MAGNETISM IN SOME Y-Ba-Cu OXIDES

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MAGNETISM IN SOME Y-Ba-Cu OXIDES

S. Mcalister, I. Davidson, W. Mckinnon, J. Morton, G. Pleizier, M. Post, L.

Selwyn

To cite this version:

JOURNAL DE PHYSIQUE

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

MAGNETISM I N SOME Y-Ba-Cu OXIDES

S. P. McAlister, I. J. Davidson, W. R. McKinnon, J. R. Morton, G. Pleizier, M. L. Post and L. S. Selwyn Division of Chemistry, N.R.C., Ottawa, Ontario, Canada, K I A OR9

A b s t r a c t . - Magnetic measurements are reported on insulating Y-Ba-Cu oxides with metal ratios 2:l:l (green phase) and 1:3:2 (brown phase). The green phase is antiferromagnetic below 3 1 f 1 K with a moment of 1.14zk0.10 ,UB

/

CU. The brown phase is paramagnetic with a moment of 1.5 f 0.2 ,UB / Cu (oxygen-rich sample) or 1.75 f 0.2 p~

/

Cu (oxygen-depleted sample).

The first reported 90 K superconducting Y-Ba- Cu oxides often contained other phases which did not affect the superconducting transition temperature greatly but did cause difficulties in studying the ma- terials in the normal state. Notably the samples often showed evidence of green material on the surface or in the bulk. This is the "green phase" with a Y-Ba- Cu ratio of 2:1:1, first reported by Michel and Raveau [I]. Here we report magnetic measurements on pure 2:l:l material as well as on two 1:3:2 phase samples, the so-called "brown phase" [2]. Magnetic measure- ments of the pure insulating and non-superconducting phases may help avoid confusion when studying mul- tiphase superconducting material [3] and lead to an understanding of the role of magnetic interactions in these ceramic materials.

The samples were prepared by heating appropriate amounts of Yz03, BaCos and CuO at 950 C for at least 24 h followed by a slow cooling to room temper- ature. Two brown phase samples have been studied one which is oxygen-rich and a second which is de- pleted in oxygen. The oxygen-depleted brown phase was made by heating oxygen-rich material at 750 C under vacuum

(lod3

pa) for 24 h in a stainless steel container. An X-ray study of the green phase material showed that it was single phase. The structure of the brown phase material has not been completely deter- mined yet, although it has been found that the cell volume increases by approximately 2.5 % in the de- pleted sample [4]. The oxygen positions have not been determined. Recently there has been a claim that the brown phase is only stable because of the presence of C in the structure [5] - this awaits confirmation by others. The brown phase would have an oxygen sto- ichiometry of 6.5 if it contained only cu2' but 7.5 if just cu3+.

The magnetization of powdered samples was mea- sured with a VSM which was calibrated with Ni. To determine the magnetic moment in the paramagnetic regime we used the method of Danielian (see [6]) which enables the moment to be determined from an inter- cept rather than the slope of a plot of the inverse of the magnetization. We assume that the moments are entirely localized on the Cu atoms. Although the two

brown phase samples do not not have the same oxy- gen content, not knowing it exactly has forced us to assume, as a first approximation, that it has the same value in both cases. We have taken it to be 7. From the preparation technique we know that the difference in oxygen stoichiometry between the two samples is less than 0.8.

In figure 1 we show the temperature dependence of the magnetization of the green phase in an applied field of 15 kOe. The sample shows an antiferromag- netic transition at 31 f 1

K.

This transition is smeared

0.2

-

0 50 100 150 200 Temperature ( K )

Fig. 1. - Temperature dependence of the magnetization in 15 kOe for the 2:l:l (green phase) material. The magneti- zation M (T) has been scaled t o t h e value a t the maximum Mmax

out in less pure samples and appears at a lower temper- ature. (No anomaly is seen at 90 K which would haye indicated contamination of the sample with 1:2:3 phase material.) In an ESR study [7] a marked decrease of the signal was found a t low temperatures which was ascribed to the onset of magnetic order. From the paramagnetic regime we obtain a moment of 1.24 f 0.10 p , ~

/

CU, a more accurate value than that from an

ESR spin count.

