Photonic crystals in biology

Poster Session, Tuesday, June 15
Theme A1 - B702
Modeling of Atomic Evol utions in Equiatomic FeCo–Au Nanoalloys
Muratahan Aykol, 1 * Amdulla O. Mekhrabov 1 and M. Vedat Akdeniz 1
Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06531, Turkey
1
Abstract-In this study, influences of varying particle size and shape on atomic ordering of equiatomic B2-type FeCo nanoalloys with Au
additions are revealed as a function of temperature by pseudopotential theory combined with Monte Carlo simulation. The bulk limit is
determined as 5 nm above which long range order does not differ much from that of bulk alloy, but due to free boundaries, all nanoalloys show a
diffuse B2-disorder transition (ODT), in contrast to the well defined ODT temperature of ~1000 K of bulk FeCo. When alloyed with
Au, B2rface-initiated
disordering effect. In addition, Au atoms tend to segregate near particle surface as degree of order diminishes.
FeCo nanoparticles with very high magnetic moments have
potential use in biomedical applications such as drug delivery,
hyperthermia treatment of cancer and magnetic resonance
imaging. Although synthesis of FeCo nanoparticles with
controllable size and shape distributions [1,2] is made
possible, degree of retained long range order (LRO) and
nature of order disorder transition (ODT) still remain intact
[3]. Magnetic anisotropy can be significantly affected by the
LRO, which is controlled by particle size, shape and thermal
history. Moreover, to overcome the biocompatibility issues,
particles are generally coated by Au atoms, which can also
alter the LRO significantly.
Canonical ensemble Monte Carlo (MC) method with a rigid
B2 lattice and an Ising-type Hamiltonian being composed of
ordering energies of pairs are utilized to reveal the response of
degree of LRO to varying particle size and shape in near
equiatomic B2-(Fe 0.5 Co 0.5 ) 100-X Au X nanoparticles (a = 2.87
Å) from room temperature to higher temperatures. The
exchange energies of atoms (ordering energies) are evaluated
via pseudopotential theory [4] and its combination with MC
methodology was successfully applied to bulk FeCo-V alloys
by the author’s group recently [5]. First five coordination
spheres are taken into account since exchange interactions are
relatively weak in FeCo system in the first coordination
sphere. Bulk simulations involve periodic boundaries while
nanoparticles have free boundaries and no additional treatment
was conducted for surface potentials. The number of MC-steps
to attain equilibriu m varies for each particle, ranging between
10 4 and 10 7 steps. Since the quality of generated random
numbers is of utmost importance in MC simulations for the
convergence of the system, an extremely powerful pseudorandom
number generator of periodicity of 2 19377 called
Mersenne-Twister was utilized for generation. ODT
temperature (T odt ) was determined from inflection point of
LRO vs. T plots.
In pure FeCo nanoparticles, T odt drop is not extensive (See
Figure 1) as also experimentally observed by Turgut et al [2].
Xu and Wang [1] synthesized FeCo-Au nanoparticles in which
a FeCo core – Au shell type structure exists, but there is
considerable amount of Au dissolved in the core as observed
from their concentration profiles. Thus, nanoparticles are
supersaturated with Au. The result of such Au supersaturation
in B2-FeCo nanoparticles is shown in Figure 1. Since a finite
system cannot show a distinct phase transition thus LRO of
particles show diffuse responses to temperature, T odt
decreasing with decreasing particle size. Au has a strong
disordering effect on B2-FeCo structure as also shown in
Figure 1. Cubic particles have a higher disordering tendency
with increasing Au content.
There is a great tendency of Au-atoms to locate near surface
of particles. When LRO is lost, Au-self-clustering increases
inside particles. In cubic particles, Au atoms show higher
surface segregation tendencies. This might be beneficial for
Au coating. However, it seems that if nanoparticles are Audoped
with supersaturation, due to long range interactions
present, Au atoms cannot escape to surfaces readily as
expected. They tend to segregate near surface but cannot form
a layer over surface. This phenomenon becomes more clarified
since even at high temperatures which lacks the LRO and
avoids any preferential site occupation for Au, a layer
formation is hindered for entrapped Au and a clustering
slightly inwards form the surface forms up.
Figure 1. ODT temperatures (T odt ) for cubic and spherical FeCo
nanoparticles with varying Au contents and dimensions. “Size” is
edge length for cubes and diameter for spheres.
The outcome of the current study can be summarized as
follows. First of all, rate of loss of order with increasing Audoping
is very high and no LRO can be expected if dissolved
Au is more than ~6 at.%. For all part icles, T odt decreases with
decreasing particle size but particles larger than 5 nm have
ODT behavior not much different than corresponding bulk
alloys. Cubic particles are much more disorderable than
spherical ones, due to sharp corner defects. In the ordered
states, Au atoms prefer Fe sites since Co-Au interactions are
stronger. Moreover, if the synthesis of FeCo-Au nanoalloys
employs a state which is Au-supersaturated, Au atoms tend to
segregate near surface but may not escape outside easily due
to long range interactions. This may also cause Au leakage
from shell to core in Au-coated nanoparticles and result in loss
of LRO. These should be taken into account in direct synthesis
of FeCo-core/Au-shell type particles for biocompatibility and
organic molecule attachments.
*Corresponding author: e129665@metu.edu.tr
[1] Y. Xu and J. Wang, Appl. Phys. Lett. 91, 233107 (2007).
[2] Z. Turgut et al., J. Appl. Phys. 81, 4039 (1997).
[3] A. Hütten et al., J. Magn. Magn. Mat. 293, 93 (2005).
[4] A.O. Mekhrabov and M.V. Akdeniz, Acta Mater. 47, 2067
(1999).
[5] M . Aykol, A.O. Mekhrabov and M. V. Akdeniz, Intermetallics, in
press (2010).
6th Nanoscience and Nanotechnology Conference, zmir, 2010 227