Anomalous surface impedance in reentrant ferromagnetic superconductors

We report evidence which suggests surface ferromagnetism in a ferromagnetic superconductor arising from the competition between the Meissner current shielding effect and the fluctuations of the localized magnetic moments. We have measured the surface impedance penetration depth Λ of ErRh4B4 and Er0.5Ho0.5Rh4B4 by employing a spectrometer operated at 9 GHz. For ErRh4B4, owing to the critical spin fluctuations just above Ts (>Tc2), the critical temperature at which surface ferromagnetism appears, Λ−1 decreases smoothly as temperature decreases toward Ts. For Er0.5Ho0.5Rh4B4, the decrease of Λ−1 arising from spin fluctuations for T>Tc2 is very small, and Λ−1 decreases abruptly at Tc2. We will discuss the theory and show how the predicted values of Λ(T) compare well with the observed values.