A review on advances in synthesis, composition, structural and microwave properties of U-type hexaferrites nanoparticles

U-type hexaferrites represent a specific category of ferrimagnetic materials with general composition formula Ba₄Me₂Fe₃₆O₆₀ and have the largest as well as complex structural unit cell among all hexagonal ferrites. These materials have fascinated the researchers ever since they were discovered (1964) because of their extraordinary structural, dielectric, and magnetic properties. Modifying the composition of these materials by incorporating different elements (transition, or rare earth, or their suitable combination) can impact their crystalline structure and hence change their characteristics. Consequently, these changes can influence the functional properties of these materials to absorb electromagnetic (EM) waves. Microwave absorption by U-type hexaferrites primarily originates from the FMR (ferromagnetic resonance) phenomenon. Ferromagnetic resonance frequency (f_r) is linked closely to the magnetocrystalline anisotropy (MCA) fields. This review paper provides a detailed description regarding the major studies in the field of U-type hexaferrites on single platform. Key research outcomes are briefly presented in the form of tables and graphs. Among various synthesis techniques, SSR (solid-state reaction) method is mostly used for synthesizing U-type hexaferrites. The specific features of U-type hexaferrites have led to their widespread use in cutting-edge applications. These materials demonstrate excellent microwave-absorbing properties and antenna application due to their attractive dielectric and magnetic behavior. The combination of U-type fillers and polymer matrix to form composites can be potentially applied for EMI shielding.