  The magnetically ordered states of the A-site ordered perovskite manganites
LaBaMn2O6 and YBaMn2O6 have been investigated by muon spin relaxation in zero
external magnetic field. Our data reveal striking differences in the nature of
the magnetically ordered state of these materials. For LaBaMn2O6, the muSR
time-spectra in the ferromagnetic state below ~ 330 K reveal a strongly
inhomogeneous phase, reminiscent of a Griffiths phase. Within this magnetically
inhomogeneous phase, an antiferromagnetic state develops below 150 K, which
displays well defined static internal magnetic fields, but reaches only 30% of
the volume fraction at low temperatures. A broad distribution of muSR
relaxation rates is inferred down to the lowest temperatures. This behavior is
similar to the A-site disordered La0.5Ba0.5MnO3. On the other hand, for
YBaMn2O6, the muSR time spectra for both (i) the charge and orbital ordered and
(ii) the paramagnetic phases reveal rather homogeneous states, namely, an
exponential relaxation in the paramagnetic state and well defined muon spin
oscillation in the antiferromagnetic state.
