부산대학교 도서관

Iron sulfide nanoparticles FeS with the spinel-type crystal structure were synthesized by the polyol mediated process. The particle size depends on preparation conditions and varies from 9 to 20 nm. Mössbauer data have revealed that the dominating fraction of iron ions in the 9-nm sample is in the high-spin ferric state. This implies an occurrence of the cation vacancies in nonstoichiometric greigite. The stoichiometric phase of greigite FeS dominates in the 18-nm-size nanoparticles. Magnetic measurements have shown a ferrimagnetic behavior of all samples at temperatures between 78 and 300 K. The estimated value of magnetic moment of the stoichiometric greigite nanoparticles is about 3.5 μ per FeS unit. The Mössbauer spectra indicate a superparamagnetic behavior of small particles, and some fraction of superparamagnetic phase is observed in all samples synthesized which may be caused by the particle size distribution. The blocking temperatures of T ≈ 230 and 250 K are estimated for the 9 and 14 nm particles, respectively. The Mössbauer parameters indicate a great degree of covalency in the Fe-S bonds and support the fast electron Fe ⇆ Fe exchange in the B-sites of greigite. An absence of the Verwey transition at temperatures between 90 and 295 K is established supporting a semimetal type of conductivity. The temperature and magnetic field dependences of the magnetic circular dichroism (MCD) of optical spectra were measured in FeS for the first time. The spectra differ substantially from that of the isostructural oxide FeO. It is supposed that the MCD spectra of greigite nanoparticles result from the collective electron excitations in a wide band with superimposed peaks of the d- d transitions in Fe ions. [ABSTRACT FROM AUTHOR]