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A strong effect of sample size on magnetic Jc(H) was observed for bulk MgB2when Jc is obtained directly from the critical state model. Thus obtainedzero-field Jc (Jc0) decreases strongly with the sample size, attaining aconstant value for the samples larger than a few millimetres. On the otherhand, the irreversibility field (Hirr) defined at Jc = 100 A/cm2 increases withthe sample size. The decrease of Jc0 is described in terms of voids in the bulkMgB2 samples and superconducting screening around the cells of superconductingmaterial between these voids (35 micro-m), because of concentration of thecurrent in the narrow bridges connecting the cells. For samples larger than afew millimetres, the value of magnetic Jc is in agreement with the transport Jcand it is restricted by the voids. The critical state model is not suitable forobtaining Jc for small bulk MgB2. The increase of Hirr with the sample size isan artefact of defining Hirr by the value of Jc at which an additionalsuperconducting screening on 1mm scale dominates Dm

Effect of sample size on magnetic Jc for MgB2 superconductor