Insights into the Magnetic Properties of Sub-10 nm Iron Oxide Nanocrystals through the Use of a Continuous Growth Synthesis

Most studies on iron oxide nanocrystals (NCs) suggest that the magnetic properties depend strongly on size for diameters below 10 nm, but there is less agreement about how the structure of the NC surface influences magnetic properties. Because the magnetic properties of iron oxide NCs hold promise for applications from cancer detection and therapeutics to environmental remediation, it is imperative to understand how size influences those properties. In most cases, the effective magnetic size is significantly lower than the measured physical size, a finding attributed to spin canting or disorder at the NC surface. A complicating factor is that the reaction conditions used to produce samples influence their magnetic properties. Thus, we employed a continuous growth method involving layer-by-layer addition of precursor to produce single-crystalline, spherical cores with subnanometer precision over a range of sizes under the same reaction conditions. Analysis of the NCs by small-angle X-ray scattering, transmission electron microscopy, and powder X-ray diffraction showed that the NCs possess the spinel structure (primarily maghemite) and are crystalline, defect-free, and uniform in size. The saturation magnetization values for a series of eight distinct diameters between 4 and 10 nm increase smoothly with increasing size, from 55 to 78 Am2/kg. Magnetic sizes of the NCs determined by fitting magnetization curves to the Langevin function are nearly identical to the physical sizes, suggesting low levels of strain-producing defects and a very thin nonmagnetic surface layer on the NCs. The results suggest that syntheses that permit slower growth at reduced temperatures through a single reaction mechanism can enhance, and offer fine control over, magnetic properties.