Composite nanoplatelets combining soft-magnetic iron oxide with hard-magnetic barium hexaferrite

Abstract

By coupling two different magnetic materials inside a single composite nanoparticle, the shape of the magnetic hysteresis can be engineered to meet the requirements of specific applications. Sandwich-like composite nanoparticles composed of a hard-magnetic Ba-hexaferrite (BaFe₁₂O₁₉) platelet core in between two soft-magnetic spinel iron oxide maghemite (γ-Fe₂O₃) layers were synthesized using a new, simple and inexpensive method based on the co-precipitation of Fe³⁺/Fe²⁺ ions in an aqueous suspension of hexaferrite core nanoparticles. The required close control of the supersaturation of the precipitating species was enabled by the controlled release of the Fe³⁺ ions from the nitrate complex with urea ([Fe((H₂N)₂CO)₆](NO₃)₃) and by using Mg(OH)₂ as a solid precipitating agent. The platelet Ba-hexaferrite nanoparticles of different sizes were used as the cores. The controlled coating resulted in an exclusively heterogeneous nucleation and the topotactic growth of the spinel layers on both basal surfaces of the larger hexaferrite nanoplatelets. The direct magnetic coupling between the core and the shell resulted in a strong increase of the energy product |BH|_max. Ultrafine core nanoparticles reacted with the precipitating species and homogeneous product nanoparticles were formed, which differ in terms of the structure and composition compared to any other compound in the BaO–Fe₂O₃ system.