The temperature $T$ and magnetic field $H$ dependence of anisotropic in-plane $\rho_{ ab}$ and out-of-plane $\rho_{ c}$ resistivities have been investigated in single crystals of the bilayer manganite La$_{ 1.2}$Sr$_{ 1.8}$Mn$_{ 2}$O$_{ 7}$. Below the Curie transition temperature $T_c=$ 125 K, $\rho_{ ab}$ and $\rho_{ c}$ display almost the same temperature dependence with an up-turn around 50 K. In the metallic regime (50 K $\leq T \leq$ 110 K), both $\rho_{ ab}(T)$ and $\rho_{ c}(T)$ follow a $T^{ 9/2}$ dependence, consistent with the two-magnon scattering. We found that the value of the proportionality coefficient $B_{ ab}^{ fit}$ 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 non-metallic regime ($T<$ 50 K), { \it both} the in-plane $\sigma_{ ab}$ and out-of-plane $\sigma_{ c}$ 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$_{ 1.2}$Sr$_{ 1.8}$Mn$_{ 2}$O$_{ 7}$ at $T<T_c$.
Categories: cond-mat.mtrl-sci cond-mat.str-el