NEUTRON DIFFRACTION STUDY OF THE MAGNETIC ORDERING IN GdBa2Cu3O7-δ

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NEUTRON DIFFRACTION STUDY OF THE

MAGNETIC ORDERING IN GdBa2Cu3O7-δ

T. Chattopadhyay, H. Maletta, W. Wirges, K. Fischer, P. Brown

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, decembre 1988

NEUTRON DIFFRACTION STUDY OF THE MAGNETIC ORDERING IN

GdBa2

C U ~ O ~ - ~

T. Chattopadhyay (I)', H. Maletta (I), W. Wirges ( I ) , K. Fischer (') and P. J. Brown (2)

(') Institut fur Festk6rperforschung der KFA Julich, 5170 Julich, F.R.G.

(2) Institut Laue-Langevin, BP 156X, 38042 Grenoble Cedex, France

Abstract. - Neutron diffraction investigations on a single crystal of GdBa2Cu307-6 (6 = 0.5) show that the Gd moments order at TN = 2.2 K with a wave vector (1/2, 1/2, 0). The Gd moments are antiferromagnetically coupled to their nearest neighbours in the a

-

b plane but are ferromagnetically coupled along the c-axis in contrast to the case of GdBa2Cu307 where the coupling along this direction is antiferromagnetic.

Following the recent discovery of high-temperature superconductivity in YBa2Cu307-6 (Tc = 90 K) [I] it has been recognized that superconductivity appears to be remarkably insensitive to the full substitution of

Y ions by several magnetic rare earth ions [2]. Mag- netic properties of these compounds show that a t high temperatures the rare earth ions exhibit the full magnetic moments expected for trivalent ions and order a t low temperatures [3]. Neutron powder diffraction investigations have established [4] that Gd moments in GdBa2Cu307 (Tc = 90 K) order a t TN = 2.2 K to a structure with the wave vector (112, 112, 112). The Gd moments are antiferromagnetically coupled both in the a

-

b plane and along the c-axis. To examine whether the oxygen deficiency 8 which destroys superconductivity in going from 6 = 0 to 6 = 1 in this class of compounds [5] also plays any significant role in the magnetic ordering, we have undertaken a single crystal neutron diffraction investigation of GdBa&u306.5.

Single crystals of GdBazCu3O.i.-6 were grown by the flux method from a flux composition of 0.70Cu0 f

0.26

BaO

+

0.04Gd01.5. Magnetization measurements on a single crystal of dimensions 2.5 x 2.5 x 0.3mm3

as a function of temperature with an applied magnetic field of 50 G parallel t o [OOl] direction showed that the crystal has a superconducting transition temperature Tc = 40 K. From this the value of the oxygen occupancy has been estimated [5] to be 6.5. The same plate-shaped single crystal was mounted on the neutron diffractometer Dl5 a t the Institut Laue-Langevin in Grenoble. The (001) plane of the crystal (the plane of the plate) was set vertical with the [I101 direction parallel to the w-axis of the digractometer in a helium cryostat. To deduce the effect of strong ab- sorption by the Gd atoms the shortest available wave- length X = 0.85

A

was used. The lattice constants were determined from five centered reflections t o be

No magnetic intensity was found in the (112 112 1/2.) Bragg peak a t 1.5 K. This shows that the magnetic structure of G d B a ~ C u ~ 0 6 . ~ is different from that of GdBa2Cu307. We observed a Bragg peak at (112, 1/2,0) a t 1.5 K whose intensity drops t o zero at TN = 2.2 K confirming its magnetic origin. The ordering temperature is the same as that determined for GdBa2Cu307.

Intensities of lo'magnetic reflections together with a few nuclear reflections were measured a t 1,5 K. The magnetic structure derived from the wave vector k = (1/2,1/2,O)and as well as from the intensities of the magnetic reflection is shown in figure 1. In the same figure we also display the magnetic structure of GdBa&uaO.r determined by McK Paul et al. [4]. The

a = 3.874(3)

A,

c = 11.66(1)

A

at about 80 K. Ex-

Fig. 1.

-

Schematic representation of the magnetic struc- perimental resolution and possible twinning effect did _ture_of_(a)_{superconducting}_GdBazCu306,s_(Tc_{= 40}_K) not permit us t o determine any small orthorhombic _{and (b) superconducting GdBa2Cu307 (Tc}= 90 K). Only distortion if it exists. the Gd atoms and their moment directions are shown.

'Present address: Centre d'Etudes NuclCaires, DRF/SPh-MDN, 85X 38041 Grenoble Cedex, France.

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JOURNAL DE PHYSIQUE

magnetic moments in both structures are oriented parallel t o the c-axis and coupled antiferomagnetically in the a

-

b plane. However, the magnetic coupling between the Gd ions along the c-axis is antiferromagnetic in GdBazCu307 whereas in G d B a 2 C ~ ~ 0 6 . ~ it is ferro- magnetic. More details of the experimental results are given in reference [6].

