.

3,1 MOSSBAUER _{STUDY OF A} QUASI-STOICHIOMETRIC SAMPLE. We have

prepared

_a 0.$%

~~Fe-doped La2Cu04 sample

where the _{oxygen excess} has been removed

by annealing

under argon

atmosphere

at 700 °C for 16. This

sample,

^{where the} _c parameter ^is

13,138(2)1,

is

referred _as

quasi-stoichiometric

in the

following.

Its Ndel temperature TN _f~ 317 K is similar to the

highest

TN value observed

by Saylor

et

al.,

which shows that the 0.S$io Fe

doping

has

no influence _on TN

Figure

3 shows the M6ssbauer spectra in this

sample

at 323

K,

295 K and 4.2

K,

and fitted parameters are

given

in table II.

R(Tf R(3001

is

x₌ o.05

1o ^{x =}

o-off

x=o.13

x = o-lo o_30 0.5

o

o So loo lso loo TlKl

Fig. 2. Resistivity measurements

(normalized

at room

temperature)in (La2-zsrz)(Cuo.99s ~~feo.oos)

for diierent

~ values.

~~~~

~~~

I f l

i

_g

~

of~ ,i~

,,o

~

~,

f~

.9 ^~~~~

~ ~'

~

~~.

C'4 '

~

~ o

OJ I

~

~

4.2K

-lo -8 -fi -4 -2 o 2 4 _fi 8 lo

v

(mm/S)

Fig. 3. ~~Fe _{M6ssbauer spectra at 323} K, 295 K and 4.2 K in _a quasi- stoichiometric sample of

La2(Cuo,995 ~~feo_oos)04

(TN ci 317

K).

Fitted parameters are given in table II.

In the

paramagnetic phase,

_a ^{well defined}

quadrupole

doublet

corresponding

to _a

single

^~~Fe

site is observed

(see

^the

spectrum

at 323 K in

Fig. 3).

The absence of

magnetic

^relaxation

broadening

in this spectrum ^{shows that the}

spin

fluctuations within the 2D quantum

spin

fluid state, as observed

by

Shirane et al,

by

neutron diffusion

experiments

in

La2Cu04

above TN

[19],

are too fast

(>

^10~°

s~~)

to be detected

by

M6ssbauer spectroscopy.

At 295

K,

_a

hyperfine

sextet

corresponding

to the

antiferromagnetic phase

is observed.

N°7

MbSSBAUER

_{STUDY OF} ~~Fe _DOPED La2-~Sr~C~I04 1411

Table II. -~~Fe fitted M6ssbauer parameters ^{at 323}

K,

295 K and 4.2 K in _a

quasi-

stoichiometric

sample

of

La2(Cuo.995 ~~feo.oos)04 (see

spectra ⁱⁿ

Fig.3).

_E : isomer shift rel- ative to iron metal. A ₌

~~~~

l

+q2/3

_:

quadrupole

_energy G _: linewidth FWHM

(for

2

sextets, ^{G is}

successively given

for the three line

pairs 3-4,

2-5 and 1-6

).

H:

magnetic hyperfine

field. S _:

angle (OZ,H),

where OZ is the main axis of the electric field

gradient (EFG).

_{q :}

EFG asymmetry parameter

(fitted assuming

H in the YOZ symmetry

plane

of the

EFG).

P:

relative

weights

of the

paramagnetic

fraction

(doublet)

and of the

antiferromagnetic

fraction

(sextet ).

doublet _sextet

T _e G P _E A G II I _q P

(K) (mm/s) (mm/s)

_(%)

(mm/s) (mm/s) (Tesla) (deg) (il)

323

~~

~~'~~

^0.30

0.37

295 0.30 1.73 0.68 21 0.30 1.73 0.58 19.2 86 0 79

+0.01 +0.02 +0.01

(fixed)

0.94 +0.5 +4

(fixed)

4.2 0.43 1.84

0.27

0.29 48.7 86 0.08 100

+0.02 +0.02 0.32 +0.2 +4 +0.04

A residual fraction of the _parama netic

phase

also contributes _a

quadrupole

doublet with _a

splitting (Al

₌

~~~~ @@

= l.73 ~ 0.02

mm/s.

