ANTIFERROMAGNETIC ORDERING AND PHASE DIAGRAM OF YBa2Cu3O6+x

(1)

HAL Id: jpa-00229231

https://hal.archives-ouvertes.fr/jpa-00229231

Submitted on 1 Jan 1988

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

ANTIFERROMAGNETIC ORDERING AND PHASE

DIAGRAM OF YBa2Cu3O6+x

J. Rossat-Mignod, P. Burlet, M. Jurgens, C. Vettier, L. Regnault, J. Henry, C.

Ayache, L. Forro, H. Noel, M. Potel, et al.

To cite this version:

(2)

Colloque C8, Supplkment a u no 12, Tome 49, d h m b r e 1988

ANTIFERROMAGNETIC ORDERING AND PHASE DIAGRAM OF YBa2Cn306,, J. Rossat-Mignod ( I ) , P. Burlet (I), M. J. Jurgens (I)*, C. Vettier (,), L. P. Regnault (I),

J. Y. Henry (I), C. Ayache (I), L. Forro (I), H. Noel (3), M. Potel (3), P. Gougeon (3) and J. C. Levet (3)

(I) Centre d7Etudes Nucle'aires de Grenoble, De'partement de Recherche Fondamentale, 8 5 X , 38041 Grenoble Cedez, France

(") Institut Laue Langevin, 156 X , 38042 Grenoble Cedeq France

(3) Laboratoire Chimie Minerale B, Universitk de Rennes I, 35042 Rennes, France

Abstract. - Neutron scattering experiments on single crystals have allowed an accurate determination of the magnetic phase diagram of YBa2C~306+x. With increasing oxygen content the antiferromagnetic ordering of YBa2Cu306 and

TN

remain unchanged up to x = 0.2. For x

>

0.2 both

TN

and the ordered moment of c u 2 + ions in the Cu(2) site decrease and vanish abruptly around z z 0.40. The CU+ ions in the Cu(1) site do not order for any oxygen content. For x = 0.30

a reentrant behaviour is observed. Preliminary inelastic neutron scattering experiments on a rather large single crystal (0.2cm3) with x z 0.3 are reported.

I n t r o d u c t i o n 1101. They have shown as powder experiments [ l l ] that Since the discovery of superconductivity in

(LaBa), Cu04(Tc ci 40 K) [l] and in Y B a 2 C ~ ~ 0 6 . 9 (T, = 90 K) [2] many theoretical models have been developped t o account for the observed high T, val- ues (see proceedings of the Interlaken Conference [3]). However, up t o now, there is no unanimity on the un- derstanding of the pairing mechanisms which would lead t o superconductivity in these high T, materials. Several models for the electron-electron attrac- tion have been proposed involving phonon excitations, like in the conventional BCS theory, or the strong coupling approximation, or charge/spin fluctuations. This confusing situation will be clarified by accurate experimental results. Photoemission studies have established the strong electron correlations on both Cu and 0 sites, and the oxygen-2p hole nature of the charge carriers [3]. Apart from the classical magnetic and transport measurements, the neutron scattering tech- niques together with local probe experiments, as NMR and pSR, have also led t o important results. The most exciting ones were the discovery of antiferromagnetic

(.4F) ordering in pure La2Cu04 below

TN

= 240 K [4] and the existence of large AF correlations above

TN and in Sr-doped samples [5]. More~ver this AF ordering was found to be very sensitive t o both the Sr content [5] and the oxygen stochiometry:

TN

can be as high as 300 K 16, 71 when oxygen p-holes are no longer present in C u 0 2 planes. So on similar experiments were undertaken in the YBa2Cu3o6+, system in order t o understand the difference in T, values between the two systems.

-4 similar AF ordering within C u 0 2 planes was dis- covered in the insulating compound YBa2CII306 18, 91 which persists up t o

TN

= 415

K

[a]. We have undertaken systematic magnetic scattering studies of YBa2Cu306+, compounds as a function of tb.e oxygen content using single crystal samples. In this paper we present the results of a detailed investigaion of the magnetic phase diagram. Preliminary single crystal results where reported a t the Interlaken Conference

t h e long range 3d AF ordering disappears suddenly for x

=

0.40. So crystals with several oxygen contents around x

=

0.4 were carefully investigated. The experiments which are presented in this paper allow us to conclude that the addition of oxygen weakens the 3d A F ordering by introducing some static disorder, first in the stacking of AF planes and then within the AF planes. .4t the CEN-Grenoble we initiated a programme t o grow large single crystals of YBazCu306+,. Crystals of about 0.2 cm3 have been obtained, which have allowed us t o perform inelastic neutron scattering experiments. Preliminary results are also presented.

