Transport properties of Na doped La_1−xCa_x−yNa_yMnO₃ measured in a pulsed magnetic field

Temperature-dependent magnetization and magnetoresistance of the Na doped La_1−xCa_{x −y}Na_yMnO₃ type samples with x = 0.3 and 0 ≤ y ≤ 0.15, showing a rhombohedrally distorted perovskite structure with space group Rc, have been studied under a pulsed magnetic field. With the increase in temperature from the low temperature ferromagnetic phase, the samples exhibit a sharp metal to insulator transition around T_p accompanied by a ferromagnetic (metallic) to paramagnetic (semiconducting) phase transition with a well defined Curie temperature T_C (almost equal to T_p). Interestingly, small Na doping largely increases the conductivity and T_p, which is associated with a decrease of the (e–ph) interaction constant (γ). Doping of monovalent Na in the Ca site of La_1−xCa_x−yNa_yMnO₃ drives the system from a high resistivity regime with lower T_p to a lower resistivity regime with higher T_p values. Low temperature resistivity data fit well the relation ρ = ρ₀ + ρ₂ T², signifying the importance of the electron–magnon scattering process (the ρ₂ T² term). On the other hand, the high temperature (T > T_p up to 320 K) conductivity data satisfy the variable range hopping (VRH) model. For T > 320 the small polaron hopping model is more appropriate than the VRH model. Even with a very small change of y, the density of states at the Fermi level N(E_F) changes considerably. The resistivity of these materials measured under pulsed and continuous DC magnetic fields behaves in an identical fashion. The relaxation time (decay time of the magnetic pulse within the sample) varies with field strength, which indicates that, with a change of magnetic field, the ordering of spin in the ferromagnetic regime changes.