Understanding The Formation of Ferromagnetic CuCr₂Se₄ Nanocrystals

Spinels (AB₂X₄) are a unique class of materials that exhibit a range of magnetic, optical, and magnetooptical properties tunable through choice of A and B site cations. While many of the oxides have been synthesized at the nanoscale, research into nanocrystalline chalcogenide spinels is lacking. Amongst these the copper chromium chalcogenides are of particular interest for their high Curie temperatures (>350K) and large magneto-optical Kerr effects. Only a handful of reported syntheses for these materials exist, with conflicting magnetic properties (e.g. superparamagnetic vs. ferromagnetic), minimal mechanistic understanding, and no optical data.<span style="font-size:10.8333px"> </span>Here, we delve deeper into the synthesis of this material. Binary copper selenide forms as an intermediate followed by Cr diffusion into the lattice. Upon ramping the reaction temperature, magnetism is slow to turn on as evident by magnetic susceptibility measurements. After approximately 40 minutes at 340°C ferromagnetic ordering appears concurrently with a change in lattice parameter, crystallite size, and stoichiometry suggesting annealing and cation diffusion. Magnetic circular dichroism data of nanocrystal films is similar to magnetooptical Kerr rotation spectra of single crystals exhibiting a NIR feature that overlaps with telecommunications wavelengths. Current and future directions involving doping, cation exchange, and spectroelectrochemistry are also discussed.