Abstract
The effect of Cu substitution for Mn in layered manganese oxides ${\mathrm{La}}_{1.4}{\mathrm{Sr}}_{1.6}{\mathrm{Mn}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{O}}_{7}$ on magnetic and transport properties has been investigated. It is interesting that the undoped samples undergo complex transitions with lowing temperature. They transform from the paramagnetic state to the two-dimensional (2D) short-range ferromagnetic (FM) order at \ensuremath{\sim}370 K and then enter the three-dimensional (3D) long-range ferromagnetic state at ${T}_{C}\ensuremath{\sim}100\mathrm{K}.$ At last, they display the metastable canted antiferromagnetic (AF) state and semiconductor behavior below ${T}_{N}\ensuremath{\sim}50\mathrm{K}.$ It is suggested that the semiconductor behavior is correlated to the coexistence of the FM order and the canted AF order below ${T}_{N}.$ With increasing Cu doping level, the 3D long range FM order and then the 2D short range correlation disappears in turn and the conduction at $T>{T}_{C}$ transforms from the near-neighbor hopping process to the variable-range hopping process.
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