Synthesis and characterisation of magnetic iron sulfide nanocrystals

Fe1−xS and Fe3S4 nanocrystals with a variety of morphologies and average sizes were synthesised by the reaction of iron(II) acetylacetonate (Fe(acac)2) and elemental sulfur in oleylamine. Reaction at 200 °C for 240 min produced extremely thin Fe3S4 sheets, which displayed low coercivities (14 kA m−1) suggestive of pseudosingle-domain or multidomain particles. Reaction temperatures ≥300 °C for 30 min produced 70 nm Fe1−xS nanocrystals with hexagonal plate and hexagonal prism morphologies, which displayed high magnetic coercivities (110 kA m−1) characteristic of single magnetic domain particles. Rapid injection of sulfur solution at 280 °C followed by immediate cooling produced a mixture of Fe1−xS nanocrystals and spherical, polydisperse ∼5 nm Fe3S4 nanocrystals, which displayed superparamagnetism above an average blocking temperature of 55 K.