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X-ray absorption and magnetic circular dichroism spectra at both the Fe and Pt L-3,L-2 edges were measured on wet-chemically synthesized monodisperse Fe50Pt50 particles with a mean diameter of 6.3 nm before and after complete removal of the organic ligands and the oxide shell covering the particles by soft hydrogen plasma resulting in a pure metallic state. After thermal treatment of the metallic particles, the coercive field increased by a factor of 6, the orbital magnetic moment at the Fe site increased by 330% and is reduced at the Pt site by 30%, while the effective spin moments did not change. A decrease of the frequency of oscillations in the extended x-ray absorption fine structure at the Pt L-3,L-2 edges provides evidence for crystallographic changes towards the L1(0) phase.
Enhanced Orbital Magnetism in Fe50 Pt50 Nanoparticles
C. Antoniak, J. Lindner, M. Spasova, D. Sudfeld, M. Acet, and M. Farle
Experimentalphysik —AG Farle, Fachbereich Physik, Universita
¨t Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany
K. Fauth
Experimentelle Physik IV, Universita
¨rzburg, Am Hubland, 97074 Wu
¨rzburg, Germany,
and MPI fu
¨r Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart, Germany
U. Wiedwald, H.-G. Boyen, and P. Ziemann
Abteilung Festko
¨rperphysik, Universita
¨t Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany
F. Wilhelm and A. Rogalev
European Synchrotron Radiation Facility (ESRF), 6 Rue Jules Horowitz, BP 220, 38043 Grenoble Cedex, France
Shouheng Sun
Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA
(Received 28 September 2005; published 13 September 2006)
X-ray absorption and magnetic circular dichroism spectra at both the Fe and Pt L3;2edges were
measured on wet-chemically synthesized monodisperse Fe50 Pt50 particles with a mean diameter of 6.3 nm
before and after complete removal of the organic ligands and the oxide shell covering the particles by soft
hydrogen plasma resulting in a pure metallic state. After thermal treatment of the metallic particles, the
coercive field increased by a factor of 6, the orbital magnetic moment at the Fe site increased by 330% and
is reduced at the Pt site by 30%, while the effective spin moments did not change. A decrease of the
frequency of oscillations in the extended x-ray absorption fine structure at the Pt L3;2edges provides
evidence for crystallographic changes towards the L10phase.
DOI: 10.1103/PhysRevLett.97.117201 PACS numbers: 75.75.+a, 61.10.Ht, 61.46.w, 78.70.Dm
The magnetic properties of nanoparticles with diameters
<7nmmay differ from those of the corresponding bulk
materials due to finite size effects, different crystal struc-
tures, and large surface contributions. Microscopic quan-
tities like the orbital and spin magnetic moments are
expected to reflect changes in crystal and electronic struc-
ture in a very sensitive way. In the case of bimetallic alloys
like FePt, also the differences in composition at the surface
and in the volume may lead to characteristic changes of the
magnetic moments in Fe and Pt in a nanoparticle. From the
technological perspective, FePt has become one of the
most intensely researched nanostructured materials (see,
e.g., [16]), since its large magnetic anisotropy of Keff
6106J=m3[7,8] and associated large coercivity makes
it the prime candidate for new ultrahigh density magnetic
storage media or biomedical applications [6]. Organo-
metallic synthesis of FePt nanoparticles with subsequent
heat treatment has been tried as an inexpensive route to
obtain L10ordered FePt particles with diameters around
4nm[7]. Later analysis, however, has unambiguously
shown that the thermal treatment led to agglomerated large
crystals [9]. Here, we focus on the determination of the Fe
and Pt magnetic moments in nonagglomerated wet-
chemically synthesized FePt particles. The changes from
the as prepared ligand covered state, over the pure metallic
chemically disordered fcc state without a ligand shell, to a
partially chemically ordered fct state after annealing are
examined. At all stages the size distribution of the particles
was monitored by high-resolution scanning electron mi-
croscopy (SEM). Specifically, we demonstrate the follow-
ing: (i) pure metallic 6 nm Fe50 Pt50 particles self-
assembled on a Si wafer can be derived from the wet-
chemical synthesis after a soft hydrogen plasma treatment;
(ii) subsequent annealing at 600 Cyields structurally
modified nanoparticles and a dramatic enhancement
(330%) of the orbital magnetic moment lat the Fe site
while lat the Pt site is slightly reduced and the effective
spin magnetic moments remain unchanged after annealing;
(iii) the structural transformation after annealing is evi-
denced by the lower frequency of the extended x-ray
absorption fine structure (EXAFS) oscillations. In addition,
the increase of the coercive field after thermal treatment
indicates the partial formation of the chemically ordered
The as prepared sample consists of monodisperse nano-
particles with a mean diameter of d6:3nmand a stan-
dard deviation of 0:9nmof a chemically disordered
Fe50Pt50 alloy surrounded by a ligand shell of oleyl amine
and oleic acid. The particles, suspended in hexane, were
deposited by spin coating onto a naturally oxidized Si
substrate to form a monolayer. SEM images show a cover-
age of 10% on a 510 mm2area and the formation of
PRL 97, 117201 (2006) PHYSICAL REVIEW LETTERS week ending
0031-9007=06=97(11)=117201(4) 117201-1 ©2006 The American Physical Society
monolayer islands of about 2104nm2consisting of
hexagonally arranged particles with a mean center-to-
center distance dcc 9nm.
