Single-Domain Fe₂CoAl Nanoparticles and Their Crystallite Size-Dependent Properties

Abstract

Single-domain Fe 2 CoAl nanoparticles exhibiting an Xtype ordered Heusler alloy structure have been prepared by a simple template-less chemical process. Near-stoichiometric Fe 2 CoAl nanoparticles with three different average crystallite sizes of 41, 45, and 51 nm exhibit soft ferromagnetic behavior with high saturation magnetization (>95% of the theoretical estimate), high effective magnetic anisotropy (∼10 7 erg/cc), and a high Curie temperature (∼1225 K). The property variation of the nanoparticles with crystallite size has been explained using well-established theoretical models. Ab initio calculations performed in this study, apart from establishing the preferred crystal structure, also help to understand the magnetic and half-metallic properties of the alloy. Critical size estimations authenticate the formation of single-domain Fe 2 CoAl nanoparticles.

Single-domain Fe2CoGa0.5Al0.5 Heusler alloy nanoparticles with enhanced properties

“…The lattice parameter ( a ) obtained from the Rietveld refinement and the average crystallite size ( D v ) estimated using Schërrer's equation 25 for the Fe 2 CoGa 0.5 Al 0.5 nanoparticles are 5.745 Å and 42 ± 1 nm, respectively. Comparison of the a obtained for Fe 2 CoGa 0.5 Al 0.5 nanoparticles with the previously reported a values of similar sized Fe 2 CoGa (5.764 Å with D v of 42 nm) and Fe 2 CoAl (5.723 Å with D v of 41 nm) 20 nanoparticles, show a decreasing trend in a with increasing Al content. Similar trend was observed in bulk Ru 2 VGa x Al 1− x (0 ≤ x ≤ 1) alloys, 8 NiMnGa 2 –NiMnAl 2 alloys, 9 and Fe 50 Mn 25 Ga x Al 25− x (0 ≤ x ≤ 10) HA.…”

“…18 Moreover, the stability of X -type structure has already been confirmed in Fe 2 CoGa 19 and Fe 2 CoAl. 20 However, the XRD patterns of Fe 2 CoGa 0.5 Al 0.5 HA simulated for L 2 1 and X -type unit cells as depicted in Table 1, are nearly indistinguishable. Rietveld refinements of the experimental XRD data based on L 2 1 and X -type unit cells also could not resolve the issue due to similar values of goodness of fit parameter χ 2 .…”

“…Table 1 clarified that intensities of (111) and (200) reflections are <1% of that of (220) which make them non-detectable in the experimental XRD pattern above the noise level, as also observed in the cases of Fe 2 CoGa and Fe 2 CoAl. 19–23 This task becomes more arduous in a quaternary alloy with more probability of intermixing, apart from smearing effect due to surface defects and finer crystallites. 22,24 In order to resolve this, FETEM studies were performed on Fe 2 CoGa 0.5 Al 0.5 nanoparticles.…”

“…Both the end compositions Fe 2 CoGa and Fe 2 CoAl are known to be non-half-metallic as shown in our previous studies. 19,20 However, some earlier studies have demonstrated superior half-metallic characteristics and spin polarization ( P ) in intermediate quaternary compositions such as Co 2 FeGa x Ge 1− x as compared to the ternary end compositions. 36 Therefore, ab initio calculations were performed on Fe 2 CoGa 0.5 Al 0.5 as per the procedure described in the experimental and computational section using the experimentally determined lattice constant a .…”

“…Fe 2 CoGa 0.5 Al 0.5 was also found to be non-half-metallic similar to the stoichiometric end compounds. An expanded view of the TDOS about the Fermi level ( E F ) is depicted for end compounds Fe 2 CoGa, 19 Fe 2 CoAl 20 and intermediate quaternary compound Fe 2 CoGa 0.5 Al 0.5 , in Fig. 3(b), (d) and (c), respectively.…”