Metamagnetic behavior in a heavy fermion compound YbCo₂Zn ₂₀

We studied an electronic state at low temperatures by measuring the specific heat and ac-susceptibility under magnetic fields for a heavy fermion compound YbCo₂Zn₂₀. The magnetic specific heat in the form of C_mag/T increases steeply and exhibits an extremely large value 8 J/(K²-mol) below 0.2 K, which corresponds to an electronic specific heat coefficient with a very low Kondo temperature of 0.2-1 K. The magnetic entropy increases rapidly at higher temperatures up to 10 K and reaches Rln8 at 50-60 K, which corresponds to a small crystalline-electric-field (CEF) splitting energy. In addition, we found a metamagnetic anomaly at H_m 5.7 kOe below the characteristic temperature T_χmax =0.32 K, where the temperature dependence of the ac-susceptibility shows a broad peak. Interestingly, the metamagnetic behavior was observed as the double peak at 4.0 and 7.5 kOe in the magnetic field dependence of the specific heat C/T at 95 mK.