For the determination of the element specific magnetic
moments, x-ray absorption spectroscopy (XAS) spectra of
the identical sample of photon energy were taken at the Fe
L3;2and Pt L3;2edges in the total electron yield (TEY)
mode at the PM-3 bending magnet beam line of the
BESSY II synchrotron facility in Berlin, Germany, in fields
of 2:8Tand in the hard x-ray regime in the fluorescence
yield mode at the undulator beam line ID12 of the ESRF in
Grenoble, France, in fields of 0:6T. After each scan,
either the magnetic field or the polarization of the x rays
was flipped.
Figure 1shows the normalized XAS, the corresponding
x-ray magnetic circular dichroism (XMCD), and the inte-
grated XMCD signal at the Fe and Pt L3;2edges in the
hydrogen plasma treated and annealed states of the parti-
cles. To separate the electron excitations to unoccupied d
states from those to the continuum, a standard two-step-
like function is subtracted in the case of the spectra at
the Fe L3;2edges [10]. Since the error in performing such
a separation for the Pt L3;2edges is large due to the not
well pronounced absorption peaks, the adjusted isotropic
absorption spectra of Au were subtracted instead [11].
The XAS of the sample in the as prepared state at the
Fe L3;2edges showed that the ligand shell does not pro-
tect the surface of the particles from oxidation, confirm-
ing previous results [12,13]. After the sample was exposed
to a soft hydrogen plasma at room temperature and a
pressure of 5 Pa, pure metallic XAS at the Fe L3;2edges
was obtained [Fig. 1(a)]. At the carbon Kedge, no absorp-
tion peaks were observed, since the hydrogen plasma does
not only reduce Fe oxides but also removes the organic
ligands as is also known for reactive oxygen plasma
SEM images reveal that the number and the arrangement
of particles have not changed within the error bars with
respect to the as prepared state. In order to achieve the
chemically ordered phase, the plasma treated particles
have been annealed in hydrogen atmosphere of 5 Pa for
30 min at 600 C. At this temperature the disorder-order
transformation in FePt nanostructures takes place (see,
e.g., [7,1619]). Since the hard magnetic, chemically or-
dered L10phase is known to have a large coercivity
[13,20,21], element specific magnetization curves [22]at
the Fe L3edge were measured to provide evidence for the
formation of the L10state. The normalized dichroic signal
at T15 K as a function of the external magnetic field is
shown for perpendicular and grazing incidence, i.e., for
magnetization parallel to the normal of the sample plane
(0) and 75(Fig. 2). The signals are normalized
to the white line absorption. Before thermal treatment, the
coercive field is 0Hc36 5mT, and the magnetiza-
tion is favored in plane due to weak dipolar interactions.
After annealing, the coercive field is not only enhanced,
but also found to depend slightly on the angle :
0Hc0292 8mT,0Hc75228 8mT.
Note that the sample may still not be fully saturated at
0H2:8Tfor both geometries [Fig. 2(b)]. At 300 K, a
clear hysteresis was observed with a coercive field of
0Hc35 5mT. The values of the coercivity are in
agreement to reported data for well-separated chemically
ordered FePt particles of the same size with a random
distribution of anisotropy axes [23] and are much smaller
than the one for cosputtered FePt pancakelike particle
layers for which a coercive field of 2 T at room temperature
has been reported [8]. The ratio of remanence-to-saturation
magnetization is 0.5, as predicted for randomly oriented,
noninteracting particles by the Stoner-Wohlfarth theory,
confirming that the dipolar coupling of the particles is
negligible with respect to the large magnetocrystalline
anisotropy of every single particle after annealing.
The size distribution obtained from a statistical analysis
of many SEM images confirms that the median value of the
diameter distribution does not change within the error bar
Fe L3,2 Pt L3,2
(a) (b)
(c) (d)
FIG. 1. Isotropic XAS, XMCD, and
integrated XMCD spectra at 15 K for
Fe50Pt50 nanoparticles after Hplasma
treatment (a),(b) and after annealing in
Hplasma (c),(d) measured at the Fe L3;2
edges (left) and at the Pt L3;2edges
(right). The thin lines in panels (b) and
(d) are Au reference spectra with a
shifted and stretched energy scale.
XMCD spectra at the Pt L3;2edges are
scaled up by a factor of 4.
PRL 97, 117201 (2006) PHYSICAL REVIEW LETTERS week ending
and that more than 80% of the annealed sample consist of
well-separated nanoparticles.
