Article

Composition-dependent ratio of orbital-to-spin magnetic moment in structurally disordered FexPt1-x nanoparticles

Abstract and Figures

The ratio of orbital-to-spin magnetic moment mu(L)(eff)/mu(S)(eff) averaged over the element-specific contributions of Fe and Pt has been measured for 3-nm FexPt1-x nanoparticles at room temperature using the multifrequency electron paramagnetic resonance method for different concentrations of Fe. From a detailed g-factor analysis we determine that the ratio decreases from mu(L)(eff)/mu(S)(eff)=0.049 for x=0.43 to mu(L)(eff)/mu(S)(eff)=0.016 for x=0.70 which is much smaller than the bulk iron value (mu(L)(Fe)/mu(S)(Fe)=0.045). The observed concentration dependence is much stronger than the one calculated for FexPt1-x bulk samples and reveals likely changes of the confined electronic structure of the nanoparticle system. The ratio mu(L)(eff)/mu(S)(eff) takes the lowest value at the concentration (x=0.70) where the magnetic anisotropy energy vanishes in bulk alloys. For x>0.72 a phase transition from a fcc to the Fe bcc structure occurs resulting in the increased bulk ratio again.
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Composition-dependent ratio of orbital-to-spin magnetic moment
in structurally disordered FexPt1Àxnanoparticles
M. Ulmeanu,1C. Antoniak,1U. Wiedwald,1M. Farle,1Z. Frait,2and S. Sun3
1Institut fu
¨r Physik, Universita
¨t Duisburg-Essen, Lotharstrasse 1, D-47048 Duisburg, Germany
2Institute of Physics, Academy of Science of the Czech Republic, Na Slovance, 18221 Prague 8, Czech Republic
3IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA
Received 12 September 2003; revised manuscript received 3 December 2003; published 20 February 2004
The ratio of orbital-to-spin magnetic moment
L
eff/
S
eff averaged over the element-specific contributions of Fe
and Pt has been measured for 3-nm FexPt1xnanoparticles at room temperature using the multifrequency
electron paramagnetic resonance method for different concentrations of Fe. From a detailed g-factor analysis
we determine that the ratio decreases from
L
eff/
S
eff0.049 for x0.43 to
L
eff/
S
eff0.016 for x0.70 which is
much smaller than the bulk iron value (
L
Fe/
S
Fe0.045). The observed concentration dependence is much
stronger than the one calculated for FexPt1xbulk samples and reveals likely changes of the confined elec-
tronic structure of the nanoparticle system. The ratio
L
eff/
S
eff takes the lowest value at the concentration (x
0.70) where the magnetic anisotropy energy vanishes in bulk alloys. For x0.72 a phase transition from a
fcc to the Fe bcc structure occurs resulting in the increased bulk ratio again.
DOI: 10.1103/PhysRevB.69.054417 PACS numbers: 76.50.g, 81.07.b
Much of the interest in nanoparticle systems of transition-
metal compounds like FePt, FePd, and CoPt is related to
their large magnetocrystalline anisotropy energy MAE,
which is of the order of 0.5 meV per magnetic 3datom
(106J/m3),1and makes these materials potential building
blocks for ultrahigh-density magnetic recording media and
permanent magnets.2The understanding and the control of
MAE in these nanoscale building blocks is a crucial task to
design new devices with desired magnetic properties. Unfor-
tunately, it is not evident that bulk properties can be directly
applied to nanoparticle systems. The confinement and the
complicated interplay of s,p, and dstates in a 4-nm com-
posite nanoparticle may result in dramatic changes3in the
magnetic properties of Fe and Pt which have been
measured4–6 and calculated7–9 in bulk systems. This in turn
may alter the magnitude and anisotropy of the orbital mag-
netic moment which is related to the MAEand also the
magnitude of the induced magnetic moment on the Pt site.
Furthermore, also the surface magnetic anisotropy is of high
importance in nanoparticles with up to few nanometers in
diameter, since the ratio of surface-to-volume atoms ap-
proaches 30–40% in a 4-nm particle. Consequently, a study
of the intrinsic magnetic properties like the ratio of orbital-
to-spin magnetic moment and a microscopic understanding
of the mechanisms contributing to the large magnetocrystal-
line anisotropy of these materials are of fundamental interest.
FexPt1xhas a continuous range of solid solutions, and
both stoichiometric and nonstoichiometric alloys with vari-
ous degrees of order can be prepared. The phase diagram is
rather complex, showing three ordered phases as a function
of Fe concentration,10 i.e., ferromagnetic Fe3Pt and antifer-
romagnetic FePt3with cubic Cu3Au-type or L21) structure,
while ferromagnetic FePt presents a tetragonal CuAuI-type
or L10) structure with the largest value for MAE of 58
106J/m3. One should note that this extremely large value
is almost by two orders of magnitude larger than the one of
the disordered fcc phase that the alloys presents in the
nonannealed state.3
It has been widely accepted that the MAE is related to the
magnitude and anisotropy
Lof the orbital magnetic
moment.11 In an itinerant binary system consisting, for ex-
ample, of a 3danda5delement the total moment cannot be
attributed to one or the other element alone. Hybridization
and polarization effects have to be taken into account. X-ray
magnetic circular dichroism has been considered to be the
appropriate method of choice to determine the element spe-
cific orbital and spin magnetic moment in composite materi-
als with high precision. However, recent results indicate that
the application of the so-called ‘‘sum rules’’ may lead to
erroneous results due to hybridized end states in a binary
system without first-principle theoretical support.12 A
method which directly measures the ratio of the effective
orbital
L
eff and spin
S
eff magnetic moments inherent in the
hybridized band structure of both elements is the para- and
ferromagnetic resonance.13,14 In this classical technique one
measures the precession of the coupled magnetic moments of
Fe and Pt in the crystal potential and a detailed analysis15
yields the gfactor which is related to the effective ratio
L
eff/
S
eff .
