The effect of columnar defects on the pinning properties of NdFeAsO0.85 conglomerate particles

An oxypnictide superconductor NdFeAsO0.85 sample with transition temperature of 46 K was irradiated with 2 GeV Ta ions at a fluence of 5 x 10(10) ions cm(-2). High resolution transmission electron microscopy study revealed that the irradiation produced columnar-like defects. The effect of these defects on the irreversible magnetization in polycrystalline randomly oriented fragments was studied as a function of field angle and field sweep rate. We find that the critical current density is moderately enhanced at fields below the matching field (similar to 1 T). The pinning enhancement is anisotropic and maximum along the defect direction at 35 K, but becomes more isotropic as the temperature is decreased. The creep rate is suppressed at 35 K and at fields below the matching field, indicating that the columnar defects are efficient pinning sites under these H and T conditions.