To address the question of whether structural distortions
associated with the L10phase occur, we analyzed the
difference of the frequency of the EXAFS oscillations at
the Pt L3;2edges of the metallic particles before and after
annealing [Figs. 1(b) and 1(d)]. Using a Au reference
spectrum, we determined a distance of nearest neighbor
atoms consistent with the lattice constant of 3:85 0:02
of the fcc disordered particles before annealing and a
reduced one by 21%after annealing in agreement to
the corresponding decrease known in FePt bulk alloy
By applying the XMCD sum rules [25] it is possible to
determine the effective spin magnetic moment eff
sand the
orbital magnetic moment l.eff
ss7tconsists of
the spin magnetic moment sand the magnetic dipole
moment twhich accounts for the asphericity of the spin
moment distribution and might not cancel out especially in
the case of nanostructured materials. Since the spin-orbit
coupling in FePt is large, tcannot be eliminated by
angular dependent XMCD measurements [26], and even
for randomly oriented crystallographic axes in the nano-
particle ensemble twill not average out [27]. Hence, we
can quote effective spin moments eff
Values of land eff
sobtained for Fe and Pt, which
include a TEY saturation correction [28,29] of 3% for eff
and 30% for lin the case of Fe moments, are listed in
Table I. Absolute values were calculated using the theo-
retical numbers of dholes ~
hwith nPt
h0:75, and nFe
h3:705 [30] for both the dis-
ordered and the ordered states. Note that the different peak
intensities in the XAS reveal a small decrease (<10%)of
hafter annealing, indicating a change in the electronic
structure. The Pt moments of the annealed particles have
been obtained by extrapolation to the saturation moments,
since in the applied magnetic field of 0.6 T the XMCD is
only about 74% of the one in the magnetically saturated
state. For a better comparison, we also present in Table I
the theoretical values (b) obtained from band structure
calculations for a L10structure and the values (a) derived
from calculated XMCD spectra by application of the sum
rules [30]. The later values include t.
First, we discuss the spin moment of Fe and Pt. We
found no significant change of eff
sat both the Fe and Pt
sites before and after annealing (Table I), in agreement to
theory [31]. However, the absolute values of the spin mo-
ments at the Fe sites are about 15% smaller than the one
obtained from band structure calculations (2:87B,
[30,31]) for bulk materials. A reduction of the effective
spin magnetic moment caused by a noncollinear spin
structure at the particles surface [14] is assumed to be
unlikely due to the dominating exchange energy between
all spins in such small particles. One may note that for Fe
nanoclusters a decrease of eff
swith decreasing size was
associated with a negative contribution of t[32], which
may be present in the FePt nanoparticles as well. Another
reasonable explanation is the formation of small Fe clus-
ters of a few atoms in the nonannealed particles resulting in
a low-spin Fe state. In the case of Fe chains epitaxially
grown at the steps of Pt(997) such a low-spin state was
identified for a coverage above the monochain limit re-
flecting a strong dependence of eff
son the Fe-Fe coordi-
nation [33]. Additionally, in the case of the annealed
particles the magnetic moments measured in 2.8 T may
not be fully saturated (Fig. 2).
Now we turn to the discussion of the orbital contribu-
tions of the magnetic moments. At the Pt site, lin the
annealed particles is enhanced by about 100% compared to
the calculated one for bulk. This is most likely due to the
contribution of the increased orbital magnetic moments at
the surface [34]. On the other hand, it is reduced by 30%
with respect to the plasma treated (disordered) state. This
may indicate local composition variations within the par-
TABLE I. Element specific effective spin and orbital magnetic
moments in B. Theoretical values derived from band structure
calculations (b) and from calculated XMCD spectra (a) for the
L10structure are also listed [30].
Fe Pt
Plasma treatment 2.48(25) 0.056(10) 0.41(2) 0.054(6)
Annealed 2.59(26) 0.240(18) 0.41(3) 0.042(6)
Theory (a) 2.50 0.064 0.41 0.020
Theory (b) 2.87 0.072 0.33 0.046
FIG. 2. Hysteresis loops of the XMCD for Fe50 Pt50 nanopar-
ticles at 15 K measured at the Fe L3edge for perpendicular (
0, solid lines) and grazing (75, dotted lines) incidence
(a) after Hplasma treatment and (b) after 30 min annealing at
600 C. The insets show SEM images of the sample before and
after annealing.
PRL 97, 117201 (2006) PHYSICAL REVIEW LETTERS week ending
ticle. One may argue that in the chemically disordered state
a local segregation of a few atoms of Pt polarized by the Fe
may effectively acquire a larger ldue to the larger spin-
orbit coupling in Pt while sremains the same. This effect
may even be pronounced if the Pt enrichment occurs at the
surface of the disordered particles. At the Fe site the orbital
magnetic moment is strongly enhanced after annealing,
namely, 330% with respect to the nonannealed state. The
same dramatic increase is found when we consider just the
ratio of orbital-to-spin magnetic moments, which is inde-
pendent of the number of unoccupied dstates. At the Fe
site the ratio increases from 2% to 9% after annealing and
is nearly identical to the ratio at the Pt site (10%) This
indicates a noncubic environment of the Fe and Pt atoms as
expected for the L10structure.