Summarizing, the magnetic structure of both compounds GdBa2Cu307-6 with 6 = 0 and 6 = 0.5 is determined by the strong antiferromagnetic exchange interaction which exists between nearest Gd neighbours, so that the nearest neighbour Gd spins are always oriented antiparallel within the a

-

b planes. Any three- dimensional magnetic order must be established by the coupling between Gd moments along the c-axis of the orthorhombic cell where the Gd-Gd distance is about three times larger (about 12

A)

than the in-plane sepa- ration. Hence one would expect two-dimensional mti- ferromagnetism in these compounds. Indeed, the temperature dependence of the sublattice magnetization below TN resembles more a curve for a two-dimensional

magnetic system in both samples (4, 61. As described above, however, neutron diffraction unambigously pro- vides evidence for three-dimensional magnetic ordering in GdBazCu30.1-&. In GdBa2CuaO~ the coupling along c is evidently such as t o make the Gd moments antiparallel t o each other, whereas in G d B a z C ~ ~ 0 , j . ~ it is parallel. The three-dimensional (NBel) transition temperature TN remarkably is nearly the same in both

compounds. These observations raise the question of the mechanism of the magnetic interaction between the Gd ions in GdBazCu3O.r-a along the c-axis - a magnetic exchange path across the copper-oxygen layers which are responsible for superconductivity.

Mossbauer (isomer shift) investigations [7] show that the electronic charge density a t the Gd nucleus is neg- ligibly small. This suggests that the Ruderman-Kittel- Kasuya-Yosida (RKKY) interaction is rather weak in this compound. Several authors [8] have pointed out the importance of dipolar interactions in these compounds which have low values for Nkel temperature. However, dipolar interaction cannot by itself explain the coupling between RE ions separated by about 12

A

along the c-axis. Energy calculations [4] assuming dipolar interaction alone lead to a Nee1 temperature of only about 0.6 K for GdBa2Cu307 which is much less than the actual ordering temperature of 2.2 K. More- over dipolar interaction cannot explain the dependence of the sign of the magnetic exchange along the c-axis on the oxygen occupancy which is found in the present experimeni. This suggests that a multi-step superex- change along c via several oxygen atoms may be in-

volved in the three-dimensional magnetic ordering in these compounds. The observation of different stack- ing arrangements of the Gd spins between the planes in both samples, but with nearly unchanged TN values is not yet understood in detail. In this respect, a

somewhat similar behaviour was found in RbMnFc by Birgeneau et al. [9]

We have not observed any evidence for the ordering of the copper moments in the present investigations. We are unable t o associate the differences between the observed and calculated magnetic structure fac- tors with magnetic scattering from the copper atoms. The intensities of 1/2 1/2 0 and 1/2 1/2 1 reflections drop to zero at 2.2 K showing that any copper moments contributing t o these reflections order a t or below this temperature. However, it is still possible that the copper moments order with a different wave vector although we were unable to detect intensity in the 112 0 0 and 1/2 112 112 reflections a t 1.5

K.

However, the absence of the magnetic ordering of the copper

moments in the present single crystal G ~ B ~ ~ C U ~ O ~ . ~

(T, = 40 K) is not surprising; Although magnetic ordering of the copper moments has been observed in the non-superconducting YBa~Cu306+, [lo], so far no evidence for magnetic ordering in the superconducting YBazCu3O.r-6 exists.

[I] Wu, M. K., Ashburn, J. R., Torng, C. J., Hor, P. H., Meng, R. L., Gao, L., Huang, E. J., Wang, Y. Q. and Chu, C. W., Phys. Rev. Lett. 58

(1987) 908.

[2] Hor, P. H., Meng, R. L., Wang, Y. Q., Gao, L., Huang, Z. J., Bechtold, J., Forster, K., Chu; C. W., Phys. Rev. Lett. 58 (1987) 1891. [3] Brown, S. E., Thompson, 3. D., Willis, J. O.,

Aikin, R. M., Zirngiel, E., Smith, J. L., Fisk, Z. and Schwarz, R. B., Phys. Rev. B 36 (1987) 2298. [4] McK Paul, D., Mook, H. A., Hewat, A. W., Sales, B. C., Boatner, L A., Thompson, J. R. and Mostoller, M., Phys. Rev. B 37 (1988) 2341.

[5] Taakagi, H., Uchida, S., Iwabuchi, H., Eisaki, H., Kishio, K., Kitazawa, K., Fueki, K. and Tanaka, S., Physica B 148 (1987) 349.

[6] Chattopadhyay, T.,,Maletta, H., Wirges, W., Fis- cher, K. and Brown, P. J., Phys. Rev.

B

38 (1988) 838.

[7] Meyer, C., Borneman, H. J., Schmidt, H., Ahrenst, R., Ewert, D., Renker, B. and Czjzek, G., J. Phys. F 17 (1987) L345.

[8] Lynn, J. W., Li, W. H., Li, Q., Ku, H. C., Yang, H. D. and Shelton, R. N., Phys. Rev. B 36 (1987) 2374.

191 Birgeneau, R. J., Guggenheim, H. J. and Shirane,

G., Phys. Rev. B 1 (1970) 2211.