Tuning ofL10atomic order in Co-Pt nanoparticles:Ab initioinsights

Abstract

Monte Carlo simulation of the atomic configurational behavior of Co-Pt nanoparticles is performed in wide temperature-composition-size ranges. The simulation is based on bulk and local (inhomogeneous) cluster expansions calculated from first principles. A sharp drop of equilibrium L1 0 order is predicted for small equiatomic particles at low temperatures. This drop is explained by the interplay of two effects. First, the strong Pt and Co segregation to the first and second surface layers, respectively (core/Co/Pt or "onion2shell" profile) cause a depletion of Pt atoms within the core of small Co-Pt particles. Second, L1 0 is an adaptive structure on the fcc-restricted ground-state phase diagram of Co-Pt. Nevertheless, we show that a remarkable degree of L1 0 order can be restored by tuning the total composition of small Co-Pt particles.

Investigations of the Co-Pt alloy phase diagram with neutron diffuse scattering, inverse cluster variation method, and Monte Carlo simulations

“…Temperature mainly affects atomic shells farther than the fourth neighbor distance yielding a 100 K variation of the calculated order-disorder transition temperature. The comparison of EPI with those reported in the literature within the cluster expansion 12 and the tight binding Ising model 22 shows that the TBIM EPI provide a relatively good description of the alloy thermodynamics at temperatures close to ordering temperature (short-range order and temperature of phase transformation), even if they strongly differ from the EPI determined in this study. µ.…”

“…The EPI obtained from our measurements are listed in the upper part of Tab. I (ICVM) as well as those reported in the literature 12,22 and based on rigid FCC lattice calculations. The values determined from the neutron diffuse scattering measurements significantly differ from those obtained from the cluster expansion approach (CE) and the tight-binding Ising model (TBIM).…”

“…At the CoPt composition, the derivation of EPI from the formation energy of stable and metastable phases led to a transition temperature which was about half of the measured one 12 . This suggests that some effects not taken into account in the ab initio calculations and in the cluster expansion (CE) formalism 13 prevent the accurate prediction of the transition temperature.…”

Ordering and surface segregation inCo1−cPtcnanoparticles: A theoretical study from surface alloys to nanoalloys

“…One crucial question was to characterize the order-disorder transition temperature as a function of the cluster size since theoretical models agree to conclude that it decreases with cluster size [43,44,47,50], which has been experimentally confirmed by high-resolution transmission electron microscopy [33] and by synchrotron X−ray diffraction [28]. Moreover, the nature of the transition displays a first order type as in the bulk as shown by the experimental [28] and theoretical [53] evidences of phase coexistence between the ordered and disordered phases even though there is a smoothing with decreasing cluster size [42,44,47,52].…”

“…For example, the effect of the chemical order on the electrocatalytic activity of model PtCo electrodes used in fuel cells has been evident recently [39,40]. Theoretical investigations have been performed on alloy surfaces [7][8][9][10][11], and nanoalloys of Fe-Pt or Co-Pt [41][42][43][44][45][46][47][48][49][50][51][52][53]. The studies on nanoparticles have been mostly concentrated on the equiatomic composition, because of the high magnetic anisotropy of the L1 0 ordered phase in (Fe,Co)-Pt systems, which is expected to compensate the superparamagnetism of nanoparticles used for the nextgeneration recording media for ultrahigh density data storage.…”