• Saura-Múzquiz, Matilde
• Christensen, Mogens
• Shyam, Priyank
• Eikeland, Anna
• Ahlburg, Jakob Voldum

Abstract

Rare-earth based permanent magnets (such as Nd2Fe14B3 magnets) exhibit superior performance characteristics but are limited by their high costs and low corrosion resistance. Volatile geopolitical circumstances and high environmental costs associated with rare-earth mining are additional concerns. These factors have given impetus to the development of permanent magnets that are free of rare-earth elements. Magnetic ferrites have emerged as viable alternatives, with the hexaferrite SrFe12O19being an excellent candidate. While SrFe12O19 has high coercivity (due to pronounced magnetocrystalline anisotropy) – making it a ‘hard magnetic’ phase, but it lacks a high magnetic saturation value. Spinel ferrites (AB2O4 type) on the other hand, are ‘soft magnetic’ phases i.e. low coercivities, but potentially strongly magnetic. Mixing the hard and soft phases at the nanoscale level results in an exchange-spring nanocomposite magnet where the soft phase enhances magnetization of the composite and the hard phase stabilizes the composite against demagnetization.The resultant magnetic properties of such composites would be hierarchically emergent – arising from the underlying atomic structure, via the nanoscale morphology of the individual particles, to the microscopic structural coupling of the different phases. While various studies have focused on the synthesis of exchange-spring magnets and their magnetic characterizations, detailed structural investigations are limited.In the present study, we report a comparative investigation on nanocomposites of SrFe12O19 (hard phase) and Zn0.2Co0.8Fe2O4 (soft phase) prepared by two different techniques: mechanical powder mixing and sol-gel coating. Hysteresis loops from VSM magnetometry showed a dependence of the exchange-coupling behavior on the technique used for nanocomposite formation. Crystallographic and magnetic structure of the composites (and the parent phases) was obtained by combined Rietveld refinement of data from synchrotron X-ray diffraction (SR-XRD performed at MS X04SA beamline at the Swiss Light Source) and thermal neutron powder diffraction (NPD performed using the HRPT diffractometer at SINQ spallation source, Paul Scherrer Institute). The difference in the scattering interaction for X-rays and neutrons allows for complementary, robust & accurate structural analysis. Combined Rietveld refinement of SR-XRD and NPD data of the nanocomposites enabled extraction of accurate values for lattice parameters, atomic positions, thermal motion, cation distribution, magnetic moments and microstructure. As the crystallographic and magnetic structures of exchange-spring nanocomposite systems dictate their observed magnetic properties, a detailed understanding of the intertwined magnetostructural properties would be a key enabler towards engineering better permanent magnets.

Topics

• nanocomposite
• microstructure
• morphology
• corrosion
• phase
• extraction
• mass spectrometry
• neutron diffraction
• magnetization
• coercivity
• synchrotron radiation X-ray diffraction