In conclusion, we determined the element specific mag-
netic moments, land eff
s, in pure metallic 6.3 nm
Fe50Pt50 particles and found a strongly enhanced value of
lat the Fe site after annealing for 30 min at 600 C.
Evidence for local composition variations within the par-
ticles based on a comparison of all contributions to the
magnetic moments is discussed. Additionally, changes of
the EXAFS oscillations and the enhanced coercivity after
thermal treatment indicate the partial formation of the L10
We thank A. Trounova, A. Schlachter, N. Friedenberger,
and S. Stienen for assistance in the measurements. This
work was supported by the BMBF (05 ES3XBA/5), the
ESRF, the DFG (SFB 445, SFB 569, Zi 21-1), and the
EU under Contract No. MRTN-CT-2004-005567
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... All the spectra have been acquired at the saturation regime, respectively 2 T and 5 T before and after annealing. The maximum intensity of the XMCD spectrum decreases for annealed FePt nanoparticles (figure 2(b)) reflecting a reduced magnetic moment as theoretically expected for L1 0 FePt compared to the A1 phase [39,66]. ...
... After annealing, the spin moment is lower and is similar to the value found in the literature for FePt nanoparticles [38,66]. However, m L is surprisingly low compared to previous studies [66,72], including our results on 3 nm FePt nanoparticles embedded in carbon matrix (0.18 µ B /at) [38]. ...
... After annealing, the spin moment is lower and is similar to the value found in the literature for FePt nanoparticles [38,66]. However, m L is surprisingly low compared to previous studies [66,72], including our results on 3 nm FePt nanoparticles embedded in carbon matrix (0.18 µ B /at) [38]. m L is very close to the value of L1 0 bulk (0.07 µ B /at [39,71]), which is unexpected, since it is usually assumed that the orbital moment in nano-objects is higher than in bulk due to the broken symmetry. ...
The moiré pattern created by the epitaxy of a graphene sheet on an iridium substrate can be used as a template for the growth of 2D atomic or cluster arrays. We observed for the first time a coherent organization of hard magnetic preformed FePt nanoparticles on the 2D lattice of graphene on Ir(111). Nanoparticles of 2 nm diameter have been mass selected in a gas phase and deposited with low energy on the hexagonal moiré pattern. Their morphology and organization have been investigated using grazing incidence small angle x-ray scattering, while their magnetic properties have been studied by x-ray magnetic circular dichroism, both pointing to a FePt cluster-graphene surface specific interaction. The spatial coherence of the nanoparticles is preserved upon annealing up to 700 °C where the hard magnetic phase of FePt is obtained.
... The phase diagram of the Fe-Pt system also reveals several types of magnetic orders such as ferromagnetism in Fe3Pt, FePt and FePt3 and antiferromagnetism in FePt3 [109,110]. In the alloys, the magnetic moments of Fe in the FePt alloy ranges from 2.8 to 3.5 µB/atom [111][112][113][114]. The different chemical composition and fabricating temperature of the alloys lead to different magnetic states and thus there is dependence of TC or TN on composition of Fe-Pt system. ...
... The other set of films was finally applied RTP heat treatment at 750°C, at 50°C/s, and in various annealing time. The annealed FePt films grown with an oop crystallographic c-textured reflections perpendicular with film planes, and diffraction reflections from other texture, i. e., (111) peaks barely observed, are compared in Figure III.33a, which is indicated by the predominance of the superstructure at 24.02° and fundamental reflections at 49.18° from [129,226]. They illustrate that achieving ≥ 95 % fraction transformed from A1 to L10 phase in 10 nm film, at 700°C of annealing, followed by: (i) the Pt-rich compositional film (as in this work), requires considerably longer time (~5×10 0 s -10 3 s) to achieve a certain fraction transformed from A1 to L10 phases compared with the Fe-rich film (~10 1 s -10 3 s depending on the model being used); (ii) the annealing in conventional furnace also requires longer time compared to laser annealing (~10 -3 s -10 -2 s). ...
... Moreover, there is a shift in the peak positions of both (111) and (002) orientations. This shift becomes higher and higher when the Fe65Co35 content increases. ...
