Dissipative Effects of Vortex Movements in YBa 2 Cu 3 O 7 Measured by Magnetothermal Effects
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DISSIPATIVE EFFECTS OF VORTEX MOVEMENTS INYBa 2Cu 30 7 MEASURED BY MAGNETOTHERMAL EFFECTS Oh. SIMON 0 , I. ROSENMAN°, L. LEGRAND°, G. COLLIN+ "°Groupe de Physique des Solides, Universit• Paris 7, Laboratoire 17 associ• au Centre National de Ia Recherche Scientifique, 75251 Paris Cedex 05, France +Laboratoire de Physique des Solides, Bat 510, Universit• Paris sud, 91405 Orsay Cedex, France. ABSTRACT We have measured the temperature and the magnetization of singlecrystalline YBaCuO samples in a variable magnetic field under quasi adiabatic conditions. By varying the sweep rate of the field, the thermal link to the bath, the size of the sample, the critical temperature and the sample temperature, we have measured the time dependence of the vortex relaxation (the penetration of the critical state, flux jumps, and the instability regime). This magnetothermal instability controls the value of the effective critical current. INTRODUCTION In the high Tc superconductors the superconducting coherence length • is very small (10 A or less), inducing smaller pinning energy for vortices [1] than in "old" superconductors (• = 3000 A). Since the Tc itself is larger, there exists a range of temperature where the ratio of the pinning energy versus kBT varies over a wide range and allows a study of the pinning models which was impossible in classical materials. Existing theories such as that of KimAnderson [2] can be really tested. The existence of a finite critical current in classical superconductors causes an irreversibility of the flux penetration and hence of the magnetization. This effect is well described by the Bean model [3] which assumes that the critical current is established on the edges of the sample and screens the variations of the magnetic field. This simple model is also valid in high Tc superconductors. The irreversibility line (the critical current vanishes and the magnetization becomes reversible above a given temperature) observed in many experiments in torque or critical current [4] measurements is usually interpreted by a melting of the vortex lattice. The time relaxation of the vortex lattice was also studied by magnetization measurements [5] which shows that the magnetization relaxes to the equilibrium as the logarithm of the time, following the Kim-Anderson model of the creep. The flux jump phenomenon is known as a strong limitation for the use of superconductors. It is an instability of the superconductor: the flux starts to relax to the equilibium state (reversible) and hence heats the sample, so the flux movement is more easy so it heats more, till it reaches the normal state. Few flux jumps were reported in new superconductors [6,7]. More generally, the stability of the superconducting state relative to magnetothermal instabilities is discussed in detail by Tinkham [8]. EXPERIMENTAL TECHNIQUE We present here a new approach to this problem by measuring at the same time the temperature and the magnet~ation in a high Tc superconductor. This very powerful technique allows one to follow both aspects of magne
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