The paper is arranged as follows. The sample prepa- ration and characterization and the experimental con- ditions are described first, and then we report and discuss the elastic and preliminary inelastic neutron scattering results.

S a m p l e p r e p a r a t i o n a n d characterisation The neutron scattering experiments presented in this paper where first performed on a single crystal of about 4mm3 (2.5 x 1.5 x 1 mm3, named N) grown at the University of Rennes by a kind of mineralization method [lo]. In the meantime a programme of single crystaI growth was initiated a t the Centre d'Etudes Nuclkaires de Grenoble. We have successfully grown crystals of increasing sizes using a grain growth tech- nique. Three crystals were used with volumes 20 mm3 (HN.4), 60 mm3 (HNB) and 200 mm2 (HNC). The latter one has a size (20 x 7 x 2.1 mm3) which allows t o undertake inelastic neutron scattering experiments. The single crystals of Y B ~ ~ C U ~ O ~ + ~ , grown both at the University of Rennes and at the CEN-Grenoble, were of good quality with a mosaic spread of about 0.3 degree. Moreover a big advantage of these crystals is that the oxygen content can easily be changed from x = 0 t o x = 0.93 even in the largest one (HNC). -4ctually this makes the Y B ~ ~ C U ~ O ~ + ~ system easier to investigate because the complete range can be covered, from the AF t o the superconducting state with the same crystal.

* ~ r o m Kamerlingh Onnes Laboratory, Leiden, The Netherlands.

(3)

C8 - 2120 JOURNAL DE PHYSIQUE

The quality of these crystals can be demonstrated by resistivity measurements. In figure 1 we report the basal-plane and c-axis resistivities measured on a CEN-Grenoble type crystal. In YBazCusOs.93, the superconducting transition, T, ci 92 K is very

sharp (AT, E 0.2 K) and the resistivities measured along both directions decrease with temperature in sharp contrast with,the behaviour observed for YBa2Cu306.75 The up turn of the resistivity along the c-axis is clearly related to off-stoichiometry in the Cu-0 chains.

For a detailed investigation of the magnetic phase diagram it is mandatory to introduce well defined quan- tities of oxygen in the crystal which was achieved by following a well defined procedure. For each investigated oxygen content, the crystal was first reduced to YBa2CusO6, together with a weighted amount ( N 2 g) of powder, by heating under vacuum at 675 OC during 12 hours. Then the crystal and the powder are intro- duced, at room temperature, into a closed tube with a well-controlled mixture of oxygen and argon gas. To determine the oxygen concentration of the samples we used several methods, which agree rather well: measurements of nuclear Bragg intensities, of the lattice parameter c and the change in mass of the crystals during the heat treatment. The actual oxygen concentrations which where investigated are x = 0,0.15,0.20, 0.30, 0.33, 0.36, 0.38, 0.40, 0.42 and 0.75. The error in the determination of x is about f 0.03, but relative values are known with a better accuracy, especially for

x = 0.38 and e = 0.42.

Neutron experiments were performed on the three- axis spectrometers IN8 and IN20, the four circle spec- trometer Dl0 and the two-axis diffractometer Dl5 of the high flux reactor of the Institut Laue Langevin. Most of the measurements were carried out with a wavelength of 2.36

A

provided by a Ge(ll1) crystal monochromator. To eliminate further higher order contamination PG filters and often a Ge(ll1) analyser were used, giving a X/2 contamination smaller than The crystals were mounted with a (110) axis vertical in a standard ILL cryofurnace which allows to vary the temperature from 1.5 K to 550 K. Then any direction in [110], [001] reciprocal plane could be investigated. For the YBazCu306 measurements the crystal was mounted inside an evacuated quartz tube to prevent any oxygen absorption, which was confirmed after the experiments.

figure 2. The absence of intensity at (112, 112, 0) sub- stantiates the AF coupling between c u 2 + moments in the Cu(2) Bravais sublattices at (00z) and (OOX) and confirms the non-magnetic CU+ state of Cu(1) site at (0, 0, 0). The AF direction lies within the basal plane, as established unambiguously by magnetic intensity measurements on single crystals [lo, 121. More recent magnetic intensity measurements yield a low temperature moment values mo = 0.64

rt

0.03 p~ and an accurate study as a function of temperature indicates a second order phase transition with an ordering temperature TN = 415

f

5 K. Crystals with various oxygen contents where investigated in detail as a function of temperature. The temperature dependence of magnetic Bragg peak intensities is reported on a normalized scale in figure 3.

