Antiferromagnetic order in the two-dimensional spin system Cu₃B₂O₆

In order to reveal the ground state of Cu₃B₂O₆, we synthesized the powder samples of (Cu_1-xZn_x)₃B₂O₆ and investigated the magnetic susceptibilities and the magnetization processes. ¹¹B nuclear magnetic resonance (NMR) measurements were also performed for Cu₃B₂O₆. The ground state of this compound has been clearly proved to be an antiferromagnetically ordered one by ¹¹B NMR. It has been also found that the ground state of this compound is the spin-density-wave (SDW) state where the spin amplitude is modulated, not a simple antiferromagnetically ordered state. The hyperfine fields at the boron sites are distributed from zero to the finite value of about 1.49 kOe at 4.2. K. The maximum field develops with decreasing temperature. The magnetization curve increases nonlinearly above the field at the spin-flop transition of H_c=9.5 T for x=0. The Néel temperature of T_N=11 K for x=0 decreases several times as rapid as that of other two-dimensional (2D) or 1D spin system with substitution by nonmagnetic element. These behaviors are considered to be due to the characteristic SDW state of this compound.