Field-Induced Ordering in the Heavy Fermion Compound YbCo₂Zn₂₀

DC Magnetization of the heavy-fermion compound YbCo₂Zn₂₀ was examined at very-low temperatures down to 0.07 K in the field up to 14.5 T. In addition to the nearly-isotropic metamagnetic behavior at 0.6 T, a new metamagnetic behavior is observed in the field variation of magnetization M(H) at 6 T only for H || [111]. For the same field direction, a clear kink appears in the temperature dependence of the magnetization M(T) for fields above 6 T. These results strongly suggest the existence of a new ordered phase induced by magnetic fields. The H - T phase diagram as well as the anisotropy in M(H) above ~ 2 T are best explained by a level crossing of the low-lying crystalline-electric-field states by magnetic fields.