Josephson effects in ferromagnetic superconductors: Noise effect due to critical spin fluctuations

A theory for the Josephson effect in the ferromagnetic superconductors is presented. Taking account of critical fluctuations of the localized-spin magnetization, we calculate the magnetic field dependence of the maximum Josephson current and I-V characteristics. The Josephson current is reduced by a noise effect arising from the critical spin fluctuations at low temperatures. The anomalous behaviors observed in the dc Josephson effect in ErRh4B4-LuxOy-In junctions are interpreted on the basis of present theory.