The

mixing

of both

phases

_{over a}

2

certain temperature _range

(see below)

shows the _presence of _some distribution in the values of TN

throughout

the

sample,

due to very

slight inhomogeneities

in the local _oxygen content. As the Fe

probe

has similar electronic

properties

and

occupies

the _same site in both

fractions,

we

used the measured value of the

quadrupole

interaction )A) at 295 K

(directly

obtained from the

paramagnetic subspectrum)

in the line fit of the

magnetic subspectrum. Thus,

_we obtained

an accurate evaluation of the

angle

S ₌

OZ,

H between the main axis OZ of the electric

field

gradient (EFG)

and the

hyperfine

field H in the

antiferromagnetic phase:

= 86° ~ 4°

(this

_error includes the

uncertainty

due _to

possibly

_{non zero} values of the EFG asymmetry parameter

q). Besides,

the fit shows that l§z is

unambiguously positive.

At 4.2

K,

where the

hyperfine

field has reached its saturated value

(Ho

_" 48.7 ~ 0.02

T),

the

hyperfine

sextet has

extremely

_narrow lines. We have used this

high

resolution spectrum

to evaluate the asymmetry parameter _{q of} ^{the EFG} at the Fe site in

La2Cu04.

When

fitting

q, ^we have assumed that the

hyperfine

^field Ho ^{lies in}

(or near)

_a symmetry

plane

^XOZ or

YOZ of the EFG

(note

that the

magnetic

moment of the Fe

probe

_very

probably

follows the direction of the Cu

magnetic

moments, which _are

parallel

to the orthorhombic axis a in the Abma _{space group}

[36,37].

^{Besides the}

^point

symmetry at the Cu site in

La2Cu04 implies

_a mirror in the

(a,c) plane).

We find: _q

= 0.08 + 0.04, ^{which shows that the EFG is} almost

axially symmetrical.

The isomer shift measured at 295 K _on the ~~Fe

probe

in this

sample

_{(E =} 0.30 ~0.01

mm/s,

relative _to

~~Fe-metal)

is in agreement ^{with the value}

reported by

Nishihara et al.

[38]. By

comparison

to a reference isomer shift data table for ~~Fe

[39],

we may

assign

the

high-spin (S

=

5/2) ^Fe~+ charge

state to the M6ssbauer

probe

in this

compound.

This

assignment

is confirmed

by

the value of the saturated

hyperfine

field Ho which is observed at low temperature.

However,

_as both ₆ and Ho are somewhat reduced

compared

to pure ionic

Fe~+ high-spin

state

values,

_{we can}

conjecture

that

covalency

effects _are present to _a certain extent. Ho is

also reduced

by

_zero

point spin fluctuations,

which _are

large

in 2D

magnetic

systems. ^These effects have

already

been invoked for

Fe~+

_{in the}

layered perovskite compound LaSrFe04 by Soubeyroux

et al.

[40].

^{It should be}

emphasized

that the

high-spin Fe~+ ion,

which is almost

magnetically isotropic,

is _a suitable

magnetic probe

for

following

the local

magnetic

orientation in the _presence of

magnetic ordering

in the

Cu02 Planes.

For

high-spin ^Fe~+

ions with _zero

orbital moment, ^{the electric field}

gradient (EFG) experienced by

the ~~Fe nucleus in _a

given

matrix is

only

due to the lattice

contribution,

whose thermal variation is

usually

small. As

a

matter of

fact,

the observed

quadrupole

_{energy on} ^~~Fe increases

by only

about 6$io between

295 K and 4.2 K in the various

compounds

of the

La2-~Sr~Cu04

series. A nearest

neighbour point charge

calculation

performed

at the Cu site in

La2Cu04 Predicts

_a

quadrupole splitting

A of about 1.8

mm/s

_on ~~Fe, a

positive

value for

l§z

and _a

negligible

asymmetry parameter _q of the EFG. Our

experimental

measurements _are in

good

agreement with all these

predictions.

