High-field magnetization of the S=1/2 two-leg spin-ladder antiferromagnets CuAX₂ (A=Diazine, X=Halogen)

We have performed magnetic susceptibility and high-field magnetization measurements on powder samples of Cu(DEP)Br₂ (DEP=2,3-diethyl- pyrazine) and Cu(DMP)Cl₂ (DMP=2,3-dimethyl-pyrazine) which are regarded as spin-1/2 two-leg spin-ladders in which the exchange constants are given by J _∥ along the leg and J _⊥ along the rung. The temperature dependences of magnetic susceptibilities of these compounds show a round peak at 11 and 18 K, respectively, and are compared with those calculated with a density matrix renormalization group method for α=J _∥/J _⊥=0.45-0.55. The exchange constants of the former and the latter compounds are evaluated to be J/k_B=16.5 K and J/k_B=28.4K, respectively. No magnetization at 1.3 K appears in magnetic fields of up to about 5 T for Cu(DEP)Br ₂ and about 14 T for Cu(DMP)Cl₂, and then the magnetization increases monotonically to saturate at around 25 T and 38 T for respective compounds. Both magnetization curves are also compared with those calculated for α=0.45-0.55 using the same exchange parameter values as in the susceptibilities.