Structure, microstructure and magnetic investigation of the hexagonal δ-FeSe nanophase produced by mechanochemical synthesis.

We present a systematic structural, microstructural and magnetic characterization of the hexagonal δ-FeSe nanophase produced by a simple one-step mechanochemical synthesis route, by using conventional X-ray powder diffraction (XRPD), Rietveld refinement, transmission electron microscopy (TEM) and magnetometry techniques. We observed the simultaneous formation of tetragonal β-FeSe and δ-FeSe after 3 h of milling (with minor amounts of unreacted iron), followed by complete β-FeSe → δ-FeSe phase transition as milling time increases to 6 h (no unreacted iron). The average crystallite size of the δ-FeSe phase of about 16 nm after 3 h milling time decreases by about 31% up to the final milling time (24 h). TEM images and electron diffraction patterns confirm the nanometric size of the crystalline domains in the irregularly-shaped agglomerated particles. Two ferromagnetic phases with distinct coercivity spectra were assumed here by considering an assembly of randomly-oriented weakly-anisotropic ferromagnetic particles, mixed at a 4 to 6 volume ratio with other randomly-oriented ferromagnetic grains. Four years after synthesis, the aged samples milled for less than 9 h revealed a certain amount of the β-FeSe phase that slightly affects the δ-FeSe (micro)structure but causes some variations (decreasing) in magnetic parameters. Milling times as long as 12 h were shown to be necessary to guarantee the δ-FeSe nanophase stability and to retain its magnetic properties over time.