Temperature and particle-size dependence of the equilibrium order parameter of FePt alloys

Abstract

First, second and third nearest neighbor pair mixing potentials for equiatomic FePt alloys were calculated from first principles by the Connolly-Williams method within the canonical cluster expansion formalism. It was demonstrated that the Connolly-Williams potentials (based on completely ordered states) and the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) potentials (based on completely disordered state) can be brought into very close correspondence to each other simply by increasing the magnitude of the strain-induced interactions added to the KKR-CPA potential. Using the mixing potentials obtained in this manner, the dependency of equilibrium L10 ordering on temperature was studied for bulk and for (approximately) spherical nanoparticles ranging in size from 2.5 to 6nm. The order parameter was calculated using Monte Carlo simulation and the analytical ring approximation. The calculated order-disorder temperature for bulk (1495-1514 K) was in relatively good agreement (4% error) with the experimental value (1572K). For nanoparticles of finite size, the (long range) order parameter changed continuously from unity to zero with increasing temperature. Rather than a discontinuity indicative of a phase transition, we obtained an inflection point in the order as a function of temperature. This inflection point occurred at a temperature below the bulk phase transition temperature and decreased as the particle size decreased. Our calculations predict that 3.5nm diameter particles in configurational equilibrium at 600• C (a typical annealing temperature for promoting L10 ordering) have an L10 order parameter of approximately 0.84 (compared to a maximum possible value equal to unity). According to our investigations, the experimental absence of (relatively) high L10 order in 3.5nm diameter nanoparticles annealed at 600• C or below is primarily a problem of kinetics rather than equilibrium.

Microstructures and magnetic alignment of L10 FePt nanoparticles

“…The compositions of these particles would place it beyond the bulk compositional phase field for the L1 0 ordering. 16 This composition variation could inhibit the phase transformation and is the subject of further work. Although the total volume fraction of FePt nanoparticles with A1 phase is small, it will give a soft component to the M-H curve as shown in Fig.…”

Dynamics and stability of icosahedral Fe–Pt nanoparticles

“…[19][20][21] Relevant information about structural stability and electronic properties of the Fe-Pt nanoclusters have been obtained by the theoretical studies based on either semiempirical potentials or ab initio techniques. [22][23][24][25][26][27] The Monte Carlo simulations showed that a continuous transformation between the ordered structure L1 0 (tP4) and a disordered phase in Fe-Pt NPs occures at temperatures lower than the bulk melting temperature (1572 K). 23,24 Disordering processes in nanoalloys are enhanced due to finite-size and surface effects, e.g.…”

First principles study of Ag, Au, and Cu surface segregation in FePt-L1

“…The phenomenon has been explained with size, kinetic, and surface segregation effects. [7][8][9][10][11][12][13] One of the possible approaches to achieve high L1 0 order and a correspondingly high magnetic anisotropy is by doping FePt nanoparticles with a third element. 1,3 Atomic species such as Ag, [14][15][16][17][18][19][20] Au,17,[19][20][21][22][23][24][25] and Cu 25-27 have been previously tested.…”