  A new protocol of the zero-field-cooled (ZFC) magnetization process is
studied experimentally on an Ising spin-glass (SG)
Fe$_{0.50}$Mn$_{0.50}$TiO$_3$ and numerically on the Edwards-Anderson Ising SG
model. Although the time scales differ very much between the experiment and the
simulation, the behavior of the ZFC magnetization observed in the two systems
can be interpreted by means of a common scaling expression based on the droplet
picture. The results strongly suggest that the SG coherence length, or the mean
size of droplet excitations, involved even in the experimental ZFC process, is
about a hundred lattice distances or less.
