  We report vibrating coil magnetometry of the spin ice system Ho2Ti2O7 down to
~0.04 K for magnetic fields up to 5 T applied parallel to the [111] axis.
History dependent behavior emerges below T*_0 ~ 0.6 K near zero magnetic field,
in common with other spin ice compounds. In large magnetic fields we observe a
magnetization plateau followed by a hysteretic metamagnetic transition. The
temperature dependence of the coercive fields as well as the susceptibility
calculated from the magnetization identify the metamagnetic transition as a
line of first order transitions terminating in a critical endpoint at T*_m ~
0.37 K, B*_m ~ 1.5 T. The metamagnetic transition in Ho2Ti2O7 is strongly
reminiscent of that observed in Dy2Ti2O7, suggestive of a general feature of
the spin ices.
