Role of weak uncorrelated pinning introduced by BaZrO3 nanorods at low-temperature in (Y,Gd)Ba2Cu3Ox thin films

REBa2Cu3Ox (REBCO) films can achieve remarkably high critical current density Jc values by the incorporation of insulating nanoparticles. A particularly interesting case concerns BaZrO3 (BZO) nanorods which strongly enhance Jc at high temperatures and fields up to a few tesla. Here we investigate the full angular transport Jc over a much broader range of field (up to 31 T) and temperature (4.2-77 K), a range suitable for new very high field magnet applications. We show that the correlated c-axis pinning of BZO nanorods becomes progressively less obvious at lower temperatures and indeed at 4.2 K up to 31 T, the only visible correlated pinning is for fields parallel to the film plane. Jc at 4.2 K is still, however, strongly enhanced by the BZO nanorods, as is clearly manifested by a remarkably high bulk pinning force density Fp ? 900 GN/m3 for H?c, because the BZO nanorods add weak but very dense pins which contribute up to half of Jc in the low temperature limit. Above about 30 K these dense but weak pins lose their effectiveness and Jc then becomes dominated by the sparse but strong BZO pins. This strong temperature dependence of the dominant pinning effects of multiple pin types makes broad range characterization essential to reveal the great flexibility of pinning in modern REBCO films that is almost certainly not yet fully optimized.