G. Chouteau (I), M. Potel (4), P. Gougeon (4), H. No61 (4), J. C. Levet (4), M. Guillot (') and J. L. Tholence (3). (I) Service National des Champs Intenses, CNRS, ...
JOURNAL DE PHYSIQUE Colloque C8, Supplkment au no 12, Tome 49, dbcembre 1988
HIGH TEMPERATURE AND HIGH FIELDS MAGNETIC PROPERTIES OF A HoBa2Cu307 SINGLE CRYSTAL G. Chouteau (I), M. Potel (4), P. Gougeon (4), H. No61 (4), J. C. Levet (4), M. Guillot (') and J. L. Tholence (3) (I) Service National des Champs Intenses, CNRS, B P 116X, 38042 Grenoble Cedex, France1 (') Laboratoire Louis Ndel, CNRS, B P 166X, 38042 Grenoble Cedex, FranceL (3) Centre de Recherche sur les TrCs Basses TempCratures, CNRS, B P 166X, 38042 Grenoble Cedeq France1 (4) Laboratoire de Chimie Mindrale B, Universitk de Rennes, CNRS, U.A. 254, Avenue du Gknkrale Leclerc, 35042 Rennes Cedex, France Abstract. - In HoBa2Cu307-, the magnetic moment per Ho is far from the maximum moment of the H O ~ +ion. A magnetic anomaly depending on the cooling rate is observed around 60 K. It is attributed to a structural transition.
Introduction In the REBa&u307-, compounds due to the large distance between the RE ions, the magnetic ordering temperature is in the Kelvin range or even below in the case of holmium [I]. The insensitivity of the superconducting state to the presence of the RE indicates that the spin-dependent exchange interaction with the super conducting electrons is weak. In this paper we present a study of the high field high temperature magnetic properties of the compound with holmium. Special attention was devoted to the crystalline field effect. We have also studied the influence of the cooling rates.
Experimental The magnetization of a single crystal was measured with the field parallel and perpendicular to the c-axis between 4 K and 300 K in two different apparatus: in the first one the field is produced by a Bitter coil and can be varied from zero up to 18 T with various rates ranging from 30 min to several hours in the second one, a superconducting coil producing a maximum field of 11 T is used and the field variation is slow: six hours were necessary to obtain the magnetization curves. 1.SUPERCONDUCTING STATE. - The figure 1 shows the hysteresis curves obtained at 4.2 K. In the case H // c the flux jumps are much more numerous when the field is rapidly varied (Bitter coil, Fig. 1 continuous curve) than when the field varies slowly (superconducting coil Fig. 1dots). As we mentioned earlier 121 this effect is of thermal origin. In bulk materials the energy flow associated with the vortice motion is not easily evacuated because of the low thermal diffusivity.
Fig. 1. - Hysteresis cycles at 4.2 K up to 18 T (rapid field sweep) and 11 T (slow field sweep).
2. PARAMAGNETIC STATE. - Above the critical field Hcz the magnetization is proportional t o the field up to 11 T. No saturation tendency is 0bserve.d in both directions ( H parallel or H perpendicular to the caxis). For the case H perpendicular t o c, the pinning is very low and the superconducting part of the magnetization is small. Tbus the averaged magnetization M = (M+ + M-) / 2 (where M+ is the magnetization measured by increasing the field and M- the magnetization measured by decreasing it) gives a good way of obtaining the paramagnetic contribution even below H,z. We also find it proportional to the field. Above 100 K a slight positive curvature is observed below 3 teslas. The anisotropy of M is small. In figure 2 we have plotted the quantity X T as a function of T where X is the static "susceptibility" M / H measured in a 10 teslas field for the case H perpendicular to the c-axis. We proceed as follows: first of all the sample is cooled down to 4 K in zero field then heated up to 300 K under the measuring field and cooled down again to 4 K under the same field. Figure 2 (dots) shows the
'~aboratoiresassocibs B l'Universit.6 Joseph Fourier, Grenoble, France.
