High-field magnetization processes of quantum double spin chain systems KCuCl₃, TlCuCl₃ and NH₄CuCl₃

High-field magnetization processes of S = 1/2 double spin chain systems KCuCl₃, TlCuCl₃ and NH₄CuCl₃ have been measured up to 39 T at helium temperatures using single crystals. A transition from a singlet ground state with an excitation gap to a gapless magnetic state occurs at H_c ∼ 20 and 6 T for KCuCl₃ and TlCuCl₃, respectively. The excitation gap δ is evaluated as Δ/k_B = 31.1 K for KCuCl₃ and 7.5 K for TlCuCl₃. Although the antiferromagnetic interactions in TlCuCl₃ are stronger than those in KCuCl₃, which can be seen from the slope of the magnetization curves above H_c, the excitation gap in TlCuCl₃ is about a quarter of that of KCuCl₃. In contrast to KCuCl₃ and TlCuCl₃, NH₄CuCl₃ has a gapless magnetic ground state. It is found that NH₄CuCl₃ undergoes five phase transitions in high magnetic fields. Two wide plateaus are observed at a quarter and three quarters of the saturation magnetization.