For increasing oxygen contents up to a: = 0.2 the AF structure remains unchanged, mo and TN staying constant. For x

>

0.20 both TN and mo decrease, rather smoothly up to x ci 0.35 and more abruptly

above resulting in a sharp disappearance of the long range AF ordering at x, ci 0.413~ 0.02 (see Fig. 4). The

steepness of the magnetic - nonmagnetic transition is evidenced in figure 5 which indicates 3d AF ordering for x = 0.38 (TN = 150 K) and the absence of ordering for x = 0.42.

In the oxygen concentration range x = 0.30

-

0.33 the temperature dependence of the intensities of the magnetic Bragg peaks (112, 1/2, 1) and (112, 1/2, 2), given in figure 6, exhibits a reentrant behaviour below T = 50

K.

As the ratio of these two intensities remains constant, the decrease of the intensities corresponds actually t o a decrease of the ordered moment at low temperatures. Since these measurements have been carried out with an analyser set in the elastic position, this reduction in the ordered moment values implies the appearance of some static disorder over an energy scale narrower than 0.5 meV as we will see later. Actually in figures 6 and 7 we can observe that

this disordered component gives rise to a diffuse scattering only along (112, 112, 1) rods indicating that the disorder occurs mainly in the stacking of the AF- planes. It is noteworthly (see Fig. 7) that no diffuse scattering is observed around the (112, 112, 0) scattering vector indi&ting that the AF coupling between the two CuO2 layers is not affected (m2 = - m l ) . This result shows that the direct AF exchange interaction J, between c u 2 + moments of adjacent layers of the unit cell is much larger than the indirect AF exchange interaction J' betyeen Cu2+ moments via the CU+ ions (see Fig. 1). Therefore the magnetic ordering in Y B a 2 C ~ 3 0 6 + ~ consists of AF bilayers and the disor- M a g n e t i c p h a s e d i a g r a m _{der takes place only in the stacking of these bilayers.} As it has already been reported [8, 91 YBa2CusOs Moreover no Bragg peaks could be observed at (112, orders with an antiferromagnetic structure described 1/2, 0) and (112, 112, 112) at any x value, indicat- by a wave vector k = (112,112, O), i.e. corresponding that there is no ordered moment (mo

<

0.03 p ~ )

(4)

Fig. 1. - Resistivites along the c-axis and within the basal plane as a function of temperature for YBaaCusOs+, with

x = 0.93, 0.75 and 0.33.

Fig. 2. - Antiferromagnetic structure of Y B a 2 C ~ 3 0 6 + ~ .

Fig. 3. - Intensity of the magnetic Bragg peak (112 112 1) as a function of temperature of Y B a 2 C ~ 3 0 6 + ~ for various oxygen contents. Intensities have been normalized in order t o reflect the square of the ordered moment.

0 100 200 300 400 500

Oxygen content : x 1 ( r.I.u.1 Temperature ( K

Fig. 4. - Ordered moment and ordering temperature as a function of the oxygen content for YBa2Cu306+,. Fig. 5. - Elastic scans along [1/2 112 I ] for YBa2Cu306+, with x = 0.38 and 0.42.

Fig. 6. - Temperature dependence of magnetic Bragg peak intensities and of the [1/2 112 I] diffuse scattering.

ment with Cu-NMR measurements [13], but not with some other neutron experiments [14,15] which have revealed some ordering of Cu(1) magnetic moments and a doubling of the magnetic unit cell along the c-axis. Clearly results are sample dependent. In our case, we have investigated many oxygen concentrations and we have never observed any moment in Cu(1) site. We suspect that some ordered magnetic moment could ap- pear in Cu(1) site if oxygen atoms in chain planes are rather randomly distributed between the different oxy-

gen sites, because in our case the oxygen intercalation was done following a well defined procedure ensuring a rather good ordering of oxygen atoms.

For oxygen contents closer to the magnetic - nonmagnetic transition, i.e. x = 0.36 and 0.38, surpris- ingly no reentrant behaviour is observed. However for

(5)

C8 - 2122 _{JOURNAL DE PHYSIQUE}

Fig. 7. - Elastic scans along [1/2 112 11 for YBa2C~306.3 at T = 300 and 1.5 K, showing the Id-diffuse scattering originating from the reentrant behaviour.

some static disorder both within and between the Cu- planes, which develops just below T N .

