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)

Abstract

While La_0.7Ba_0.3MnO₃ is a ferromagnetic metal (T_C=340 K)with long-range ordering, Nd_0.7Ba_0.3MnO₃ shows a transition around 150 K with a small increase in magnetization, but remains an insulator at all temperatures. Gd_0.7Ba_0.3MnO₃ 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 Nd_0.7Ba_0.3MnO₃, and a complex behaviour with different ordering/freezing transitions at 62, 46 and 36 K in the case of Gd_0.7Ba_0.3MnO₃, 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 Nd_0.7Ba_0.3MnO₃ is a cluster glass below 150 K, a situation similar to that found for La_1−xSr_xCoO₃. Gd_0.7Ba_0.3MnO₃, however, shows non-equilibrium dynamics characteristic of spin glasses, below 36 K. The difference in nature of the glassy behaviour between Gd_0.7Ba_0.3MnO₃ and Nd_0.7Ba_0.3MnO₃ 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 Nd_0.7Ba_0.3MnO₃ and Gd_0.7Ba_0.3MnO₃ suggest that the magnetic insulating states often reported for rare earth manganates of the type Ln_1−xA_xMnO₃ (Ln=rare earth, A=alkaline earth) are likely to be associated with glassy magnetic behaviour.