The aim of the present work is to investigate the influence
of the composition on the spin and orbital magnetic moments
of the fcc disordered phase of FexPt1xnanoparticles. An
experimental determination of the orbital-to-spin ratio for
different compositions will be given. The contribution of the
orbital magnetism, which is responsible for the MAE, is
found to decrease as a function of increasing Fe content
much stronger than in bulk disordered alloys, indicating the
importance of surface and finite-size effects in these compos-
ite nanoparticles.
Monodisperse FexPt1xnanoparticles with a particle size
distribution
5% were synthesized by the high-
temperature solution phase decomposition of Fe(CO)5and
reduction of Pt(acac)2in the presence of oleic acid, oley-
lamine, and 1,2-hexadecanediol forming a protective ligand
shellas described elsewhere.16 The composition of the
PHYSICAL REVIEW B 69, 054417 2004
0163-1829/2004/695/0544175/$22.50 ©2004 The American Physical Society69 054417-1
FexPt1xnanoparticles was controlled by tuning the molar
ratio of the metal precursors. The resulting solution was
dried forming an air-stable powder, which could be redis-
persed in hexane at any concentration.
To determine the size distribution of the particles, a drop-
let of the hexane solution was evaporated onto a transmission
electron microscope TEMamorphous carbon covered Cu
grid. The arrays were examined with a Philips CM12 Twin
120 kVTEM to determine the particle diameter, the inter-
particle spacing and the degree of ordering. The nanopar-
ticles exhibit spherical morphology, for two compositions
Fe58Pt42 and Fe70Pt30 the mean particle diameter is 2.6 nm
with a standard deviation of 0.1 nm. In Fig. 1 we show one
example of many TEM micrographs for Fe43Pt57 that pre-
sents nanoparticles with sizes in the range of 3.60.1 nm.
Also, energy dispersive x-ray analysis EDXin the TEM
and Rutherford Backscattering RBSmeasurements were
performed to determine the chemical composition of the
FexPt1xnanoparticles. A detailed statistical analysis of the
EDX spectra recorded for many particles showed that the
nominal Fe43Pt57 ,Fe
58Pt42 and Fe70Pt30 particle composition
varied by up to 3% around the nominal composition Table
I. From our high-resolution HR-TEM investigations, there
is no indication of an ordered phase in the as prepared state.
A typical HRTEM image of one Fe70Pt30 nanoparticle is
shown in the inset of Fig. 1 as an example.
To check for the presence of Fe oxides in the Fe70Pt30
nanoparticles, we performed near-edge x-ray-absorption fine
structure NEXAFSspectroscopy at the Fe L3and L2ab-
sorption edges in total electron yield mode at beam line
D1011 at MAX laboratory synchrotron facility, Lund, Swe-
den. Our results are in excellent agreement with those of
Anders et al.17 who reported that the presence of a very thin
Fe oxide layer of less than 0.4 nm has to be assumed to
explain the observed spectra. Also, in agreement with this
work, we find that annealing of the particles removes some
of the oxides and produces a more metallic spectrum.
The magnetic properties of the nanoparticles were studied
using a superconducting quantum interference device
SQUIDmagnetometer. The Fe70Pt30 nanoparticles exhibit
superparamagnetic behavior at room temperature and have a
blocking temperature in the range 50–75 K.
The samples for the magnetic resonance measurements
were prepared by redispersing 1 mg of the nanoparticle pow-
der in a hexane solvent. Many layers of nanoparticles for
each composition were formed on a quartz substrate by dry-
ing the solution in air yielding a noise-free resonance signal
as depicted in Fig. 2.
Ferromagnetic resonance FMRmeasurements were per-
formed at 9, 24, and 79 GHz at room temperature. The mag-
netic field was applied in the plane or perpendicular to the
plane of the sample. No angular dependence was observed
for the magnetic response of the nanoparticles, indicating
that the multilayered samples are isotropic. In a period of
time of over eight months the same samples and freshly pre-
pared ones have been measured three times in order to check
the reproducibility and the accuracy of the g-factor determi-
nation. No changes within the given error bars were observed
indicating no long term degradation effects for the FexPt1x
nanoparticles protected by a ligand shell. Additional high-
sensitivity measurements on a single particle layer of
Fe70Pt30 obtained by drying a dispersion with a much smaller
particle concentration on silicon yielded the same angular
independent gfactor indicating that at 300 K additional lay-
ers do not modify the local magnetic field in which the par-
ticle’s magnetic moment precesses. This indicates that the
magnetostatic interaction for the ligand separated nanopar-
FIG. 1. TEM micrographs of the Fe43Pt57 nanoparticles dried on
a conventional TEM grid. The inset shows a typical high-resolution
image of a Fe70Pt30 nanoparticle revealing the perfect crystallinity
of the chemically disordered particle.
