High-temperature redistribution of cation vacancies and irreversible magnetic transitions in the Fe1−x S nanodisks observed by the Mössbauer spectroscopy and magnetic measurements

The hexagonal pyrrhotite Fe1−xS nanodisks with the NiAs-type structure were synthesized by thermal decomposition of ferrous chloride and thiourea in oleylamine. The Mossbauer spectroscopy and magnetic measurements data indicate that a mixture of antiferromagnetic (AFM) and ferrimagnetic (FRM) phases with the NC (N ≥ 3) and 2C-type superstructures is present in the Fe1−xS compound at temperatures between 80 K and Neel temperature TN. At T 370 K, a redistribution of iron vacancies takes place, and the vacancy ordering transforms from the NC (N ≥ 3) to 2C-type which essentially increases the magnetization with maximum value at 470 K. Heating the sample above the Neel temperature 565 K leads to a random distribution of vacancies, and this state is quenched upon subsequent cooling of the sample to 300 K. This gives rise to a pure AFM structure with a zero magnetic moment due to a total compensation of the moments in neighboring iron layers. Thus, the high-temperature redistribution of cation vacancies leads to irreversible magnetic transformations in the Fe1−xS nanoparticles.