LOW-TEMPERATURE MAGNETIC-BEHAVIOR OF CUFESE2 FROM NEUTRON-DIFFRACTIONDATA

Measurements of neutron diffraction spectra were made at temperatures 4.2, 60, 75, 90, 150 and 300 K on powdered synthetic CuFeSe2. The data showed that a magnetic transition occurs in CuFeSe2 at approximately 80 K, graphs of integrated intensity for the magnetic lines extrapolat ing to zero at this temperature, Rietveld analysis using GSAS showed t hat at the temperatures above 80 K, a good fit to the data could be ob tained with a tetragonal unit cell and the space group taken as <P(4)o ver bar 2c>, as reported previously by Delgado et al. For temperatures below 80 K, the positions of the magnetic lines indicated that the ma gnetic unit cell was larger, with the a lattice parameter twice that o f the chemical unit cell. GSAS analysis including magnetic contributio ns showed that the Fe magnetic moments had components along all three reference axes, giving the overall space group as triclinic, P1. The r esultant magnetic moment has a magnitude of similar to 1.75 mu(B) and a direction at an angle of similar to 15 degrees to the y-z plane. The Fe atoms on the 2e and 2a sites (of the tetragonal cell) have slightl y different magnetic moments, resulting in a weak ferrimagnetic behavi our, as observed from magnetic susceptibility measurements. Analysis o f the nuclear lines below 80 K indicated that the Debye-Waller factor U-iso fell rapidly below the magnetic transition. Conversion to a Deby e theta(D) showed that theta(D) increased by similar to 25% below the transition, and this is attributed to a large value of partial derivat ive J/partial derivative a in this material.