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Low field magnetization measurements on single crystals of superconducting YBa2Cu3O7-δ

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HAL Id: jpa-00210593

https://hal.archives-ouvertes.fr/jpa-00210593

Submitted on 1 Jan 1987

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Low field magnetization measurements on single crystals of superconducting YBa2Cu3O7-δ

J. Hammann, M. Ocio, A. Bertinotti, D. Luzet, E. Vincent, A. Revcolevschi, J. Jegoudez

To cite this version:

J. Hammann, M. Ocio, A. Bertinotti, D. Luzet, E. Vincent, et al.. Low field magnetization mea- surements on single crystals of superconducting YBa2Cu3O7-δ. Journal de Physique, 1987, 48 (10), pp.1593-1597. �10.1051/jphys:0198700480100159300�. �jpa-00210593�

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1593

Low field magnetization measurements on single crystals of superconducting YBa2Cu3O7-03B4 J. Hammann, M. Ocio, A. Bertinotti, D. Luzet, E. Vincent, A. Revcolevschi+ and J. Jegoudez+

Service de Physique du Solide et de Résonance Magnétique, CEN-Saclay, 91191 Gif-sur-Yvette Cedex,

France

+Laboratoire de Chimie des Solides, Université Paris-Sud, Orsay, France

(Reçu le 9 juillet 1987, accepti le Q0 juillet 1987)

Résumé.-Des mesures d’aimantation par magnétométrie SQUID à champ faible ont été effectuées sur des monocris- taux d’YBa2Cu3O7-03B4 supraconducteurs en dessous de Tc = 95K. Elles révèlent un comportement anisotrope de

l’aimantation. L’effet Meissner (toujours inférieur à 10% ) est 2 à 3 fois plus important suivant la direction c que dans le plan (a,b). Une transition unique à 95K est observée lorsque le champ est parallèle à c. Perpendiculairement à c,

une singularité probablement liée au passage d’une seconde ligne de transition apparait sur la courbe d’aimantation à 84K pour 3 G. Dans les trois cristaux étudiés, on a détecté une trés faible polarisation ferromagnétique dirigée approximativement suivant c.

Abstract.-Low field (H ~ 3G) magnetization measurements have been performed on small single crystals of super-

conducting YBa2Cu3O7-03B4 (Tc ~ 95K) using a SQUID magnetometer. They revealed anisotropic properties in the temperature dependence of the magnetization. The Meissner effect (always smaller than 10 % ) is 2 to 3 times larger along the c direction than along one of the perpendicular directions. A sharp unique transition at 95K is observed with the field parallel to c. In the perpendicular direction a second transition line seems to be crossed at T = 84K with

an applied field of 3G. In all three crystals studied, a very small ferromagnetic polarization, pointing approximately along the c direction, is observed.

J. Physique 48 (1987) 1593-1597 OCTOBRE 1987,

Classification .

Physics Abstracts

74.70 - 74.30C

LE JOURNAL DE PHYSIQUE

Very few experiments have been published to

date on single crystals of the extensively studied high Tc superconductors. In order to check the mecha-

nisms leading to the high temperature transitions,

such experiments are indeed very important, but due

to the small sizes of the available crystals, the mea-

surements are quite difficult.

Large anisotropies have already been pointed out respectively in the coherence lengths for TmBa2Cu3 07-6 [1] and in the higher critical field Hc2 for YBa2 Cu30x [2]. These systems are characterized by a cris- tallographic arrangement favouring a two dimensional behaviour. They display an orthorhombic structure

with nearly equal values of the lattice parameters a and b, and a much larger value for c [3 - 5]. The

observed anisotropies appear between directions par- allel and perpendicular to the cristallographic c direc-

tion. The published results stem from measurements in large applied magnetic fields. We present in this

paper a study of the low field magnetic properties of single crystals of YBa2Cu307-6 using SQUID mag-

netometry.

