Matilde Saura-Múzquiz

Magnetic Properties of Strontium Hexaferrite Nanostructures Measured with Magnetic Force Microscopy.

Magnetic property is one of the important properties of nanomaterials. Direct investigation of the magnetic property on the nanoscale is however challenging. Herein we present a quantitative measurement of the magnetic properties including the magnitude and the orientation of the magnetic moment of strontium hexaferrite (SrFe12O19) nanostructures using magnetic force microscopy (MFM) with nanoscale spatial resolution. The measured magnetic moments of the as-synthesized individual SrFe12O19 nanoplatelets are on the order of ~10−16 emu. The MFM measurements further confirm that the magnetic moment of SrFe12O19 nanoplatelets increases with increasing thickness of the nanoplatelet. In addition, the magnetization directions of nanoplatelets can be identified by the contrast of MFM frequency shift. Moreover, MFM frequency imaging clearly reveals the tiny magnetic structures of a compacted SrFe12O19 pellet. This work demonstrates the mesoscopic investigation of the intrinsic magnetic properties of materials has a potential in development of new magnetic nanomaterials in electrical and medical applications.

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.

Enhancement of magnetic properties through morphology control of SrFe12O19 nanocrystallites

Nanocrystallites of the permanent magnetic material SrFe12O19 were synthesised using a conventional sol-gel (CSG) and a modified sol-gel (MSG) synthesis route. In the MSG synthesis, crystallite growth takes place in a solid NaCl matrix, resulting in freestanding nanocrystallites, as opposed to the CSG synthesis, where the produced nanocrystals are strongly intergrown. The resulting nanocrystallites from both methods exhibit similar intrinsic magnetic properties, but significantly different morphology and degree of aggregation. The nanocrystallites were compacted into dense pellets using a Spark Plasma Sintering (SPS) press, this allows investigating the influence of crystallite morphology and the alignment of the nanocrystallites on the magnetic performance. A remarkable correlation was observed between the crystallites morphology and their ability to align in the compaction process. Consequently, a significant enhancement of the maximum energy product was obtained after SPS for the MSG prepared sample (22.0 kJ/m3), compared to CSG sample, which achieved an energy product of 11.6 kJ/m3.

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.

Functional and Energy Materials

Neutron News Volume 27 • Number 1 • 2016 7 T topic of functional and energy related materials started relatively late in the conference. Nevertheless, there were a number of instrumental and neutron scattering technique talks of great interest for the functional and energy materials community. In particular Bragg edge imaging should be mentioned. This technique can be expected to grow signifi cantly in the coming years as new instruments at spallation sources with dedicated timeof-fl ight detectors become available. The topic was introduced on Tuesday morning by the speakers Takenao Shinohara (Japan Atomic Energy Agency), Eberhard Lehmann (Paul Scherrer Insitut), and Pavel Trtik (Paul Scherrer Insitut), collectively giving a very good introduction to imaging techniques and the opportunities provided by the use of Bragg edge enhancement to create elemental contrast to obtain spatially resolved phase identifi cation. In addition to conventional imaging and tomography, the time-of-fl ight technique offers access to strain and texture. Several examples were given to illustrate how imaging can be used to investigate fuel cells and Li-ion batteries. The fi rst talk dedicated to functional and energy materials was given by Jean-Marie Tarascon (Collège de France), with focus on the structure-to-property relationship of electrode materials in re-chargeable Li-ion batteries. The talk was very interesting from a materials perspective, but the amount of neutron related science was limited. However, it was interesting to learn that the oxidation state of oxygen can be different from 2giving an additional handle on the valence state in battery materials. Also, a look into the economic cycle for Li-ion battery materials was provided, from which it was surprising to fi nd out that batteries must be cycled more than 3000 times before they add positively to the energy balance. On Wednesday morning there was a session dedicated to Chemistry of Materials where the talks were focused on hydrogen dynamics in various catalytic and proton conducting systems. Stewart Parker (Rutherford Appleton Laboratory) gave an introduction to the application of inelastic scattering for chemical analysis using TOSCA at ISIS. On Wednesday afternoon Christian Masquelier (LRCS, Université de Picardie Jules Verne) gave an interesting talk on operando studies using X-rays and neutron diffraction for investigating Li-ion batteries. Powder diffraction was specifi cally used to observe the orthorhombic to cubic phase transition in LiMn2O4, while cycling the battery in operando. The fi rst session dedicated to functional materials was held Thursday afternoon, starting with a talk given by Gwenaelle Rousse (UPMCCollège de France) about new potential positive electrode materials for Li-ion batteries. The aim was to replace PO4 3with SO3 2and Fto create a compound with the chemical composition LiFeSO3F, which has a very high electrochemical potential of 3.9 eV. Later in the session Martin J. Mühlbauer (Karlsruhe Institute of Technology) spoke about spatially resolved neutron imaging of a commercial battery. The last talk in the session was given by Maths Karlsson (Chalmers University of Technology) related to proton conducting oxides, more specifi cally a family of Ba-In oxides, which contain a rich number of structural modifi cations depending on temperature. Both diffraction and in-elastic scattering have been used in the characterization of these compounds. On Friday morning there were two sessions in parallel, which were interesting with respect to functional and energy related materials, namely functional materials and engineering applications. The keynote speaker in “Engineering Applications” Giovanni Bruno (Bundesanstalt Materialforschung und Pruefung) gave an excellent talk about the industrial use of neutrons. For industry, neutrons are but one of many techniques applied, when investigating engineering problems of commercial materials. In conclusion, the main topics at ECNS2015 concerning functional and energy related materials were Li-ion batteries and proton conducting materials. These topics coincide well with the strength of neutron scattering in relation to dynamic processes of hydrogen and with the ability to visualize and locate lithium atoms. The use of new imaging techniques taking advantage of the time structure at spallation sources are likely to become increasingly used for characterization of functional and energy materials.