Enhancement of critical current density in helium ion irradiated Ba(Fe, Co) ₂As ₂ thin films

The effect of 600 keV He ⁺ ion irradiation on the temperature and magnetic field dependence of the critical current density J _C in high quality BaFe _1.84Co _0.16As ₂ (Co-doped Ba122 type) thin films is investigated. The films are prepared by pulsed-laser-deposition (PLD) on CaF ₂ (00) substrates. The irradiation dosages are varied between 1 × 10 ¹³ and 1 × 10 ¹⁶ cm ^-2. Upon irradiation, the superconducting transition temperature T _C drops slightly from 25 K for the unirradiated sample to about 20 K for the sample with the highest irradiation level. The J _C values of the thin film samples are calculated by using the Bean critical state model. The results showed that J _C could be enhanced substantially. The maximum J _C value at 4.5 K temperature is enhanced up to 2.4 MA cm ^-2 under 1 T field. The analysis of pinning force dependence on magnetic field shows that the pinning behavior is not changed in the irradiated samples, suggesting more pinning centers of similar nature to those of presented in the unirradiated samples are introduced by the irradiation process. The results indicate that the irradiation of light element ions He ⁺ with relatively low energy could increase the critical current density in iron based superconductors.