Polymer-Coated Magnetic Nanoparticles as Ultrahigh Verdet Constant Materials: Correlation of Nanoparticle Size with Magnetic and Magneto-Optical Properties

Abstract: The Faraday effect and Faraday rotation are important magneto-optical phenomena, where the polarization direction of linearly polarized light can be controlled by the application of a magnetic field along the direction of light propagation. Transmissive, magneto-optical materials of sufficient thickness can achieve large Faraday rotation angles, where the intrinsic magneto-optical activity of a substance is described by the Verdet constant of the material. High Verdet constant materials are critical for a wide range of magneto-optical devices, such as optical isolators, optical circulators, and modulators. State of the art in Faraday rotation devices such as optical isolators most widely employs inorganic garnet materials, which possess excellent optical transparency and robust thermomechanical properties. However, these materials possess fairly low Verdet constants (∼10^3–4 °/T·m). In this report, we demonstrate the use of polymer-coated magnetic cobalt nanoparticles (CoNPs) to afford ultrahigh Verdet constant materials (−2.2 × 10⁵ to −2.5 × 10⁶ °/T·m at 1310 nm) with 2–3 orders of magnitude greater Verdet constants than classical inorganic garnets and earlier polymer-magnetic NP materials. Furthermore, the polymer coating on magnetic NPs affords excellent colloidal dispersion that enables solution or melt processing of these materials into multilayered thin films or free-standing films. The ability to prepare CoNPs of varying sizes further enabled structure–property correlations of magnetic NP size with both bulk magnetic and magnetic-optical properties, which previously has not been conducted.