Paramagnetic Meissner effect in YBa₂Cu₃O_7−δ: A non-resonant microwave absorption study

The low-field dependence of non resonant microwave absorption (MWA) in polycrystalline samples of YBa₂Cu₃O_7−δ with 0.1 ≤ δ ≤ 0.5 has been measured at T < Tc along with simultaneous monitoring of paramagnetic impurities in the chain copper (Cu-I) sites by EPR spectroscopy. MWA exhibits a cross-over from the normal zero-field minimum to an anomalous zero-field maximum accompanied by the gradual appearance and growth of the EPR signal as δ is changed from 0.1 to 0.5. It is to be noted that in the δ = 0.3 − 0.4 composition range, oxygen vacancies are disordered giving rise to magnetic moments due to isolated Cu²⁺ impurity ions. This correlation of the anomalous MWA with isolated paramagnetic Cu²⁺ impurities is consistent with the presence of π junctions, earlier implicated in the paramagnetic Meissner effect (PME), which in turn is related to the anomalous MWA.