The measurements were done in fields always

smaller than 3G, applied either parallel or perpendic-

ular to the c direction. Two experimental procedures

were used. First the sample was cooled down into its

superconducting state in zero field, the field was then

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198700480100159300

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1594

applied and the magnetization recorded as a function

of increasing temperatures (ZFC curves). Second, the

field was applied at a temperature larger than the

transition temperature Tc and again the magnetiza-

tionwas measured at constant field as a function of temperature (FC curves). These procedures yield re- spectively the shielding and the Meissner effects.

Three different single crystals have been inves-

tigated. They were extracted from sintered single phase preparations of YBa2Cu307-6 characterized

by a Tc value of 95K. Two of the crystals (# 1 and

# 3) came from the same preparation. All of them

were checked by X-ray diffraction. They present the orthorhombic structure expected for the supercon-

ducting phase [3] with lattice parameters of a = 3.833 A, b = 3.88Q A,c =11.67oA. The diffraction pat-

terns show that they consist of several crystals twin-

ned along the (a, b) plane with a well defined common c axis. Crystals # 1 and # 3 are platelets with char-

acteristic sizes of respectively, 0.2 x 0.2 x 0.1 mm and

0.3 x 0.35 x 0.13 mm. Crystal # 2 has the shape of

a truncated pyramid of size 0.18 x 0.18 x 0.16. This nearly cubic shaped crystal is in fact not strictly sin-

gle, a principal twinned crystal being associated with

some microcrystals showing a distribution of c direc- tions within an angle of 6°. This presents no draw- back for the present experiments since the crystals

have only been visually oriented with an accuracy of that order.

Figure 1 and Figure 2 show the temperature de- pendences of the ZFC and FC magnetizations in the

case of the largest crystal (# 3) with an applied field

of 2.85G respectively parallel and perpendicular to

the c axis. The reported values are corrected for the contribution of the sample holder which has been measured in a separate run. The magnetic moment

of the sample is determined at each temperature by extracting the sample from the pick-up coils and mea- suring the flux variation. The change of position is

small enough so that the field applied to the sam- ple does not change by more than 0.3 % . The scale

in the figures corresponds to arbitrary units. The

experiment has only,been roughly calibrated by mea- suring the shielding effect of a lead droplet (diameter

~ 0.12 mm) below its superconducting transition and

assuming that it corresponds to the maximum shield-

ing x = - 4 (emu).

Along the c direction (Fig.1), a unique sharp

transition is seen to occur at 95K. The shielding ef-

fect is much larger than the Meissner effect. Taking

into account the demagnetizing factor, the shielding

amounts to about 44 % whereas the Meissner effect is only 3.3 % . In crystal #1 the shielding rate is the

same but the Meissner effect is more than twice as

large, being equal to 7.6% . This last value is about

Fig.l.- Zero field cooled (open circles) and field cooled (crosses) magnetization versus temperature for crystal #3 in an applied

field of 2.85 G parallel to c.

Fig.2.- Zero field cooled (open circles) and field cooled (crosses) magnetization versus temperature for crystal #3 in an applied

field of 2.85 G perpendicular to c.

the average value measured on the sintered material from the same preparations [6].

Perpendicular to the c direction (Fig.2), the

sharp decrease appearing at 95K is followed at 84K by another singularity. The occurence and the lo-

cation of the singularity has been checked in a sec-

ond run and the behaviour was quite reproducible. It

seems that for the applied field of 2.85 G two transi-

tion lines are crossed as the temperature varies. The overall shielding rate amounts in this case to 37%,

the demagnetizing factor being taken into account.

This value is approximately the same as along c. The

Meissner effect is here only 1.7 % which is almost half

the value measured in the parallel case.

The results corresponding to the different sam-

ples studied are gathered in table I. The anisotropy of

the Meissner effect could only be checked in crystals

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Table IJ Main results observed on the difi’erent crystals, parallel and perpendicular to the cristallographic c direction,

in an applied field of !.85 G. (crystals # 1 and # 3 are from the same preparation).

