Understanding the Formation of Colloidal Ferrimagnetic CuCr ₂ Se ₄ Nanocrystals with Strong Room-Temperature Magnetic Circular Dichroism

The ongoing development and eventual implementation of magnetic nanocrystals in devices requires not only syntheses that can bring bulk compositions down to the nanoscale but also a deep understanding of their formation such that size, morphology, and composition can be finely tuned. Chromium chalcogenide spinels are a class of materials that epitomize this dilemma; their unique magnetic and magneto-optical properties make them promising for applications in spintronics, data storage, and quantum information sciences, but only a few compositions have been synthesized as colloidal nanocrystals. Furthermore, these few existing reports lack mechanistic understanding and demonstrate little control over the physical characteristics of the final products. Here, we set forth to understand the synthesis of CuCr2Se4 nanocrystals by examining how the structure, composition, and magnetic properties evolve over the course of the reaction. We find that the material proceeds through binary copper selenide nanocrystal intermediates followed by Cr incorporation via diffusion. This process results in polycrystalline CuCr2Se4 nanocrystals that do not exhibit magnetic ordering until Cu incorporation modifies their stoichiometry and defects are annealed, which takes approximately 40 min at 340 °C to achieve. The resulting CuCr2Se4 nanocrystals show a strongly enhanced magnetic circular dichroism signal at the bulk plasma frequency of ℏωpl ∼ 1.0 eV with a field dependence that reflects magnetization of the Cr3+ spin sublattice. These results highlight the possibility of solution processing strong near-IR magneto-optical materials for future device integration.