Nanotechnology involves the study of nature at a very small scale, searching new properties and applications. The development of this area of knowledge affects greatly both biotechnology and medicine disciplines. The use of materials at the nanoscale, in particular magnetic nanoparticles, is currently a prominent topic in healthcare and life science. Due to their size-tunable physical and chemical properties, magnetic nanoparticles have demonstrated a wide range of applications ranging from medical diagnosis to treatment. Combining a high saturation magnetization with a properly functionalized surface, magnetic nanoparticles are provided with enhanced functionality that allows them to selectively attach to target cells or tissues and play their therapeutic role in them. In particular, iron oxide nanoparticles are being actively investigated to achieve highly efficient carcinogenic cell destruction through magnetic hyperthermia treatments. Hyperthermia in different approaches has been used combined with radiotherapy during the last decades, however, serious harmful secondary effects have been found in healthy tissues to be associated with these treatments. In this framework, nanotechnology provides a novel and original solution with magnetic hyperthermia, which is based on the use of magnetic nanoparticles to remotely induce local heat when a radiofrequency magnetic field is applied, provoking a temperature increase in those tissues and organs where the tumoral cells are present. Therefore, one important factor that determines the efficiency of this technique is the ability of magnetic nanoparticles to be driven and accumulated in the desired area inside the body. With this aim, magnetic nanoparticles must be strategically surface functionalized to selectively target the injured cells and tissues.

The crystallographic structure and the magnetic order of the distorted perovskite ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$ $(0.10\mathrm{}<~x<~0.30)$ has been studied by neutron diffraction, high-resolution electron microscopy, and magnetic-susceptibility measurements. The results give direct evidence for an inhomogeneous distribution of the ${\mathrm{Sr}}^{2+}$ ions and the segregation of the material into hole-rich ferromagnetic regions and a hole-poor semiconducting matrix at lower values of x. The holes introduced by Sr doping are attracted to the ${\mathrm{Sr}}^{2+}$ ions where they stabilize to lowest temperatures an intermediate-spin state at neighboring trivalent cobalt. The antibonding e electrons so stabilized increase the mean unit-cell volume and are delocalized over the cobalt atoms of the cluster where they couple the localized $t^5$ configurations ferromagnetically. Long-range ferromagnetic order between clusters is realized even for Sr doping as low as $x=\mathrm{0.10.}\mathrm{}$ The transition to a spin glass state is observed only for Sr concentrations smaller than 0.10. The volume of a hole-rich cluster grows in a magnetic field, and the origin of the large negative magnetoresistance observed near $T_C$ for $0.15<~x<~0.25$ appears to be due to a growth of the clusters to a percolation threshold. For $x=0.30,$ the ${\sigma }^{*}$ band of the intermediate-spin state below $T_C$ is at the threshold of a transition from itinerant to polaronic conduction and, above $T_C,$ the system transforms smoothly to a cluster state.

Using a criterion given by Banerjee [Phys. Lett. 12, 16 (1964)] to distinguish first-order magnetic transitions from second-order ones, we demonstrate that not all the title perovskites exhibit a first-order transition at their Curie temperatures $T_C.$ We observe that, although ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ exhibits a first-order magnetic transition at $T_C,$ ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ exhibits a second-order one.

Using a sol‐gel method we have prepared monodispersive particles of La_0.67Ca_0.33MnO₃ in the range from 20 to 110 nm at temperatures from 540 to 1000 °C. A magnetoresistance above 10% was obtained in a field of 1 kOe for all the particles sizes. These results may be relevant for future applications.

We use Monte Carlo simulations to study the influence of dipolar interaction and polydispersity on the magnetic properties of single-domain ultrafine ferromagnetic particles. From the zero field cooling (ZFC)/field cooling (FC) simulations we observe that the blocking temperature $T_{\mathrm{B}}$ clearly increases with increasing strength of interaction, but it is almost not effected by a broadening of the distribution of particle sizes. While the dependence of the ZFC/FC curves on interaction and cooling rate are reminiscent of a spin glass transition at $T_{\mathrm{B}}$, the relaxational behavior of the magnetic moments below $T_{\mathrm{B}}$ is not in accordance with the picture of cooperative freezing.

Exceptional magnetic properties of magnetite, Fe₃O₄, nanoparticles make them one of the most intensively studied inorganic nanomaterials for biomedical applications. We report successful gram-scale syntheses, via hydrothermal route or controlled coprecipitation in an automated reactor, of colloidal Fe₃O₄ nanoparticles with sizes of 12.9 ± 5.9, 17.9 ± 4.4, and 19.8 ± 3.2 nm. To investigate structure–property relationships as a function of the synthetic procedure, we used multiple techniques to characterize the structure, phase composition, and magnetic behavior of these nanoparticles. For the iron oxide cores of these nanoparticles, powder X-ray diffraction and electron microscopy both confirm single-phase Fe₃O₄ composition. In addition to the core composition, the magnetic performance of nanoparticles in the 13–20 nm size range can be strongly influenced by the surface properties, which we analyzed by three complementary techniques. Raman scattering and X-ray photoelectron spectroscopy (XPS) measurements indicate overoxidation of nanoparticle surfaces, while transmission electron microscopy (TEM) shows no distinct core–shell structure. Considered together, Raman, XPS, and TEM observations suggest that our nanoparticles have a gradually varying nonstoichiometric Fe₃O_4+δ composition, which could be attributed to the formation of Fe₃O₄–γ-Fe₂O₃ solid solutions at their outermost surface. Detailed analyses by TEM reveal that the hydrothermally produced samples include single-domain nanocrystals coexisting with defective twinned and dimer nanoparticles, which form as a result of oriented-attachment crystal growth. All our nanoparticles exhibit superparamagnetic-like behavior with a characteristic blocking temperature above room temperature. We attribute the estimated saturation magnetization values up to 84.01 ± 0.25 emu/g at 300 K to the relatively large size of the nanoparticles (13–20 nm) coupled with the syntheses under elevated temperature; alternative explanations, such as surface-mediated effects, are not supported by our spectroscopy or microscopy measurements. For these colloids, the heating efficiency in magnetic hyperthermia correlates with their saturation magnetization, making them appealing for therapeutic and other biomedical applications that rely on high-performance nanoparticle-mediated hyperthermia.