When the oxygen content of the superconducting 1:2:3 coinpounds is reduced superconductivity disap- pears. Neutron diffraction studies have shown that antiferromagnetic order occurs when the oxygen sto-

C8 - 2160 JOURNAL DE PHYSIQUE

ichiometry is reduced to below 6 [8]. The magnetic ordering temperatures are in excess of 400 K. The mo- ments at the Cu sites are not all equal because some Cu atoms are surrounded by only 2 orygen atoms giv- ing c u l + , which does not have a moment. From these studies it is clear that there is an interplay between the presence of magnetic order and the absence of su- perconductivity. This is not well understood. The magnetic ordering discussed above is not to be con- fused with that of [3] where a superconducting sample was claimed to be antiferromagnetic below 20 K. The sample was most probably a mixture of at least two phases

-

the green phase and a superconducting one.

In the green phase the interactions between the mo- ments, assumed to be localized on Cu ions, is indirect through at least 3 oxygen atoms in a triangle to a neighbouring Cu which is also surrounded by oxygen atoms, as shown in figure 2. (This is in contrast to the layered superconducting material where there is a single intermediate oxygen atom, which is believed to play an important role in the appearance of supercon- ductivity.) The positions of the Y and Ba atoms in

Figure 2.

-

A unit cell of the green phase Y2BaCu06. Symbols are: filled circles Y, open circ1,es Ba, and concentric circles Cu. Each square pyramid has 0 atoms at each of its 5 corners. Unit cell atom positions were taken from [I].

the structure are not as important in mediating the exchange interactions between the Cu ions as the oxy- gen atoms. Since the oxygen polyhedra around the Cu atoms are not connected to each other the superex- change between the Cu atoms is more complicated than normal. Without neutron diffraction data it is not clear from the crystal structure alone whether the alignment of the moments would be along the c axis

or not.

Both brown phase samples were paramagnetic with no evidence of magnetic ordering down to 4 K. No anomalies were seen a t the ordering temperatures

TN

or

Tc

of the other two phases, indicating that the sam- ples were not contaminated with those phases. The de- pleted sample yielded a moment of 1.75

f

0.2 , u ~ /Cu compared with 1.5 f 0.2 - , u ~

/

Cu for the oxygen-rich material. The errors reported are absolute values and do not come from scatter in the data alone-such errors are less. Thus we claim that the moment difference is a real effect. The assumption of identical oxygen compositions for the two samples could only produce a difference of a few percent in the calculated moment. Our results confirm the importance of the oxygen con- tent t o the properties even in the non-superconducting compounds. This is reflected in a more fundamental way as differences in unit cell volume. The decrease in effective moment as the oxygen content increases could be interpreted as an increasing proportion of cu3+ ions in the crystal. Confirmation of such a possibility and determination of the magnetic structure of the green phase awaits further investigation.

[l] Michel, C. and Raveau, B., J. Solid State Chem.

43 (1982) 73.

[2] Frase, K. G., Liniger, E. G. and Clarke, D. R., J.

Am. Ceram. Soc. 70 (1987) C204.

[3] Sun, J. Z., Webb, D. J., Naito, M., Char, K., Hahn, M. R.,, Hsu, J. W. P., Kent, A. D., Mitzi, D. B., Oh, B., Beasley, M. R., Geballe, T. H., Hammond, R. H. and Kapitulnik, A., Phys. Rev.

Lett. 58 (1987) 1574.

[4] Davidson, I. J., private communication.

[5] Roth, R. S., Bull. Am. Phys. Soc. 33 (1988) 377. [6] McAlister, S. P. and Strobel, P., J. Magn. Magn.

Matls. 30 (1983) 340.

[7] McKinnon, W. R., Morton, J. R., Preston, K. F. and Selwyn, L. S., Solid State Commun. 65 (1988) 855.

[8] Tranquada, J. M., Cox, D. E., Kunnmann, W., Moudden, H., Shirane, G., Suenaga, M., Zolliker, P., Vaknin, D., Sinha, S. K., Alvarez, M. S., Ja- cobson, A. J. and Johnson, D. C., Phys.

Rev.