TABLE I. Composition, diameter, and gfactor of the nanopar-
ticles. The gfactor (g2.09) of bulk bcc Fe is given also for ease
of comparison.
Fe content
at %
Mean diameter
nmgfactor
433 3.60.1 2.0980.015
583 2.60.1 2.0700.015
703 2.60.1 2.0320.010
100 2.090
FIG. 2. The magnetic resonance spectra for Fe70Pt30 nanopar-
ticles at: a
9.829 GHz, b
24.121 GHz, and c
79.344 GHz.
M. ULMEANU et al. PHYSICAL REVIEW B 69, 054417 2004
054417-2
ticles is very weak at room temperature. The FMR spectra at
three microwave frequencies for the Fe70Pt30 sample mea-
sured at room temperature are shown in Fig. 2. All spectra
were fitted with Lorentzian line shapes and the resonance
field Bres was determined. The asymmetry of the FMR spec-
trum measured at 79 GHz is due to a small misalignment of
the sample in the experimental setup.
Paramagnetic resonance describes the resonant absorption
of microwaves which matches the Zeeman energy levels
splitting in a paramagnetic material. Crystal-field effects
modify the electronic states, and in a cubic crystal the orbital
moment is nearly completely quenched.18 In a classical pic-
ture the magnetization precesses around the direction of an
effective static magnetic field and the torque acting on the
magnetization is a direct measure of the local magnetic field
composed of the external and all intrinsic magnetic fields. In
the case of superparamagnetic particles the particle’s effec-
tive magnetic moment precesses uncorrelated to neighboring
particles as long as the measurement is performed high
above the blocking temperature of the particle. In this case,
the intrinsic magnetic fields due to the dipolar interaction
between particles is very weak and averages out due to the
thermal fluctuations of the particles over the time window of
the measurement nanoseconds. This offers the unique op-
portunity to observe in single crystalline ferromagnetic nano-
particles the magnetic resonance undisturbed by the presence
of large intrinsic magnetic fields, thus facilitating the deter-
mination of the gfactor. In Refs. 19–22 the following rela-
tion between the gfactors measured by magnetic resonance
in ferromagnetic FMRcompounds and the magnetization
contributions due to spin and orbital angular momentum was
derived, which for small orbital contributions is given by19
L
S
g2
2.1
Note that in binary, strongly exchange coupled systems
like FePt with an induced polarization at the Pt site the ef-
fective orbital and spin contributions are measured.
Usually the determination of the gfactor based on FMR
measurements is very complicated due to large intrinsic mag-
netic anisotropy fields which are temperature dependent. In
ensembles of superparamagnetic nanoparticles above their
blocking temperature, however, the intrinsic magnetic fields
become negligibly small due to thermal fluctuations. A
straight forward gfactor analysis in terms of the paramag-
netic resonance condition for FexPt1xalloys becomes now
possible.
The gfactor for the case of paramagnetic solids can be
directly deduced from the slope of the linear dependence of
the resonance field versus the Larmor frequency according to
the following equation:
h
g
BBres ,2
where
Bis the Bohr magneton, his the Planck constant,
and Bres is the resonance field obtained from the derivative of
the absorptive part
of the complex rf susceptibility
which is conventionally measured at constant microwave fre-
quency
as a function of the external magnetic field.
Frequency-dependent measurements yield an accurate g
value according to the standard paramagnetic resonance con-
dition 2.15,22 The number of magnetic moments which is
detectable in an FMR experiment is of the order of
1010–1014 depending on the linewidth of the signal.13
We find that the resonance fields for each composition
depend linearly on the microwave frequency as expected ac-
cording to Eq. 2. No difference between in-plane and out-
plane resonance fields was observed as stated above. In Fig.
3, we show the result for the Fe43Pt57 and Fe70Pt30 composi-
tion for clarity only, since the data points for the intermediate
concentration fall in between the plotted data. One can see
that there are only small changes in the slope and only the
large range in frequencies allows to distinguish the small
differences between the samples. This linear dependence
clearly shows that no ferromagnetic contributions that is to
say additional internal magnetic fieldshave to be considered
in this multilayered thin film sample. Such–even small—
internal fields due to for example demagnetization fields of
dipolar origin would lead to a difference between in-plane
and out-of-plane measurements as observed, for example, in
layers of Co/CoO core shell nanoparticles.23
The gfactors Table Iare directly calculated from the
slopes of the data in Fig. 3 for the different compositions
according to the paramagnetic resonance condition. The error
bars for the determination of the gfactor and the respective
concentrations are given in Fig. 4 and Table I. One finds that
only for the highest Pt concentrations, i.e., Fe43Pt57, the g
factor is larger than the one of bulk bcc Fe (g2.09),14
while for the other two compositions, Fe58Pt42 (g2.070)
and Fe70Pt30 (g2.032), the gfactor is lower. Before dis-
cussing the composition dependence, we would like to point
out that there is a size difference between the Fe43Pt57 di-
ameter 3.6 nmand the other two compositions diameter 2.6
nm. From our results we cannot exclude that the ratio
L/
Sfor the 3.6-nm nanoparticle could be slightly smaller
than the one for 2.6-nm particles due to the smaller surface
contribution.24 However, for these diameters 2.6 nm vs 3.6
nm corresponding to more than 1000 atoms per clusterthe
possible size dependent reduction of the ratio averaged over
the particle is very small, and one may conclude that the
composition dependence is the more important factor.
