Top-down and bottom-up approaches to obtain magnetic nanoparticle of Fe3O4 compound: Pulsed laser deposition and chemical route

Our goal in this work was to prepare nanoparticles (NPs) of Fe3O4 using two approaches: top-down and bottom-up. In the former two wavelengths (1064 and 532 nm) were used in the pulsed laser deposition (PLD) technique to produce NPs from α-Fe bulk, with subsequent heat treatment to produce Fe3O4, and in the latter the NPs were produced by chemical co-precipitation route (CR).Transmission Electron Microscopy (TEM) revealed that the as-deposited NPs exhibit a spherical morphology with core/shell structure and with diameters of (8 ± 4) nm (1064 nm) and (16 ± 5) nm (λ = 532 nm) for PLD approach, while for the CR, diameters of 8.0 ± 0.3 and 18.0 ± 0.3 nm were obtained for pure and SiO2-coated samples, respectively. X-Ray diffraction technique with Rietveld method was used to quantify the phases of oxidized NPs, and the percentages obtained were: 58% of Fe3O4 and 42% of α-Fe (532 nm) and 85% of Fe3O4 and 15% of α-Fe (1064 nm). For the samples obtained by CR, no spurious phase was observed in the diffractograms, only a very broad peak of SiO2 was observed around 25°, which indicates that the NPs were coated with SiO2, and this layer is amorphous. From Rietveld refinement, we found that the SiO2-coated sample has the highest Fetet-O-Feoct bond angle, which indicates a greater magnetic coupling between Fe atoms for these samples. Measurements of AC magnetization in different frequencies were carried out for all NPs and a detailed analysis of the dependence of the χ′ peak with T(K) helped to find out the relaxation time of the sample coated with SiO2 is 8.7 × 10−6 s, while for the other samples it was on the order of 10−9 s. Based on the results, it was possible to conclude that the coated sample presents a super spin-glass behavior, while in the other samples a magnetic interaction between NPs and oriented ferromagnetic clusters may be present.