Exchange-Coupling and Nano-Structuring As a Means to Improve the Magnetic Performance of Ferrite Nanoparticles: <i>In Situ </i>and <i>Ex Situ </i>Powder Diffraction Studies

Permanent magnets (PMs) are indispensable constituents of electronic devices, consumer appliances, transportation components, military hardware, etc. They are also integral part of clean energy technologies, such as wind turbine generators or hybrid vehicle regenerative motors, as well as basis of several state-of-the-art biomedical applications.[1] Despite rare-earth (RE) magnets offering the best known magnetic performance, RE metals are critical raw materials, subject to worldwide market fluctuations and intermittent shortage. Therefore, optimization of RE-free PMs is of paramount importance and an area of intense research activity today.The magnetic properties of a material change dramatically when the size of its constituent particles is reduced to the nanometer scale. This opens a wide range of opportunities to fabricate high-performance magnetic materials. In the present work, the extraordinary behavior at the nano-scale is exploited to produce SrFe12O19 and CoFe2O4 magnets with optimized magnetic performance. In situ and ex situ powder X-ray diffraction (PXRD) data have been combined to characterize the materials during and after their preparation, respectively. Quantitative information on the material structure and micro-structure has been obtained from Rietveld analysis of the PXRD data, and the magnetic properties have been analyzed in this context.The coercivity (Hc) is a measure of the resistance of the material against demagnetization. For most magnetic materials, the Hc is found to increase as the particle size is reduced, reaching a maximum value at the critical single-domain size.[2] The first half of the studies presented in this dissertation are devoted to control the size of SrFe12O19 as a means to increase Hc. SrFe12O19 is a low-cost, non-toxic, chemically stable, hard magnetic material with a relatively strong uniaxial magnetocrystalline anisotropy and a very high Curie temperature. Furthermore, SrFe12O19 tends to form particles in the shape of platelets, which has been shown to be ideal for achieving a high-degree of alignment upon subsequent compaction into a dense magnet.[3,4] Here, SrFe12O19 nanoparticles were hydrothermally synthesized while the process was monitored in situ using synchrotron radiation. The real-time information on the crystallization of the nanoparticles provided the know-how to control the size of the produced material. Subsequent ex situ studies allowed magnetic characterization and correlation between micro-structure and magnetic performance.[5]The energy product (BHmax) is a measure of the magnetic energy stored by a magnet, and it is the most commonly used figure of merit for evaluating the performance of PMs. The novel strategy of exchange-coupling was used here to enhance the BHmax of CoFe2O4. The theory predicts a significant BHmax enhancement when magnetic phases of different nature (hard/soft) are magnetically coupled at the atomic-level.[6] The second half of this thesis is dedicated to the optimization of CoFe2O4 (hard)-CoFe2 (soft) nano-composites, prepared through a partial reduction of CoFe2O4 nanoparticles.[7] In situ and ex situ PXRD studies were combined for structural characterization. The magnetic properties, measured at both low and room temperature, are discussed in terms of the composition and the size of the different phases, and the degree of exchange-coupling evaluated