Magnetic materials are vital in technologies from spintronics to biomedicine. Coupling magnetism with optical responses broadens their utility to sensing, magneto-optical memory, and optical isolation.  Chromium chalcogenide spinels display particularly rich magnetism and magneto-optical properties. Colloidal nanocrystals (NCs) offer routes to solution-processing, heterointegration, and property modulation through size, shape, or heterostructure control, but many chalcogenide spinels have never been synthesized at the nanoscale, and little control over size or morphology has been demonstrated.

This work examines CuCr₂Se₄ NC synthesis, revealing that precursor injection first forms copper selenide (Cu_2-xSe) NCs, which then template spinel formation. Slow Cr and Se in-diffusion forms polycrystalline CuCr₂Se₄ at the NC edges, and colloidal annealing then yields single-crystalline CuCr₂Se₄ NCs with bulk-like ferrimagnetism at room temperature. Spin-coated NC films show strong magnetic circular dichroism (MCD) at the bulk plasma frequency (ℏω_pl ~ 1.0 eV), reflecting bulk-like metallicity and optical properties. MCD intensities track NC magnetization, saturating at low fields. These CuCr₂Se₄ NCs thus offer a path for introducing magneto-optical functionality into next-generation spin-photonic architectures.