Quantum effect in the high-field phase of CsCuCl₃

A small magnetization jump at B_c = 12.5 T in CsCuCl₃ found in 1978 has been a mystery for a long time. In this compound, the magnetization was supposed to be linear because of smooth tilt of the magnetic moments toward the field direction for B || c-axis. Then a jump was completely unexpected by the molecular field model. Recently, Shiba proposed a model where the 120°-structure at low field changes to the coplanar type above B_c due to a quantum effect. This is the first example which shows a quantum effect in high fields very clearly in a magnetic system with strong frustration. This structure is confirmed by high field neutron diffraction and high field submillimeter wave ESR. But the plateau observed in the magnetization for B ⊥ c is still an open question.