Large Enhancement of J C in YBacuo Single Crystals Induced by Proton Irradiation
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LARGE ENHANCEMENT OF J, IN YBaCuO SINGLE CRYSTALS INDUCED BY PROTON IRRADIATION L. CIVALE, A.D. MARWICK, M.W. McELFRESH, A.P. MALOZEMOFF AND F. HOLTZBERG, IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598-0218
ABSTRACT We have used magnetization measurements to study the effect of proton irradiation on the critical current density of a YBaCuO single crystal. We have obtained a maximum value of 1.5x l05 A/cm 2 at 77 K and 1 T at a fluence of 2 x 1016 cm-2 , representing a 50-fold increase with respect to the unirradiated sample.
INTRODUCTION The low values of the critical current density Jc of bulk high Tc superconductors is a significant limitation to their potential technological application. Since the discovery of these materials, several attempts have been made to increase Jc by introducing flux pinning centers with various types of radiation. Most work has been on neutron irradiation, and large enhancements of J, have been observed in both ceramics [1] and crystals [2,3], with a recent report [4] of a 100-fold increase of J, for a YBaCuO single crystal at 77 K and 9kOe. Proton irradiation offers some advantages including the use of monoenergetic particles, a low level of induced radioactivity and short irradiation times. Although the low penetration range of protons precludes their use with thick samples, they can be used to generate defects in YBaCuO single crystals of typical thickness (-30pm). The report of van Dover et al. [5] and our work presented here represent the first studies of the effect of proton irradiation on the critical current of YBaCuO single crystals. At T = 77 K and H = I T, we have obtained a maximum J, of 1.5x105 A/cm 2, representing a 50-fold increase with respect to the original value. EXPERIMENTAL RESULTS AND DISCUSSION We have measured magnetization as a function of applied field, M(H), and have determined Jc as a function of proton fluence using the critical state Bean model. The studies were performed on a platelet-like YBaCuO single crystal of dimensions 0.6x0.7x0.03 mm 3 with the c axis parallel to the shorter dimension. The initial Tc was 93.4 K, having a transition width of about 0.3 K, as determined by a.c. susceptibility measurements. Details of sample preparation and characterization have been reported previously [6]. The crystal was irradiated at room temperature with a scanned beam of 3 MeV protons at a flux of 3 x 1012 cm-2 sec-' . The sample was fixed with silver paint on a copper plate in order to obtain adequate heat sinking. The temperature of the copper plate was measured several times during the irradiation, and no significant heating was observed. Measurements of M(H) were made in a Quantum Design SQUID magnetometer in the Hujc-axis configuration, with T = 5 K and 77 K and fields of up to 5.5 T. Figure 1-a and b show the M(H) curves of the crystal at T = 5 K and 77 K before and after irradiation to a dose of 2 x 101" cm-2 . A large enhancement of the hysteresis is apparent from these plots. We have experimentally found that if M(H) is measured too s
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