Crossover phase diagram in heavy fermion compound YbCo₂Zn ₂₀

We studied the electronic state and metamagnetic behavior in the heavy fermion compound YbCo₂Zn₂₀ by measuring the specific heat, ac-susceptibility, and electrical resistivity under magnetic fields. The magnetic specific heat in the form of C_mag/T increases steeply with decreasing temperature below 10 K and becomes constant with an extremely large value 8 J/(K²-mol) below about 0.2 K, indicating a Fermi-liquid nature in this temperature range. The magnetic entropy increases rapidly below 10 K and reaches Rln8 around 60 K, suggesting a small crystalline-electric-field (CEF) splitting energy. From the precise electrical resistivity measurements, together with the C(H)/T and_χac(T) measurements, it was found that there are two crossover lines at low temperatures in the H-T phase diagram for H || 〈100〉. The Fermi-liquid state evolves at lower temperatures than these crossover lines.