MAGNETIC-PROPERTIES OF LOW-DIMENSIONAL SYSTEMS (PER)(2)M(MNT)(2) (M=CU AND NI)

The magnetic properties of the two-chain compounds based on perylene ( Per) and metal-bis(maleonitriledithiolate) (M(mnt)(2)) with general fo rmula (Per)(2)M(mnt)(2), M = Cu and Ni, either alpha or beta polymorph s, 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 alph a and beta phases, respectively) with g values close to those of peryl ene, consistent with a full charge transfer (Per)(2)(+)[M(mnt)(2)](-) and a diamagnetic organometallic species. The EPR spectra of the Ni co mpounds show wider (300 and 180 G at room temperature for the alpha an d beta phases, respectively) and anisotropic lines with temperature-de pendent g values, denoting spin exchange between the electrons on the perylene cations and the Ni(mnt)(2)(-) units. The highly conducting al pha-Cu phase has a magnetic susceptibility due to a small and almost t emperature-independent contribution of the perylene conduction electro ns that vanishes at the metal to insulator transition at 33 K. The alp ha-Ni compound, in addition to a similar Paul-like contribution, has i ts magnetic susceptibility dominated by a larger term due to the chain s of Ni(mnt)(2)(-) spins, also vanishing at the metal to insulator tra nsition (25 K). The magnetic susceptibility of the semiconducting P-ph ases of the Cu and Ni compounds shows a distinct behavior with a conti nuous increase of susceptibility upon cooling following a T--alpha law , with alpha similar or equal to 0.8, indicative of disorder. These re sults are compared with those of the other members of this family of c ompounds, and for the a-compounds, they are discussed- in terms of the instabilities occurring in the conduction perylene chains (Peierls) a nd in the Ni(mnt)(2) spin chains (spin-Peierls).