Lattice Monte Carlo simulations of FePt nanoparticles: Influence of size, composition, and surface segregation on order-disorder phenomena

Müller, Michael; Albe, Karsten

doi:10.1103/physrevb.72.094203

Cited by 126 publications

(97 citation statements)

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“…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.…”

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confidence: 99%

“…The fact agrees with the Pt segregation tendency previously observed and calculated in FePt. [9][10][11][12][13][35][36][37][38][39] The data show that We perform a comparison between the most-outer-layer surface segregation energies of Ag, Au, and Cu in L1 0 -FePt and those in pure Fe or Pt as previously calculated with the Korringa-Kohn-Rostoker atomic sphere and local density approximation ͑KKR-ASA-LDA͒ method. 40,41 For this task, we restrict Eq.…”

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

Chepulskii

Curtarolo

2010

Applied Physics Letters

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Doping FePt nanoparticles could be a possible approach to achieve high L1 0 order and magnetic anisotropy. To address stability, first-principles studies of surface segregation of dilute Ag/Au/Cu solutes at and near the ͑001͒/͑100͒/͑111͒ surfaces of FePt-L1 0 are performed. It is found that a strong surface segregation tendency at first outer layer is present in all the cases. For Cu, segregation is less than half of Ag and Au. Ag and Cu segregate to Fe sites at surfaces and preferentially substitute for Fe in the bulk, whereas Au substitutes for Fe at surfaces and for Fe and Pt in the bulk. © 2010 American Institute of Physics. ͓doi:10.1063/1.3522652͔In the past decade, FePt nanoparticles have been extensively investigated in view of possible future applications as an ultrahigh-density information-storage medium and highperformance permanent magnets. [1][2][3][4][5][6] The critical issue for information-storage application is the presence of magnetic anisotropy offering sufficiently large thermal stability. In FePt, a high magnetic anisotropy is guaranteed by an ordered L1 0 phase. However, experimental observations 4-6 have shown a difficulty in obtaining a high degree of L1 0 order in FePt nanoparticles annealed at T Շ 600°C with diameter less than ϳ4 nm. 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. To follow this approach, it is imperative to determine whether dopants concentrate inside the particle cores or segregate at the surfaces. In the latter case, the additional surface vacancies generated by the atoms rearranging on the surface during segregation upon annealing might increase Fe and Pt mobilities and help the particles to reach their L1 0 equilibrium. 16,21,27 Moreover, modeling and explaining surface segregation phenomena are important to understand catalysis, oxidation, and corrosion in nanostructured systems with high surface to volume ratio.In this letter, we study the segregation of dilute Ag, Au, and Cu solutes on and near the ͑001͒, ͑100͒, and ͑111͒ surfaces of FePt crystals with perfect L1 0 order ͑directions are defined with respect to Ref. 9͒. The results can also be used to draw conclusions regarding surface segregation in FePt nanocrystals with ͑001͒ and ͑111͒ facets such as cubes, tetrahedrons, octahedrons, cubeoctahedrons, truncated cubes, and the truncated octahedrons experimentally observed. [28][29][30] We consider ͑001͒, ͑100͒, and ͑111͒ slabs with perfect L1 0 order. For the ͑100͒ and ͑111͒ cases we choose the systems to have eight layers, while for the ͑001͒ slabs we consider nine layers ͑see Fig. 1 of Ref. 9͒. The vacuum thickness between the periodic replica of the slabs is fixed at ϳ12 Å. One substitutional impurity of ...

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

Chepulskii

Curtarolo

2010

Applied Physics Letters

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“…[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.…”

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Dynamics and stability of icosahedral Fe–Pt nanoparticles

Jochym

Łażewski

Sternik

et al. 2015

Phys. Chem. Chem. Phys.

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“…The transformation to this phase in FePt NPs smaller than 4 nm has never been unambiguously identified, and the possibility of its formation in small NPs has been questioned 1,2 based on the influence of surface composition and segregation. 3 Commonly in these discussions, a uniform crystal structure across the NP is assumed that is the influence of a possible relaxation of the surface layers as is known at the surface of bulk crystals is not considered. Here, we show that the layer-resolved surface relaxations of a few percent indeed exist in icosahedral FePt NPs, which may be driven by the formation of Pt-enriched surface layers.…”

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confidence: 99%

FePt Icosahedra with Magnetic Cores and Catalytic Shells

et al. 2009

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Surprisingly oxidation resistant icosahedral FePt nanoparticles showing hard-magnetic properties have been fabricated by an inert-gas condensation method with in-flight annealing. High-resolution transmission electron microscopy (HRTEM) images with sub-Angstrom resolution of the nanoparticle have been obtained with focal series reconstruction, revealing noncrystalline nature of the nanoparticle. Digital dark-field method combined with structure reconstruction as well as HRTEM simulations reveal that these nanoparticles have icosahedral structure with shell periodicity. Localized lattice relaxations have been studied by extracting the position of individual atomic columns with a precision of about (0.002 nm. The lattice spacings of (111) planes from the surface region to the center of the icosahedra are found to decrease exponentially with shell numbers. Computational studies and energy-filtered transmission electron microscopy analyses suggest that a Pt-enriched surface layer is energetically favored and that site-specific vacancies are formed at the edges of facettes, which was experimentally observed. The presence of the Pt-enriched shell around an Fe/Pt core explains the environmental stability of the magnetic icosahedra and strongly reduces the exchange coupling between neighboring particles, thereby possibly providing the highest packing density for future magnetic storage media based on FePt nanoparticles.

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Lattice Monte Carlo simulations of FePt nanoparticles: Influence of size, composition, and surface segregation on order-disorder phenomena

Cited by 126 publications

References 19 publications

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

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

Dynamics and stability of icosahedral Fe–Pt nanoparticles

FePt Icosahedra with Magnetic Cores and Catalytic Shells

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