Low-temperature electrical transport in bilayer manganite La_1.2Sr_1.8Mn₂O₇

The temperature T and magnetic-field H dependences of anisotropic in-plane ρ_ab and out-of-plane ρ_c resistivities are investigated in single crystals of the bilayer manganite La_1.2Sr_1.8Mn₂O₇. Below the Curie transition temperature T_c = 125 K, ρ_ab and ρ_c display almost the same temperature dependence with an up-turn around 50 K. In the metallic regime (50 K ≤ T ≤ 110 K), both ρ_ab(T) and ρ_c(T) follow a T^9/2 dependence, consistent with the two-magnon scattering. We found that the value of the proportionality coefficient B^fil_ab and the ratio of the exchange interaction J_ab/J_c, obtained by fitting the data, are in excellent agreement with the calculated B_ab based on the two-magnon model and J_ab/J_c deduced from neutron scattering, respectively. This provides further support for this scattering mechanism. At even lower T, in the nonmetallic regime (T <50 K), both the in-plane σ_ab and out-of-plane σ_c conductivities obey a T^1/2 dependence, consistent with weak-localization effects. Hence this demonstrates the three-dimensional metallic nature of the bilayer manganite La_1.2Sr_1.8Mn₂O₇ at T<T_c.