Magnetic phase diagram of a molecule-based ferrimagnet: Weak ferromagnetism and multiple dimensionality crossovers

A detailed study of the magnetic behavior of the molecule-based magnet, [MnOEP][HCBD], (OEP=meso-octaethylporphyrinato, HCBD=hexacyanobutadiene) from 1.7 to 20 K was performed. The earlier reported magnetic transition at 19.6 K, ascribed to a crossover from a one-dimensional Heisenberg-like ferrimagnet to a two-dimensional Ising-like antiferromagnet, is further probed via ac-dc magnetic studies consisting of de magnetization as a function of field at various temperatures, and magnetization as a function of temperature with both field cooling and zero-field cooling. In addition, the ac susceptibility was measured as a function of temperature and applied de field. The appearance of a nonzero out-of-phase component of the ac susceptibility in zero de field at 8 K accompanied by a shoulder in the in-phase component indicates the presence of a magnetic transition near that temperature. Irreversibilities and a spontaneous moment observed below 4.2 K indicate an additional lower temperature transition. The ac and de data allow a determination of the temperature-held phase boundaries around these transitions. Evidence of a tricritical point at 2 kOe and 19.6 K and a multicritical point at 9.5 kOe and 8 K is presented. The nature of the ordered states, along with the possible mechanisms responsible for the transitions, including dipole-dipole interactions, are analyzed.