Perpendicular magnetic anisotropy in Co–Pt granular multilayers
Magnetization hysteresis curves have been measured on Co granular multilayers, (Al₂O₃/Co/Pt)N (N = 1 and 25), with the applied magnetic field parallel and perpendicular to the substrate plane. In all samples perpendicular magnetic anisotropy was observed. For Co particles with average diameter 3 n...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2012
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| Cite this: | Perpendicular magnetic anisotropy in Co–Pt granular multilayers / J. Bartolomé, A.I. Figueroa, L.M. García, F. Bartolomé, L. Ruiz, J.M. González-Calbet, F. Petroff, C. Deranlot, F. Wilhelm, A. Rogalev, N. Brookes // Физика низких температур. — 2012. — Т. 38, № 9. — С. 1053-1057. — Бібліогр.: 15 назв. — англ. |
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| author | Bartolomé, J. Figueroa, A.I. García, L.M. Bartolomé, F. Ruiz, L. González-Calbet, J.M. Petroff, F. Deranlot, C. Wilhelm, F. Rogalev, A. Brookes, N. |
| author_facet | Bartolomé, J. Figueroa, A.I. García, L.M. Bartolomé, F. Ruiz, L. González-Calbet, J.M. Petroff, F. Deranlot, C. Wilhelm, F. Rogalev, A. Brookes, N. |
| citation_txt | Perpendicular magnetic anisotropy in Co–Pt granular multilayers / J. Bartolomé, A.I. Figueroa, L.M. García, F. Bartolomé, L. Ruiz, J.M. González-Calbet, F. Petroff, C. Deranlot, F. Wilhelm, A. Rogalev, N. Brookes // Физика низких температур. — 2012. — Т. 38, № 9. — С. 1053-1057. — Бібліогр.: 15 назв. — англ. |
| collection | DSpace DC |
| container_title | Физика низких температур |
| description | Magnetization hysteresis curves have been measured on Co granular multilayers, (Al₂O₃/Co/Pt)N (N = 1 and
25), with the applied magnetic field parallel and perpendicular to the substrate plane. In all samples perpendicular
magnetic anisotropy was observed. For Co particles with average diameter 3 nm, the coercive field at low
temperature is μ₀HC = 0.5 T. HC decreases for increasing temperature and disappears at ≈200 K. A soft magnetic
component is also present in all samples up to the freezing temperature Tf = 365 K. Co and Pt XMCD measurements
at the L2,3 edges were performed, yielding to the orbital mL and spin mS contributions to the total magnetic
moment of the system. These results, in addition to XANES ones, indicate the presence of CoxPt₁₋x alloy. Particles
conformed of CoPt alloy, embedded in Pt and coupled magnetically by dipolar or RKKY interaction, may
explain the phenomenology observed in these systems.
|
| first_indexed | 2025-12-07T15:38:27Z |
| format | Article |
| fulltext |
© J. Bartolomé, A.I. Figueroa, L.M. García, F. Bartolomé, L. Ruiz, J.M. González-Calbet, F. Petroff, C. Deranlot, F. Wilhelm, A. Rogalev,
and N. Brookes, 2012
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 9, pp. 1053–1057
Perpendicular magnetic anisotropy in Co–Pt granular
multilayers
J. Bartolomé, A.I. Figueroa, L.M. García, and F. Bartolomé
Instituto de Ciencia de Materiales de Aragón, Departamento de Física de la Materia Condensada
CSIC/U de Zaragoza, Zaragoza, Spain
E-mail: barto@unizar.es
L. Ruiz and J.M. González-Calbet
Departamento de Química Inorgánica, Universidad Complutense de Madrid, E-28040 Madrid, Spain
F. Petroff and C. Deranlot
Unité Mixte de Physique, CNRS/Thales, Route Départementale 128, 91767 Palaiseau Cedex, France, and
Université Paris-Sud, 91405 Orsay (Paris), France
F. Wilhelm, A. Rogalev, and N. Brookes
European Synchrotron Radiation Facility, BP220, F-38043, Grenoble, France
Received April 20, 2012
Magnetization hysteresis curves have been measured on Co granular multilayers, (Al2O3/Co/Pt)N (N = 1 and
25), with the applied magnetic field parallel and perpendicular to the substrate plane. In all samples perpendicu-
lar magnetic anisotropy was observed. For Co particles with average diameter 3 nm, the coercive field at low
temperature is μ0HC = 0.5 T. HC decreases for increasing temperature and disappears at ≈200 K. A soft magnetic
component is also present in all samples up to the freezing temperature Tf = 365 K. Co and Pt XMCD measure-
ments at the L2,3 edges were performed, yielding to the orbital mL and spin mS contributions to the total magnetic
moment of the system. These results, in addition to XANES ones, indicate the presence of CoxPt1–x alloy. Parti-
cles conformed of CoPt alloy, embedded in Pt and coupled magnetically by dipolar or RKKY interaction, may
explain the phenomenology observed in these systems.
PACS: 75.30.–m Intrinsic properties of magnetically ordered materials;
75.70.–i Magnetic properties of thin films, surfaces, and interfaces.
