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Magnetic nanoparticles (NPs) are especially interesting for several biomedical applications due to their chemical surface, especially for targeted cancer imaging and therapeutics. In order to realize these applications, it is important to know their magnetic properties among other complementary properties that help to improve the understanding of the synthesis process. In this work, we report the magnetic properties of polyethyleneimine-coated magnetite (PEI-Fe3O4) NPs synthesized by a one-step method via the co-precipitation method and using PEI as a stabilizer. Transmission electron microscopy (TEM) images revealed agglomerated magnetic nanoparticles with an average size of ∼10 nm; meanwhile, the X-ray diffraction (DRX) analysis confirmed a pure magnetite phase. The study of magnetic properties shows a superparamagnetic system with coexistence of non-interacting single NPs with a low blocking temperature (∼35 K) and interacting NPs in the aggregates with a higher blocking temperature (>150 K), in which the interparticle interactions of magnetic cores dominate over surface spin disorder. The interaction between the surface spin-disorder layer and NP core was found to be weak, related to a weak exchange bias effect. A maximum specific loss power (SLP) value of 70 W g−1 was obtained (f = 571 kHz and H = 23.87 kA m−1) indicating that the magnetic response plays a crucial role in determining the heating efficiency for future applications.

This study reports on the synthesis and characterization of oleic acid (OA)-coated Fe3O4 nanoparticles (Fe3O4@OA) and AO plus essential oil (EO)-coated Fe3O4 nanoparticles (Fe3O4@OA/EO). The EO was extracted from Croton cajucara Benth (CCB) leaves; a plant from the Brazilian Amazon region. Structural and morphological characterizations were carried out using X-ray diffraction (XRD) and transmission electron microscopy (TEM) images, respectively. Additionally, thermogravimetric analysis and magnetization measurements (hysteresis cycle, zero field-cooled-ZFC, field-cooled-FC, and AC susceptibility) were used to assess thermal and magnetic properties of the as-fabricated samples. Rietveld refinement of XRD pattern confirmed the formation of magnetite phase with no extra phases, whereas TEM images revealed spherically-shaped nanoparticles in the Fe3O4@OA and (Fe3O4@OA/EO) samples with a mean physical size of 8.5 nm and 10.1 nm, respectively. ZFC and FC curves revealed the occurrence of blocked/frozen state below the maximum peak (Tmax) at ∼81 K and ∼40 K for the Fe3O4@OA and (Fe3O4@OA/EO) samples, respectively. Moreover, low-temperature AC susceptibility vs. T curves recorded in the range of 0.2–1000 Hz showed that the OA coating of the Fe3O4 nanoparticles leads to a spin-glass-like behavior credited to the strong particle-particle interactions; meanwhile, the double layer (AO + EO) coating of the Fe3O4 nanoparticles remarkably quenches the particle-particle interaction leading to a superparamagnetic-like behavior.

Surface coating magnetic nanoparticles can be realized using organic moieties, such as essential oil (EO) extracted from Croton cajucara Benth leaves, a common plant from the Brazilian Amazon region successfully used for infusion in popular medicine. Although, there are reports in the literature indicating the protective and healing actions of the EO on gastric lesions, the inclusion of the magnetic core to control (targeting and retaining) the MNPs in specific sites can improve the performance of the EO molecules for the treatment of gastric disorders. Therefore, in this work, we report on fabrication and characterization of surface-coated magnetite nanoparticles (MNPs), aiming thermal-assisted site delivering EO. Firstly, oleic acid (OA) coated MNPs (sample M1) were fabricated using a thermal decomposition route. Secondly, the OA-coated MNPs were dressed with EO (sample M2). Meanwhile, x-ray diffraction and transmission electron microscopy data confirmed formation of spherically-shaped magnetite core while successful single OA coating and double OA + EO coating of MNPs was confirmed by infrared spectroscopy and thermogravimetric analysis (TGA). TGA results indicated release of the EO at ∼50 °C, whereas the OA is released only above ∼380 °C. Blocking (TB) and irreversible (TIRR) temperatures of samples M1 (TB = 210 K; TIRR = 210 K) and M2 (TB = 35 K; TIRR = 215 K) were assessed from magnetic measurements data analysis. In sample M2, the difference TB-TIRR (180 K) was related to inhomogeneous partial shifting of OA molecules by EO molecules whereas the remarkable reduction of MS with respect to sample M1 was assigned to a dead surface layer dominated by ferrous ions in low-spin state. Moreover, the lower TB value we have found in sample M2 (∼35 K) indicates negligible particle-particle interactions due to the successful double coating (OA plus EO) of the MNPs, in contrast to that obtained in sample M1. In sample M2 weakening of particle-particle interaction warrants superparamagnetic properties which can be used to obtain magnetic resonance image and for guiding the MNPs to targeting sites using gradients of external magnetic field. Therefore, the combination of the improved magnetic properties plus the detachment of EO at ∼50 °C (while keeping stable the OA coating) via magnetohyperthermia makes the double coated system (sample M2) a very promising candidate for site delivering of EO to treat gastrointestinal disorders.