Since the gfactor is related to the ratio of the orbital-to-
FIG. 3. Resonance field as a function of the microwave fre-
quency for Fe43Pt57 and Fe70Pt30 particles at room temperature. The
error bar for resonance fields is smaller than the symbol size.
COMPOSITION-DEPENDENT RATIO OF ORBITAL-TO-... PHYSICAL REVIEW B 69, 054417 2004
054417-3
spin magnetic moment we can discuss the results directly in
terms of the variation of this ratio as a function of Fe con-
centration. At this point one has to remember that the aver-
aged moments of the binary alloy enter the experimentally
determined ratio. As mentioned before, the measured ratio
does not reflect the ratio of the individual Fe or inducedPt
moment alone but rather the averaged collective response of
both elements at their respective concentrations, i.e., xfor Fe
and (1x) for Pt. Hence the ratio Rshould be defined by
Rx
L
Fe1x
L
Pt
x
S
Fe1x
S
Pt
L
eff
S
eff g2
2,3
which takes into account that the effective magnetic moment
depends on the type and number of nearest neighbors. One
should note that by calculating the effective ratio according
to Eq. 3we implicitly assume that the induced orbital mo-
ment of Pt couples more strongly to the one of Fe than to its
own spin moment. This can be paraphrased as a type of
Russel-Saunders LScoupling versus a JJ-type of coupling
for which one would calculate first the ratios
L/
Sfor Fe
and Pt individually and add the two ratios to determine the
effective experimentally measured value. As shown below
we find reasonable evidence that the ‘‘LS’’ type of coupling
Eq. 3兲兴 is the appropriate coupling scheme for the moment
contributions in the FePt binary alloy.
In Fig. 4 we show the experimental result for Rright
scaleobtained from Eq. 3together with the ratio calcu-
lated according to Eq. 3from the experimental and theo-
retical orbital and spin magnetic moments of Fe and Pt given
in Refs. 7 and 8. The coupling of the Pt and Fe moments is
assumed ferromagnetic, which in general has been theoreti-
cally predicted and experimentally confirmed in the concen-
tration range x30%. Only ordered FePt3crystals show an
antiferromagnetic AFMcoupling between adjacent Fe
planes, which may involve a frustrated AFM coupling be-
tween the induced Pt moment and the one of Fe. Recently, a
near energetic degeneracy between ferro- or antiferromag-
netic in alternating Fe layers has been calculated for the FePt
composition in bulk samples.25 The tendency for ferromag-
netic order has been found to become more favorable when
the chemical disorder is increased which corresponds to our
samples. So, the assumption of ferromagnetic coupling in
our particles seems well justified. Considering the possibly
modified electronic structure in our nanoparticles another
type of coupling than in bulk samples cannot be completely
excluded. Also, one may notice, that in other 3d/5dinter-
faces, the alignment of orbital and spin magnetic moments
has been found to violate Hund’s third rule allowing for a
parallel alignment of the induced spin moment while the
induced orbital moment is oppositely oriented.26–28 After cal-
culating all combinations of parallel and antiparallel combi-
nations of Fe and Pt orbital and spin magnetic moments7,8 we
find the best correlation of the concentration dependence of
the literature bulk data open symbols in Fig. 4to our ex-
perimental data when we assumed parallel alignment of the
moments e.g., FePt3:8
S
Fe3.14
B,
L
Fe0.1
B,
S
Pt
0.30
B,
L
Pt0.05
B; FePt:7
S
Fe2.89
B,
L
Fe
0.11
B,
S
Pt0.35
B,
L
Pt0.05
B;Fe
3Pt:8
S
Fe
2.51
B,
L
Fe0.09
B,
S
Pt0.29
B,
L
Pt0.05
B).
It is obvious that the element-specific moments do not
vary much as a function of composition, however, the
weighting by the atomic percentage xpresent in the sample
yields the composition dependence of the averaged ratio,
which is consistent with our experimental data. The differ-
ence between the literature and the experimental ratio may
have two reasons. First, all calculations were done for infi-
nite samples and the finite geometry of the nanoparticles
including the large surface contribution was not taken into
account. Recently, it was shown by resonant magnetic dif-
fraction experiments at the Pt L3absorption edge5of Fe38Pt62
and Fe56Pt44 nanoparticles of approximately 5 nm diameter
that the Pt magnetic moment is significantly smaller than for
the respective bulk alloys. Second, the implicit assumption
of collinear magnetic moments in our analysis of the experi-
mental gfactor may only be a good approximation. To
clarify these issues additional measurements to determine the
orbital and spin magnetic moment at the Fe and at the Pt
edge of monodisperse nanoparticles are highly desirable.