Keywords: magnetic thin films, magnetic multilayers, perpendicular magnetic anisotropy, platinum alloys,
granular materials.
1. Introduction
Granular Co thin film multilayers constitute a family of
nanoparticle systems which have been very useful for un-
derstanding size effects in magnetism. The reason is that
Co clusters of nearly spherical shape can be produced by
sputtering of Co on amorphous alumina, previously depos-
ited on a Si substrate. Capping with a noble metal enables
to modify the matrix that surrounds the particle, thus also
modifying its magnetic properties. When capping with Cu,
Ag or Au the particles behave as superparamagnetic with
anisotropy constants that depend on the metal used [1–3].
Instead, capping with Pt has a completely different effect.
In this case Co particles are strongly coupled via the polar-
ized Pt [4]. Besides, in a recent work [5] a small increase
in anisotropy was detected on Co particles capped with Pt,
which was related to the formation of CoPt3 clusters. In
this paper we show that the Co–Pt granular multilayers
present strong perpendicular magnetic anisotropy (PMA)
to the substrate plane.
PMA has been observed in Co/Pt thin film multilayers,
when the Co thickness is below a threshold Co thickness of
J. Bartolomé et al.
1054 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 9
11 ML, and has been assigned to a strong increase in the
surface magnetocrystalline anisotropy [6]. On the other
hand, Co1–xPtx alloy films have also shown PMA [7]. In
the case of (111) CoPt3 thin films, the PMA was estab-
lished by means of x-ray magnetic circular dichroism
(XMCD) measurements at both the Co and the Pt L2,3 edg-
es. They could correlate to the existence of anisotropic
structural effects induced during the codeposition process
[8]. So, the origin of PMA may be associated to interface
or alloying effects, or both. In the present work we show
that in the case of Co nanoparticles capped with Pt, alloy-
ing plays the dominant role.
2. Experimental
2.1. Sample preparation and morphology
The samples measured in this work were prepared by
sequential sputtering on a Si substrate at room temperature.
The multilayers are formed by a number N of
(Al2O3/Co/Pt) trilayers, each formed by a buffer layer of
alumina, another layer of Co, and a third of Pt. The Co
coalesces into clusters such that the mean particle diameter
is proportional to the nominal thickness, tCo, of the depos-
ited Co. In this work we present results on samples with
tCo = 0.7 nm, which produces Co particles of 3 nm average
diameter when deposited on amorphous alumina. Two se-
ries of multilayers, with N = 1 and 25 have been measured
[1]. The capping Pt film has tPt = 1.5 nm depth and the
separation between layers was tAl2O3 = 3 nm (see Fig. 1 for
N = 25). The capping metal tends to alloy with the Co par-
ticle and fills the interparticle spaces with the excess Pt, as
can be observed in the Fig. 1 and inset.
2.2. Magnetic measurements
Magnetization measurements were performed with a
SQUID magnetometer equipped with the high resolution
option. For tCo = 0.7 nm the M(T), measured with a bias
field μ0Hdc = 0.02 T in the direction parallel to the sub-
strate plane, shows a maximum at the freezing temperature
Tf = 365 K (see also Fig. 2). The M(H) were measured with
the field applied parallel and perpendicular to the substrate
plane at several temperatures. In Fig. 3 we show the data
for tCo = 0.7 nm and N = 1 measured at T = 5 K. Above Tf
the curves have the characteristic superparamagnetic be-
havior, which can be fitted to a Langevin curve of the
nominal Co particle average diameter [4]. At low tempera-
tures the hysteresis curves measured in the perpendicular
direction have a characteristic step near H = 0, and a se-
cond step when the switching to the reverse orientation
takes place (Fig. 3). The former step indicates that there is
a magnetically soft component present in the sample, while
the latter shows the presence of a ferromagnetic hard com-
ponent with a rather high coercive field μ0HC = 0.5 T at
T = 5 K. HC decreases with temperature till it disappears at
about 200 K (Fig. 4). On the other hand, in the direction
Fig. 1. TEM cross-section of a granular multilayer with N = 25,
tCo = 0.7 nm, tPt = 1.5 nm, and tAl2O3 = 3 nm (intermediate stripes
of light grey). Inset: Fourier transform showing Pt crystalline
morphology in the selected area. The circle indicates a Co–Pt
particulate morphology.
Fig. 2. Magnetization as a function of temperature measured for a
trilayer of (Al2O3/Co/Pt) N = 25, tCo = 0.7 nm and tPt = 1.5 nm,
with a bias field of μ0Hdc = 0.02 T. ( , ) H perpendicular to the
substrate, ( , ) H parallel to the substrate.
100 200 300 400
0.5
1.0
1.5
2.0
ZFC substrate⊥
FC substrate⊥
ZFC || substrate
FC || substrate
T, K
Tf
0
χ,
e
m
u/
cm
C
o
3
μ0Hdc = 0.02 T
Fig. 3. Hysteresis loops at T = 5 K for a trilayer of (Al2O3/Co/Pt)
N = 1, tCo = 0.7 nm and tPt = 1.5 nm. (▲) H perpendicular to the
substrate, (■) H parallel to the substrate.
