The results of a search for pair production of the lighter scalar partners of top quarks (t~1) in 2.05 fb -1 of pp collisions at 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 (e, μ) 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 (χ~10) is the next-to-lightest supersymmetric particle. Scalar top quark masses up to 310 GeV are excluded for 115GeVmZ. © 2012 CERN.