High-Temperature Magnetism as a Probe for Structural and Compositional Uniformity in Ligand-Capped Magnetite Nanoparticles

To investigate magnetostructural relationships in colloidal magnetite (Fe 3 O 4 ) nanoparticles (NPs) at high temperature (300-900 K), we measured the temperature dependence of magnetization ( M ) of oleate-capped magnetite NPs ca. 20 nm in size. Magnetometry revealed an unusual irreversible high-temperature dependence of M for these NPs, with dip and loop features observed during heating-cooling cycles. Detailed characterizations of as-synthesized and annealed Fe 3 O 4 NPs as well as reference ligand-free Fe 3 O 4 NPs indicate that both types of features in M ( T ) are related to thermal decomposition of the capping ligands. The ligand decomposition upon the initial heating induces a reduction of Fe 3+ to Fe 2+ and the associated dip in M , leading to more structurally and compositionally uniform magnetite NPs. Having lost the protective ligands, the NPs continually sinter during subsequent heating cycles, resulting in divergent M curves featuring loops. The increase in M with sintering proceeds not only through elimination of a magnetically dead layer on the particle surface, as a result of a decrease in specific surface area with increasing size, but also through an uncommonly invoked effect resulting from a significant change in Fe 3+ /Fe 2+ ratio with heat treatment. The interpretation of irreversible features in M ( T ) indicates that reversible M ( T ) behavior, conversely, can be expected only for ligand-free, structurally and compositionally uniform magnetite NPs, suggesting a general applicability of high-temperature M ( T ) measurements as an analytical method for probing the structure and composition of magnetic nanomaterials.