Therefore,

the results _are consistent with _a substitution of the ~~Fe Mdssbauer

probe

in the Cu octahedral site. Further considerations relative to the local symmetry _are

developped

in

section 3.3.

Magnetic hyperfine

field measurements _on ~~Fe in the

quasi-

stoichiometric

sample

_{are rep-}

resented between 4.2 K and 275 K in

figure

4

(full circles). They

_are rather similar to those obtained in the _same temperature _range

by Tang

et al. in _a

sample

annealed under _argon with TN _Ci 300 K [41]. ^This

particular

variation of

H(T),

which shows _an almost linear decrease

above about 80

K,

has been

interpreted by

these authors in _terms of the thermal excitation of

spin-waves

in _a

highly anisotropic antiferromagnet, resulting

in _{a crossover} from _a 3D be-

haviour to a

quasi

2D behaviour for

increasing

temperatures

[41,42].

^Within this

model,

^the

normalized

hyperfine

field

H(T) /H(0)

_on the ~~Fe~+

probe

is assumed to follow the normalized Cu sublattice

magnetization M(T)/M(0),

_as

given by

_a theoretical

expression (Eq.(5)

of Ref.

[42])

^which contains two

adjustable

parameters

JM(0)

and

r/M(0).

J is related to the

planar exchange

_energy

Jp by

J ₌

Jp (I

+

r~/2),

where r~

measures the ratio of effective

interlayer

magnetic coupling

to

planar coupling:

_{r =} 0 refers _to 2D systems, ^while r = I refers to

isotropic

3D systems. We obtained _a

good

fit _{to our} data

(full

line _curve in

Fig. 4) using

J ₌ 1600 K

and _r

= 0.011

(assuming M(0)

=

0.5),

_as in references

[41,42].

We have

investigated

ⁱⁿ _some ^{detail the transition}

region

_near TN ^{in the}

quasi-stoichiometric sample. Figure

5 shows the thermal variations above 250 K of the

magnetic hyperfine

field H _on

~~Fe in the

antiferromagnetic phase

and of the relative

weight

P of the

paramagnetic

^fraction.

A

rapid

increase of the

paramagnetic

fraction is observed between 310 K and 320 K. At 315

K,

where this fraction exceeds

2/3

of the

sample,

the

hyperfine

field remains rather well defined in the residual

antiferromagnetic phase

and its value is still

large (about

1.2

T).

At first

sight,

this behaviour looks like _a first-order type

magnetic

^transition with _some dhtribution of the Ndel

temperature.

