M. Abuín

Growth, structure and magnetism of ε-Fe2O3 in nanoparticle form

We present a novel and easy synthetic path to prepare e-Fe2O3 (∼90%) with a small portion of α-Fe2O3 nanoparticles embedded in an amorphous silica matrix. An exhaustive structural study reveals the higher relative concentration of the e-phase, with an average particle size of 17 nm. Confocal Raman microscopy and X-ray absorption spectroscopy are presented as novel techniques to characterize the e-polymorph. The magnetic properties have been studied in a wide range of temperatures (5–1000 K), detecting blocking effects (∼135 K), collapse effects (50–125 K), Morin (∼268 K) and Neel (∼505 K) transitions.

Origin of the magnetic transition at 100 K in ε-Fe2O3 nanoparticles studied by x-ray absorption fine structure spectroscopy

We present a study of the correlation between the magnetic phase transition and the structural distortion observed at 100 K in ε-Fe2O3. For this purpose, we have designed a novel one-pot sol-gel method assisted by glycerol, which reproducibly provides samples with a nominal 100% concentration of ε-Fe2O3 nanoparticles embedded in a SiO2 matrix. The high crystallinity of the samples and the absence of other iron oxide polymorphs has allowed us to perform, for the first time, temperature-dependent X-ray absorption fine structure spectroscopy experiments, with the aim of investigating the origin of the magnetic quenching anomaly observed at 100 K. The deformation of the structure at a local scale, where the tetrahedral and octahedral Fe sites undergo distortions of different intensities, has been simulated to fulfill the long-range order. Our results point to a local structure distortion accompanied by the magnetism quenching through a magneto-elastic coupling.The current study unveils the structural origin of the magnetic transition of the ε-Fe2O3 polymorph from an incommensurate magnetic order to a collinear ferrimagnetic state at low temperature. The high crystallinity of the samples and the absence of other iron oxide polymorphs have allowed us to carry out temperature-dependent x-ray absorption fine structure spectroscopy experiments out. The deformation of the structure is followed by the Debye-Waller factor for each selected Fe-O and Fe-Fe sub-shell. For nanoparticle sizes between 7 and 15 nm, the structural distortions between the Fete and Fe-D1oc sites are localized in a temperature range before the magnetic transition starts. On the contrary, the inherent interaction between the other sub-shells (named Fe-O1,2 and Fe-Fe1) provokes cooperative magneto-structural changes in the same temperature range. This means that the Fete with Fe-D1oc polyhedron interaction seems to be uncoupled with temperature dealing with these nanoparticle sizes wherein the structural distortions are likely moderate due to surface effects.

Tuning the magnetic properties of FeCo by pulsed DC magnetron sputtering

We report a comparison of the magnetic properties of Fe53CO47 grown with DC and pulsed DC (PDC) sputtering sources. While no remarkable differences concerning the structure and composition are found, films grown using the PDC source exhibit reduced coercivity by several times compared to the samples grown using the DC source under similar conditions. In addition, PDC films exhibit hard axis rocking effects and a better defined anisotropy axis. By means of EXAFS, we have measured the atomic distances in the FeCo matrix and bcc-Co clusters. We show that the small changes within the FeCo matrix are directly related to the reduction in coercivity. The tuning of these properties by simply choosing the source or the power opens a simple path to tailor the magnetic properties.

Multifunctional core–shell Co–SiO2 nanowires via electrodeposition and sol–gel techniques

In this work, we propose a new strategy for the synthesis of multifunctional nanowires using a combination of sol–gel and electrodeposition techniques, based on a two-step procedure. First of all, nanotubes of SiO2 are synthesized via a sol–gel technique using polycarbonate membranes as templates. Homogenous nanotubes are obtained after centrifugation and thermal annealing. Afterwards, a ferromagnetic cobalt core is grown using potentiostatic electrodeposition. Finally, the core–shell Co–SiO2 nanowires are released by dissolving the template using wet-etching. These nanodevices can be used for many detection and sensing purposes. As a proof of concept, we have developed a pH nanosensor by including a pH-sensitive organic dye in the SiO2 shell. The sensing principle is based on the optical response of the organic dye towards pH when added to a solution. The magnetic core allows the recovery of the nanosensors after use. These nanowires can therefore be used as recoverable pH nanosensors. By changing the dye molecule to another molecule or receptor, the procedure described in the paper can be used to synthesize nanodevices for many different applications.