ANISOTROPIC SUPERCONDUCTIVITY OBSERVED FOR Y1Ba2Cu3Oy SINGLE CRYSTALS BY TORQUE MAGNETOMETRY

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ANISOTROPIC SUPERCONDUCTIVITY OBSERVED

FOR Y1Ba2Cu3Oy SINGLE CRYSTALS BY TORQUE

MAGNETOMETRY

H. Miyajima, H. Tomita, Y. Otani, F. Yonezama, S. Chikazumi, H. Takeya,

H. Takei

To cite this version:

H. Miyajima, H. Tomita, Y. Otani, F. Yonezama, S. Chikazumi, et al..

ANISOTROPIC

SUPERCONDUCTIVITY OBSERVED FOR Y1Ba2Cu3Oy SINGLE CRYSTALS BY TORQUE

MAGNETOMETRY. Journal de Physique Colloques,

1988,

49 (C8),

pp.C8-2185-C8-2186.

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JOURNAL DE PHYSIQUE

Colloque C8, SupplBment au no 12, Tome 49, ddcembre 1988

ANISOTROPIC SUPERCONDUCTIVITY OBSERVED FOR Y1Ba2Cu30y SINGLE

CRYSTALS BY TORQUE MAGNETOMETRY

H. Miyajima ( l ) , H. Tomita (l), Y. Otani (l), F. Yonezama (l), S. Chikazumi (l),

H.

Takeya (2)

and H. Takei (')

(l) Department of Physics, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1,

Yokohama 223, Japan

(') Institute for Solid State Physics, University of Tokyo, Roppongi 7-22-1. Tokyo 106, Japan

Abstract. - Magnetic torques were measured on Y1Ba2Cu30y single crystals over the temperature range from 4.2 K to

RT

in magnetic fields up to 10 kOe. The torque curve exhibited a large rotational hysteresis. The anisotropic behaviors are discussed in terms of magnetic flux pinning effect.

1. Introduction

The superconductive properties for Y1Ba2Cu307 compound strongly depend on the crystallographic direction. Therefore, the magnetic torque L, which is given by an angular derivative of the internal magnetic free energy of superconductor, is exerted on a specimen upon the application of a magnetic field. In this study, the magnetic torque in the plane including the c-axis of Y1Ba&usO, single crystals was measured in magnetic fields up to 10 kOe. The temperature and the magnetic field dependences of the torque curves were interpreted in terms of the movement of fluxoid trapped in the specimens.

2. Experimental procedure

The single crystal YlBa~Cu30, grown by a flux method [l], where the value of y was undetermined, was a thin plate of 1 mm in thickness and 18 mm3 in volume. The crystallographic orientation was determined with an X-ray diffraction and Laue method, and the c-axis was found t o be almost perpendicular to the surface of the plate. In the specimen, there seems t o exist the a - b

twin structure. The superconducting transition temperature, T,, of this specimen was confirmed t o be 89 K

by means of DC and AC susceptibility measurements. The magnetic torque exerted on the specimen was measured in the following manner. At first, the specimen was gradually cooled down from 300 to 4.2

K

in the absence of an external field, and after then a stron- ger magnetic field than Hcl was applied. The torque curve was measured in the plane including the c-axis in a constant magnetic field with rotating an electro- magnet a t 0.3 rpm. The initial curve depends on the direction of the initially applied magnetic field with respect t o the crystallographic direction, but it is tr- ansformed to a common shape through the sequential rotation of the magnet. The temperature dependence

of the torque curve was measured with increasing temperature from 4.2 K.

The torque measurement was also carried out at

77

K

on a sintered polycrystalline Y1Ba2Cu30y com- pounds, but it was too small t o be observed. The magnetization parallel and perpendicular t o the c-axis was measured by means of a magnetometer.

3. Results and discussions

The isothermal torque curve is shown in figure 1, where 0 is the rotating angle from the c-plane and the subscripts "fwd" and "rev" of

L

refer to the curves obtained from forwarded and reversed rotation of the magnet, respectively. The curves depend on the rotational direction of the magnet and show a large hysteresis. Both Lfwd and L,,, exhibit 180' symmetry, but their shape distorts in low temperatures. The distortion is disappeared with increasing temperature below

Tc.

