Role of cationic size mismatch and effect of disorder in mixed valent manganites

Comparative studies of structure, magnetism, and magnetoresistance (MR) have been carried out in A-site ordered NdBaMn2O6 (O-NB), A-site disordered NdBaMn2O6 (D-NB) and A-site disordered NdCaMn2O6 (D-NC). O-NB, where A-site cations, Nd3+ and Ba2+ (of different ionic sizes) are arranged periodically, undergoes structural transition with temperature, while no structural change is present in D-NB where A-site cations are arranged randomly. However, structural transitions are observed in D-NC where Nd3+ and Ca2+ have similar ionic sizes. Magnetization (M) data shows O-NB has AFM ground state associated with a lower structurally symmetric phase and an FM ground state is observed for D-NB with higher structural symmetry. However, AFM ground state is observed in D-NC similar to that of O-NB. Both the disorder systems exhibit semiconductive transport characteristics over the entire temperature range. The resistivity data of disorder compounds have been fitted with different theoretical models to elucidate the conduction process in these systems. Further, MR studies depict a three times higher value of MR in both disorder compounds compared to that of order one. However, the behavior of MR with H is different for D-NB and D-NC, implying a different origin of this large MR in these compounds. We believe that the different magnetic ground state of D-NB and D-NC is the possible origin of their distinct MR behavior to the magnetic field.

Automated summary

Comparative studies were conducted on the structure, magnetism, and magnetoresistance of A-site ordered NdBaMn2O6 (O-NB), A-site disordered NdBaMn2O6 (D-NB), and A-site disordered NdCaMn2O6 (D-NC). The study found that O-NB undergoes a structural transition with temperature, while D-NB does not show any structural change. However, D-NC exhibits structural transitions. Magnetization data showed that O-NB has an antiferromagnetic (AFM) ground state, while D-NB has a ferromagnetic (FM) ground state. D-NC exhibited an AFM ground state similar to O-NB. Both disorder compounds showed semiconductive transport characteristics. Resistivity data of the disorder compounds were fitted with different theoretical models to understand the conduction process. Additionally, the study revealed a three times higher magnetoresistance value in both disorder compounds compared to the ordered one, but the behavior of magnetoresistance with applied magnetic field was different for D-NB and D-NC, suggesting a different origin for their large magnetoresistance. We propose that the different magnetic ground states of D-NB and D-NC may be the possible origin of their distinct magnetoresistance behavior to the magnetic field.