Long-distance super-exchange and quantum magnetic relaxation in a hybrid metal-organic framework

The hybrid metal-organic framework (CH₃)₂NH₂Fe(HCOO)₃ with a perovskite-like structure exhibits a variety of unusual magnetic behaviors at low temperatures. While the long-distance super-exchange through the Fe-O-CH-O-Fe exchange path leads to a canted antiferromagnetic ordering at T_N ～ 19 K, a second transition of magnetic blocking develops at T_B ～ 9 K. The stair-shaped magnetization hysteresis loops below T_B resemble the behaviors of resonant quantum tunneling of magnetization in single-molecular quantum magnets. Moreover, the magnetic relaxation also exhibits several features of resonant quantum relaxation, such as the exponential law with a single characteristic relaxation time, and the nonmonotonic dependence of relaxation rate on the applied magnetic field with a much faster relaxation around the resonant fields. The origin of quantum tunneling behaviors in the (CH₃)₂NH₂Fe(HCOO)₃ metal-organic framework is discussed in terms of magnetic phase separation due to the modification of hydrogen bonding on the long-distance super-exchange interaction.