Two-dimensional magnetic correlation and transport behavior of layered manganite La1.4Sr1.6Mn2-xCuxO7

The effect of Cu substitution for Mn in layered manganese oxides La1.4Sr1.6 Mn2-xCuxO7 on magnetic and transport properties has been investigated. It i s interesting that the undoped samples undergo complex transitions with low ing temperature. They transform from the paramagnetic state to the two-dime nsional (2D) short-range ferromagnetic (FM);order at similar to 370 K and t hen enter the three-dimensional (3D) long-range ferromagnetic state at T(C) similar to 100K. At last, they display the metastable canted antiferromagne tic (AF) state and semiconductor behavior below T(N)similar to 50 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 dopin g level, the 3D long range FM; order and then the 2D short range correlatio n disappears in turn and the conduction at T>T-C transforms from the near-n eighbor hopping process to the variable-range hopping process.