Temperature and field dependence of resistivities and anisotropy of layered manganites La2-2xSr1+2xMn2O7

In-plane rho (ab) and out-of-plane rho (c) resistivities of L alpha 1.2Sr1. 8Mn2O7 and LaSr2Mn2O7 have been measured by a multiterminal technique. We f ocus on the effects that magnetic and charge ordering transitions have on t he anisotropy sigma (ab)/sigma (c) which reflects the ratio of the phase co herence lengths in the respective directions. Both resistivities of La1.2Sr 1.8Mn2O7 decrease dramatically below the Curie temperature T-C approximate to 135 K. Anisotropy changes with temperature nonmonotonically and displays a peak close to T-C. In LaSr2Mn2O7, rho (c) and rho (ab) display a broad p eak close to the paramagnetic-antiferromagnetic transition temperature T-N approximate to 165 K. The anisotropy exhibits an even more complex behavior than in (La12Sr1.8Mn2O7)-Sr-., displaying a maximum at the charge ordering temperature and a minimum at T-N.