^The same observation _was made

by

Uemura et al. [43] ^based on the _muon

precession frequency signal

and

by Lutgemeier

and

Pieper

from

~~~La-NQR

measurements

[44].

We

emphas12e

however that _our results _are also

compatible

with

a second-order type ^transition

(with

also _some distribution of TN

values) provided

that the

magnetization

_curve presents _a

very

sharp drop

at TN This latter

interpretation

is _more

likely,

_{as an}

abrupt

variation of

M(T)

near TN is _{a common} feature in 2D

magnetic compounds:

low values of the critical exponent

fl

in the _power law

M(T)/M(0)

=

(1- T/TN)~

_are

usually

^{observed in such}

compounds (for

instance,

^the

expected

value of

fl

is

1/8

in 2D

Ising lattices).

N°7

M6SSBAUER

_{STUDY OF} ~~Fe _DOPED La2-~Sr~C~I04 1413

,

",

(0 '

',

~

'~,

g ,

w ~m

~ ',

,,

~ ~m

o ioo 200

T (Kl

Fig. 4. Thermal dependence of the hyperfine field H _on ~~Fe _in

La2(Cuo.995 ~~feo.oos)04.

Circles and full line: sample annealed under argon, with TN Cf 317 K

(q~lasi-stoichiometric sample).

Squares

and dashed line: sample cooled in air, with TN_~bS 230 K. Curves _are fitted using the theory of reference [42]

(see text).

50 ioo

10 '

I

% 30

fl

~~

f_

j

~

~

~

~ ~~

/

,

~

~~

,,o~""

,o~"'

0 ^"'

250 300

T lKl

Fig. 5. La2_(C~10.995

~~feo oos)04

quasi-stoichiometric sample (TN ci 317

K)

_: thermal variations of the magnetic hyperfine field H _on ~~Fe _{in the} antiferromagnetic fraction above 250 K

(full

circles and full

line)

and of the relative

weight

P of the

paramagnetic

fraction

(open

circles and dashed

line).

3. 2 MbSSBAUER STUDY OF THE SAMPLE ANNEALED IN AIR. In the standard

~~Fe-doped

La2Cu04+y sample,

^which has been

prepared by sintering

and

cooling

in

air,

the _average Ndel temperature

(TN

Cf 230

K)

is much lower than in the

quasi-stoichiometric sample. According

to the

dependence

of TN _{versus y} in

La2Cu04+y>

as determined

by Saylor

et al.

[35],

^the

JOURNAL DE PHYSIOUE I -T 2. N'?-JULY 1992 51

oxygen excess in this

sample

is _{y ci} 0.018.

At 295

K,

the observed

quadrupole

doublet has broadened lines and it is better fitted

using

a

superimposition

of _two different

quadrupole splittings (see Fig.

6 and Tab.

III).

The

larger splitting (Amax

₌ 1.77 ~ 0.02

mm/s)

_can be identified with the value of A observed _in the

quasi~stoichiometric sample

and thus it relates to ~~Fe

probes

substituted in the standard site.

We

assign

the additional doublet with smaller

splitting (Amin

₌ 1.45 ~ 0.02

mm/s)

to those

~~Fe

probes

which _are located in the

vicinity

of _oxygen interstitials

(and

not oxygen

vacancies,

as was first

conjectured

in Ref.

[18]).

Table III. -~~Fe _{fitted M6ssbauer} parameters at 295 K in the series

(La2-zsr~)

(Cuo.995 ~~feo.oos)04

for several _z values

(spectra

in

Fig. 6).

₆ isomer shift relative to iron metal. )A)

quadrupole splitting. Gi,G2

linewidths

(FWIfM)

of

respectively

lines I

(left

hand

line)

and 2

(right

hand

line).

P: relative

weights,

when two doublets _are fitted

(z

= 0

sample).

Standard

experimental

errors are ~0.01 mm

Is

_{on E} and ~0.02 _mm

Is

_on

)A).

z E

Gi G2

P

(mm Is) (mm Is)

~ 0.314 1.77 0.25 0.25 30

0.311 1.45 0.42 0.42 70

0.306 1.72 0.30 0.32

0.14 0.281 1.57 0.35 0.35

0.267 1.49 0.37 0.37

0.228 1.27 0.35 0.40

The temperature _range over which _a

mixing

of

paramagnetic

and

antiferromagnetic

fractions occurs, is much

larger

in this

sample

than in the

quasi~stoichiometric sample.

This behaviour is related to _a

larger degree

of oxygen

inhomogeneity.

In

addition, magnetic hyperfine

field

distributions _are observed in the

antiferromagnetic fraction,

which reveal local

heterogeneities

in the

exchange

interactions. Mean values of the

magnetic hyperfine field,

measured between 4.2 K and 200

K,

_are

represented

in

figure

4