In summary, by multifrequency magnetic resonance ex-
periments we have shown that the averaged ratio of orbital-
to-spin magnetic moment of FexPt1xnanoparticles de-
creases as a function of increasing Fe content. At the Fe rich
composition x0.70 which corresponds to the range where
the magnetic anisotropy energy in disordered bulk alloys
vanishes we find a very small nearly negligible ratio indicat-
ing the expected relation between MAE and orbital moment
anisotropy. It is demonstrated that paramagnetic resonance of
superparamagnetic particles well above the blocking tem-
perature is a valuable tool to determine microscopic mag-
netic properties of nanoparticles with high precision and high
sensitivity.
We thank M. Spasova, B. Rellinghaus, and M. Cerchez
for helpful discussions, and S. Stappert and H. Za
¨hres for
help in the TEM measurements. This project was supported
by the European Community, Contract No. HPRN-CT-1999-
00150, the Access to Research Infrastructure Action of the
Improving Human Potential Programme, and the Deutsche
Forschungsgemeinschaft.
FIG. 4. Experimental gfactor and ratio of orbital-to-spin mag-
netic moment as a function of Fe concentration xof the nanopar-
ticles solid triangles. The value for bulk Fe (x100) is also given
as a reference. For comparison the ratio calculated from literature
data see textis also shown open squares.
M. ULMEANU et al. PHYSICAL REVIEW B 69, 054417 2004
054417-4
1G. H. O. Daalderop, P. J. Kelly, and M. F. H. Schuurmans, Phys.
Rev. B 44, 12 054 1991.
2S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science
287, 1989 2000.
3B. Stahl, J. Ellrich, R. Theissmann, M. Ghafari, S. Bhattacharya,
H. Hahn, N. S. Gajbhiye, D. Kramer, R. N. Viswanath, J.
Weissmuller, and H. Gleiter, Phys. Rev. B 67, 014422 2003.
4S. Imada, T. Muro, T. Shishidou, S. Suga, H. Maruyama, K.
Kobayashi, H. Yamazaki, and T. Kanomata, Phys. Rev. B 59,
8752 1999.
5O. Robach, C. Quiros, S. M. Valvidares, C. J. Walker, and S.
Ferrer, J. Magn. Magn. Mater. 264, 202 2003.
6H.-G. Boyen, G. Ka
¨stle, F. Weigl, P. Ziemann, K. Fauth, M.
Heßler, G. Schu
¨tz, B. Stahl, J. Ellrich, H. Hahn, F. Banhart, M.
Bu
¨ttner, M. G. Garnier, and P. Oelhafen, Adv. Funct. Mater. 13,
359 2003.
7A. B. Shick and O. N. Mryasov, Phys. Rev. B 67, 172407 2003.
8V. N. Antonov, B. N. Harmon, and A. N. Yaresko, Phys. Rev. B
64, 024402 2001.
9R. Hayn and V. Drchal, Phys. Rev. B 58, 4341 1998.
10H. Okamoto, in Binary Phase Diagrams ASM International,
Cleveland, OH, 1996.
11 F.Wilhelm, P. Poulopoulos, P. Srivastava, H. Wende, M. Farle, K.
Baberschke, M. Angelakeris, N. K. Flevaris, W. Grange, J.-P.
Kappler, G. Ghiringhelli, and N. B. Brookes, Phys. Rev. B 61,
8647 2000.
12A. Scherz, H. Wende, K. Baberschke, J. Mina
´r, D. Benea, and H.
Ebert, Phys. Rev. B 66, 184401 2002.
13M. Farle, Rep. Prog. Phys. 6, 755 1998.
14C. Kittel, Phys. Rev. 76, 743 1949; J. Pelzl, R. Meckenstock, D.
Spoddig, F. Schreiber, J. Pflaum, and Z. Frait, J. Phys.: Condens.
Matter 15, S451 2003, and references therein.
15A. N. Anisimov, M. Farle, P. Poulopoulos, W. Platow, K. Baber-
schke, P. Isberg, R. Wappling,A. M. N. Niklasson, and O. Eriks-
son, Phys. Rev. Lett. 82, 2390 1999.
16S. Sun, E. E. Fullerton, D. Weller, and C. B. Murray, IEEE Trans.
Magn. 37, 239 2001.
17S. Anders, M. F. Toney, T. Thomson, J.-U.Thiele, B. D. Terris, S.
Sun, and C. B. Murray, J. Appl. Phys. 93, 7343 2003.
18G. Y. Guo and H. Ebert, Phys. Rev. B 53, 2492 1996.
19Ch. Kittel, Phys. Rev. 76, 743 1949.
20J. H. Van Vleck, Phys. Rev. 78, 266 1950.
21R. A. Reck and D. L. Fry, Phys. Rev. 184, 492 1969.
22A. J. Meyer and G. Asch, J. Appl. Phys. 32, 330S 1961.
23U. Wiedwald, M. Spasova, E. L. Salabas, M. Ulmeanu, M. Farle,
Z. Frait, A. Fraile Rodriquez, D. Arvanitis, N. S. Sobal, M.
Hilgendorff, and M. Giersig, Phys. Rev. B 68, 064424 2003.
24C. Binns, S. H. Baker, K. W. Edmonds, P. Finetti, M. J. Maher, S.
