Evidence of a field-induced ordering in YbCo₂Zn₂₀ in a [111] magnetic field

DC magnetization (M) measurements were done on a single crystal of YbCo₂Zn₂₀ at very low temperatures (T) down to 0.07K in magnetic fields H || [111]. In addition to the metamagnetic behavior at μ0Hm = 0.6 T already known, we found a new metamagnetic transition at μ0H'm = 6 T whose critical field moves to higher fields with increasing T. Moreover, in fields above 6 T, temperature dependence of the magnetization M(T) shows a kink below 0.6 K. These anomalies indicate the presence of a new field-induced ordered phase. In addition, a broad peak is observed in M(T) in fields above 3 T outside the ordered phase. These results are best explained by a level crossing of the low-lying crystalline-electric-field state with the ground state, possibly assisted by a valence change in magnetic fields.