Superparamagnetic and spin glass behavior in mesoporous niobium oxide bis(cyclopentadienyl)nickel composites

Herein we report the synthesis and characterization of a new family of meso porous niobium oxide bis(cyclopentadienyl)nickel composites which exhibit s uperparamagnetic or spin glass behavior depending on the different loading levels of the organometallic in the pores. XRD and nitrogen adsorption stud ies confirm the retention of the mesostructure in these new materials upon introduction of the guest species. The Nb 3d region of the XPS provides evi dence for the slight reduction of the Nb(V) oxide walls while the Ni 2p reg ion shows the presence of both neutral and cationic bis-cyclopentadienyl ni ckel species with peaks at 856.3 eV and 861.9 eV respectively, as well as t wo more highly oxidized species at 865.5 eV and 867.3 eV. The presence of n eutral and cationic bis(cyclopentadienyl)nickel is further confirmed by EPR . SQUID magnetic measurements show superparamagnetic behavior for the mater ial with a Ni:Nb ratio of 0.07:1, while at a ratio of 0.1:1 the material sh ows spin glass behavior, a phenomenon normally associated with amorphous me tal alloys. The difference in magnetic behavior is rationalized by an incre ased density of unpaired spins in the pore structure.