(full squares).

We obtained _a

good

fit to these data

(dashed line)

with the model of reference [42],

assuming

^the _same value for the parameter J _as in the

quasi-stoichiometric sample (J

₌ 1600

K)

but

using

_a smaller value for the parameter r

(r

₌ 0.0033 instead of

0.011). Thus,

the effective

interlayer magnetic coupling,

which varies _as

r~,

is about 12 times smaller in the

sample

cooled in air than in the

quasi-stoichiometric sample.

An estimate of the Ndel temperature ^{in the model of reference} [42] ^{leads to:} kBTN

~w

J/In (r~

_~)

Using

this relation and the fitted values of J and _r, the ratio of TN values in the _two

samples

is evaluated to be

1.27,

in

rough

agreement with the

experimental

ratio 1.38. We mention that the 3D

long

_range

ordering

temperature TN is also

supposed

to vary as

(fla)~,

where

f

is the

spin

correlation

length

in the

Cu02 Planes

near TN and _a is the lattice parameter [45].

The

lowering

of both the effective

interlayer magnetic coupling

and the Ndel temperature in the

sample

annealed in

air,

with respect to the

quasi-stoichiometric sample, probably

reflects the reduction of the

spin

correlation

length f by

the holes induced in the

Cu02 Planes by

the oxygen interstitials. We note however that the

efficiency

of oxygen interstitials for

inducing

holes is lower than the

efficiency

of

Sr,

since _a Sr substitution level _{z ci} 0.02

practically destroys

the

long

_range

magnetic ordering (see

Section

5)

whereas _an oxygen excess of _{y ci} 0.018

only

N°7

M6SSBAUER

_{STUDY OF} ~~Fe DOPED La2-zSrzC~104 1415

x=0.00

x=005

~ o

U X*0,14

h

Qd 0D J~

~

x=0.20

x=0.30

-z -i o i z

v

lmm/sl

Fig.

6.-~~Fe

_{M6ssba~ler absorption spectra at 295K for different}

z values in

(La2-zsrz)

(C~lo.995

~~feo.oos)04 (samples

cooled in

air).

Each spectrum ^{is fitted} to _a q~ladr~lpole doublet,

except for _{z =} 0 where two slightly different doublets

are used

(Tab. III).

reduces TN ^from 317 K to 230 K.

As _a

conclusion,

the _presence of _oxygen in terstitials evidenced in the

quadrupole

data above TN

clearly

enhances the 2D behaviour in

La2Cu04.

Another evidence for this enhancement is the lower value of the saturated

hyperfine

field

Ho

in the

sample having

the lower Ndel

temperature (see Fig. 4),

^{which reflects}

larger zerc-point spin-wave

fluctuations.

3.3 LOCAL SYMMETRY AT THE ~~Fe _{SITE IN}

La2Cu04

AND Sr SUBSTITUTED COM-

POUNDS. In order to

clarify

the controversial

question

of the orientation of the EFG ten-

sor on ~~Fe _in the

La2-zSr~Cu04 series,

_we have

recently performed

detailed M6ssbauer and

X-ray experiments

_on oriented

powders

of ~~Fe and

~~Cc-doped La2Cu04

and of

~~Fe-doped Lai_8Sr0.2CU°4

_(~fij.

We have shown that the control

procedure

of the

grain

orientation in

previous experiments [18,47]

was not

appropriate

and _{our new} data contradict the

previous

results. We find that the

tilt-angle fl

₌

(d, OZ)

of the main EFG axis OZ _on the ~~Fe

probe,

relative _to the

crystal

axis _c, is 26° ~ 5° in the La2-zSr~Cu04 ^series and that the orientation

angle

_{~7 =}

(c, H)

of the

hyperfine

field H, measured _on ~~Fe in

La2Cu04,

lies in the _range 75° < _~7 < 90°.

The latter result relative to the

angle

_~7is

compatible

with the _common

assumption

that the

magnetic

moment of the

isotropic

^~~Fe

probe

in

La2Cu04

follows the

magnetization

direction of the Cu

sublattice,

which lies

parallel

to the

crystal

axis d

(using

the Abma _space

group).

Then,

since OZ is

roughly perpendicular

to H in this

compound _(S

₌

(OZ, H)

₌ 86°

~4°),

it is

probable

that OZ is

tilted,

with respect to the axis d, in

(or near)

the

(b, c) plane.

We have _no definite _answer

concerning

the

origin

of the

appreciable tilt~angle fl

of the EFG axh OZ which

we have observed _at the standard Fe site in the series. This

