The Magnetic Properties of FeSe_x with the NiAs Structure

Abstract

The thermal and X-ray analyses were carried out for iron-selenium system from 48.8 to 57.4 atomic percent of selenium, such range being previously reported to be ferrimagnetic.It was concluded from the experimental results that at room temperature the concentration range from 48.8 to 53.1% selenium is the two phase region α+β; α has the PbO type structure and β the NiAs type. These two phases transform through a eutectoid reaction into a γ phase with the NiAs structure of slightly different lattice parameters in comparison with those of the β phase, the eutectic temperature being 350°C. The solubility limit in iron side of the phase lies at 53.1% selenium. It is remarkable that the substance with stoichiometric concentration (50% Se) does not lie in the single phase region of NiAs type. It was also found that the ferrimagnetism of this binary system originates from the phase of NiAs type (β phase) and the α phase is weakly paramagnetic or antiferromagnetic.The Curie point of the compound containing 53.1% selenium is 174°C and the spontaneous magnetization at absolute zero of temperature is 0.20 Bohr magneton. This magnitude of magnetization is explained by assuming a kind of the ordered arrangement of ferric ion in the crystal lattice.