Josephson plasma in RuSr2βGdCu2βO8β,
Ru1βxβSr2βGdCu2+xβO8β (x = 0.3), and
RuSr2βEu2βxβCexβCu2βO10β (x = 0.5) compounds is
investigated by the sphere resonance method. The Josephson plasma is observed
in a low-frequency region (around 8.5 cmβ1 at T βͺTcβ) for
ferromagnetic RuSr2βGdCu2βO8β, while it increases to 35 cmβ1
for non-ferromagnetic Ru1βxβSr2βGdCu2+xβO8β (x = 0.3), which
represents a large reduction in the Josephson coupling at ferromagnetic
RuO2β block layers. The temperature dependence of the plasma does not shift
to zero frequency ({\it i.e.} jcβ = 0) at low temperatures, indicating that
there is no transition from the 0-phase to the Ο-phase in these compounds.
The temperature dependence and the oscillator strength of the peak are
different from those of other non-magnetic cuprates, and the origins of these
anomalies are discussed.Comment: to appear in Phys. Rev.B Rapid Com