μSR investigation of magnetically ordered states in the A-site ordered perovskite manganites RBaMn2O6 (R = Y and La)

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 between these materials. For LaBaMn2O6, the mu SR time spectra in the ferromagnetic state below similar or equal to 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 mu SR relaxation rates is inferred down to the lowest temperatures. This behavior is similar to that in the A-site disordered La0.5Ba0.5MnO3. On the other hand, for YBaMn2O6, the mu SR 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.