Development of pulsed magnetic field and study of magnetotransport properties of K-doped La1-xCax-yKyMnO3CMR materials

Temperature-dependent magnetization, magnetoresistance and magneto-thermoelectric power of the K doped La1−xCax−yKyMnO3 type samples with x=0.3 and 0⩽y⩽0.15 has been studied. All the samples exhibit sharp metal–insulator transition (MIT) around Tp accompanied by a ferromagnetic (metallic) to paramagnetic (semiconducting) phase transition with a well-defined Curie temperature TC (almost equal to Tp). Doping of monovalent K in the divalent Ca site of La1−xCax−yKyMnO3 drives the system from a high resistivity regime with lower Tp to a lower resistivity regime with higher Tp. Systematic increase of Curie temperature with increase of K doing is observed from the magnetization measurement down to 5 K. Low temperature resistivity (ρ) and thermoelectric power (Seebeck coefficient, S) data well fit the relations ρ=ρ0+ρ2T2 and S=S0+S3/2T3/2+S4T4, respectively, signifying the importance of electron–magnon scattering process (ρ2T2 and S3/2T3/2 term). On the other hand, the high temperature (T>Tp upto 320 K) conductivity data satisfy the variable range hopping (VRH) model. For T>320 K small polaron hopping model is more appropriate than the VRH model. High temperature thermoelectric power (TEP) data also indicates the formation of thermally activated small polarons. Even with very small change of y, the density of states at the Fermi level N(EF) changes considerably. The magnetotransport properties have been measured under pulsed magnetic field of microsecond duration. The decay time of the magnetic pulse within the sample (ι) varies with field strength, which indicates that with change of magnetic field, ordering of the spin in the ferromagnetic regime changes.