A reliable crystallographic model of Ni1−xZnxFe2O4 is presented using a combination of different methods; TEM, STEM-HAADF, and powder diffraction data from different sources (in-house, synchrotron and neutron).

Temperature treatment of magnetic Mn-Zn ferrites with the composition Mn 0.6 Zn 0.2 Fe 2.2 O 4 up to 1100 °C results in a tremendous enhancement of the saturation magnetization by more than 60%. Employing a robust combined Rietveld refinement of powder X-ray and neutron diffraction (PXRD and NPD) data, it is revealed how a reordering of the cations takes place during the annealing step, the extent of which depends on the annealing temperature. While Zn(II) exclusively occupies tetrahedral sites throughout the whole temperature range, as the annealing temperature increases up to 700 °C, the Mn(II) cation distribution shifts from 80(7)% of the total Mn content occupying the octahedral sites (partly inverse spinel)to Mn only being present on the tetrahedral sites (normal spinel). Above 700 °C, pronounced crystallite growth is observed, followed by an increase of the saturation magnetization. Complementary techniques such as energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM) confirm an even cation distribution and the particle growth with annealing temperature. The structural changes caused by annealing of spinel ferrites directly alter the magnetic properties of the materials, thus serving as an easy handle for enhancing their magnetic properties.values typically ranging between 23 and 85 Am 2 kg -1 . [15][16][17] The wide range of values in the literature can be attributed to modifications of microstructure and cation distribution. Despite these characteristics being key to explaining the differences in the magnetic performance of Mn-Zn substituted spinel ferrites, there is a general lack of investigations addressing the microstructure and cation distribution. This is partly because the cation distribution effect is particularly difficult to unravel in the case of neighboring atoms in the periodic table when using laboratory X-rays or a standard synchrotron experiment. Even though powder Xray diffraction (PXRD) allows modeling the site occupancies of all cations in the crystallographic structure, the reliability of a model is compromised when elements are in the vicinity of each other in the periodic table. This is a challenge in complex spinel ferrites, as the transition metals have a similar number of electrons and therefore similar atomic form factors, e.g. Mn (Z = 25) and Fe (Z = 26). A way to circumvent this is to collect neutron powder diffraction (NPD) data. When using NPD, strong contrasts between cations can be achievable, as the scattering lengths vary erratically with atomic number, e.g. Mn (b Mn = -3.73 fm) and Fe (b Fe = 9.45 fm). This facilitates modeling the cation distribution in spinel ferrites reliably.The cation distribution of a sample depends on the crystal field stabilization energies of the individual cations, their charge and their ionic radius. 18 Annealing can introduce changes in the cation distribution.However, not only the heating step is decisive in modifying the cation distribution, but also the cooling procedure is crucial in controlling it and ther...

Nanocrystalline platelets of Sr hexaferrite, SrFe 12 O 19 , were prepared by four techniques (two hydrothermal and two sol-gel techniques) and calcined at 1000 °C for 1 h, 2 h, 4 h, 8 h and 16 h. The microstructure of these samples was analyzed using Rietveld refinements of high-resolution synchrotron powder X-ray diffraction (PXRD) data, and the obtained results were correlated with the magnetic properties obtained from vibrating sample magnetometer (VSM) measurements. The calcination treatment causes the crystallites to preferentially grow along the c-axis, leading to more isotropic crystallites. Moreover, the microstructural changes induced by calcination alter the magnetic properties, yielding higher saturation magnetizations in all samples. The attained coercivity is correlated with the crystallite size along the width of the platelets. Despite the pronounced changes of the microstructure and the magnetic properties after calcination, the calcination duration has a minor effect on the properties, i.e. in most cases steady state is obtained after 1 hour. The starting material has a profound impact on the microstructural change during calcination, despite the high calcination temperature.

Platelet shaped strontium hexaferrite (SrFe12O19) crystallites were hydrothermally synthesized in an autoclave to study the effect of (a) Fe/Sr molar ratio, (b) choice of base NaOH/KOH and (c) base concentration. The influence of these parameters on the final product is evaluated with regards to phase composition, structure and magnetic properties. Rietveld refinements were performed on powder x-ray diffraction (PXRD) data to determine the phase composition, structural changes, crystallite sizes, and preferred orientation, while the magnetic properties were measured using a vibrating sample magnetometer. When NaOH is used as the base, the samples consist mostly (>95 wt.%) of SrFe12O19 up to the same molar ratio of Fe/Sr = 8, independent of the concentration of the base. In contrast, when using KOH, the phase composition depends on both the molar ratio of Fe/Sr and the concentration of KOH. High concentrations of Sr2+ and OH− (Fe/Sr = 1 and OH−/NO3 − = 4) result in the growth of wide crystallites (>400 nm). The thickness of the crystallites are in all cases around 40 nm causing the crystallites to have an anisotropic shape, which can align without applying an external magnetic field. In the case of KOH as base instead of NaOH, an expansion of the unit cell is observed, which can be attributed to K+ substituting Sr2+ in the structure. This is corroborated by increasing microstrain when increasing the KOH/NO3 − ratio. Variations in the observed coercivity may be attributed to substitution of Sr2+ by K+. The present study illustrates that meticulous control of all reaction parameters and a meticulous analysis of the crystal structure is key for preparing and understanding hard-magnetic SrFe12O19.

Nano-structuring is a crucial step in optimizing permanent magnet materials, where both the size and morphology of the individual particles heavily affect the properties of the compacted bulk magnet. The size is tuned to ensure magnetic single-domain particles, which is crucial for optimizing the coercivity of the magnet (the field strength required to flip the magnetic orientation), as this is lowered by the mobility of walls between domains. Generally, the minimum required coercivity of a permanent magnet is half of the saturation magnetization, at which point the remanence (the magnetization at zero field) is the limiting factor. The remanence, in turn, is primarily tuned by the alignment of the particles, i.e. the texture of the bulk magnet. One way to control the alignment is through the morphology of the nano-particles, as the right particle shape leads the powder to selfalignment during compaction and sintering into dense pellets, thus optimizing the alignment of the magnetic domains in the resulting bulk magnet [1] . The overall figure-of-merit for permanent magnets is evaluated from the volume-weighted energy product (a.k.a. the BHmax), reported in kJ/m 3 (MGOe in cgs-units). It follows that the density of the final bulk magnet is an important parameter, which again is correlated with the particle microstructure. The combination of crystallite size, texture, and density emphasizes the compaction process when going from powder to bulk, which also often includes a sintering step. Powder diffraction is a powerful technique for studying the compaction and sintering processes, as proper refinement of the data can provide valuable information about phases, crystallite size, texture, and, in the case of neutron diffraction, the magnetic moments, as they develop during the compact. Neutron powder diffraction also comes with the benefit of a large probing volume, which is useful for studying bulk behavior.Using parametric refinements allows us to get the most out of our neutron powder diffraction data! [1] Stingaciu, Marian, et al. "Optimization of magnetic properties in fast consolidated SrFe12O19 nanocrystallites."