Low temperature magnetic behaviour of CuFeSe 2 from neutron diffraction data

Abstract

Measurements of neutron diffraction spectra were made at temperatures 4.2, 60, 75, 90, 150 and 300 K on powdered synthetic CuFeSe 2. The data showed that a magnetic transition occurs in CuFeSe 2 at approximately 80 K, graphs of integrated intensity for the magnetic lines extrapolating to zero at this temperature. Rietveld analysis using GSAS showed that at the temperatures above 80 K, a good fit to the data could be obtained with a tetragonal unit cell and the space group taken as P 4 2c, as reported previously by Delgado et al. For temperatures below 80 K, the positions of the magnetic lines indicated that the magnetic unit cell was larger, with the a lattice parameter twice that of the chemical unit cell. GSAS analysis including magnetic contributions showed that the Fe magnetic moments had components along all three reference axes, giving the overall space group as triclinic, P1. The resultant magnetic moment has a magnitude of ∼ 1.75 μ B and a direction at an angle of ∼ 15° to the y— z plane. The Fe atoms on the 2e and 2a sites (of the tetragonal cell) have slightly different magnetic moments, resulting in a weak ferrimagnetic behaviour, as observed from magnetic susceptibility measurements. Analysis of the nuclear lines below 80 K indicated that the Debye-Waller factor U iso fell rapidly below the magnetic transition. Conversion to a Debye θ D showed that θ D increased by ∼25% below the transition, and this is attributed to a large value of ∂J/ ∂a in this material.