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3. THE CRITICAL FIELD. - We have determined the critical field H as the field at which the hysteresis of the magnetization begins. This criterion is not very accurate because the slope d M / cLH tends to zero near Hc2. This is probably the reason why our values are lower than the values determined by the resistive method [7]. However, as shown in the figure 3, the curvature of the curve Hc2(T) is positive near Tc in good agreement with the numerous results of the literature and confirms the fact that this property is intrinsic and does not depend on the measuring method.
Fig. 2. - X.T versus T, with X = M/H, H = 10 T. Dots: slow cooling, crosses: quenching. The insert shows the anomaly around 60 K with H // c and H perp. t o c.
curve obtained after a slow cooling (6 hours from 300 K down to 4 K): it exhibits a plateau above 100 K and a downwards curvature below this temperature. It is worthnoticing that since the critical temperature at H = 10 T is 60 K this curvature cannot be attributed t o the superconductivity. The value of XT at the plateau, 9 emu/mole, is smaller than the high temper~ '(14 emu/mole). ature Curie constant of the ~ 0 ions This fact associated with the absence of saturation of the magnetization in 10 T shows that the crystal field ~ ' in this high Tc compound are levels of the ~ 0 ions spread out over a wide range of temperature, the lowest levels being non magnetic, in good agreement with previous results [3, 41. In figure 2 (crosses) we have plotted the same quantity after a rapid cooling (four minutes from 300 K down to 4 K). A broad anomaly can be observed between 150 and 250 K. In this range, contrary to the first experiment, the behavior is hysteretic. Above 250 K the two curves (crosses and dots) superimpose. In the insert we show the variation of X T deduced from the magnetization curves in both directions. This corresponds to a rapid cooling. A maximum is clearly present around 60 K. It cannot be associated with a magnetic order because it occurs at a too high temperature. It should be noted that, in this case, X is obtained by a root mean square determination of the slope d M / d H below 10 T , and thus differs from the quantity M / H measured in the above experiments. It probably includes a part of diamagnetism due to superconductivity. Since the susceptibility depends on the cooling rate we should invoke the existence of a structural transition between 60 and 300 K leading to a change in the distribution of the low lying crystal field ehergy levels. The existence of a structural transition has been recently suggested in the YBaCuO compounds at 240 K [5] and a lattice distortion was observed around Tc [6].
Fig. 3. - Critical field Hc2 versus T,deduced from the hysteresis of the magnetization curves.
Conclusion ~ ' are in a non We have confirmed that the ~ 0 ions magnetic state in the HoBaCuO compound. A magnetic anomaly occurs around 60 K when the sample is quenched from room temperature. It does not appear on very slow cooling. We think that it could be the signature of a structural transition occurring well above 60 K and below the tetragonal-orthorhombic transition.
[I] Shimizu, S., Friedberg, S. A., Hayri, E. A. and Greenblatt, M., Phys. Rev. B 36 (1987) 7129. [2] Tholence, J. L., N&l, H. Levet, J. C., Potel, M., Gougeon, P., Chouteau, G. and Guillot, M., In. Conf. Dn High Tc Superconductors Interlaken (Feb. 23-March 3) Switzerland, to appear in Physica B. [3] Hulliger, F. and Ott , H. R., 2. Phys. B. 67 (1987) 291. [4] Shelton, R. N., McCallum, R. W., Darnento, M. A., Gschneider Jr., K. A., Ku, H. C., Yang, H. D., Lynn, J. W., Li, W.-H. and Li, Q., Physica B 148 (1987) 285. [5] Beal-Monod, M. T., J. Phys. France 49 (1988) 295. [6] Horn, P. M., Keane, D. T., Held, G. A., JordanSweet, J. L., Kaiser, D. L. and Holtzbzerg, F., Phys. Rev. Lett. 59 (1987) 2772. [7] Iye, Y., Tamegai, T., Takeya, H. and Takei, H., Jpn J. Appl. Phys. 26 (1987) L1850.
