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

High-field magnetization processes of S=1/2 double spin chain systems KCuCl3 and TlCuCl3 have been measured up to 39 T at 4.2 and 1.7 K for both systems and at 1.0 and 0.5 K for TlCuCl3 using single crystals. A transition from a singlet ground state with an excitation gap to a gapless magnetic state with finite susceptibility occurs at H-c similar to 20 T and 6 T for KCuCl3 and TlCuCl3, respectively. The critical field H-c depends on the external field direction due to the anisotropy of the g-factor. The excitation gap Delta is evaluated as Delta/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. It is observed that the transition field H-c increases with increasing temperature.