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Anisotropic magnetic behavior of GdBa_2Cu_3O_{6+y} single crystals

Magnetic properties of high-quality Al-free nonsuperconducting GdBa_2Cu_3O_{6+y} single crystals grown by flux method have been studied. The magnetic anisotropy below the Néel temperature T_N~2.3K corresponds to the direction of Gd^{3+} magnetic moments along the tetragonal c-axis. At T < T_N clear indications of spin-flop transitions for H||c have been observed on magnetization curves at H_{sf}~10kOe. Magnetic phase diagrams have been obtained for H||c as well as for H||ab. A pronounced anisotropy in the magnetic susceptibility (unexpected for Gd-based compounds) has been found above T_N.