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 system s KCuCl3, TiCuCl3 and NH4CuCl3 have been measured up to 39 T at helium temperatures using single crystals. A transition from a singlet groun d state with an excitation gap to a gapless magnetic state occurs at H -c similar to 20 and 6 T for KCuCl3 and TlCuCl3, respectively. The exc itation gap Delta is evaluated as Delta/kappa(B) = 31.1 K for KCuCl3 a nd 7.5 K for TlCuCl3. Although the antiferromagnetic interactions in T lCuCl3 are stronger than those in KCuCl3, which can be seen from the s lope of the magnetization curves above H-c, the excitation gap in TlCu Cl3 is about a quarter of that of KCuCl3. In contrast to KCuCl3 and Tl CuCl3, 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 sat uration magnetization. (C) 1998 Elsevier Science B.V. All rights reser ved.