Electronic properties of 4-nm FePt particles

The structural, electronic and magnetic properties of 4 nm metallic FePt particles were studied by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), X-ray photo electron spectroscopy (XPS), magnetometry and Mössbauer spectroscopy. At low temperatures, the Mössbauer data reveal an unusually high and well defined magnetic hyperfine field compared to FePt multilayer or bulk samples. The magnetic anisotropy of the as-prepared FePt particles embedded in a layer of oleic acid molecules arises from surface contributions. The distribution of anisotropy under the constraint of a constant particle moment and volume is reflected in the hysteresis loop as a function of external field at 4.3 K. Due to faceting as seen in the high resolution TEM images, the magnetization reversal does not follow the simple Stoner-Wohlfarth switching. The annealing experiments show that at a size of 4 nm the high-temperature fcc phase is stable up to at least $560°\mathrm{C}$ as long as agglomeration and particle growth is prevented.