Magnetic behavior of Ni nanoparticles and Ni2+ ions in weakly loaded zeolitic structures

Magnetic nanocomposites with low Ni content (nominal 1 wt%) were produced by a process involving thermal treatments of Ni-exchanged zeolite precursors of different type and investigated by XRPD, TEM and dc magnetic techniques. Ni-rich nanoparticles of size in the 10-80 nm range (depending on parent zeolite and thermal treatment) are observed at zeolite grain boundaries and/or surfaces, while a fraction f o of Ni2+ ions are present inside the grains. The blocking temperature of nanoparticles is above room temperature. At high temperatures (75 K <= T <= 300 K) the sample magnetization is dominated by nanoparticles; below, the paramagnetic signal of Ni(2+ )ions begins to be observed. A new procedure of magnetic data analysis is proposed and applied to find the residual ionic fraction f(0) in the two limit cases of full/no quenching of the orbital momentum on Ni2+ ions; f(0) turns out to be in the range 1.5-14.5%, depending on type of parent zeolite and thermal treatment. The temperature behavior of the high-field magnetization in the 2-300 K range and the variation of room-temperature magnetization are both explained taking into account the ionic and nanoparticle fractions estimated using the proposed method. Clustering of weakly interacting Ni2+ ions appears at low temperature in the sample with the highest ionic concentration. (C) 2019 Elsevier B.V. All rights reserved.