Unidirectional pinning in irradiated Bi₂Sr₂CaCu ₂O₈ (invited)

Bi₂Sr₂CaCu₂O₈ crystals were irradiated with heavy ions (Pb, Xe) to produce columnar defects along the c direction or at 45°with respect to it. We describe the dependence of the critical currents, irreversibility fields, pinning force, and magnetic relaxation rates on the dose and on the type of ions. The irreversible properties of the Pb-irradiated samples are more enhanced than those of the Xe-irradiated sample at low temperatures, and vice versa at high temperatures. At low temperatures the efficiency of the columnar defects is enhanced with the ion dose but, at high temperatures, the maximum dose (2.5×10¹¹ ions/cm²) is less efficient than intermediate doses. The unidirectional nature of the columnar pinning centers serves to demonstrate that the vortices maintain linelike features. For most of the irradiated samples, the pinning force density data are reduced to a single curve below 45 K. The relaxation rates at low temperatures reflect thermal reduction of the effective pinning barriers.