Furthermore for x N 0.38 the magnetic intensity ex-

hibits a quite unusual temperature dependence (see Fig. 8), actually close to that found in some off - sto-

ichiometric LazCu04 single crystals [5]. Of course we can argue that this behaviour could result from a slight inhomogenity in the oxygen content, but we think that it is more likely an intrinsic behaviour resulting from a large decrease of the effective interplanar coupling due t o important in-plane disorder. For x = 0.42 and

x = 0.75 no magnetic Bragg peak is observed down to T = 1.5 K and 30 K, respectively, indicating the absence of long range 3d AF ordering.

Temperature ( K )

Fig. 8. - Intensities of Bragg peaks (112, 112, 1) and (112, 112, 2) as a function of temperature for YBa2C~~06.38. Inelastic n e u t r o n s c a t t e r i n g results

Preliminary inelastic neutron scattering experiments have been carried out in YBa2Cu306+, for an oxygen content x = 0.3, i.e. in the AF state. At T = 4.2 K , the energy scan performed a t Q =

(0.6,0.6,1.6) shows only a nuclear incoherent signal while a t Q = (0.5,0.5,1.6) extra contributions can be observed. A narrow quasi-elastic magnetic contribu-

tion results from a weak interplanar disorder. While the energy resolution is not gery good (HWHM 21

0.5 meV) this contribution can be interpreted as aris- ing from some topological static disorder of the AF direction between magnetic bilayers. This contribution results from the reentrant behaviour described above. The other contribution is strongly inelastic and corresponds to a large energy scale because the intensity decrease results from resolution effects (kf3 cotg QA) due t o the constant-k; procedure. Actually this inelastic contribution is better investigated by performing Q scans a t fixed energy as shown in figure 9.

At T = 200 K , scans around Q = (0.5,0.5,1.6) along the (q, q, 0) direction a t h w = 3 meV and 6 meV, given in figure 9, show a clear magnetic contribution, which decreases as a function of the energy transfer. The q-width of the observed peak increases from Aq =

0.03 r.1.u. (0.07 A-') for 3 meV t o Aq = 0.042 r.1.u. (0.10 A-') for 6 meV. This @dependence and the asymmetry of the peak at fiw = 3 meV can be ex- plained by a spin-wave contribution. The dispersion curve being very steep, the q-resolution is not good enough t o get two well-defined peaks and resolution effects explain the assymmetry, the low q side being defocussal whereas the high-q side is focussal. This result is quite similar to what was observed in La2Cu04 [5]. A q-width Aq

--

0.040 r.1.u. (0.07

A-')

was reported a t AW = 4 meV, so the spin wave velocities are quite similar in both compounds (v 11 0.4

~ v A )

.

Furthermore the temperature dependence of the

h-

tensity at Q = (0.5,0.5,1.2) for an energy transfer

hw = 2 meV follows quite well Bose statistics up to

T = 60 K (see Fig. 10) but above this temperature the intensity becomes almost temperature independent. Discussion

We will discuss successively the large Nee1 temperatures, the magnetic moment values and the electron transfer process. The first striking result is the high value of the ordering temperature found in Y B s ~ ~ C U ~ O ~ (TN = 415 f 5 K) and in compounds with a small amount of oxygen (x

<

0.2) while magnetic couplings exhibit a pronounced 2d character. This TN

value is larger than the one found in LazCuO4, but actually the difference is not as large as was reported [lo] because TN values of about 300 K were found in a slightly de-oxygenated sample 16, 71. The value

TN = 230 K corresponds t o a slightly off-stoichiometric sampIe, i.e. containing a small amount of oxygen- p holes. In YBa2Cus06 TN is 40 % larger than in La2CuO4. This difference cannot be accounted for by a change of the in-plane AF coupling because neutron inelastic scattering experiments yield about the same in-plane spin wave velocity in both compounds

( J N 500 K). Differences in the interplanar couplings

(6)

Heisenberg behaviour. Then for a S = 112 system the 3d-ordering temperature (T;~)

_.

