Charge transport and magnetic properties of bilayer manganites

This dissertation is a study of the charge transport and magnetic behavior of two bilayer manganites: La_1.2Sr_1.8Mn₂O ₇ and LaSr₂Mn₂O₇. The temperature T and magnetic field H dependences of the anisotropic in-plane ρ_ab and out-of-plane ρ_c resistivities are investigated on these two systems in order to probe the different conduction and scaling mechanisms. Both ρ_ab(T) and ρ_c( T) of La_1.2Sr_1.8Mn₂O₇ single crystals follow a T9/2 dependence in the metallic regime (50 ≤ T ≤ 110 K), consistent with the two-magnon scattering. In the nonmetallic regime (T < 50 K), both the in-plane σ_ab(T) and out-of-plane σ_c(T) conductivities obey a T1/2 dependence, consistent with weak-localization effect. These results demonstrate the three-dimensional nature of the bilayer manganite La_1.2Sr_1.8Mn₂O ₇ for T < T_C.; In the paramagnetic state (T > 218 K), the zero field in-plane resistivity of La_1.2Sr_1.8Mn₂O ₇ single crystals obeys the adiabatic small polaron hopping mechanism, while the out-of-plane resistivity can be ascribed by an Arrhenius law with the same activation energy as the one for the in-plane resistivity. Considering the magnetic character of the polarons and the close correlation between the resistivity and magnetization, we developed a model which allows the determination of ρ_ab,c(H,T). The excellent agreement between the calculations and the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of La_1.2Sr_1.8Mn₂O₇.; Both the in-plane and out-of-plane resistivities for LaSr₂Mn ₂O₇ single crystals follow the variable-range-hopping behavior in the temperature range from 220 K to 300 K. The magnetoresistivity ln[ρ( T, H,0)/ρ(T,0)] is proportional to H2 and sin2 $4$. These results provide convincing evidence of the SE-type variable-range-hopping conductivity in half-doped manganites.; We performed magnetization measurements on LaSr₂Mn₂O ₇ single crystals as a function of temperature, and magnetic field over a wide T range and in magnetic fields up to 5 T. We found that this manganite displays a spin glass behavior at temperatures below 20 K. The susceptibility follows the standard scaling theory; i. e., χ ∼ H1/δ−1 for 20 K ≤ T ≤ 100 K. The susceptibility is field-independent and only temperature-dependent above 100 K.