An investigation of Molecular Magnets Synthesized from Ni(cod)(_2) and the Organic Acceptors TCNQ and TCNQF(_4)

Abstract

Recently a new molecular magnet, Ni(_2)TCNQ, was reported to be ferromagnetic at room temperatures, with a Curie temperature of 400K(^1) Ni(_2)TCNQ is synthesized through a wet chemical route using the starting materials, Ni(cod)(_2) (bis(1,5-cyclooctactadiene)nickel) and TCNQ (7,7,8,8-tetracyanoquinodimethane). This work focuses on the synthesis of this molecular magnet and subsequent characterisation of the magnetic properties. Several organic acceptor molecules were also examined with the intention of synthesising a new molecular magnet. The chosen organic was TCNQF’(_4), (2,3,5,6- tetrafluoro-7,7,8,8-tetracyanoquinodimethane) due to its small size and magnetic properties. The resulting magnet, Ni(_2)TCNQF(_4), was synthesized following the same procedures as for Ni(_2)TCNQ with the substitution of TCNQ for TCNQF(_4).The magnetic properties for the nickel samples are qualitatively similar and both show ferromagnetic behaviour at room temperature. More specifically the materials exhibit two magnetic phases, in high magnetic fields the materials are paramagnetic and in low magnetic fields exhibit nanoparticulate behaviour. At low temperatures in the paramagnetic phase the materials have been compared to a Brillouin function. This revealed weak ferromagnetic interactions between the spins in Ni(_2)TCNQ and weak antiferromagnetic interactions in Ni(_2)TCNQF(_4).In the nanoparticulate phase the materials are superparamagnetic above the blocking temperature, T(_h), and below exhibit single domain behaviour. The Curie temperature for Ni(_2)TCNQ was found to be much higher than previously reported(^1) at approximately (625±5) K. The Curie temperature for Ni(_2)TCNQF’_4’ was (620±5) K. This suggests that the ferromagnetic phase observed in these materials arises from the nickel nanoparticles present in the material. This conclusion is also supported by XRD and microscopy measurements.