  While La0.7Ba0.3MnO3 is a ferromagnetic metal (TC = 340 K) with longrange
ordering, Nd0.7Ba0.3MnO3 shows a transition around 150 K with a small increase
in magnetization, but remains an insulator at all temperatures. Gd0.7Ba0.3MnO3
is non-magnetic and insulating at all temperatures. Low field dc magnetization
and ac susceptibility measurements reveal the presence of a transition at
around 150 K in Nd0.7Ba0.3MnO3, and a complex behavior with different
ordering/freezing transitions at 62, 46 and 36 K in the case of Gd0.7Ba0.3MnO3,
the last one being more prominent. The nature of the field dependence of the
magnetization, combined with the slow magnetic relaxation, ageing and memory
effects, suggests that Nd0.7Ba0.3MnO3 is a cluster glass below 150 K, a
situation similar to that found for La_{1-x}SrxCoO3. Gd0.7Ba0.3MnO3, however,
shows non-equilibrium dynamics characteristic of spin glasses, below 36 K. The
difference in nature of the glassy behavior between Gd0.7Ba0.3MnO3 and
Nd0.7Ba0.3MnO3 probably arises because of the larger disorder arising from the
mismatch between the sizes of the A-site cations in the former. Our results on
Nd0.7Ba0.3MnO3 and Gd0.7Ba0.3MnO3 suggest that the magnetic insulating states
often reported for rare earth manganates of the type Ln1-xAxMnO3 (Ln = rare
earth, A = alkaline earth) are likely to be associated with glassy magnetic
behavior.
