Glassy behaviour of the ferromagnetic and the non-magnetic insulating states of the rare earth manganates Ln_0.7Ba_0.3MnO₃(Ln = Nd or Gd)

While La0.7Ba0.3MnO3 is a ferromagnetic metal (TC = 340 K)with long-range 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 behaviour 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 La1−xSrxCoO3. Gd0.7Ba0.3MnO3, however, shows non-equilibrium dynamics characteristic of spin glasses, below 36 K. The difference in nature of the glassy behaviour 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 behaviour.