Ultrasound-assisted synthesis and magnetic investigations of Ni0.4Cu0.4Zn0.2GaxGdxFe2-2xO4 (0.00 ≤ x ≤ 0.04) nanosized spinel ferrites

This study focused on the impact of Ga-Gd co-substitution on the structural, magnetic features and hyperfine interactions of nanosized NiCuZn spinel ferrites [Ni0.4Cu0.4Zn0.2GaxGdxFe2-2xO4 (0.00 <= x <= 0.04) NSFs] synthesized via sonochemical approach using ultrasonic irradiation. X-ray powder pattern (XRD) analyses confirmed cubic spinel structures of products. Crystallites and particle sizes of all samples are found to be between 9-14 nm and 18-22 nm, respectively. The NSF morphology and chemical composition were investigated by transmission and scanning electron microscope (TEM and SEM) along with energy-dispersive X-ray (EDX). The cation distribution of all ions was evaluated by Mossbauer spectroscopy. The high spin Fe3+ ions determined the isomer shift (I.S.) values. For lower rare-earth dopant ratios, we found that Gd3+ ions occupy B sites, while Ga3+ ions reside in A sites, and thereafter, some Ga3+ ions start to enter B sites. Magnetometry measurements were used to evaluate the effect of magnetic field (H) and temperature (T) on the magnetization of products. For each sample, both coercivity (H-c) and saturation magnetization (M-s) increase when the T decreases below ambient temperature. All substituted samples display M-s values higher than those of non-substituted ones; however, H-c values diminish for x content up to 0.02 and thereafter enlarge. Different magnetization measurements revealed superparamagnetic-to-ferromagnetic transition below the blocking temperature (T-B).