  We study electron spin decoherence in a two-electron double quantum dot due
to the hyperfine interaction, under spin-echo conditions as studied in recent
experiments. We develop a semi-classical model for the interaction between the
electron and nuclear spins, in which the time-dependent Overhauser fields
induced by the nuclear spins are treated as classical vector variables.
Comparison of the model with experimentally-obtained echo signals allows us to
quantify the contributions of various processes such as coherent Larmor
precession and spin diffusion to the nuclear spin evolution.
