Electronic Material Properties - und Geowissenschaften ...

Structural stability of multiply twinned FePt nanoparticles
Michael Müller and Karsten Albe
The prospect of realizing magnetic nanoparticles that can be applied in high-density
recording or medical applications has driven a large number of research activities in
recent years. Among all candidate materials FePt in the face centered tetragonal L10
phase has attracted much attention. This structure is characterized by alternating Fe
and Pt layers in c-direction and possesses a high uniaxial magneto-crystalline
anisotropy energy (MAE). Because of the high MAE, single crystalline FePt
nanoparticles with a diameter as small as 4 nm can maintain a stable magnetization
direction on a time scale of 10 years. When prepared by gas-phase synthesis
processes, however, no single crystalline FePt particles are obtained. Instead,
multiply twinned shapes in the form of icosahedral or decahedral particles are
predominant. In order to understand the occurrence of the different structural
motives, a detailed knowledge of the energetics of FePt particles in the various
conformations is necessary. In our work we investigated the structural stability of
FePt nanoparticles by atomic scale and continuum model calculations.
In conjunction with a recently developed interatomic FePt potential, the molecular
statics method has been applied for calculating the fully relaxed energy of FePt
nanoparticles in single crystalline, multiply twinned icosahedral and multiply twinned
decahedral morphologies. The particle energy as a function of size is shown in Fig. 1.
In general, even for the smallest particle sizes studied, the single crystalline shapes
are energetically favored over multiply twinned particles by at least 10 meV/atom.
Also, icosahedral particles exhibit a much higher energy than decahedral ones
because of their higher internal strain and larger twin boundary areas. The large
energy differences between single crystalline particles on the one hand and
icosahedral and decahedral particles on the other hand, however, can mainly be
attributed to a large twin boundary energy predicted by the interatomic potential.
Compared to electronic structure calculations, an overestimation of the twin boundary
energy by a factor of two is possible.
Fig. 1: Average potential energy for FePt nanoparticles in different morphologies. Data points
denote molecular statics calculations, curves are predictions of the continuum model Eq. (1).
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