Hysteretic and non-hysteretic a.c. losses in detwinned YBa2Cu3O7−δ single crystals

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PHYSICA 8 Physica C 282-287

ELSEVIER

Hysteretic

and non-hysteretic

(1997) 2035-2036

a.c. losses in detwinned

YBa$u307_s

single crystals

D. Bracanovica, S. Gordeeva, S. Pinfolda, M. Oussenaa, P.A.J. de Groota, R. Gagnonb and L. Tailleferb a Physics Department, University of Southampton, Southampton SO17 IBJ, UK b Physics Department, McGill University, Montreal, Quebec H3A 2T8, Canada The amplitude of the 2” peak is found to be strongly temperature dependent in a wide temperature region below the melting line. We explain this effect by a crossover from a hysteretic (at low T) to non-hysteretic (at high T) mechanism of the ac. losses. 1. INTRODUCTION

3. RESULTS AND DISCUSSION

The critical state (CS) model [l] is widely used to interpret the a.c. susceptibility data of superconductors. We have demonstrated that in YBa2Cu307_dsamples with weak bulk pinning, the low frequency a.c. losses have a large contribution from a non-hysteretic mechanism. Therefore, the applicability of the CS model is limited.

The peak in the imaginary part of the a.c. susceptibility, x”, marks the coincidence of the flux (current) penetration depth with the relevant sample dimension (times a geometrical factor) [3]. According to the CS model the height of the peak xp”=O.241 is temperature independent and occurs at h,P=0.97Jc/d (for a disk shaped sample), where d is the sample thickness and Jc is the critical current density [4]. This formula is often used for estimation of the Jc values,

2. EXPERIMENTAL The ac. susceptibility measurements were performed using a miniature, 3 mm internal diameter, coaxial mutual-inductance system. It consists of a primary excitation field coil and a secondary pick-up coil pair wound in serial opposition so that the mutual inductance of each with respect to the field coil is the same. All measurements were performed in d.c. magnetic field Hd,=2.2 T and superimposed small ac. field (haf) parallel to the crystalline c-axis. The amplitude of the ac. field was varied between 3 and 500 pT, and the modulation frequency v was between 39.9 Hz and 4 kHz. The sample was field cooled with data collected on warming. The YBazCu30,_s single crystal used in our studies (dimensions 1.69x1.08x0.09 mm3) was grown by a self-flux method and detwinned under an uniaxial pressure as described in [2].

0921-4534/97/$17.00 0 Elsevier Science B.V. All rights reserved. PI1 SO921-4534(97)01106-4

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1. Temperature dependence of x” at &,=2.2T for several values of ac. field (50, 62.5. 75, 87.5, 100, 125, 150, 175, 200, 250, 300 and 4OOpT). The dashed arrow shows the direction of h,, increase. T, is the melting temperature obtained from resistivity measurements. hax” and xp” are maximum and peak values, respectively, for T=79.72 K. Figure

D. Bracanovic

2036

et al./Physica

Fig. 1 presents x”(T) dependence at &.c=2.2T for several amplitudes of h,,. The sharp x” onset marks the melting temperature T,,,. Our transport measurements show a sharp drop of ohmic resistivity at the same temperature. As seen from fig. 1, the peaks which occurred at lower temperature had higher amplitude xp”. As a result, the amplitude of the peak does not represent the maximum x” value for a given temperature (kax”). In fig. 2 we plot the values of the applied ac field, as a function of temperature, at which the xp” and x”- were observed. It can be seen from the figure that these two criteria give different h,, values at higher temperatures (TCT,,,). At low temperatures, where the height of the peak becomes close to the value predicted by the CS model (0.241) the two curves converge. We conclude that the CS model is applicable at sufficiently low temperatures. -

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REFERENCES

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explained in terms of a continuous transition from reversible vortex behaviour at high temperature to irreversible behaviour at lower temperatures. The model of Matsushita et al. [5] cannot explain the results given in this paper. It predicts frequency independent x” behaviour whereas our results, as illustrated in the insert of fig. 2, show a clear increase of x” with frequency. This frequency dependence is a typical signature of the eddycurrent dissipative regime, for which the penetration depth is F-[p(T)lv]‘” (where p(T) is temperature dependent ohmic resistivity). Hence, the x”(T) behaviour shown in fig. 1, can be explained as follows. The initial onset of dissipation below T, is caused by eddy current losses. As the temperature decreases the behaviour of the vortices becomes more and more irreversible, leading to an increases in the magnitude of xp”. At sufficiently low temperatures the dissipation is caused by hysteretic a.c. losses and the CS model can be applied.

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(1997) 2035-2036

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80 82 84 86 88 T(K) Figure 2. Temperature dependence of the ac. field, at which the x”(T) dependence has maximum (b”) or peak (xp”) value. The inset shows the x”(T) dependence for two different modulation frequencies v (393 and 2978 Hz). The dashed arrow shows the direction of v increases. The ac. field is h,,= 25 pT. One of the possible explanations for the increase in the magnitude of xp” with decreasing temperature is that given by Matsushita et al. [5]. They showed that the reversible fluxoid motion within individual pinning wells can reduce dissipation. Incorporating the Campbell [6] and CS models they demonstrated that the anomalous behaviour of the x” peak magnitude can be

1. C. P. Bean, Rev. Mod. Phys. 2 (1964) 3 1. 2. R. Gagnon et al., Phys. Rev. B 50 (1994) 3458. 3. V. B. Geshkenbein et al., Phys. Rev. B 43 (1991) 3748. 4. J. R. Clem and A. Sanchez, Phys. Rev. B 50 (1994) 9355. 5. T. Matsushita et al., Physica C 182 (1991) 95. 6. A. M. Campbell, J. Phys. C 4 (1971) 3186.