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Abstract

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
¨tWu
¨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
L10state.
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
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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
[14,15].
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.
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117201-2
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
A
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
[13,24].
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
sonly.
Values of land eff
sobtained for Fe and Pt, which
include a TEY saturation correction [28,29] of 3% for eff
s
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 ~
nhnPt
hnAu
hwith nPt
h
1:73,nAu
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
nFe
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
eff
sleff
sl
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
100nm
(b)
100nm
(a)
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
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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
structure.
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
(SyntOrbMag).
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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. ...
Article
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. ...
Thesis
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). ...
Article
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.
Thesis
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.
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