关键词: 混价态, 相变, 负磁化, 磁极翻转, 交换偏置

Although N-type ferrimagnets exhibit negative magnetization under positive magnetic fields, compounds that could maintain negative magnetization behavior under strong magnetic field (e.g. 1 T) are still rare. In this work, a mixed-valence metal-formate [CH₃NH₃]_n[Fe^IIIFe^II(HCO₂)₆]_n (1) was constructed by the reaction of FeCl₃·6H₂O, formic acid and N-methyl formamide at 140 ℃ for two days. At room temperature, 1 crystallizes in the space group P31c, in which a three-dimensional anionic niccolite topological framework is constructed by Fe^II, Fe^III ions, and anti,anti formate. The guest CH₃NH₃⁺ cations fill in the cavities of the framework as the charge balancer with the N atoms displaying threefold disorder and the atoms having a twofold disorder. The heat capacity measurement shows two different peaks, being the signatures of phase transitions. The change in heat capacity at 136 K corresponds to the phase transition triggered by the order-disorder phenomena of the CH₃NH₃⁺ cations. At low temperature phase, 1 has a symmetry of 2/m in space group C2/c, in which the threefold disorder of the N atoms of the CH₃NH₃⁺ was freezed. The order-disorder phase transition also results in dielectric relaxation in the temperature range 130~200 K at 500 Hz~1 MHz. The change of heat capacity at 40 K is associated with the ferromagnetic order of the antiferromagnetically coupled Fe^III and Fe^II sublattices. 1 is a N-type ferrimagnet with negative magnetization below T_N on cooling under the applied field, and thermo-driven magnetic poles reversal could be found in 1 with large applied field of 1 T. Furthermore, positive field regulated switchable magnetic dipoles switching of the magnetization, together with obvious huge positive exchange bias is also observed in 1. These results reveal the significant magnetic anisotropy in 1, and the guest in the framework not only can tune the structural phase transitions but also modulate the anisotropy of the host framework leading to different magnetism.

Key words: mixed-valence, phase transitions, negative magnetization, switchable magnetic dipoles, exchange bias