High field ESR measurements of Cu 3 (CO 3 ) 2 (OH) 2 have been performed in the frequency region from 50 to 900 GHz and in the temperature region from 1.8 to 265 K using the pulsed magnetic field up to 36 T. The large g-shifts below 23 K and the characteristic temperature dependence of the linewidth are discussed in connection with the two maxima of magnetic susceptibility observed at 5 and 23 K. Our ESR results together with the magnetization and theoretical results suggest that the ground state of the system is in the spin fluid (SF) phase at low field and low temperature. From the frequency-field dependence measurements at 1.8 K, the direct transition, which is closely related to the magnetization plateau from 16 to 26 T at 1.5 K, is observed for the first time. The observed frequency-field diagram suggests that the exchange interaction in the dimer is estimated to be about 50 K (1057 GHz). Recently, the study of quantum spin systems with competing interactions, that is, frustration and quantum fluctuation, has attracted great interest. The diamond chain system, as shown in Fig. 1(a), is one of such systems, and it has attracted much theoretical interest, particularly after the suggestion by Ishii et al. that Cu 3 Cl 6 (H 2 O) 2 Á2H 8 C 4 SO 2 may be the model substance for the S ¼ 1=2 distorted diamond chain with the spin gap.1) Takano et al. studied the S ¼ 1=2 diamond chain model in the case of J 1 ¼ J 3 , and showed that the ground state is in the ferrimagnetic (FRI), tetramer-dimer (TD) and dimer-monomer (DM) phases for J 2 =J 1 < 0:909, 0:909 < J 2 =J 1 < 2 and 2 < J 2 =J 1 , respectively.2) It was extended to the S ¼ 1=2 distorted diamond chain model with J 1 , J 2 and J 3 , as shown in Fig. 1(a) by Okamoto et al. and Tonegawa et al., and the ground state phase diagram in the case of H ¼ 0, composed of ferrimagnetic (FRI), dimerized (D) and spin fluid (SF) phases as schematically shown in Fig. 1(b), were determined.3,4) It turned out that the TD and DM states were special cases of the twofold degenerate D and SF states, respectively. Tonegawa et al. also studied the magnetization curve numerically. 4,5) However, it turned out recently that the compound assumed to be Cu 3 Cl 6 -(H 2 O) 2 Á2H 8 C 4 SO 2 was actually Cu 2 Cl 4 Á2H 8 C 4 SO 2 which can be described by the S ¼ 1=2 double-spin chain model.
6)Therefore, the model substance for the S ¼ 1=2 distorted diamond chain is desired strongly from the experimental point of view.Recently, Kikuchi et al. proposed that Cu 3 (CO 3 ) 2 (OH) 2 might be the first model substance for the S ¼ 1=2 diamond chain. 7) Cu 3 (CO 3 ) 2 (OH) 2 is a well-known natural mineral azurite, which has a dark blue color, and it often contains the natural mineral malachite Cu 2 (OH) 2 (CO 3 ), which has a light green color. However, little was known about the magnetic properties of Cu 3 (CO 3 ) 2 (OH) 2 previously, except for the antiferromagnetic order at T N ¼ 1:9 K from the NMR and magnetic susceptibility measurements 8,9) in the restricted low-temperature region between 1.55...