_,is much smaller than the mean field value (TFF = 25) according to

T:~/T?~ = x/Ln

(J/J')

[16] where J and J' are the

in-plane and interplanar couplings, respectively. The difference in TN between YBa2Cu306 and La2CuOc could be accounted for by a J' value larger by one order of magnitude (J'/J of the order of and respectively). However this simple model may not ap- ply properly t o these high T, materials. First of all La2Cu04, in the monoclinic phase, has a small Ising- type anisotropy revealed by susceptibility anisotropy. Recently, Friedel [17] has shown that for X Y systems

TN remains proportionnal t o Ln

(J/J')

because of the existence of vortices parallel to the planes. Sec- ondly, as was deduced from the reentrant behaviour. " .

in YBazCusOs+, the two inter-plane interactions have quite different values, and they may differ by more than one order of magnitude. Therefore YBa2Cu306 has to be considered as a bilayer system. For an XY

model Friedel argues that TN of a n-layer system must be proportionnal to the number of layers [18]. This is another possibility of explaining the larger TN value in YBa2Cu306 (n = 2) compared with LazCu04 (n = 1) (the Ising character of LazCuOc would enhance T;~).

A determination of the spin wave spectrum is clearly needed t o get quantitative information on the various exchange interactions.

The second interesting feature is the reentrant be-

0.40 OX5 0.50 0.55 0.60 Reduced wave vector q(r: 1.u.)

Fig. 9. - Q-scans performed for an energy transfer fuJ = 3

and 6 meV around (112, 112, 1.6) for YBa2Cu306.3 at T = 200 K.

Fig. 10. - Neutron intensity at Q = (0.5, 0.5, 1.2) and

fiw = 2 meV as a function of temperature. The dashed line is calculated according to Bose statistics.

Fig. 11. - Value of the low temperature ordered

moment as a function of the ordering temperature for Y B ~ ~ C U ~ O ~ + ~ .

haviour observed around x E 0.30-0.33 which results

in the building up of some static disorder between bilayers a t low temperatures. For this oxygen content, the concentration of 2 p-oxygen holes transfered to the C U O ~ planes is quite small. Then they can be considered as localized (within a length lo) and so they undergo a strong AF exchange interaction with neigh- bouring Cu spins yielding a singlet ground state. This magnetic hole introduces some perturbation in the AF direction [19] which may be at the origin of the reentrant behaviour. Actually the perturbation affects first the interplanar ordering and then for larger hole concentrations the in-plane ordering.

It is worthwhile noting that the low temperature value of the ordered moment varies linearly with the ordering temperature (see Fig. 11). The largest value

mo

"

0.64 p ~ , found in YBazCu306, is strongly reduced from the expected mean field value (- 1.1 p ~ )

.

However we do not need to invoke large hybridization effects to explain this reduction but only quantum fluctuations resulting from the 2d character of the spin wave spectrum [20].

Let us discuss the charge transfer mechanisms from the analysis of the magnetic phase diagram. We can distinguish different regimes as a function of the oxygen content x. First, for 0

<

x

<

0.2, the filling of oxygen in the Cu(1) planes induces electron transfers only in the Cu(1) plane, no electrons are transfered from Cu(2) planes. The CuO2 planes contain no 2p-oxygen

01 . ' I . I . I . I I

0 100 2CQ 300 LOO

(7)

C8 - 2124 JOURNAL DE PHYSIQUE

holes whereas Cu(1) planes contain a mixture of CU', c u 2 + and cu2+-0- species. For 0.20

<

x

<

0.35 only a very few 2p-holes are created in CuO2 planes resulting in a weakening of the interlayer coupling.

However for x

>

0.35 an appreciable amount (a few percent) of 2p-holes is transfered t o CuO2 planes leading. to a destruction of the AF ordering, a behaviour quite similar t o that found in Sr-doped La2Cu04 system [5]. In this range of oxygen concentration also occurs a tetragonal orthorhombic struc- tural transition and around x = 0.40 the insulating metallic transition takes place together with the appearance of superconductivity. We have t o keep in mind that around x c 0.5 an oxygen ordering has been found in the Cu(1) plane [12] corresponding to an alternation of filled and empty Cu-0 chains. then around x 11 0.4, Cu(1) planes contains mainly CU+ in

empty chains and cu2+-0- in filled chains whereas Cu(2) contains mainly c u 2 + and a small amount of 2p-holes.

For 0.40

<

x

<

0.55 the electron transfer occurs mainly from CuO2 planes, creating a large amount of holes in these planes and a sharp increase of

T,

up to a plateau of about 60 K [3]. While further increasing the oxygen content, oxygen atoms will occupy vacant sites of the empty Cu chains and then electron transfers will occur mainly within these chains, i.e. in Cu(1) planes. This mechanism will occur up to x 11 0.75. There-

fore from x

=

0.55 up t o x rr 0.75 there will be no sub- tantial increase of the 2p-hole concentration in CuOa planes, which explains the existence of the plateau for Tc. Above x c 0.75 again electrons are transfered from the C U O ~ planes yielding to an increase of T, from 60 K to 92 K.

