Among iron oxide phases, magnetite (Fe₃O₄) is often the preferred one for nanotechnological and biomedical applications because of its high saturation magnetization and low toxicity. Although there are several synthetic routes that attempt to reach magnetite nanoparticles (NPs), they are usually referred as “IONPs” (iron oxide NPs) due to the great difficulty in obtaining the monophasic and stoichiometric Fe₃O₄ phase. Added to this problem is the common increase of size/shape polydispersity when larger NPs (D > 20 nm) are synthesized. An unequivocal correlation between a nanomaterial and its properties can only be achieved by the production of highly homogeneous systems, which, in turn, is only possible by the continuous improvement of synthesis methods. There is no doubt that solving the compositional heterogeneity of IONPs while keeping them monodisperse remains a challenge for synthetic chemistry. Herein, we present a methodical optimization of the iron oleate decomposition method to obtain Fe₃O₄ single nanocrystals without any trace of secondary phases and with no need of postsynthetic treatment. The average dimension of the NPs, ranging from 20 to 40 nm, has been tailored by adjusting the total volume and the boiling point of the reaction mixture. Mössbauer spectroscopy and DC magnetometry have revealed that the NPs present a perfectly stoichiometric Fe₃O₄ phase. The high saturation magnetization (93 (2) A·m²/kg at RT) and the extremely sharp Verwey transition (at around 120 K) shown by these NPs have no precedent. Moreover, the synthesis method has been refined to obtain NPs with octahedral morphology and suitable magnetic anisotropy, which significantly improves the magnetic hyperthemia performance. The heating power of properly PEGylated nano-octahedrons has been investigated by AC magnetometry, confirming that the NPs present negligible dipolar interactions, which leads to an outstanding magnetothermal efficiency that does not change when the NPs are dispersed in environments with high viscosity and ionic strength. Additionally, the heat production of the NPs within physiological media has been directly measured by calorimetry under clinically safe conditions, reasserting the excellent adequacy of the system for hyperthermia therapies. To the best of our knowledge, this is the first time that such bulklike magnetite NPs (with minimal size/shape polydispersity, minor agglomeration, and exceptional heating power) are chemically synthesized.

Neutron-diffraction, magnetic and transport measurements, and Mössbauer spectroscopy have been used in order to study the structural and magnetic changes of the magnetoresistive ${\mathrm{La}}_{0.7}{\mathrm{Pb}}_{0.3}{\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_3$ $(x=0,$ 0.05, 0.1, 0.2, 0.3) perovskitelike compounds. Samples were prepared by the sol-gel low-temperature method. In all cases the exact stoichiometry is different from the nominal one probably due to the presence of cation vacancies. All the phases exhibit ferromagnetic behavior except the compositions with the highest Fe contents $(x=0.2,$ 0.3), which show spin-glass order. A 10% Fe contribution lowers the $T_C$ by about 130 K. The magnetic moment continuously decreases with the increasing amount of iron. The magnetoresistance $[\mathrm{MR}=\Delta R/R(0\left)\mathrm{}\right],$ between 0 and 6 T applied fields, has been measured only in the $x=0.05\mathrm{}$ and 0.1 compositions, being of about 65% around the magnetic transition in the case of the 10% Fe content sample. For higher Fe contents, the samples show clearly insulating trends. These results can be interpreted in terms of a hindering of the double-exchange mechanism by ${\mathrm{Fe}}^{3+}$ ions, which gives rise to the localization of the electronic carriers and to a subsequent frustration of the long-range magnetic order.

In this work polycrystalline perovskite with composition ${\mathrm{La}}_{\mathrm{0.7}}{\mathrm{Ca}}_{\mathrm{0.3}}{\mathrm{Fe}}_{\mathrm{0.05}}{\mathrm{Mn}}_{\mathrm{0.95}}{\mathrm{O}}_{\mathrm{3}}$ has been produced by standard ceramic method. A considerable change occurs in magnetic, transport and magnetoresistance properties with respect to the classic composition without Fe. The introduction of a different metal, Fe, in Mn-O layer causes a decrease of about 50 K in the value of $T_C.$ In the same way, a decrease of about 10–15% in the average magnetic moment measured at 1 T is also observed. On the other hand, the introduction of Fe does not cause any appreciable change in the value of the lattice parameter. This new compound presents 60% of giant magnetoresistance ratio at 200 K. Magnetization vs. temperature measurements in zero field cooling and field cooling show clear differences at applied fields below 80 ${\mathrm{kA\hspace{0.17em}m}}^{\text{−1}}$ . Such a behavior, also observed in ${\mathrm{La}}_{\mathrm{0.7}}{\mathrm{Ca}}_{\mathrm{0.3}}{\mathrm{MnO}}_{\mathrm{3}}$ sample, is not recognizable as a simple ferromagnetic one. A qualitative discussion of the effect in the magnetic and transport properties of these compounds, caused by the substitution of some Mn atoms by Fe is given.