Magnetic properties of the low-dimensional systems (Per)2M(mnt)2 (M = Cu and Ni)

Abstract

The magnetic properties of the two-chain compounds based on perylene (Per) and metal-bis(maleonitriledithio1ate) (M(mnt)z) with general formula (Per)zM(mnt)z, M = Cu and Ni, either a or @ polymorphs, were studied by EPR and static magnetic susceptibility in the range 4-300 K. The EPR spectra of single crystals of the Cu compounds show a relatively narrow line (0.3 and 4 G at room temperature for the a and @ phases, respectively) with gvalues close to those of perylene, consistent with a full charge transfer (Per>,+[M(mnt)$and a diamagnetic organometallic species. The EPR spectra of the Ni compounds show wider (300 and 180 G at room temperature for the a and @ phases, respectively) and anisotropic lines with temperature-dependent g values, denoting spin exchange between the electrons on the perylene cations and the Ni(mnt)z- units. The highly conducting a-Cu phase has a magnetic susceptibility due to a small and almost temperature-independent contribution of the perylene conduction electrons that vanishes at the metal to insulator transition at 33 K. The a-Ni compound, in addition to a similar Pauli-like contribution, has its magnetic susceptibility dominated by a larger term due to the chains of Ni(mnt)z- spins, also vanishing at the metal to insulator transition (25 K). The magnetic susceptibility of the semiconducting @-phases of the Cu and Ni compounds shows a distinct behavior with a continuous increase of susceptibility upon cooling following a F law, with a N 0.8, indicative of disorder. These results are compared with those of the other members of this family of compounds, and for the a-compounds, they are discussed in terms of the instabilities occurring in the conduction perylene chains (Peierls) and in the Ni(mnt)z spin chains (spin-Peierls).