Finite-size effects occur in nanostructured materials as thin films, nanoparticles and nanowires. The control of their morphology and functionalities at the nanoscale is a prerequisite for some applications. One relevant research field nowdays is the use of magnetic nanoparticles as nanovectors for drug delivery. Spherical empty nanocapsules are appealing for such applications since they could store larger amounts of drug than solid NPs of the same size. On the basis of their enhanced surface/bulk atomic ratio, magnetic Hollow NanoSpheres (HNS) provide an excellent system to study the competition between surface and bulk magnetism at nanoscale level and open up new perspectives for experimental and theoretical developments. The magnetic behavior of the surface atoms is characterized by the lack of symmetry and broken chemical and exchange bonds which introduce structural and magnetic disorder and originate an enhancement of the site-by-site magnetic anisotropy.
Semiconductor and magnetic micro-, submicro- and nano-spheres have been already reported in the literature, being produced by different techniques with distinct morphological characteristics. Regarding the iron oxides, HNS with narrow diameter dispersion were produced by a controlled post-synthesis oxidization of Fe or Fe3O4 nanoparticles.
The magnetic properties of these systems are so far not well explored. It is known that the post-synthesis oxidization process increments the crystalline order of the system, influencing the magnetic effects resulting from the core/surface competition. In this way, a single-step and controlled procedure to synthesized magnetic HNS would enable us to produce systems with particular magnetic properties. A single-step method for obtaining monodisperse ferrite HNS, with no subsequent oxidization process, can yield samples with high crystalline disorder. The resulting unusual magnetic behavior is related to the large surface/bulk atomic ratio due to both inner and outer surfaces of a hollow sphere.
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