Combined characterization approaches to investigate magnetostructural effects in exchange-spring ferrite nanocomposite magnets

Abstract

Development of lanthanide-free permanent magnets has been the focus of intensive research for the past decade. Magnetic ferrites are viable alternatives but are limited by their magnetic characteristics. Exchange-spring nanocomposite magnets composed of interacting magnetic hard and soft phases are predicted to exhibit superior performance. In this study, we investigated the effects of nanoscale mixing by two techniques (physical powder blending and chemical nanocoating) on nanocomposites of SrFe₁₂O₁₉ (hard) and Zn_0.2Co_0.8Fe₂O₄ (soft). Detailed atomic-/nano-scale structural characterizations of these nanocomposites were performed by combining neutron and X-ray powder diffraction analysis with transmission electron microscopy. Macroscopic magnetic properties and exchange-coupling (recoil magnetization) were investigated by vibrating sample magnetometry. The powder-blend nanocomposite lacked exchange coupling while the chemically prepared nanocomposite showed exchange-spring behavior, albeit with reduced magnetic performance. Insights gained from the detailed structural characterization were employed to understand and explain the macroscopic magnetic performance. We also present a hitherto unreported degradation of the hard phase during the chemical processing. Our findings highlight potential red-flags for future investigations on exchange-spring nanocomposites where safeguards to protect the constituent phases from degradation during synthesis/processing should be considered and employed to maximize the magnetic performance.