C. Louch, S. S. Dhesi, and N. B. Brookes, Physica B 318, 350
2002.
25G. Brown, B. Kraczek, A. Janotti,T. C. Schulthess, G. M. Stocks,
and D. Johnson, Phys. Rev. B 68, 052405 2003.
26F. Wilhelm, P. Poulopoulos, H. Wende,A. Scherz, K. Baberschke,
M. Angelakeris, N. K. Flevaris, andA. Rogalev, Phys. Rev. Lett.
87, 207202 2001.
27F. Wilhelm, P. Poulopoulos, H. Wende,A. Scherz, K. Baberschke,
M. Angelakeris, N. K. Flevaris, andA. Rogalev, Phys. Rev. Lett.
90, 129702 2003.
28H. Wende, A. Scherz, F. Wilhelm, and K. Baberschke, J. Phys.:
Condens. Matter 15, S547 2003.
COMPOSITION-DEPENDENT RATIO OF ORBITAL-TO-... PHYSICAL REVIEW B 69, 054417 2004
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Nanomagnetic CoPt truncated octahedral nanoparticles (TONPs) were successfully synthesised through a facile one-pot strategy. These single crystal CoPt TONPs with an average size of about 8 nm exhibit excellent electrocatalytic performance of both activity and stability for methanol oxidation reaction (MOR). The mass and specific activities of CoPt TONPs is 8 and 6 times higher than that of standard commercial Pt/C, respectively. After accelerated durability test (ADT), the loss of electrochemical surface area (ECSA) for CoPt TONPs is only 18.5%, which is significantly less than that of commercial Pt/C (68.2%), indicating that CoPt TONPs possess much better stability than commercial Pt/C in the prolonged operation. The Curie temperature of CoPt TONPs down to 8 nm is as high as 350 K with weak ferromagntism at room temperature (RT), which is greatly valuable for recycling in the eletrocatalytic applications.
... For higher Fe content, i.e. for larger clusters with lower fraction of surface atoms, the spin contribution for the moment is lower; consequently one would expect a large 〈B hf 〉 value since the orbital and spin contributions to the magnetic hyperfine field are of opposite sign, while moments are adding. Similar behavior has been found by Ulmeanu et al. [24] in Fe-Pt nanoparticles, where the average ratio of orbital to spin magnetic moment decreases as a function of Fe concentration. Thus, the spin and orbital moments are involved in an enhancement of total magnetic moment of the clusters. ...
... FePt NPs have been prepared by different approaches mainly using colloidal chemistry [5,24], gas-phase preparation techniques [25,26], or thin films of few monolayers under ultrahigh vacuum conditions at elevated temperatures leading to dewetting [23]. Here, we have chosen the so-called micellar approach delivering well-separated and size-tuneable FePt NPs on flat supports [10,11], which is of special interest for the present experiments since particle coalescence, growth or Ostwald ripening by annealing can be completely avoided [15]. ...
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Texture formation and epitaxy of thin metal films and oriented growth of nanoparticles (NPs) on single crystal supports are of general interest for improved physical and chemical properties especially of anisotropic materials. In the case of FePt, the main focus lies on its highly anisotropic magnetic behavior and its catalytic activity, both due to the chemically ordered face-centered tetragonal (fct) L10 phase. If the c-axis of the tetragonal system can be aligned normal to the substrate plane, perpendicular magnetic recording could be achieved. Here, we study the orientation of FePt NPs and films on a-SiO2/Si(001), i.e., Si(001) with an amorphous (a-) native oxide layer on top, on MgO(001), and on sapphire(0001) substrates. For the NPs of an approximately equiatomic composition, two different sizes were chosen: "small" NPs with diameters in the range of 2-3 nm and "large" ones in the range of 5-8 nm. The 3 nm thick FePt films, deposited by pulsed laser deposition (PLD), served as reference samples. The structural properties were probed in situ, particularly texture formation and epitaxy of the specimens by reflection high-energy electron diffraction (RHEED) and, in case of 3 nm nanoparticles, additionally by high-resolution transmission electron microscopy (HRTEM) after different annealing steps between 200 and 650 °C. The L10 phase is obtained at annealing temperatures above 550 °C for films and 600 °C for nanoparticles in accordance with previous reports. On the amorphous surface of a-SiO2/Si substrates we find no preferential orientation neither for FePt films nor nanoparticles even after annealing at 630 °C. On sapphire(0001) supports, however, FePt nanoparticles exhibit a clearly preferred (111) orientation even in the as-prepared state, which can be slightly improved by annealing at 600-650 °C. This improvement depends on the size of NPs: Only the smaller NPs approach a fully developed (111) orientation. On top of MgO(001) the effect of annealing on particle orientation was found to be strongest. From a random orientation in the as-prepared state observed for both, small and large FePt NPs, annealing at 650 °C for 30 min reorients the small particles towards a cube-on-cube epitaxial orientation with a minor fraction of (111)-oriented particles. In contrast, large FePt NPs keep their as-prepared random orientation even after doubling the annealing period at 650 °C to 60 min.
