EFFECT OF SURFACE BARRIERS AND LOWER CRITICAL FIELDS TO MAGNETIZATIONOF TYPE-II SUPERCONDUCTORS

To determine the effect of the surface barrier Delta H and the lower c ritical field H-cl :o magnetization of type-II superconductors, we hav e calculated the initial magnetization curves and full hysteresis loop s within the framework of the modified Kim-Anderson critical-state mod el, where Delta H and H-cl are explicitly taken into consideration. A sample is located in an external field H(omega t)=H-dc + H-ac cos(omeg a t), where H-dc (greater than or equal to 0) is a DC bias field and H -ac (> 0) is an AC field amplitude. Derivations of the magnetization e quations were carried our on the assumption that the critical-current density J(c) is a function of the local internal magnetic-flux density B-i, J(c)(B-i)=k/(B-0 + \B-i\), where k and B-0 are constants. In add ition, we used the effective magnetic field inside the specimen H-eff for evaluations of B-i, where H-eff is assumed to have the form H-eff= H-(H/\H\)H-cl-[(dH/dt)/\dH/dt\]Delta H. We consider an infinitely long cylinder with radius a and the applied field along the cylinder axis. Denoting the maximum and minimum values of H by H-A (= H-dc + H-ac) a nd H-B (= H-dc-H-ac), respectively, four types of hysteresis loops app ear, depending on the magnitude of H-A. Among these, three types are f urther classified into several cases, depending on the magnitude of H- B. To describe completely the descending and ascending branches of the loops for all the cases, 113 stages of H are considered. To verify th e present derivations, all the equations were confirmed to be continuo us at their end points. Some typical hysteresis loops computed using t he appropriate magnetization equations are demonstrated. From the resu lts, we recognize the effect of Delta H and H-cl on the hysteresis loo ps. Delta H merely expands the M(H) curves up and down with the increa se of Delta H, while H-cl introduces a step-like feature into the hyst eresis loops. From measurements of the difference in level of the magn etization under the applied field of opposite directions, one can prec isely determine H-cl. (C) 1998 Elsevier Science B.V.