Cecilia Granados-Miralles

Unraveling structural and magnetic information during growth of nanocrystalline SrFe12O19

The hydrothermal synthesis of magnetic strontium hexaferrite (SrFe12O19) nanocrystallites was followed in situ using synchrotron powder X-ray diffraction. For all the studied temperatures, the formation of SrFe12O19 happened through an intermediate crystalline phase, identified as the so-called six-line ferrihydrite (FeOOH). The presence of FeOOH has been overlooked in previous studies on hydrothermally synthesized SrFe12O19, despite the phase having a non-trivial influence on the magnetic properties of the final material. The chemical synthesis was successfully reproduced ex situ in a custom-designed batch-type reactor that resembles the experimental conditions of the in situ setup, while allowing larger quantities of material to be produced. The agreement in phase composition between the two studies reveals comparability between both experimental setups. Hexagonal platelet morphology is confirmed for SrFe12O19 combining Rietveld refinements of powder X-ray diffraction (PXRD) data with transmission electron microscopy (TEM). Room temperature magnetization curves were measured on the nanopowders prepared ex situ. The magnetic properties are discussed in the context of the influence of phase composition and crystallite size.

Co on Fe3O4(001): Towards precise control of surface properties

A novel approach to incorporate cobalt atoms into a magnetite single crystal is demonstrated by a combination of x-ray spectro-microscopy, low-energy electron diffraction, and density-functional theory calculations. Co is deposited at room temperature on the reconstructed magnetite (001) surface filling first the subsurface octahedral vacancies and then occupying adatom sites on the surface. Progressive annealing treatments at temperatures up to 733 K diffuse the Co atoms into deeper crystal positions, mainly into octahedral ones with a marked inversion level. The oxidation state, coordination, and magnetic moments of the cobalt atoms are followed from their adsorption to their final incorporation into the bulk, mostly as octahedral Co(2+). This precise control of the near-surface Co atoms location opens up the way to accurately tune the surface physical and magnetic properties of mixed spinel oxides.

Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

During the past decade, CoFe2O4 (hard)/Co–Fe alloy (soft) magnetic nanocomposites have been routinely prepared by partial reduction of CoFe2O4 nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a byproduct of the reduction, although it remains unclear whether the formation of this phase occurs during the reduction itself or at a later stage. Here, a novel reaction cell was designed to monitor the reduction in situ using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the in situ data yielded time-resolved information on the sample composition and confirmed that the monoxide is generated as an intermediate phase. The macroscopic magnetic properties of samples at different reduction stages were measured by means of vibrating sample magnetometry (VSM), revealing a magnetic softening with increasing soft phase content, which was too pronounced to be exclusively explained by the introduction of soft material in the system. The elemental compositions of the constituent phases were obtained from joint Rietveld refinements of ex situ high-resolution PXRD and neutron powder diffraction (NPD) data. It was found that the alloy has a tendency to emerge in a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which can explain the early softening of the magnetic material.

Enhancement of magnetic properties by spark plasma sintering of hydrothermally synthesised SrFe12O19

Platelet-shaped nano-crystallites of SrFe12O19 were hydrothermally synthesised and compacted into bulk magnets using Spark Plasma Sintering (SPS). Pellets were obtained with densities >90% of the theoretical crystallographic density. Compaction of the as-synthesised powders at 950 °C for 2 minutes causes improved alignment of the crystallites within the pellet, compared with room-temperature compaction. Applying an external magnetic field prior to the SPS compaction further increases the degree of alignment leading to an enhanced magnetic saturation and remanence, resulting in a 25% improvement of the energy product (BHmax), from 21.5 kJ m−3 obtained without magnetic alignment to 27.0 kJ m−3 with the applied magnetic field. Post-annealing of the SPS-pressed pellets improves the magnetic saturation even further leading to an increase of the energy product to 30.4 kJ m−3. Ball milling, on the other hand, diminishes the magnetic remanence, causing a reduction of the energy product.