RF-induced flux lattice annealing (RIFLA) in the electron-doped superconductor Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$.*

A strained flux lattice (FL) in a superconductor (SC) can be annealed to a lower free energy by the RF-field used to generate an NMR spin-echo signal [W.G. Clark et al., J. Phys. IV Proceedings \textbf{9}, Pr10-49-52 (1999)]. Here, this effect is reported for the SC phase of Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$. The distorted FL is prepared by rotating the sample through a small angle in a magnetic field well below the transition temperature, which leaves the FL pinned in a non-equilibrium configuration. Then, the first spin echo has almost no amplitude. Subsequent echoes have a progressively larger amplitude until a final, steady value is reached. The interpretation is that shaking of the distorted vortex lattice by the RF pulses progressively anneals it to a lower free energy configuration. The corresponding change in the local field between the de-phasing and re-phasing pulse for each echo then reduces the height of the echo.