  The results of a search for pair production of the lighter scalar partners of
top quarks in 2.05 fb-1 of pp collisions at sqrt(s) =7 TeV using the ATLAS
experiment at the LHC are reported. Scalar top quarks are searched for in
events with two same flavour opposite-sign leptons (electrons or muons) with
invariant mass consistent with the Z boson mass, large missing transverse
momentum and jets in the final state. At least one of the jets is identified as
originating from a b-quark. No excess over Standard Model expectations is
found. The results are interpreted in the framework of R-parity conserving,
gauge mediated Supersymmetry breaking `natural' scenarios, where the neutralino
is the next-to-lightest supersymmetric particle. Scalar top quark masses up to
310 GeV are excluded for the lightest neutralino mass between 115 GeV and 230
GeV at 95% confidence level, reaching an exclusion of the scalar top quark mass
of 330 GeV for the lightest neutralino mass of 190 GeV. Scalar top quark masses
below 240 GeV are excluded for all values of the lightest neutralino mass above
the Z boson mass.
