ALBA Synchrotron
Researchers have synthesized and studied by a combination of soft X-ray techniques platelets of strontium hexaferrite allowing them to establish the differences and similarities between their synthesized nanostructures and commercial powders. Most of the experiments have been performed within a collaboration among three beamlines of the ALBA Synchrotron.
At the left, Crystallographic structure of (SrFe12O19)2. At the right, (a) TEM images and SAED of SFO particles synthesized at 503 K. (b) TXM images taken with a Fresnel zone plate of 25 nm showing the absorption contrast of the same cluster of SFO particles, at three different angles (−30°, 0°, 30°). Effective pixel size = 10 nm.
Cerdanyola del Vallès, 8th January 2020.
Ferrites are ceramic materials usually made of large proportions of iron oxide (Fe2O3, rust) blended with small proportions of other metallic elements. These materials do not conduct electricity because they are insulators; and they are ferromagnetic, which means they can easily be magnetized or attracted to a magnet.
Strontium ferrites (SFO, SrFe12O19) in particular have a large magnetocrystalline anisotropy that gives it a high coercitivity, meaning that it is difficult to demagnetize. Since its discovery in the mid-20th century, this hexagonal ferrite has become an increasingly important material both commercially and technologically, finding a variety of uses and applications because of its low cost and toxicity. SFO has been used for permanent magnets, recording media, in telecommunications, and as a component in microwave, high-frequency and magneto-optical devices. Also, because they can be powdered and formed easily, they are finding their applications into micro and nano-types systems such as biomarkers, bio diagnostics and biosensors.
Researchers from the “Rocasolano” Institute of Physical Chemistry in Madrid, the Institut “Jozef Stefan” in Ljublijana, Slovenia, the Autonomous University of Madrid, the Institute of Materials Science of Madrid, the ALBA Synchrotron and the Institute of Ceramics and Glass in Madrid have synthesized single-crystal platelets of strontium hexaferrite, up to several micrometers in width, and tens of nanometers thick by a hydrothermal method. They have characterized the structural and magnetic properties of these platelets by a combination of techniques, with emphasis on Mössbauer spectroscopy, which provides information about the oxidation state and coordination of iron, and X-ray absorption based-measurements.
This is the first time that the X-ray absorption spectra at the Fe L2,3 edges of this material in its pure form have been reported. This technique allows to study the electronic structure of transition metal atoms and complexes.
Taken together, the results seem to indicate that the platelets contain a large amount of “tetrahedral/lower than octahedral coordination sites” which are mainly located at the surface. It must be taken into account that given the shape of the platelets, whose lateral dimensions (µm) are several orders of magnitude larger than their thickness (nm), the amount of sites unsaturated in oxygen which are located at the surface has to be overwhelming as compared to the number of these sites in the bulk. It would follow that the broadening observed in the X-ray diffraction (XRD) data, and at some extent in the room temperature Mössbauer data, would reflect then the various configurations arising from the distribution of iron ions which cannot complete their octahedral coordination and that can show either tetrahedral- or penta-oxygen coordination.
Among the various techniques used in this study, full-field transmission X-ray microscopy (TXM) was carried out at the MISTRAL beamline, photoemission electron microscopy (XMCD-PEEM) at the CIRCE beamline, and X-ray magnetic circular dichroism at the BOREAS beamline, all three at the ALBA Synchrotron.