... In their comprehensive studies, Shukla et al (2009) have proposed one modification for the synthesis of FePt nanoparticles in the form of nanorods that are in particular, more suited for data storage applications 1 [60]. This was therefore clearly understood by this exhibition that the magnetic behavior of FePt nanoparticles depends not only on the relative extents of iron and platinum with respect to each other but also on the respective interactions of iron and platinum within the prepared nanomaterial [62][63][64]. In a related study, Sun et al (2001) have claimed that the best composition of FePt providing optimum anisotropy and coercivity as required for data storage applications is obtained in the configuration of Fe 55 Pt 45 for magnetic data storage applications. ...
Research
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Nanomaterials have been the topic of intense industrial research for the past several years. They have unique physical, chemical, optical and magnetic properties as compared to those of their bulky counterparts. Intense research has been done on reducing the size of memory devices in the electronics industry. This requirement has made it extremely urgent to explore the materials with larger scope of functionalities on their surfaces. In this respect the conventional silicon based materials have been engineered for better and better control and modification of their surfaces but unfortunately this conventional route does not provide us with both higher functionalities as well as high speeds. In this regard, magnetic nanomaterials have been researched for interesting and far sighted effects. These have been the hot destinations for their sensitive responses in the biomedical and diagnostic applications. Magnetism at nanoscale is also significantly different from the one observed at bulk scale. Magnetic nanoparticles are normally made up of materials which have sensitive magnetic properties arising from the unpaired electrons in their d-orbitals and their coupling effect with their nuclear spins. This review explores the synthesis methods of the magnetic nanomaterials and their possible implementation in making memory based electronic storage devices. This also highlights the significant benefits and aspects that compel the use for investigating memory based potential for magnetic nanomaterials.
... The particles were assumed to be noninteracting, where a surface anisotropy was enhanced by the formation of an oxide (g -Fe 2 O 3 ) surface layer. The same system was studied by Ulmeanu et al., (2004), where frequency dependent FMR measurements were used to assess the g factor of the nanoparticles. Berger et al. (2001) considered the temperature dependence of superparamagnetic resonance in the maghemite (g -Fe 2 O 3 ) system, where again the importance of the size distribution is shown to be of fundamental importance in the evaluation of blocking temperatures and the existence of double peaked spectra. ...
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The scientific and technological exploration of three-dimensional magnetic nanostructures is an emerging research field that opens the path to exciting novel physical phenomena, originating from the increased complexity in spin textures, topology, and frustration in three dimensions. One can also anticipate a tremendous potential for novel applications with those systems in a magnetic sensor and information processing technologies in terms of improved energy efficiency, processing speed, functionalities, and miniaturization of future spintronic devices. These three-dimensional structures are distinct from traditional bulk systems as they harness the scientific achievements of nanomagnetism, which aimed at lowering the dimensions down to the atomic scale, but expand those now in a tailored and designed way into the third dimension. This research update provides an overview of the scientific challenges and recent progress with regard to advances in synthesis approaches and state-of-the-art nanoscale characterization techniques that are prerequisite to understand, realize, and control the properties, behavior, and functionalities of three-dimensional magnetic nanostructures.
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Bimetallic FexPt1-x alloys with the L10 and L12 structures have recently gained a lot of consideration in practical applications for solid-state devices, storage of ultra-high density magnetic data and biomedicine. This is due to their high magnetic and magnetocrystalline anisotropy, density, and coercivity. In order to gain knowledge on the structural, electronic and mechanical properties of the cubic and tetragonal FexPt1-x alloys, we have calculated their equilibrium lattice constants, density of states, and elastic constants at 0 K, employing firstprinciples calculations. The calculated equilibrium lattice constants were found to be in good agreement with the experimental data to within 3 %. All independent elastic constants satisfy the necessary stability conditions for both cubic and tetragonal systems, suggesting mechanical stability. The shear anisotropic factors predict that the tetragonal FexPt1-x crystals are highly anisotropic along the {001} plane than {100}. Moreover, the percentage of bulk (AB) and shear (AG) anisotropies revealed
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In recent years, magnetic nanoalloys (MNAs) have attracted many attentions from all over the world, due to their potential applications in the broad fields of magneto-optics, data storage, engineering, and biology. Among these MNAs, Pt–M (M = Fe, Co, Ni) MNAs have been considered to be the most promising ones, due to their superparamagnetism and response to a magnetic field. Here, we firstly review the experimental work on the synthesis, characterization, and magnetic properties of Pt–Fe, Pt–Co, and Pt–Ni MNAs. Then, we discuss the recent theoretical work on Pt–Fe, Pt–Co, and Pt–Ni MNAs. Moreover, we also review the main applications of Pt–Fe, Pt–Co, and Pt–Ni MNAs in the fields of biology, information storage, and magnetic separation. It is found that the size, shape, and composition of Pt–Fe, Pt–Co, and Pt–Ni MNAs play a critical role on their fundamental magnetic properties from both the experimental and theoretical points of view. It is expected that this review could be a valuable resource for both experimental and theoretical researchers, who are interested in Pt-based MNAs.
