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 10e80 nm range (depending on parent zeolite and thermal treatment) are observed at zeolite grain boundaries and/or surfaces, while a fraction f0 of Ni2þ 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 Ni2þ ions begins to be observed. A new procedure of magnetic data analysis is proposed and applied to find the residual ionic fraction f0 in the two limit cases of full/no quenching of the orbital momentum on Ni2þ ions; f0 turns out to be in the range 1.5e14.5%, depending on type of parent zeolite and thermal treatment. The temperature behavior of the high-field magnetization in the 2e300 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 Ni2þ ions appears at low temperature in the sample with the highest ionic concentration.