Transport properties of Na doped La1−xCax−yNayMnO3 measured in a pulsedmagnetic field

Abstract

Temperature-dependent magnetization and magnetoresistanceof the Na doped La1−xCax −yNayMnO3 type samples with x = 0.3 and 0 ≤ y ≤ 0.15,showing a rhombohedrally distorted perovskite structure with space group R3̄c,have been studied under a pulsed magnetic field. With the increasein temperature from the low temperature ferromagnetic phase,the samples exhibit a sharp metal to insulator transition aroundTpaccompanied by a ferromagnetic (metallic) to paramagnetic(semiconducting) phase transition with a well defined Curie temperatureTC (almostequal to Tp).Interestingly, small Na doping largely increases the conductivity andTp,which is associated with a decrease of the (e–ph) interaction constant (γ).Doping of monovalent Na in the Ca site of La1−xCax−yNayMnO3drives the system from a high resistivity regime with lowerTp to a lower resistivityregime with higher Tpvalues. Low temperature resistivity data fit well the relation ρ = ρ0 + ρ2 T2,signifying the importance of the electron–magnon scattering process (the ρ2 T2term). On the other hand, the high temperature (T > Tp upto 320 K) conductivity data satisfy the variable range hopping (VRH) model. ForT > 320 the smallpolaron hopping model is more appropriate than the VRH model. Even with a very small change ofy, the density of states atthe Fermi level N(EF)changes considerably. The resistivity of these materials measured under pulsedand continuous DC magnetic fields behaves in an identical fashion. The relaxationtime (decay time of the magnetic pulse within the sample) varies with fieldstrength, which indicates that, with a change of magnetic field, the ordering ofspin in the ferromagnetic regime changes.