Magnetic clustering of weakly interacting Ni-ions in Ni-exchanged zeolites

Divalent Nickel cations were incorporated in two commercial zeolites (Na-A zeolite and Na-X zeolite) by a process of ionic exchange marginally affecting structure, morphology and porosity of the host materials, as verified by XRD, HRTEM and physisorption measurements. Comparable amounts of magnetic ions were introduced (4.80 wt% in Na-A zeolite and 6.20 wt% in Na-X zeolite), as checked by AAS and TGA. Magnetic measurements were done between 2 and 300 K using a SQUID magnetometer up to 70 kOe. The initial susceptibility follows the Curie-Weiss law with Curie temperatures theta of 10.3 and 11.5 K. The effective magnetic moments on Ni2+ ions suggest almost complete quenching of the angular momentum. No long-range magnetic order is found below theta; however, FC/ZFC magnetization curves indicate the formation of superparamagnetic clusters of magnetic ions with blocking temperature of about 6.5 K in both zeolites. Cluster size, average number of clustered ions, effective anisotropy of clusters are evaluated. A comprehensive picture of all magnetic effects taking place over the whole temperature range is drawn by combining magnetic, structural and morphological data.