First order metamagnetic transition in Ho ₂Ti ₂O ₇ observed by vibrating coil magnetometry at Milli-Kelvin temperatures

We report vibrating coil magnetometry of the spin-ice system Ho ₂Ti ₂O ₇ down to ∼0.04K for magnetic fields up to 5 T applied parallel to the [111] axis. History-dependent behavior emerges below T0*∼0.6K 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 end point at Tm* 0.37K, B _m 1.5T. The metamagnetic transition in Ho ₂Ti ₂O ₇ is strongly reminiscent of that observed in Dy ₂Ti ₂O ₇, suggestive of a general feature of the spin ices.