Size and Magnetic-Field Dependence of MCD Spectra of Magnetite (Fe₃O₄) Nanoparticles Synthesized by an l-Proline-Mediated Solvothermal Approach: Effect of Surface Spin Disorder

Size-tunable magnetite (Fe3O4) nanoparticles from 3.9 to 15.6 nm are successfully synthesized using a solvothermal method in the presence of l-proline and tert-butyl alcohol. Strong size and magnetic-field dependence of magnetic circular dichroism (MCD) spectra are found for the nanoparticle products, which uncovers not only the formation of a maghemite γ-Fe2O3 shell but also the existence of a spin-disordered layer at the surfaces. Specifically, at a very low magnetic field, size-dependent MCD responses including both their amplitude and spectral shape are notable, and the formation of core–shell morphology in which a magnetic core of Fe3O4 is surrounded by a magnetically dead (or spin-disordered) shell layer is suggested. At a high magnetic-field strength, surprisingly, the spectral shape of MCD changes to be similar to that of γ-Fe2O3. In consideration together with the electron magnetic resonance (EMR) performance, the observed MCD behaviors can be interpreted by the emergence of spin polarization and/or correlated spin canting inside the original spin-disordered (or dead) surface layer, resulting from an increased magnetic dipolar interaction that would align the magnetic moments in a strong magnetic field.