Unusual Magnetic Properties of the D + AF and M + AF Phases in CuGe_{1-cursive Greek chi}Si_{cursive Greek chi}O₃

The thermal expansion, specific heat and magnetization of CuGe_{1-cursive Greek chi}Si_{cursive Greek chi}O₃ have been studied. Although the Néel temperature T_N of the D + AF phase, where the antiferromagnetic (AF) phase coexists with the dimerized (D) one, is proportional to cursive Greek chi up to 0.02 in H = 0 as in Cu_{1-cursive Greek chi}Zn_{cursive Greek chi}GeO₃ T_N of the M + AF phase, where the AF phase coexists with the magnetic (M) one above 10 T, suddenly increases with cursive Greek chi between cursive Greek chi = 0.005 and 0.008. The AF properties in the D + AF phase are quite unusual in contrast with the normal ones in the AF state for existence of dimerization as a background, which exists below T_SP, and the quantum effect plays an important role in this phenomenon.