Ce document est le fruit d’un travail de thèse réalisé au sein de l’institut IRCER à Limoges au cours des trois dernières années. Il présente toutes les étapes qui ont conduit à la synthèse de nano-composites ferromagnétiques composé d’une matrice ferromagnétique dure et de nanoparticules magnétiques douces. Plus généralement, ce travail de thèse s’insère dans un projet collaboratif ANR-SHAMAN sur les composites magnétiques réunissant trois laboratoires (l’Institut Néel de Grenoble, l’Institut Lumière Matière de Lyon et l’Institut de Recherche sur les Céramiques de Limoges) et une société civile (l’European Synchrotron Radiation Facility de Grenoble). Aussi, la plupart des caractérisations magnétiques présentés dans ce document ont été réalisées à/et en collaboration avec l’Institut Néel de Grenoble. Des films minces de NdFeB et de FePt ont été développés par dépôt par ablation laser (PLD) sur différents substrats (Si/SiO2, Al2O3, MgO). Le contrôle du procédé à partir d’une cible unique ainsi que la maitrise de la structure et de la microstructure des matériaux ont conduit à l’obtention de propriétés magnétiques tout à fait remarquables. Des films (150 nm) composés de grains de Nd2Fe14B1 découplés grâce à une phase riche en Néodyme présentent un couple, rémanence/coercivité, de valeurs proches des meilleurs aimants macroscopiques du marché μoHc ~1.3 T, μoMr ~1.1 T et une courbe typique d’aimantation carré sans phase secondaire. Profitant d’un processus de démouillage induit par recuit rapide, une collection de grains isolés de FePt (15 nm) réalisés par PLD présente aussi de très bonnes caractéristiques magnétiques, μoHc ~4.4 T, μoMr ~1.3 T, des phases secondaires persistantes sont toutefois à déplorer. Parallèlement à ces développements, un générateur de nanoparticules entièrement réalisé à l’institut IRCER et associé à l’enceinte principale permet la synthèse de nanoparticules ferromagnétique de Co et Fe65Co35. Les particules métalliques dont la taille varie de 2 à 5 nm de diamètres, en fonction des paramètres appliqués au générateur, sont cristallisées et magnétiquement douces. Des nano-composites, d’architectures définis, composés de grains de FePt et de nanoparticules de Fe65Co35, à 25% volumique en proportion, ont montrées une augmentation importante (+24%) de la rémanence par rapport à un film mince de FePt conventionnel, tout en préservant intact les propriétés de coercivité. La difficulté réside dans la préservation des propriétés des composés magnétiques durs et doux malgré l’application de température élevées ~750°C et des phénomènes de diffusion associés. Ces améliorations constituent une preuve expérimentale validant la théorie sur l’augmentation des propriétés magnétiques des composites basés sur une interaction de matériaux magnétiquement durs et doux. Ces travaux de thèse se situent dans la perspective d’une maitrise des architectures à l’échelle micro/nanométrique de matériaux modèles afin d’en améliorer les propriétés magnétiques.
... Since the end of the 1990s, a number of XMCD and XRMR studies focusing on Pt/3d-ferromagnetic-metal (Fe, Ni, Co, and Ni 81 Fe 19 ) junction systems have been reported and the Pt ferromagnetism in these systems has been established. [44][45][46][47][48][49][50][51][52][53][54][55][56] Recently, these techniques were applied for the Pt/magnetic-oxide systems: Pt=Y 3 Fe 5 O 12 , [29][30][31]36 37 and Pt=Fe 3 O 4 ; 37 in contrast to the Pt/3dferromagnetic-metal systems, the proximity-induced Pt ferromagnetism in these Pt/magnetic-oxide systems was found to be undetectably small when the effect of Pt oxidization is negligible. 30,31 These results suggest that small magnetization of the magnetic oxides is disadvantageous to inducing proximity ferromagnetism in the adjacent Pt layer; Y 3 Fe 5 O 12 , NiFe 2 O 4 , CoFe 2 O 4 , and Fe 3 O 4 are insulators or highly-resistive conductors [57][58][59] and their volume magnetization is much smaller than that of the 3d-ferromagnetic-metals. 60 In this paper, by means of XMCD at the Pt L 3,2 -edges, we measured induced Pt ferromagnetism in Fe 3 O 4 =Pt(t Pt 2 and 5 nm)=Fe 3 O 4 epitaxial trilayer samples at various temperatures ranging from T ¼ 300 K to 12 K, including the metal-insulator transition temperature T V of Fe 3 O 4 ; above (below) T V 114 K, Fe 3 O 4 is a resistive conductor (insulator). ...
Induced Pt ferromagnetism in Fe3O4/Pt/Fe3O4 epitaxial trilayer films has been investigated by means of X-ray magnetic circular dichroism (XMCD) at the Pt L3,2-edges at various temperatures from 300K to 12K, including the metal-insulator transition temperature of Fe3O4 (TV∼114K). At all the temperatures, we observed clear XMCD signals due to Pt ferromagnetism, the amplitude of which was determined to be 0.39μB at 300K and 0.52μB at 12K for the sample with the Pt thickness of ∼2nm. Interestingly, these values are comparable to or even greater than those in Pt/3d-ferromagnetic-metal (Fe, Ni, Co, and Ni81Fe19) junction systems. The results can be interpreted in terms of a possible Fe interdiffusion into the Pt layer and also possible Fe-Pt alloying due to its high-temperature deposition.
... We now turn to FePt on SiO 2 . Here, the magnetic moments on the Pt sites are induced by the exchange field of the 174419-2 intrinsic moments on the surrounding Fe sites [3,4,[32][33][34]. The static asymmetry of FePt on SiO 2 is shown in Fig. 2(b), the corresponding element-specific demagnetization and relaxation dynamics for a similar maximum quenching as for the CoFeB samples in Figs. ...
