Element-specific quantitative determination of the local atomic order in CoPt alloy nanoparticles: Experiment and theory

Abstract

International audienceAn annealing-driven transition from a chemically disordered A1-like structure to a chemically ordered L10-like phase has been revealed for size-selected CoPt clusters with diameters from 2 to 4 nm, from experiment to theory confrontation. For chemically ordered particles, atypical lattice-parameter ratios c/a are inferred. This ratio is found to be remarkably different depending on the approach used (c/a>1 from x-ray absorption at the Co K edge and c/a≃0.94 from the Pt dominated x-ray diffraction). A first-principles theoretical study accurately explains this astonishing feature and provides a detailed understanding of the element-specific local relaxations, which are crucial in these nanoalloys

Perpendicular magnetic anisotropy in granular multilayers of CoPd alloyed nanoparticles

“…In fact, the ratio (c/a) Pd = 2(R Co−Pd /R Pd−Pd ) 2 − 1 at the Pd atoms is now about 0.92, in agreement with the ratio (c/a) Co > 1 at the Co atoms [39]. Moreover, these values are comparable with those theoretically estimated for Co 50 Pd 50 clusters at Pd K edge [43,49].…”

“…This is larger than that measured in bulk L1 0 CoPd, c/a ∼0.97 [44], but it is similar to that found in the CoPt NP multilayers [21]. This has been explained as relaxation of the atom-atom distances because of finite size in the L1 0 structure, and DFT calculations for finite size Co-Pt relaxed clusters effectively predict a ratio c/a(EXAFS)>1 [39]. Therefore, the present CoPd NPs in the fabricated multilayers could also show this structural relaxation.…”

“…4(a)] and the white line (denoted as II) of the normalized x-ray absorption near edge structure (XANES) reflect a population rearrangement between the electronic levels of Co and Pd upon alloying [39,40]. The structure within the edge is more obvious in the first derivative of the XANES spectrum plotted in Fig.…”

“…The Debye-Waller factors are rather high for both Co-Co and Co-Pd bonds in all samples, which can be attributed to dispersion of atom-atom distances because of the finite particle size. The element specific ratio (c/a) Co = 2(R Co−Pd /R Co−Co ) 2 − 1 at the Co atoms in L1 0 unit cell [39] is ≈1.08. This is larger than that measured in bulk L1 0 CoPd, c/a ∼0.97 [44], but it is similar to that found in the CoPt NP multilayers [21].…”

First principles study of bimetallic Ni13−nAgn nano-clusters (n = 0–13): Structural, mixing, electronic, and magnetic properties

“…[1][2][3][4][5][6] During the last few decades, bimetallic nano-clusters have received special interest in the research of nano science and nano technology because of the additional degrees of freedom achievable by changing the chemical composition and ordering of its constituent atoms. Recently, several bimetallic multifunctional nano systems consisting of magnetic transition metal (M) and nonmagnetic noble metal (NM) atoms such as Fe-Pt, [7] Fe-Pd, [8] Fe-Au, [9,10] Fe-Ag, [11] CoPt, [12,13] Co-Pd, [14] Co-Au, [15] Ni-Pt, [16] Ni-Pd, [17] Ni-Au [18], have been studied both experimentally as well as theoretically. Besides their catalytic properties, the M-NM bimetallic nano-systems have also great importance in the fields of fuel cells, magnetic storage as well as in spintronics applications.…”