  A search for resonances produced in 7 TeV proton-proton collisions and
decaying into top-quark pairs is described. In this Letter events where the
top-quark decay produces two massive jets with large transverse momenta
recorded with the ATLAS detector at the Large Hadron Collider are considered.
Two techniques that rely on jet substructure are used to separate top-quark
jets from those arising from light quarks and gluons. In addition, each massive
jet is required to have evidence of an associated bottom-quark decay. The data
are consistent with the Standard Model, and limits can be set on the production
cross section times branching fraction of a Z' boson and a Kaluza-Klein gluon
resonance. These limits exclude, at the 95% credibility level, Z' bosons with
masses 0.70-1.00 TeV as well as 1.28-1.32 TeV and Kaluza-Klein gluons with
masses 0.70-1.62 TeV.
