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 ga pless magnetic state with finite susceptibility occurs at H-c similar to 20 T and 6 T for KCuCl3 and TlCuCl3, respectively. The critical fie ld H-c depends on the external field direction due to the anisotropy o f 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 antiferromagneti c interactions in TlCuCl3 are stronger than those in KCuCl3, which can be seen from the slope of tile magnetization curves above H-c; the ex citation gap in TlCuCl3 is about a quarter of that of KCuCl3. It is ob served that the transition field H-c increases with increasing tempera ture.