Shang-Da Jiang, Bing-Wu Wang,* Gang Su, Zhe-Ming Wang, and Song Gao*

     we have synthesized a single-ion magnet containing a D_y^III ion with approximate D_4d local symmetry. The complex exhibits a crossover at T _cross=8 K between thermally activated relaxation (T>T cross ) and quantum tunneling relaxation (T<T_cross) . By dilution or application of a magnetic field, the quantum tunneling is suppressed. Meanwhile, the thermally activated relaxation happens in a wider temperature range and the hysteresis loop becomes more obvious. We propose that the Ising ground states can give rise to the slow magnetic relaxation, while the various energy sublevels for Ln III can be affected by spin–orbit coupling and ligand field. The dynamic magnetic behavior is a combined result of these properties.