Temperature-dependent magnetic and structural ordering of self-assembled magnetic array of FePt nanoparticles

摘要

Hexagonal close-packed 2D assembly of monodispersed FePt nanoparticles by the co-reduction method have been synthesised with particles size ~3 nm. The X-ray diffraction (XRD) studies reveal a structural transformation from a chemically disordered faced-centred cubic (FCC) phase to a chemically ordered face-centred tetragonal (FCT) phase with heat treatment. The percentage of phase transformation from FCC to FCT has also been investigated by the XRD profile fitting. Annealing at 750 C gives 95 ± 1 % FCT phase. The structural transformation was further confirmed by the presence of two superlattice rings corresponding to (001) and (110) by the selected area electron diffraction. Transmission electron microscopy (TEM) shows the grain growth with annealing temperature. High resolution TEM shows the presence of the carbonaceous layer over FePt after annealing. This carbonaceous layer has been investigated by the Raman spectroscopy. Magnetic measurement shows the presence of magnetic hard phase with coercivity of 1 Tesla. The soft/hard compositions have also been investigated by the hysteresis measurements.