Furthermore, it should be noted that the peak position lags toward the rotational direction by

Fig. 1. - Torque curve for a single crystal YlBa2Cu30y at 4.2 K and 50 K in H,, = 8 kOe. The symbols "fwd" and "rev" refer to the torque curve for the forwarded and reversed rotation of the magnet, respectively. The angle 6 indicates the shift of the peak position at B = 7r.

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C8

-

2186 JOURNAL DE PHYSIQUE

an angle S, as indicated by arrows in the figure. This retarded angle, which is independent of the rotating speed, is about 10 degrees at 4.2

K

and decreases rapidly with increasing temperature from 4.2 K to

T,.

Hereafter we deduce a "kinetic" torque Lk and a

"static" torque L, as Lk = (1/2) (Lre, - Lfwd) and

Ls

= (1/2) (Lrev Lfwd)

.

By integrating L, with respect t o 0 from 0 to n/2,

the difference in magnetic energy between perpendicular and parallel to the c-axis is evaluated to be 5.1 X

10' erg/cm3 a t 4.2 K, whose magnitude agrees well with that of the magnetic energy of 4.2 X 10' erg/cm3 evaluated by the magnetization parallel and perpendicular t o the c-axis. The static superconducting properties should reflect on this torque Ls. In this paper,

we discuss mainly the kinetic torque Lk. It strongly

depends on the crystallographic direction and temperature (Fig. 2). The torque Lk takes maxima when the

Fig. 2. - Torque curves Lk = (112) (L,,, - Lfwd) for a single crystal YlBazCu30y at H,, = 8 kOe. (a): 17 K,

35 K, and 55 K, ( b ) : 77 K, 96 K, and 123 K (the vertical scale of (a) is 40 times larger than that of ( b ) ) .

magnetic field rotates across about the c-plane and mi- nima when the field is directed about the c-axis. The torque Lk observed above Tc is attributable to the

eddy current loss caused by anisotropic conductivity in the crystal. Figure 3 shows the temperature varia-

tion of the maximum value Lk ( c i H )

,

the minimum value Lk (C

//

H )

and the mean value, (Lk)

,

which is

given by 1 / 2 ~

1'"

Lk ( B ) do.

The Y-Ba-Cu-0 compound is a type-I1 superconductor characterized by a mixed phase in a magnetic field

Fig. 3. - Temperature dependence of the Lk (c // H )

,

Lk ( c l H ) and ( L k ) for a single crystal Y1Ba2Cu30y at He, = 8 kOe (see text).

above

HCl,

where fluxoids are trapped and pinned at imperfections. The fluxoids are injected into the specimen by applying magnetic field a t a virgin run, and then rotate with the rotation of external magnetic field but lag by an angle 6 relative t o the field rotation angle

B. As a result, a torque Lk given by

- B H

sin S, where B is the magnetic flux density of the fluxoids, is exerted on the sample. As mentioned above, since the retarded angle 6 is independent of the rotational speed of the magnet, the fluxoids are considered to be able t o move from the pinning site to other site when the magnetic energy becomes larger than that of the pinning energy. Such magnetic viscous properties of the fluxoids would cause the rotational hysteresis and Lk corresponds to a frictional resistance of the fluxoids. As L k ( c l H ) is

larger than Lk (c

//

H )

,

the fluxoids may be pinned tightly when the vortex axis is in the c-plane because of the two-dimensional superconductivity in Y-Ba-Cu-

0 systems. Recently, Wolfus and Yeshurum f2] found similar behaviors in sintered polycrystal YlBa2CusOy from the angular dependence of the magnetization and interpreted using the superconducting glass model. In this study, however, no torque was observed in such a polycrystal specimen. So the torque is attributable t o the crystallographic anisotropy and the flux pinning effect. In conclusion, the kinetic behaviors of the magnetic torque exerted on the Y1Ba2Cua0, are interpreted by the viscous motion of the fluxoids trapped in the superconductor.

[l] Takei, H., Takeya, H., Iye, Y., Tamegai, T. and Sakai, F., Jpn J . Appl. Phys. 26 (1987) L1425. [2] Wolfus, Y. and Yeshurun, Y., Phys.

Rev.

B 37