Non-resonant Microwave Absorption in Nano Nickel Added YBCO Powders: Observation of Multiple Phase Reversals

We report on the non-resonant microwave absorption in the system of a nominal 2 wt % nano nickel particles added into YBCO powders (Ni–YBCO). With this dilute mixture of nano nickel particles, one is expected to have groups of normal Josephson junctions (JJs) and π JJs due to YBCO–nickel–YBCO interparticle weak links and as nickel is ferromagnetic. We experimentally show, for the first time multiple phase reversals in the non-resonant microwave absorption (NRMA) spectra from Ni–YBCO possibly due to the formation of π JJs. We also showed that these multiple phase reversals then depend on microwave power and temperature. We argue that microwave power induced coherence among some groups of JJs and breaking of some of the weaker JJs can then lead to the disappearance of multiple phase reversals at higher microwave power levels. Further, we also reported a role of pair breaking effects that shall give a linear field dependence of the derivative microwave absorption signal, essentially which is the NRMA signal. This pair breaking effect dominates at closer temperatures to T which is expected thermodynamically.