Surface segregation in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$(001).*

The (001) clean surface of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ with nominal dopings of 0.3 $<$ x $<$ 0.4 presents a non-magnetic surface layer tentatively explained by the existence of a surface reconstruction. A surface structural investigation of this system was performed using quantitative Low Energy Electron Diffraction (LEED-IV). Results obtained discard the occurrence of a surface reconstruction. However, a strong Sr segregation is observed in these surfaces leading to an effective doping on the surface of x = 0.8 as well as a possible anti-ferromagnetic phase (in analogy with bulk). Sr segregation is believed to be induced by the creation of a surface which promotes a shift in the phase diagram.