Magnetization Process of the Kondo Insulator YbB12 in Ultrahigh Magnetic Fields

The magnetization process of the Kondo insulator YbB12 has been unveiled in ultrahigh magnetic fields of up to 120 T at 4.2 K. We have found a novel metamagnetic transition at B-c2 = 102 T in addition to the previously known transition at B-c1 = 55 T. It has also been observed that the magnetization tends to saturate at around 112 T. Within the rigid band model, the two-energy-gap structure in the density of states (DOS) explains the successive two-step metamagnetism as a result of the Zeeman effect of the DOS. The metamagnetic transition at B-c1 occurs along with an insulator-metal transition and the field-induced phase is expected to be a heavy fermion metallic state. The Kondo effect can weaken at the second transition of B-c2, as theoretically found in the successive two-metamagnetic-transition process in the Kondo semimetal CeNiSn.