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Ferromagnetic metal alloys are today commonly used in spintronic and magnetic data storage devices. These multicompound structures consist of several magnetic sublattices exhibiting both intrinsic and induced magnetic moments. Here, we study the response of the element-specific magnetization dynamics for thin film systems based on purely intrinsic (CoFeB) and partially induced (FePt) magnetic moments using extreme ultraviolet pulses from high-harmonic generation (HHG) as an element-sensitive probe. In FePt, on the one hand, we observe an identical normalized transient magnetization for Fe and Pt throughout both the ultrafast demagnetization and the subsequent remagnetization. On the other hand, Co and Fe show a clear difference in the asymptotic limit of the remagnetization process in CoFeB, which is supported by calculations for the temperature-dependent behavior of the equilibrium magnetization using a dynamic spin model. Thus, in this work, we provide a vital step toward a comprehensive understanding of ultrafast light-induced magnetization dynamics in ferromagnetic alloys with sublattices of intrinsic and induced magnetic moments.
Il existe une demande croissante d'aimants permanents de plus en plus performants, notamment dans le domaine de l'énergie. Des calculs théoriques montrent que combiner une phase magnétique à forte aimantation avec une phase magnétique à forte anisotropie magnétocristalline dans un nanocomposite (NC) devrait permettre de doubler le produit énergétique des meilleurs aimants produits à l’heure actuelle, à base du composé intermétallique Nd2Fe14B. Néanmoins les études de tels NC sont encore loin des propriétés attendues, qui nécessitent une forte concentration en nano-inclusions douces réparties de manière homogène dans la matrice dure et un bon couplage d’échange à l’interface inclusion-matrice. Dans ce travail de thèse, une synthèse de NC est réalisée par la technique de dépôt d’agrégats en faisceau de faible énergie, combinant des agrégats doux de Co de 7,9 nm de diamètre et une matrice réalisée à partir du dépôt alternatif de couches minces de FePt recuite afin d’obtenir la phase tétragonale L1_0 à forte anisotropie. Si le recuit permet l’obtention de la matrice ferromagnétique dure, il favorise aussi la diffusion à l’interface agrégat-matrice. Une étude systématique de la structure a donc été réalisée. Les analyses EXAFS et de profil chimique EDX en TEM ont permis de révéler une interface diffuse, qui donne lieu à une structure cœur-coquille en matrice, le cœur demeurant de Co pur, la coquille étant une phase cubique douce (CoFe)3Pt de type L1_2 et caractérisée par un gradient de composition. Le lien entre la microstructure et les propriétés magnétiques a été établi, en s’appuyant notamment sur des mesures magnétiques locales de XMCD, et obtenues dans un dispositif d’effet Kerr à balayage. Les analyses magnétiques montrent clairement l’intérêt des NC, dont le champ coercitif est toujours supérieur à celui des films minces homogènes de Co/Fe/Pt de composition atomique identique. Les mesures XMCD montrent une évolution opposée des moments orbitaux et des moments de spin du Fe et du Co en fonction de la concentration en inclusions douces. Des analyses FORC réalisées dans un magnétomètre à SQUID confirment la coexistence de deux phases magnétiques, en accord avec les analyses de structures.
Sedimentation stability of iron nanoparticles (FeNPs) in low viscosity, volatile organic solvents such as ethanol and MEK is crucial for the high-quality production of FeNP-based electronics. Adding dispersants represents nowadays the common strategy to improve the sedimentation stability of FeNPs, but suffers from toxicity and high costs. Here, we present a simple vacuum drying process for enhancing the sedimentation stability of FeNPs in ethanol without using dispersants. In-situ light transmission and scattering detection showed that vacuum-dried FeNPs in ethanol and MEK sedimented much slower than those of non-treated FeNPs. The cause for the enhanced sedimentation stability was explained by Stoke's law and inter-particle attraction force change due to the reduced hydroxyl groups by the vacuum annealing. A systematic analysis of the FeNP surfaces before and after annealing provided a detailed picture of the advantages to carry out a simple vacuum annealing process to improve sedimentation stability.
In nature, the properties of matter are ultimately governed by the electronic structures. Quantum chemistry (QC) at electronic level matches well with a few simple physical assumptions in solving simple problems. To date, machine learning (ML) algorithm has been migrated to this field to simplify calculations and improve fidelity. This review introduces the basic information on universal electron structures of emerging energy materials and ML algorithms involved in the prediction of material properties. Then, the structure-property relationships based on ML algorithm and QC theory are reviewed. Especially, the summary of recently reported applications on classifying crystal structure, modeling electronic structure, optimizing experimental method, and predicting performance is provided. Last, an outlook on ML assisted QC calculation towards identifying emerging energy materials is also presented.
