Off-stoichiometry LaFe5Six cubic phase induced by the reduced size of the nanoparticles produced by pulsed laser deposition

Abstract

Bulk La(Fe,Si)13 materials received much attention because of their increasing magnetocaloric potential, low cost and Tc tunable by Co or H substitution. However, the properties of these materials in nanometric dimensions have been investigated by few authors. Thus, a detailed investigation of off-stoichiometry LaFe5Si nanoparticles produced using pulsed laser deposition technique is reported in this work. From a LaFe11Si2 target, produced by arc-melting, it was possible to deposit those nanoparticles (NPs) onto substrates, and their structural, morphological and magnetic properties were investigated by using several techniques, including Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED), X-ray Energy-dispersive Spectroscopy (EDS), Electron Energy Loss Spectroscopy (EELS), Mössbauer spectroscopy and magnetic characterization. The results show that the NPs are spherical and crystallize in a cubic NaZn13 structure, which is the same as that of the bulk/target. Two main types of NPs were observed: one with a core-shell structure, where the core is crystalline and the shell is amorphous; and another completely amorphous. The mean diameter of the core-shell and amorphous NPs were 15 nm and 9 nm, respectively, and, interestingly, the ratio La/Fe is 1/5 in both the core-shell and amorphous NPs. This atomic ratio corresponds to NPs of an off-stoichiometry LaFe5Six composition. We also observed that the crystalline core of the NPs is magnetically ordered (ferromagnetic) with a Curie temperature close to 250 K, which is slightly higher than that of its bulk counterpart. Finally, our findings show that the individual LaFe5Six NP are free from the α-Fe phase.