#1 and #3. In crystal #2 the Meissner effect was too small to be measured. The existence of a singular point at 84 K in the direction perpendicular to c is apparent in all three crystals.

Figure 3 shows the detailed FC magnetization

curves for crystals 3 with the field of 2.85 G applied parallel and perpendicular to c. The singularity at

84 K for the case of the field perpendicular to c is

also well marked on the Meissner effect.

Fig.3.- Meissner effect of crystal#3 in an applied field of 2.85

G applied parallel to c (open circles) and perpendicular to c (crosses).

In figure 3, it appears very clearly that the dia-

magnetic susceptibility showing up in the supercon-

ducting state is superimposed on a positive magneti-

zation which is temperature independent above 95 K

at least up to 120 K. A very similar behaviour has been reported in Lal.8Bao.2CU04 [7] and assigned to

a paramagnetic susceptibility of copper ions in a non

superconducting phase. In our case however, a com- plementary experiment has been performed, in which

the field of 2.85 G was reversed. In this procedure,

the variation of magnetization below 95 K was re-

versed, but the base line remained positive though it

was smaller than in the direct experiment. The result

means that the positive magnetization corresponds to

a ferromagnetic polarization which slightly decreases

as the field is reversed but cannot be reversed by the

small fields applied in this work.

In crystal #3, the value of the positive magneti-

zation is almost twice as large with the field parallel

to c than with the field perpendicular to c. A mea-

surement in zero’field showed that the ferromagnetic polarization was oriented along a direction about 30°

from the c axis. The same type of measurement made

on crystal # 1 led to an equivalent result. But in the

case of crystal # 2 the polarization was found to be along c. These differences may be due to the different values of the demagnetizing factors. In crystals 1 and # 3 the demagnetizing factor along c (perpen-

dicular to the platelets) is large and the c direction is thus not a favourable direction for the polarization.

In crystal # 2 the demagnetizing factor is about the

same along all cristallographic directions. The results in zero field on crystal # 2 are plotted in figure 4. It

appears that the measured value along c remains con-

stant up to room temperature.

The absolute value of the detected polarization

is very small. The total magnetic moment of each sample is given in table I. For crystal # 1 which yields

the largest value, the polarization does not exceed 0-00114B per copper ion. It is worthy to note that the

resultant magnetic moment decreases as the volume

of the sample increases.

One possible explanation for the observed be- haviour is that a spontaneous magnetic moment de- velops at high temperatures (T > 300K). But due to

the demagnetizing field the compact sintered material is divided in domains with alternate magnetization, canceling the overall moment. The small crystals ex-

tracted from the bulk might be formed of a finite number of such quenched domains. This would lead to an uncompensation of the magnetic moment which

would be the largest in the smaller sample and could

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1596

Fig.4.- Temperature dependence (up to room temperature) of

the ferromagnetic polarization of crystal #2, in zero field, mea-

sured along c (open circles) and perpendicular to c (crosses).

explain the very low value of the measured polariza-

tion. The suggested interpretation can (and should)

of course be easily checked in a classical magnetiza-

tion experiment.

The origin of the spontaneous moment remains unclear. No magnetic ordering has been found in the superconducting phase. Indeed results of NMR

measurements did not show any such ordering [8]

in agreement with preliminary Mossbauer measure-

ments on iron doped (0.8%) samples which did not give any evidence for a magnetic polarization of the

Mossbauer probe [9]. It is difficult to assign the spon- taneous moment observed in our experiment to a par- asitic phase outside the crystals since the moment is strongly pinned to the cristallographic directions of the single crystals and no extra phase could be de-

tected by X-ray diffraction. It is however not impos-

sible that structural defects in the crystals yield lo-

calised moments on the copper which might induce

the observed effect. Further experiments are cur- rently performed in order to clear up the situation.

