Synthesis of YIG and Bi-YIG nanoparticles by coprecipitation and thermal treatment: towards magnetic liquids with controlled magneto-optical properties.

Yttrium iron garnet (Y3Fe5O12, YIG) and bismuth-doped YIG (BixY3-xFe5O12, Bi-YIG) are reference materials for magneto-optical devices. Their cubic structure gives them high transparency in the near-infrared region (wide bandgap, 2.45 eV, and forbidden d-d transitions), high Faraday rotation and excellent chemical stability, making them interesting candidates for integrated optical isolators. However, synthesizing nano-sized YIG particles (less than 20 nm in size) and obtaining a magnetic liquid based on such nanoparticles prove to be major challenges, as crystallization of the garnet phase requires temperatures higher than 800 °C. Prolonged annealing triggers coalescence and grain growth, with a consequent impact on grain size. Added to this is the stoichiometric sensitivity of garnet: an excess of oxygen or of yttrium favors the formation of secondary phases (iron and yttrium perovskite YFeO3 (YIP) or hematite α-Fe2O3) that are difficult to subsequently eliminate. Various synthesis strategies-such as coprecipitation, sol-gel synthesis, and self-combustion-are therefore aimed at lowering the garnet formation temperature to enable the production of small nanoparticles. However, simultaneous control of both particle size and purity remains limited. This forms the background of the work presented in this article, which combines controlled coprecipitation, high-temperature annealing and wet planetary milling in the simultaneous presence of a surfactant such as PEOS (3-[methoxy(polyethyleneoxy)propyl]trimethoxysilane). This approach enabled us to obtain YIG and Bi-YIG nanoparticles (d < 20 nm) and magnetic liquids displaying good crystallinity and durable colloidal stability. Magneto-optical measurements have confirmed the optical quality of the obtained ferrofluid, paving the way for its integration into optical isolators.