High-Field Magnetization Processes of Double Spin Chain Systems KCuCl_<b>3</b>and TlCuCl_<b>3</b>

Abstract

High-field magnetization processes of S =1/2 double spin chain systems KCuCl 3 and TlCuCl 3 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 TlCuCl 3 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 ∼20 T and 6 T for KCuCl 3 and TlCuCl 3 , respectively. The critical field H c depends on the external field direction due to the anisotropy of the g -factor. The excitation gap Δ is evaluated as Δ/ k B =31.1 K for KCuCl 3 and 7.5 K for TlCuCl 3 . Although the antiferromagnetic interactions in TlCuCl 3 are stronger than those in KCuCl 3 , which can be seen from the slope of the magnetization curves above H c , the excitation gap in TlCuCl 3 is about a quarter of that of KCuCl 3 . It is observed that the transition field H c increases with increasing temperature.