Transport properties of Na doped La1 xCax yNayMnO3measured in a pulsed magnetic field

Temperature-dependent magnetization and magnetoresistance of the Na doped La 1−x Ca x −y Na y MnO 3 type samples with x = 0.3 and 0 ≤ y ≤ 0.15, showing a rhombohedrally distorted perovskite structure with space group R c , 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−x Ca x−y Na y MnO 3 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 = ρ 0 + ρ 2 T 2 , signifying the importance of the electron–magnon scattering process (the ρ 2 T 2 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.