Hard magnetic colloidal nanoplates with tunable size for magneto-optical applications

Abstract

We have synthesized highly anisotropic plate-like nanoparticles of aluminum-substituted strontium hexaferrite via the crystallization of 4Na₂O×9SrO×5.5Fe₂O₃×4.5Al₂O₃×4B₂O₃ glass, achieving tunable sizes by adjusting the annealing temperature (650–750 °C). Plate sizes range from 39 nm × 4.5 nm to 90 nm × 7.1 nm. Aluminum substitution significantly increases the coercivity of the colloid particles up to 5600 Oe. These nanoparticles form stable aqueous colloids in the pH range of 2–4. The ferrofluids exhibit a strong “jalousie effect” of adjustable optical transmission in external magnetic fields. The transmission difference rises with increasing nanoplate diameter and anisotropy factor. The high remanence of the hexaferrite particles allows them to be manipulated by weak magnetic fields, providing high-frequency particle motion with available electromagnets. Tunable particle sizes facilitate specific applications: smaller particles offer higher relaxation frequencies and better stability, while larger particles provide superior light scattering and induced mechanical momentum. These properties make the nanoparticles suitable for microfluidic stirring, mechanical impacting for cancer treatment, high-frequency light modulation, optical probing of magnetic fields, and micrometer-scale viscoelasticity sensing.