In this work, Sr_0.3Ba_0.4Pb_0.3Fe₁₂O₁₉/(CuFe₂O₄)_x (x = 2, 3, 4, and 5) as strongly exchange-coupled nanosized ferrites were fabricated using a one-pot sol–gel combustion method (citrate sol-gel method). The X-ray diffraction (XRD) powder patterns of the products confirmed the occurrence of pure, exchange-coupled ferrites. Frequency dependencies of the microwave characteristics (MW) were investigated using a co-axial method. The non-linear behavior of the MW with the composition transformation may be due to different degrees of Fe ion oxidation on the spinel/hexaferrite grain boundaries and strong exchange coupling during the hard and soft phases.

Co-Tm nano-spinel ferrite with chemical formula CoTmxFe2-xO4 (0.0 ≤ x ≤ 0.08) NPs were prepared via sonochemical approach. X-ray powder diffraction patterns, microscopic images (SEM and TEM) and infrared spectra proved the formation of Co spinel ferrite. The effect of Tm3+ substituted on spinal structure was evaluated by lattice parameters, tetrahedral and octahedral bond length and cationic distribution. The band gap energy (Eg) of samples were estimated by performing UV-Vis percent diffuse reflectance (% DR) and applying the Kubelka-Munk theory. Eg values are in an interval between 1.33 eV and 1.64 eV. The analyses of magnetization were performed at room (300 K; RT) and low (10 K) temperatures. Different magnetic parameters including coercivity Hc, saturation magnetization Ms, remanence Mr, squareness ratio (SQR = Mr/Ms) and magnetic moment nB were deduced and discussed. The results showed superparamagnetic (SPM) nature at RT for x = 0.00 and 0.02 samples. However, the other products exhibit ferromagnetic (FM) nature. At 10 K, all synthesized NPs display FM behavior. An amazing increase in the magnitudes of Ms, Mr and Hc was observed at 10 K in comparison to RT, which is principally due to the reduced thermal fluctuations of magnetic moments at lower temperatures. The Tm3+ substitution affects considerably the magnetizations data. An enhancement in the Ms, Mr, and nB was detected on increasing the Tm3+ concentration. The SQR values at RT are found to be smaller than 0.5 postulating a single domain nature with uniaxial anisotropy for all produced ferrites. However, SQRs are in the range 0.66-0.76 at 10 K, suggesting the multi magnetic domain at low temperature, except the x = 0.02 product where the SQR = 0.47 indicating the single magnetic domain. The obtained magnetic results were investigated deeply with relation to structural and microstructural properties.

In the current study, Ni_0.4Cu_0.2Zn_0.4La_xY_xFe_2−xO₄ (x = 0.00 − 0.10) nanospinel ferrites (NSFs) were fabricated via an ultrasonic irradiation route. The creation of single phase of spinel nanoferrites (NSFs) was investigated by X-ray powder diffractometry (XRD) and selected area diffraction pattern (SAED). The cubic morphology of all samples was confirmed by scanning and transmission electron microscopies (SEM and TEM) respectively. The UV-Vis investigations provided the direct optical energy band gap values in a narrow photon energy interval of 1.87–1.92 eV. The ⁵⁷Fe Mössbauer spectroscopy analysis explained that the hyperfine magnetic fields of Octahedral (Oh) and Tetrahedral (Td) sites decreased with substitution. The paramagnetic properties of NPs decrease with increase of content of doped ions. Investigations of magnetic properties reveal a superparamagnetic nature at 300 K and soft ferromagnetic trait at 10 K. The M_s (saturation magnetization) and M_r (remanence) decrease and the H_c (coercivity) increases slightly with La³⁺ and Y³⁺ substitution. The observed magnetic traits are deeply discussed in relation with the morphology, structure, magnetic moments and cation distributions. The microwave characterization of the prepared NSFs showed that, dissipation (i.e., absorption) of incoming microwave energy occurs at a single frequency, for each sample, lying between 7 and 10.5 GHz. The reflection losses (RL) at these frequencies range from −30 to −40 dB and the mechanism of which is explained in the framework of dipolar relaxation and spin rotation. The best microwave properties were obtained with a LaY concentration of x = 0.08 having an RL of −40 dB @ 10.5 GHz and an absorption bandwidth of 8.4 GHz @ −10 dB. With these high values of RL and absorbing bandwidth, LaY doped NiCuZn NSF products would be promising candidates for radar absorbing materials in the X-band.