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Magnetic circular dichroism in the soft-x-ray absorption spectrum in the 2p-->3d excitation region of the transition-metal element is measured for ferromagnetic Cu3Au-type alloys, MnPt3, Fe3Pt and CoPt3. The electronic state, especially the contribution of the spin and orbital angular momenta to the magnetic moment, is discussed by analyzing the results. The condition for the orbital angular momentum to make a large contribution to the magnetic moment in a metallic system is discussed in terms of the degree of the localization of the 3d orbital, the filling of it, and the symmetry around it.
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Using the tight-binding linearized muffin-tin orbital method combined with the coherent-potential approximation (TB-LMTO-CPA) we have calculated the electronic and magnetic structure of disordered fcc FexPt1-x alloys in a broad concentration range. The total energy was determined as a function of the lattice constant and of the magnetic moment (fixed-spin moment method). For iron concentrations between x=0.10 and x=0.85 the equilibrium lattice constant, the bulk modulus, and the magnetic moment were determined in good agreement with available experimental data. No deviations of the magnetization from the Slater-Pauling curve in the Invar region were found. In that region two minima of the total energy exist, one with a high moment and a large lattice constant and the other with a zero moment and a small lattice constant, which explains qualitatively the Invar effect. Both minima become degenerate at the critical concentration, xc=0.76. A nonmagnetic ground state was found for x>xc. The energy barrier separating these two minima is two times higher in FePt Invar alloys than in the FeNi system. The relativistic effects were included within the scalar relativistic approximation.
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A discussion is given of the connection between the results of microwave resonance absorption experiments and gyromagnetic ratio experiments on ferromagnetic substances. A review of the experimental data indicates that the microwave experiments usually give g>2, while gyromagnetic measurements usually give values of the related quantity g′ which are g in the macroscopic equations of motion in the resonance experiment is justified as a consequence of the approximate mutual cancellation of the orbital and lattice angular momenta. A critical discussion is given of other attempts to explain the g≠g′ effect.
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A Reply to the Comment by R. Tyer et al.
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State-of-the-art x-ray magnetic circular dichroism measurements of V at the L2,3 edges with an excellent signal-to-noise ratio are analyzed for a Fe0.9V0.1 disordered alloy and a Fe/V3/Fe(110) trilayer which were prepared in UHV and measured in situ on a Cu(100) single crystal. The absorption fine structure and the magnetic dichroism are discussed in detail with the help of ab initio theory. Several approaches known from literature to obtain magnetic ground-state properties from experimental spectra are tested for their validity.
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Films consisting of 3D ordered arrays of FePt nanoparticles of ∼5nm in size have been investigated with X-ray diffraction. At low angles, the diffracted intensities provide some evidence that the nanoparticles have an outer shell less dense than their interior. At high angles, the superstructure reflections have been used to evaluate the order parameter. Resonant magnetic diffraction at the Pt LIII absorption edge indicates that the magnetic moment of the Pt atoms in an Fe38Pt62 nanoparticle film is significantly smaller than that from a bulk alloy with similar composition.
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The effective electronic orbital contribution M0 and the effective electronic spin contribution Ms to the spontaneous magnetization Mt for binary alloys of Fe, Co, and Ni are determined from Scott's measured magnetomechanical g′-factors using the relations M0/Mt=(2-g′)/g′ and Ms/Mt=2(g′-1)/g′ where g′ is assumed to be independent of the strength of the magnetic field and the temperature. It is believed that M0/Mt has several-fold greater accuracy when determined from the g′ factors than when determined from ferromagnetic-resonance g factors. In terms of the Bohr magneton μB, the orbital magnetic moments for the pure elements at 300°K are: for iron, (0.0918±0.0033)μB (4.22% of Mt); for cobalt, (0.1472±0.0034)μB (8.81% of Mt); and for nickel, (0.0508±0.0012)μB (8.92% of Mt). The Ms values for the fcc phases of the alloys agree well with those reported by Meyer and Asch from their g-factor data. For these alloys, M0 is much more sensitive to the crystalline structure than Ms and Mt are. The relationship g′-1+g-1=1 holds for Fe-Ni and Fe-Co alloys (with the exception of one point) within the accuracy of the g-factor measurements (∼0.1-1.0%).
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Angular-dependent x-ray magnetic circular dichroism experiments are performed on Ni/Pt multilayers at the Ni L 2,3 edges under magnetic fields up to 50 kG at a temperature of 10 K. By rotating the applied field away from the easy axis a decrease of the ratio of the orbital-to-spin magnetic moment L / S of Ni is observed for perpendicularly magnetized Ni/Pt layers which show considerable magnetic anisotropy energy 20 eV/ atom. Saturation effects in the recorded spectra, depending on both the thicknesses of Ni and Pt, are shown to lead into erroneous conclusions in the determination of L / S and they have to be considered in the analysis. An anisotropy of the Ni-orbital moment L as large as 22% of L is determined. L is demonstrated to be the origin of the magnetic anisotropy energy of 3d ferromagnetic elements. Orbital magnetism of 3d transition metals has become a topic of major interest. The spin magnetic moment S is a priori isotropic. In first approximation, by treating the spin-orbit coupling in a second-order perturbation theory, 1 simi-larly to the well-known picture of localized magnetism, a simple relation may be derived linking the magnetic anisot-ropy energy MAE to the anisotropy of the orbital magnetic moment L , i.e., the difference L between an easy-and a hard-magnetization axis: 2 MAE