angle

is neither related to the Sr

doping (since

it is

independent

of this

doping level)

_nor to Fe-Fe

proximity

effects

(since

it is observed in the

5~Co

labelled

samples)

_nor to the _presence of oxygen interstitial

neighbour (since

it is observed in interstitial free

samples).

We have

suggested

that this symmetry

breaking might

be related in _some way ^to the

charge

unbalance introduced

by

the

Fe~+ charge

state

and

/or

to _a local

displacement

of the

apical

_oxygen

neighbours

in the lattice. We note however

that, according

to recent Cu

NQR-NMR experiments by Song

et al.

[48],

the EFG at the Cu sites is

essentially

uniaxial

along

the

crystal

t axis in the

sample

Lai

84Sro.16Cu04.

4.

Pararnagnetic

M6ssbauer spectra of 5~Fe in the

La2-zSr~Cu04

series.

A

large quadrupole splitting

is observed _{at room} temperature ⁱⁿ the 0.5%

~~Fe-doped La2-~Sr~Cu04 series,

^{whatever the Sr} concentration _z

(Fig.

6 and Tab.

3). Figure

7 shows the variations of three fitted parameters _{as a} function of _{z :} the _isomer shift _E

(relative

to ~~Fe

metal),

the

quadrupole splitting

A and the linewidth G

(FWHM).

For _{z <}

0.05,

the lines _are broadened in these air cooled

samples

and the maximum linewidth G is observed for _z

= 0

(Fig. 7).

The spectra ^for z = 0, 0.02 and 0.05 _were fitted

using quadrupole splitting

A distributions. Fitted

histograms

of these distributions _are shown in

figure

8.

They actually

confirm the _presence of the two

quadrupole energies

Amax and

Amm,

which have been

respectively assigned

in section 3.2 _to standard Fe sites and to Fe sites located

near oxygen interstitials.

Besides,

the

histograms

show that

increasing

Sr

dopings

act so as to

progressively

_remove ^the lower

quadrupole

_energy

Amin,

which

disappears

almost

completely

at _{z =} 0.05. The linewidth G thus reaches _a minimum

value,

which is close to the linewidth measured in the

quasi-stoichiometric La2Cu04 sample (represented by

a square in

Fig. 7).

Therefore,

_we conclude that substitution of

Sr~+

ions for

La~+

_{decreases the number}

ofo~~

interstitials in these

samples

which have been air

cooled,

and that this number tends to _zero for _{z =} 0.05.

The

progressive disappearence

of the lower energy component Am,n between _z

= 0 and

z = 0.05 leads to _an apparent increase of the _average

quadrupole

_energy in the spectra

[18].

As _a matter of

fact,

the

quadrupole splitting

A in the standard

site,

which is

represented

in

figure 7,

^does not vary in this

composition

_range. Both A and the isomer shift _{E are}

actually independent

of _z in the

semiconducting region (z

<

0.06)

and both decrease

quasi-linearly

in the metallic

region

from _z

= 0.08 _up to _{z =} 0.30. The dominant

part

^{of the lattice EFG} at the Fe site is induced

by

the

negative charges

carried

by

_oxygen

neighbours

in the basal

plane.

Therefore,

_an

increasing

delocalized hole

density

in these

planes

is

expected

to decrease the

quadrupole

interaction A _on the 5~Fe~+

probe.

In

addition,

_an

increasing

hole

density

in 3d orbitals is

expected

to decrease the isomer shift _{E on} 5~Fe

[39]. Thus,

the

steady

decrease of both A and _E in the metallic

region

_{as z} increases

qualitatively

reflects the

regular

increase of the number of delocalized holes _{versus z} in the Cu02

Planes.

One observes in

figure

7 that the linewidth G shows _a moderate and

regular

increase between

z = 0.05

(G

ci 0.32

mm/s)

and _z

= 0.30

(G

_ci 0.38

mm/s).

For _z >

0.15,

the

quality

of the fit is

clearly improved

^when

using

_a

superimposition

of two

slightly

different and unresolved

N°7

M6SSBAUER

_{STUDY OF} 5~Fe _DOPED La2-~Sr~C~I04 1417

w

~ l-1

~f

)

o-1

0.3

0 0.10 0.20 0.30

Fig. 7.-5~Fe

_{fitted M6ssba~ler parameters}

versus z at 295K in the series

