Advanced characterisation of FePt nanoparticles using X-ray, neutron and electron analytical probes

Nanoparticles, of the same alloy, can exhibit many different chemical and physical properties, which can drastically influence their behaviour. Elucidation of these properties is a vital pre-requisite to enabling nanoparticles to be engineered and optimised for specific commercial applications, such as; next-generation high-density data storage devices, biomedical nanodevices, and high performance nano-catalytic devices. Within this study the following FePt nanoparticles with oleic acid and olelyamine surfactants were chemically synthesised at the Healthcare Biomagnetics and Nanomaterials Laboratory: ultrafine FePt nanospheres (with isotopic substitutions of surfactants); FePt nanocubes; FePt@Fe3O4 (core@shell) spherical nanoparticles; and FePt octapods. Advanced nano-characterisation methods were used—through access to state-of-the-art electron microscopy facilities, at the Japan Advanced Institute for Science and Technology—to gain a comprehensive analysis of the crystallographic structure, compositional distribution, 3D morphology, and 2D strain distribution of FePt nanoparticles. The 2D strain gradients were resolved using a geometric phase analysis method utilising an in-house software program. Bragg coherent X-ray diffraction imaging (BCDI) measurements of FePt nanoparticles and AuPd nanoparticles were conducted at beamline 34-ID-C of the Advanced Photon Source, Argonne National Laboratory, to quantitatively reconstruct the nanoparticles’ 3D morphology and strain distribution. Novel stabilisation methodologies were developed to resolve the size limitations of the BCDI technique; extending the size regime for other synchrotron users. High-energy neutron-diffraction measurements of isotopic substitutions of FePt nanoparticles in solution were conducted at the Nimrod beamline at the Rutherford Appleton Neutron Facility. Neutron pair distribution function (PDF) analysis enabled the structural properties of the surfactant layer to be extracted. Complementary highenergy powder diffraction measurements conducted at beamline I15 of the Diamond Light Source enabled the structural properties of the FePt nanoparticle to be extracted from X-ray PDF analysis. A 3D reconstruction of FePt nanoparticles and the surrounding surfactant layer was built using a nanoparticle modelling software program for future refinement with experimental data.