A layer-by-layer, in-situ H2 plasma treatment in each cycle of atomic layer deposition, referred to as “atomic layer hydrogen bombardment” (ALHB), is applied to improve electrical properties of ZrO2 high-k gate dielectrics. The H2 plasma bombardment facilitates the adatom migration due to energy delivery to each as-deposited monolayer from the H2 plasma. In addition, the H2 plasma treatment contributes to the removal of precursor ligands for the release of steric hindrance. Hence the ALHB treatment leads to film densification and suppression of oxygen vacancies of ZrO2, as evidenced by X-ray reflectivity and X-ray photoelectron spectroscopy characterizations. As a result, ~90% decrease of gate leakage current is achieved in the ZrO2 high-k gate dielectrics with capacitance equivalent thicknesses of ~1.3 nm and ~0.6 nm in metal-insulator-semiconductor and metal-insulator-metal capacitors, respectively. The results manifest that the ALHB treatment is a promising technique to enhance dielectric and electrical characteristics of nanoscale thin films, for further progress of advanced devices such as sensors, solar cells, memories, and nanoelectronics.
The discovery of the high maximum energy product of 59 MGOe for NdFeB magnets is a breakthrough in the development of permanent magnets with a tremendous impact in many fields of technology. This value is still the world record, for 40 years. This work reports on a reliable and robust route to realize nearly perfectly ordered L10‐phase FePt nanoparticles, leading to an unprecedented energy product of 80 MGOe at room temperature. Furthermore, with a 3 nm Au coverage, the magnetic polarization of these nanomagnets can be enhanced by 25% exceeding 1.8 T. This exceptional magnetization and anisotropy is confirmed by using multiple imaging and spectroscopic methods, which reveal highly consistent results. Due to the unprecedented huge energy product, this material can be envisaged as a new advanced basic magnetic component in modern micro and nanosized devices. Huge coercivity and high spontaneous polarization with high crystallinity are achieved in FePt nanoparticles. Heat treatment and solving lattice mismatches maintain the L10‐phase. Coverage with Au increases superior magnetization. The nanoparticles are realized to be an unprecedented high energy product of 80 MGOe.
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The effective magnetic anisotropy Keff of chemically disordered Fe70Pt30 particles with a mean diameter of 2.3 nm is shown to be temperature dependent between 50 K and 350 K. From the determination of the blocking temperatures by field-cooled and zero-field-cooled magnetisation measurements and ferromagnetic resonance experiments, that is in two different time windows, we find Keff = (8.4 ± 0.9)×105 J/m3 at 23 K. This is found to be one order of magnitude larger than the bulk material value for the disordered phase. This value is confirmed by quantitative simulations of the experimentally determined zero-field-cooled magnetisation and can be explained by the large contribution of surface anisotropy, small deviations from a spherical shape and the presence of an approximately one monolayer thick iron oxide shell.
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Microstructure and magnetization processes of highly ordered FePt (001) films with large perpendicular magnetic anisotropy have been studied. The film morphology was controlled from assemblies of single-domain nanoparticles to those of multidomain islands by varying the nominal thickness (tN) of the FePt films sputter-deposited on a heated MgO (001) substrate. The change in the magnetization process from magnetization rotation to domain wall displacement is clearly demonstrated by the initial magnetization curves. Huge coercivities as high as 70 and 105 kOe have been achieved in the film with single-domain particles at room temperature and 4.5 K, respectively.
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Monodisperse FePt nanoparticles were prepared using high-temperature solution phase synthesis. Polymer-mediated layer-by-layer growth leads to precise control of the particle self assembly. The narrow particle size distribution (σ⩽5%) offers the potential for increased data storage density by utilizing a smaller mean particle size and ultimately storage of one bit per individual nanoparticle. We have studied self-assembled multilayers of magnetic FePt nanoparticles. The L10 phase of FePt has a very high magnetic anisotropy that allows the magnetization of particles of about 4 nm diameter to be thermally stable at room temperature. Magnetic measurements using a vibrating sample magnetometer were combined with x-ray diffraction (XRD) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy to study the annealed FePt nanoparticle assemblies and to optimize annealing conditions. NEXAFS spectra showed that a fraction of the iron in the as-deposited particles was oxidized, and this fraction was reduced by annealing in inert or reducing atmospheres. A very thin layer (<0.4 nm) of oxide surrounding the particle is sufficient to explain the observed spectra. Structural analysis using XRD showed that a minimum temperature of 450 °C was required to start the formation of the ordered ferromagnetic phase. Annealing for longer times and at higher temperatures led to higher coercivity and a larger fraction of ordered phase but also to the onset of some agglomeration of the nanoparticles. © 2003 American Institute of Physics.
Angle-dependent x-ray magnetic circular dichroism experiments have been performed at both the Co and Pt L2,3 edges in two epitaxial (111) CoPt3 thin films grown at 690 and 800 K. The analysis of the angular variations of the 3d orbital magnetic moment shows two different magnetic behaviors: a strong perpendicular magnetocrystalline anisotropy (PMA) for the film grown at 690 K and an almost isotropic behavior for the film grown at higher temperature. The same analysis at the Pt L2,3 edges suggests that the 5d electrons play an important role in the PMA. Our results correlate the appearance of PMA with the existence of anisotropic structural effects induced during the codeposition process.