(La2-zsr~)

(C~lo.995

~~feo.005)04 (samples

cooled in

air).

_{e :} isomer shift relative to iron metal. A _: quadrupoie splitting

(for

_z < o.05, only the value Amax relative to the normal site is considered, _see

text).

G

:

full line width at half maximum

(the

_square at _{z =} o is the value in _a q~lasi-stoichiometric sample obtained by annealing in

argon).

quadrupole splittings.

The relative fraction of the

larger splitting

increases with the Sr content and _can be

assigned

to the Fe sites which have at least _one next nearest Sr

neighbour.

Thin

interpretation

is

supported by

^Cu

NMR-NQR

measurements in

La2-~Sr~Cu04 [48-52]

which also detect _two

slightly

different

quadrupole energies

in the _same

phase,

with _an

increasing

fraction of the

larger

_energy for

increasing

_z values.

5.

Magnetic hyperfine

structures.

At the lowest temperature

(T

₌ 1.5

K),

all the

samples

present _a saturated

5~Fe magnetic hyperfine

structure, due to

magnetic ordering

_or local

spin freezing.

We define the

spin

freez-

ing (or magnetic ordering)

temperature

Tm,

^with respect to _our observation

technique,

_as the

temperature

^{at which}

inhomogeneous

^line

broadenings

_appear in the

quadrupole doublet,

due

to the onset of

magnetic

interactions. Some spectra at 1.5 K _are shown in

figure

9 for five

different Sr levels _z.

Except

for

compositions

_{near z}

=

0.15,

the spectra at this temperature

xz0

~min h~~x

5i

I

~

b~

#

x=0.02

a

x=0.05

1-o 1.5 2,o

A lmm/sl

Fig. 8. Smoothed histograms of q~ladr~lpole splitting A distributions in the 5~Fe _{M6ssba~ler spectra}

at 295K in

(La2-zsr~)(C~lo.995 ~~feo.oos)04

for _z < o.05

(samples

cooled in

air).

The right ^hand peak

(Amax)

relates to normal Fe sites, while the left hand peak

(Amin)

relates to Fe _atoms having

oxygen interstitial neighbo~lrs.

are

essentially

made _up of _a

single

visible

hyperfine

sextet due to combined

magnetic

and

quadrupolar

interactions and

they

_were fitted to _a

unique

set of

hyperfine

parameters. ^One observes _on the

figure

that the

magnetc-quadrupolar

sextet is

quite

different at low Sr substi- tution levels

(z

₌

0.05)

and at

high

Sr substitution levels

(z

₌ 0.20 and 0.30

).

^For z = 0.14 and

0.16,

the

spectra

ⁱⁿ

figure

9 _are

obviously composite

and

they

_are

satisfactorily

fitted _us-

ing

_a

superimposition

of two different

hyperfine

sextets

(Tab. IV).

In _a sextet, ^{the main fitted} parameters _are the

(saturated) hyperfine

field

Ho

and the

angle

S ₌

(OZ, Ho

between

Ho

and the

principal

axis OZ of the EFG. In

addition,

three difLerent linewidths G3,4>

G2,5, Gi,6

_are

used

respectively

for the

internal,

intermediate and external

pairs

of Zeeman lines in order to account for _a certain distribution of

hyperfine

parameters. ^{As it is} not

possible

to fit

together

the two

quantities

9 and A ₌

~~)~ ^fi@,

we used for

(Al

^{the value} of the

quadrupole splitting

_as measured in the

paramagnetic region just

above

Tm.

Our fits show that l&z is

unambiguously positive

in the whole

composition

_range

(0

< _z <

0.30),

but

generally they

_are

not very sensitive _to moderate values of the asymmetry parameter _q of the EFG. We recall that _we have found _{q =} 0.08 ~ 0.04 in the

quasi-stoichiometric La2Cu04 sample (Sect. 3.I).

As _a matter of

fact,

_we have assumed that _{q was}

negligibly

small when

fitting

the

hyperfine

parameters

given

in table IV and in

figure

10.

In order to discuss the

origin

^{and the} nature of the local

magnetic ordering

^observed in the various parts ^{of the}

phase diagram,

_{we present} in

figure

10 the _z

dependence

for _a set of

parameters

^which we have measured in _the series

(La2-zsr~)(Cuo.995Feo.oos)04 first,

the

magnetic ordering

temperature Tm ^{and the}

superconducting

critical temperature

Tc; secondly,