To conclude on the anisotropy of the supercon-

ducting properties, the main result from the compar- ative measurements along and perpendicular to the c

axis concerns the difference in the amounts of Meiss-

ner effect detected and the existence, perpendicular

to c, of a second well defined singular temperature.

Whereas parallel to c the field rejection is very sharp

at the transition temperature, perpendicular to c the

field rejection occurs in two distinct steps. The be- haviour is best seen on the shielding effect perpendic-

ular to c. As the temperature increases a first sudden field penetration occurs around 84 K (H=2.85 G).

The penetration is then about 50% of the total effect.

A final sharp penetration occurs slightly below 95 K.

A behaviour of that kind has already been observed in

sintered material where all cristallographic directions

are represented (see for instance refs. [10, 11).It was

usually assigned to imperfections of the compound.

It appears however from our experiments that it is

an intrinsic behaviour which must be related to the two dimensional character of the material. The extra

singularity is possibly much more sensitive to stoi-

chiometric imperfections and to the applied field than

the first transition temperature. Measurements with different values of the field are of course needed in order to determine the shape of the transition lines crossed.

We wish to thank G. Sarma and J.P. Carton for many fruitful discussions and R. G6rard-Deneuville for his technical skill.

References

[1] NOEL, H., GOUGEON, P., PADIOU, J., LEVET, J.C., POTEL, P., LABORDE, O., MONCEAU, P., Preprint (Mai 1987).

[2] YASUHIRO Iye, TSUYOSHI Tamegai, HIROYUKI Takeya, HUMIHIKO Takei, Subm. to Jpn. Appl.

Phys. Lett. (1987).

[3] CAVA, R.J., BATTLOG, B., VAN DOVER, R.B., MURPHY, D.W., SUNSHINE, S., SIEGRIST, T., REMECKA, J.P., RIETMAN, E.A., ZAHURAK, S., ESPINOSA, G.P., Phys.Rev.Lett. 58 1676 (1987).

[4] DAVID, W.I.F., HARRISON, W.T.A., GUNN, J.M.F., MOZE, O., SOPER, A.K., DAY, P., JOR-

GENSEN, J.D., HINKS, D.G., BENO, M.A., SO- DERHOLM, L., CAPONE, D.W., SCHULLER, I.K., SEGRE, C.U., ZHANG, K., GRACE, J.D., Nature, 327,310, Mai 1987.

[5] CAPPONI, J.J., CHAILLOUT, C., HEWAT, A.W., LEJAY, P., MAREZIO, M., NGUYEN, N., RA- VEAU, B., SOUBEYROUX, J.L., THOLENCE, J.L., TOURNIER, R., Europhys.Lett. 3 1301 (1987).

[6] MONOD, P., RIBAULT, M., D’YVOIRE, F., JE- GOUDEZ, J., COLLIN, G., REVCOLEVSCHI, A.,

J. Phys. to be published (August 1987).

[7] MOODENBAUGH, A.R., SUENAGA, M., ASANO, T., SHELTON, R.N., KU, H.C., MCCALLUM, R.W., KLAVINS, P., Phys. Rev. Lett. 58 1885 (1987).

[8] LUTGEMEIER, H., PIEPER, M.W., Submitted to

Solid State Comm.

[9] HODGES, J., JEHANNO, G., IMBERT, P., Private

Communication.

[10] GALLAGHER, W.J., SANDSTROM, R.L., DINGER,

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T.R., SHAW, T.M., CHANCE, D.A., Submitted to

Solid State Commun. (1987).

[11] GRANT, P.M., BEYERS, R.B., ENGLER, E.M.,

LIM, G., PARKIN, S.S.P., RAMIREZ, M.L., LEE, V.Y., NAZZAL, A., VAZQUEZ, J.E., SAVOY, R.J., Preprint (March 1987).

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