Phase transitions in molecule-based magnets: Neutron diffraction, magnetic and specific heat studies.

摘要

The crystallographic and magnetic properties for the M[N(CN) ₂]₂ (M = Mn, Fe, Co, Ni) series have been investigated by neutron diffraction, dc magnetization, ac susceptibility and specific heat on polycrystalline samples. The crystal structures for all compounds are isomorphous in the paramagnetic regime as well as in the ordered state, and consist of discrete octahedra which are axially elongated and successively tilted in the ab-plane. The zero-field magnetic structure for the Mn and Fe compounds consists of two sublattices which are antiferromagnetically coupled and spontaneously canted, with spin orientation mainly along the a-axis. There is a small uncompensated moment along the b-axis (Mn) and the c-axis (Fe). The zero-field magnetic structure for the Co and Ni compounds is collinear ferromagnetic with spin orientation along the c-axis. The results provide the first determination of a complete magnetic structure in the ordered state for a molecule-based magnet. The ground states are characterized by large magnetic anisotropy. The application of a magnetic field induces a spin rotation transition in the Mn compound and an energy-level crossing in the Fe compound. The do magnetization and in-field specific heat studies for Cu[N(CN) ₂]₂ reveal the previously unknown ferromagnetic ordering at low temperatures. Comparisons of the magnetic structures for the isostructural M[N(CN)₂]₂ (M = Mn, Fe, Co, Ni) series suggest that the spin direction is stabilized by crystal fields and the spin canting is induced by the successive tilting of the octahedra. We propose that the superexchange interaction is the mechanism responsible for the magnetic ordering in these compounds and we find that a crossover from noncollinear antiferromagnetism to collinear ferromagnetism occurs for a superexchange angle of α_c = 142.0(5)°.