Further neutron scattering studies are required t o investigate magnetic fluctuations in oxygen rich samples. They may shed some light on the electron transfer process and clarify current ideas on the superconductivity in high Tc materials.

Part of this programme is now in progress at the Institut Laue Langevin and the Centre d'Etudes Nucleaires de Grenoble.

[I] Bednorz, J. G., Mueller, K. A., Z. Phys. B 64 (1986) 189.

[2] Wu, M. K., Ashburn, J. R., Torng, C. J., Hor, P. H., Meng, R. L., Gao, L., Huang, Z. I., Wang, Y. Q., Chu, C. W., Phys. Rev. Lett. 58 (1987) 908.

[3] Proc. of the Int. Conf. on High Temperature Superconductors (Interlaken, Switzerland) Eds. J . Miiller and J. L. Olsen, Physica C 153-155

(1988).

[4] Vaknin, D., Sinha, S. K., Moncton, D. E., John- ston, D. C., Newsam, J. M., Safinya, C. R., King, H. E., Phys. Rev. B 36 (1987) 2802.

[5] Endoh, Y., Yamada, K., Birgeneau, R. J., Gabbe,

D. R., Jenssen, H. P., Kastner, M. A., Peters, C. J., Picone, P. J., Thurston, T. R., Tran- quada, J. M., Shirane, G., Hidaka, Y., Oda, M., Enomoto, Y., Suzuki, M., Murakami, T., Phys. Rev. B 37 (1988) 7443.

Birgeneau, R. J., Gabbe, D. R., Jensen, H. P., Kastner, M. A., Picone, P. J., Thurston, T. R., Shirane, G., Endoh, Y., Sato, M., Yamada, K., Hidaka, Y., Oda, M., Enomoto, Y., Suzuki, M., Murakami, T., preprint.

[6] Yamada, K., Kido, E., Endoh, Y., Hidaka, Y., Oda, M., Suzuki, M., Murakami, T., Solid State Cornman. 64 (1987) 753.

[7] Johnston, D. C., Sinha, S. K., Jacobson, A. J., Newsan, J. M., Physica C 153-155 (1988) 572. [8] Rossat-Mignod, J., Burlet, P., Jurgens, M. J.,

Henry, J. Y., Vettier, C., Physica C 152 (1988) 19.

191 Tranquada, J. M., Cox, D. E., Kannmann, W., Moudden, A. H., Shirane, G., Suenaga, M., Zol- liker, P., Vaknin, D., Sinha, S. K., Phys. Rev. Lett. 60 (1988) 156.

[lo] Burlet, P., Vettier, C., Jurgens, M. J., Henry, J . Y., Rossat-Mignod, J., Noel, M., Potel, M., Gougeon, P., Levet, J. C., Physica C 153-155

(1988) 1115.

[ll] Tranquada, J. M., Moudden, A. H., Goldman, A. I., Zolliker, P., Cox, D. E., Shirane, G., Sinha, S. K., Vaknin, D., Johnston, D. C., Alvarez, M. S., Jacobson, J., Phys. Rev. B 38 (1988) 2477. [12] Petitgrand, D., Collin, G., Physica C 153-155

(1988) 192.

[13] Kitaoka, Y., Hiramatsu, S., Ishida, K., Asayama, K., Takagi, H., Iwabuchi, H., Uchida, S., Tanaka, S., J. Phys. Soc. Jpn 57 (1988) 737.

[14] Kadowaki, H., Nishi, M., Yamada, Y., Takeya, H., Takei, H., Shapiro, S. M., Shirane, G., Proc. of ICNS'88 (Grenoble) July 1988, in Physica B. [15] Lynn, J. W., Li, W. H., Mook, H. A., Sales, B.

C., Fisk, Z., Proc. of ICM'88 (Paris) July 1988, in J. Phys. France.

[16] Regnault, L. P., Rossat-Mignod, J., Phase tran- sitions in quasi 2d planar magnets, in Magnetic properties of layered transition metal compound, Eds. L. J. de Jongh and R. D. Willet, under press (1989).

[17] Friedel, J., Preprint, submitted to J. Phys. France.

[18] Friedel, J., Preprint, submitted to J. Phys. France.

[19] Shraiman, B. I., Siggia, E. D., Phys. Rev. Lett.

61 (1988) 467.

[20] Grempel, D., private communication.