In this paper we report a technique that can directly fabricate L10 phase FePt nanoparticles. FePt nanoparticles were generated through gas-phase aggregation using a magnetron-sputtering-based nanocluster source. Following the source chamber, an online halogen-lamp heater was used for the L10 phase formation during the particles' flight in vacuum. Transmission electron microscopy and vibrating-sample magnetometer data verified the successful fabrication of the L10 phase FePt nanoparticles. The coercivity value at 300 K is 1100 Oe for the nanoparticles with online heating. Neon carrier gas was applied to manipulate FePt nanoparticle size and to enhance particle size uniformity. The size dependence of nanoparticle ordering was investigated.
The structural, electronic and magnetic properties of 4 nm metallic FePt particles were studied by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), X-ray photo electron spectroscopy (XPS), magnetometry and Mössbauer spectroscopy. At low temperatures, the Mössbauer data reveal an unusually high and well defined magnetic hyperfine field compared to FePt multilayer or bulk samples. The magnetic anisotropy of the as-prepared FePt particles embedded in a layer of oleic acid molecules arises from surface contributions. The distribution of anisotropy under the constraint of a constant particle moment and volume is reflected in the hysteresis loop as a function of external field at 4.3 K. Due to faceting as seen in the high resolution TEM images, the magnetization reversal does not follow the simple Stoner-Wohlfarth switching. The annealing experiments show that at a size of 4 nm the high-temperature fcc phase is stable up to at least 560°C as long as agglomeration and particle growth is prevented.
Magnetic circular dichroism has been used to study the orbital and spin moments in supported nanoscale Fe clusters deposited in situ from a gas aggregation source onto highly oriented pyrolitic graphite in ultrahigh vacuum. Mass-filtered (2.4 nm, 610 atoms) and unfiltered (1–5 nm, 40–5000 atoms) clusters at low coverage have an orbital magnetic moment about twice that of bulk Fe. With increasing coverage the orbital moment of the unfiltered clusters converges to the bulk value. There is no detectable change in the spin moment as a function of coverage. Mass-filtered clusters show an increase in the magnetic dipole moment which we ascribe to distortion resulting from their higher impact energy. An increasing magnetic remanence with coverage is found.
Electronic and magnetic properties of free Fe clusters of 9 to 89 atoms are investigated theoretically within an ab initio fully relativistic framework and compared to results of crystal surfaces. It is found that the local spin magnetic moments μspin and the orbital magnetic moments μorb are enhanced for atoms close to the surface of a Fe cluster. The corresponding Friedel-like oscillations in the depth profiles of μspin and μorb are more pronounced for clusters than for crystal surfaces. The μspin in clusters and at crystal surfaces turned out to depend linearly on the effective coordination number Neff. This empirical μspin-Neff inter-relationship is able to account for some features of the experimentally measured dependence of the magnetic moment of free Fe clusters on the cluster size. The spin-polarized density of states (DOS’s) for atoms in clusters is characterized by sharp atomiclike peaks and substantially differs from the DOS in the bulk. The width of the local valence band gets more narrow if one is moving from the center of the cluster to its surface. The DOS averaged over all atoms in a cluster converges to the bulk behavior more quickly with cluster size than the DOS of the central atoms of these clusters.
Saturation effects are determined in x-ray magnetic circular dichroism spectra, acquired by electron yield techniques. It is shown that sum-rule extraction of the number of d holes, orbital moment, and spin moment are affected for Fe, Co, and Ni. In particular, errors in the extracted orbital moment values due to saturation effects can be in excess of 100% and even yield the wrong sign for films as thin as 50 Å. They are significant even for film thicknesses of a few monolayers. Errors for the derived values for the number of d holes and the spin moment are considerably smaller but may be of the order 10–20 %. Correction factors are given for quantities obtained from sum rule analysis of electron yield data of Fe, Co, and Ni as a function of film thickness and x-ray incidence angle.
There is growing evidence that FePt nanoparticles become increasingly difficult to chemically order as the size approaches a few nanometers. We have studied the chemical ordering of FePt and FePtAu nanoparticle arrays as a function of particle size. Monodisperse Fe49Pt51 and Fe48Pt44Au8 nanoparticles with a size about 6 nm were synthesized by the simultaneous decomposition of iron pentacarbonyl and reduction of platinum acetylacetonate and gold (III) acetate in a mixture of phenyl ether and hexadecylamine (HDA), with 1-adamantanecarboxylic acid and HDA as stabilizers. The nanoparticles were dispersed in toluene, films of the particles were cast onto silicon wafers from the dispersion, and the films were annealed in a tube furnace with flowing Ar+5% H2. The magnetic anisotropy and switching volumes were determined from time- and temperature-dependent coercivity measurements. By comparing with 3‐nm FePt and FePtAu nanoparticles of comparable composition, the phase transformation is easier for the larger particles. Under the same annealing conditions, the larger particles have higher anisotropy and order parameter. Additive Au is very effective in enhancing the chemical ordering in both small and large particles, with x-ray diffraction superlattice peaks appearing after annealing at 350 °C. Dynamic remnant coercivity measurements and magnetic switching volumes suggest particle aggregation at the higher annealing temperatures in both small and large particles.