The effect of columnar defects on the pinning properties of NdFeAsO _0.85 conglomerate particles

An oxypnictide superconductor NdFeAsO_0.85 sample with transition temperature of 46K was irradiated with 2GeV Ta ions at a fluence of 5 × 10¹⁰ionscm^-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 (∼1T). The pinning enhancement is anisotropic and maximum along the defect direction at 35K, but becomes more isotropic as the temperature is decreased. The creep rate is suppressed at 35K and at fields below the matching field, indicating that the columnar defects are efficient pinning sites under these H and T conditions.