Laying to Rest the Magnetically Dead Layer in Magnetic Nanoparticles

Decades of research focused on size and shape control of iron oxide nanoparticles have led to methods of synthesis that afford excellent control over physical size and shape, but comparatively poor control over magnetic properties. Synthesis methods based on thermal decomposition of organometallic precursors yield particles with mixed iron oxide phases resulting in poor magnetic properties. One feature of this is the existence of a relatively thick (several nm) “magnetically dead layer” calculated from the difference between the physical and magnetic diameters of the nanoparticles. Guided by the hypothesis that traditional thermal decomposition syntheses are carried out under conditions where oxygen is the limiting reactant, we identified conditions for the safe stoichiometric addition of molecular oxygen as a reactive species.¹ Synthesis of nanoparticles by popular routes, such as the heating-up and extended LaMer methods, with and without controlled oxygen addition into the reactor clearly shows that particles synthesized with controlled oxygen addition have magnetic properties approaching those of bulk magnetite and have thin (< 1 nm) magnetically dead layers, whereas particles synthesized in the absence of oxygen have relatively thick (up to 5 nm) magnetically dead layers, particularly for particle physical diameters above 20 nm. Furthermore, high resolution electron microscopy suggests that particles synthesized with controlled addition of oxygen consist of uniform single crystals with minimal defects. The improved magnetic properties and thin magnetically dead layers lead to improvements in functional particle properties relevant for biomedical applications.

References:

1. Mythreyi Unni, Amanda Uhl, Shehaab Savliwala, Benjamin Savitzky, Roham Dhavalikar, Nicolas Garraud, David Arnold, Lena Kourkoutis, Jennifer Andrew, and Carlos Rinaldi, “Thermal decomposition synthesis of iron oxide nanoparticles with diminished magnetic dead layer by controlled addition of oxygen.” ACS Nano, 11(2):2284-2303, 2017. [doi: 10.1021/acsnano.7b00609]