Magnetite (Fe3O4) nanoparticles coated with organic material are of considerable importance in various areas of engineering, as well as in biomedicine. Several papers show drastic changes in the magnetic properties related to the surface effects and the particle-particle interactions strength. However, there is no consensus about the origin or mechanisms that produce these changes, which could be different depending on the particle size and shape, coating efficiency and particle-particle interaction strength. Aiming to shed light on these issues, oleic acid (OA) coated Fe3O4 nanoparticles with different sizes were synthesized by a thermal decomposition method. Structural and microscopic results revealed Fe3O4 nanoparticles with good crystallinity and almost-spherical or polyhedral shapes depending on the solvent used for the synthesis. Infrared (FTIR) spectroscopy indicated OA molecules attached to the surface of Fe3O4 NPs via bidentate (chelating and/or bridging) bonds; meanwhile, thermogravimetric analysis confirmed the presence of weakly and strongly bonded OA molecules, suggesting a successful coating of Fe3O4 NPs. Magnetization (M) vs. magnetic field (H) curves are consistent with a core/shell structure formed by magnetite/defective magnetite (maghemite) phases, in consistency with FTIR spectroscopy. It was determined lower saturation magnetization values than that expected for bulk magnetite, which was assigned to the likely presence of a fraction of iron ions in low-spin (LS) states in the defective magnetite (maghemite) layer at the particles surface. The magnetic results evidence that depending on the amount of OA coating, it is possible to tune the particle-particle separation distance, avoid particles agglomeration and particle-particle interactions. The temperature dependence of magnetization reveals the presence of non-interacting and interacting NPs. Meanwhile, AC magnetic susceptibility measurements are consistent with those results and provide features related to superparamagnetic (SPM) behavior, assigned to the non-interacting NPs, and interacting SPM behavior, assigned to the interacting NPs, whose interaction strength is not enough to lead to a spin-glass like behavior.

In this work we report the formation of hollow α-Fe2O3 (hematite) microspheres by the gas-bubble template method. This technique is simple and it does not require hard templates, surfactants, special conditions of atmosphere or complex steps. After reacting Fe(NO3)3.9H2O and citric acid in water by sol–gel, the precursor was annealed in air at different temperatures between 180 and 600 °C. Annealing at 550 and 600 °C generates bubbles on the melt which crystallize and oxidizes to form hematite hollow spheres after quenching. The morphology and crystal evolution are studied by means of X-ray diffraction and scanning electron microscopy. We found that after annealing at 250–400 °C, the sample consist of a mixture of magnetite, maghemite and hematite. Single hematite phase in the form of hollow microspheres is obtained after annealing at 550 and 600 °C. The crystallization and crystal size of the hematite shells increase with annealing temperature. A possible mechanism for hollow sphere formation is presented.

We present a systematic study of core-shell Au/Fe_3O_4 nanoparticles produced by thermal decomposition under mild conditions. The morphology and crystal structure of the nanoparticles revealed the presence of Au core of = (6.9\pm 1.0) nm surrounded by Fe_3O_4 shell with a thickness of ~3.5 nm, epitaxially grown onto the Au core surface. The Au/Fe_3O_4 core-shell structure was demonstrated by high angle annular dark field scanning transmission electron microscopy analysis. The magnetite shell grown on top of the Au nanoparticle displayed a thermal blocking state at temperatures below T_B = 59 K and a relaxed state well above T_B. Remarkably, an exchange bias effect was observed when cooling down the samples below room temperature under an external magnetic field. Moreover, the exchange bias field (H_{EX}) started to appear at T~40 K and its value increased by decreasing the temperature. This effect has been assigned to the interaction of spins located in the magnetically disordered regions (in the inner and outer surface of the Fe_3O_4 shell) and spins located in the ordered region of the Fe_3O_4 shell., Comment: 12 pages, 4 figures

We report on the synthesis and characterization of uncoated and gold coated magnetite nanoparticles. Structural characterizations, carried out using X-ray diffraction, confirm the formation of magnetite phase with a mean size of ~7 and ~8 nm for the uncoated and gold covered magnetite nanoparticles, respectively. The value of the gold coated Fe3O4 nanoparticles is consistent with the mean physical size determined from transmission electron microscopy images. Mossbauer spectra at room temperature are consistent with the thermal relaxation of magnetic moments mediated by particle-particle interactions. The 77 K Mossbauer spectra are modeled with four sextets. Those sextets are assigned to the signal of iron ions occupying the tetrahedral and octahedral sites in the core and shell parts of the particle. The room-temperature saturation magnetization value determined for the uncoated Fe3O4 nanoparticles is roughly ~60 emu/g and suggests the occurrence of surface effects such as magnetic disorder or the partial surface oxidation. These surface effects are reduced in the gold-coated Fe3O4 nanoparticles. Zero-field–cooled and field-cooled curves of both samples show irreversibilities which are consistent with a superparamagnetic behavior of interacting nanoparticles.

Due to the current necessity to obtain epitaxial superconductor films at low cost, we report the growth of YBa2Cu3O7 (Y123) films by chemical deposition. The procedure involved simple steps such as precipitation of stoichiometric amounts of yttrium, barium and copper acetates in oxalic acid (H2C2O4). The precursor solution was dripped onto LaAlO3 (100) substrates with the help of a Fisher pipette. The films were annealed in oxygen atmosphere during 12 h at three different temperatures: 820 °C, 840 °C and 860 °C. After 820 °C and 860 °C annealing, X-ray diffraction (XRD) analysis revealed high intensity of the ( 00l ) reflections denoting that most of the Y123 grains were c-axis oriented. In addition, we also observed a-axis oriented grains (( h00 ) reflexion), minor randomly oriented grains and other phases (such as Y2BaCuO5 and CuO). In contrast, the sample treated at 840 °C, we noticed c – and a-axis oriented grains, very small amounts of randomly oriented grains without formation of other phases. From the magnetization versus temperature measurements, the critical temperatures were estimated at 70K and 90K for the samples annealed at 820 °C and 860 °C respectively.