Contact free transport characterization of recent REBCO films in very high magnetic field*

Studies on pinning mechanisms in high temperature superconductors often require detailed knowledge of critical current density $J_c$ as a function of magnetic field orientation as well as field strength and temperature. However, recent REBa$_{2}$Cu$_{3}$O$_{x}$ (RE=rare earth elements) thin films can achieve remarkably high critical current density values by the incorporation of nonsuperconducting nanoparticles, that often pose problems with $J_c$ measurements, due to extremely high Lorentz force, Joule heating etc. in the limited space of high field magnets. Here we demonstrate the angularly dependent $J_c$ measured by means of contact-free vector magnetization measurements in a vibrating sample magnetometer equipped with both longitudinal and transverse pickup coils. The studies complement traditional transport techniques and are readily extended to conditions of fields and temperatures where the current density is very large and transport methods become difficult. Our results clearly show an evolution of pinning from strongly correlated effects seen at high temperatures to significant contributions from dense but weak pins that thermal fluctuations render ineffective at high temperatures but which become strong at lower temperatures