–2 –1 0 1 2
–3
–2
–1
0
1
2
3
M
,
/C
o
μ B
μ0H, T
Perpendicular magnetic anisotropy in Co–Pt granular multilayers
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 9 1055
parallel to the substrate plane there is no coercive field.
The ferromagnetic behavior implies that the Co–Pt parti-
cles are strongly coupled. Besides, the spontaneous direc-
tion of the anisotropy is perpendicular to the plane. There-
fore, it is proven unambiguously that just one layer of
granular Co capped with Pt has a strong PMA below
200 K.
From 200 till 300 K the ferromagnetic soft component
subsists till its disappearance at Tf. It is interesting to note
that upon annealing the sample up to 800 K the hard mag-
netic component transforms into the soft component, so
that the coercivity is strongly reduced (down to 0.055 T)
when remeasured at low temperatures.
The multilayer (sample with N = 25) shows the same
overall behavior as the sample with N = 1 regarding the
PMA (see Fig. 5) and coercivity decay with temperature
(see Fig. 4). However the soft magnetic component is not
as evident as it is in the sample with N = 1.
2.3. XMCD measurements
The XMCD results provide evidence of hybridization
between the Co at the particle surface and the capping
metal in the cases of Cu and Au-capping [1]. Similarly, the
presence of hybridization between Co and Pt was reported
earlier [4]. In this work the Co and Pt L2,3 edges XMCD
results are presented. The Co XMCD measurements were
performed at the ESRF ID08 instrument in total electron
yield detection, and the Pt ones at the ID12 with fluores-
cence detection.
From an XMCD experiment at the L2,3 edges of Co or Pt
one obtains, through the sum rules [9], the spin mS and or-
bital mL moments averaged over the whole sample, provided
one knows by calculation or estimation the number of holes,
nh, existing involved in the 2p → nd excitations (n = 3 and
5, for Co and Pt, respectively). For Co we have taken the nh
values proposed in the case of CoPt3 thin films [8], and for
Pt we have derived nh from the white line difference with a
Pt foil at the L2,3 edges XANES (see Table 1).
Table 1. The Co and Pt orbital and spin moments for sample
with N = 25
Parameter CO L2,3 edge Pt L2,3 edge
nh 2.25a 1.80(2)
mL/mS 0.11(1) 0.19(1)
mL, μB 0.21(1) 0.027(1)
mS, μB 1.98(2) 0.14(1)
m, μB 2.20(2) 0.16(1)
a nh: number of holes taken from Ref. 8.
XMCD experiments at the Co L2,3 edges as a function
of incident angle (0° and 60°) were performed at T = 5 K
and μ0H = 1 T, looking for anisotropy of the orbital mo-
ment. However, the results did not show clear angle de-
pendence, which was unexpected. In Fig. 6 the XMCD
Fig. 4. Coercive field of the hard magnetic component in sample
(Al2O3/Co/Pt) N = 1 and 25, tCo = 0.7 nm and tPt = 1.5 nm.
0 50 100 150 200 250
0.2
0.4
0.6
= 1N
= 25N
μ 0H
C
, T
T, K
Fig. 5. Hysteresis loops at T = 5 K for a multilayer of
(Al2O3/Co/Pt) N = 25, tCo = 0.7 nm and tPt = 1.5 nm. (▲) H per-
pendicular to the substrate, (■) H parallel to the substrate.
–2 –1 0 1 2
–3
–2
–1
0
1
2
3
M
,
/C
o
μ B
μ0H, T
Fig. 6. XMCD spectra at the Co L2,3 edge of the sample at T = 5 K
and H = 1 T, at normal incidence for a multilayer (Al2O3/Co/Pt)
N = 25, tCo = 0.7 nm and tPt = 1,5 nm. Dashed line is the inte-
grated area of the spectra.
760 780 800 820 840
–6
–4
–2
0
2
–12
–8
–4
0
4
q
X
M
C
D
n
or
m
al
iz
ed
Photon energy, eV
p
X
M
C
D
in
te
gr
al
L2
Co L2,3
L3
J. Bartolomé et al.
1056 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 9
spectrum for normal incidence is shown. These spectra
have been normalized to the x-ray absorption spectra
(XAS) in high-energy limit, after background subtraction.
The value of the Co orbital moment obtained mL = 0.21 μB
(see Table 1) resembles that found for CoPt3 films deposit-
ed on a substrates at 800 K, which also showed absence of
anisotropy [8], and that of annealed Co0.5Pt0.5 NPs with
diameters of about 2.6 nm deposited on amorphous carbon
matrices [10]. The spin moment mS = 1.98 μB has been
obtained considering that the dipole term is small with re-
spect to the total mS. The value found is larger than that
found in CoPt3 films (mS = 1.44 μB) but close to that of the
annealed Co0.5Pt0.5 NPs (mS = 1.91 μB).
In a recent work, XMCD measurements at the Co L2,3
edges on Co and Co–Pt particles have been reported
[11,12]. They find systematically larger ratios of mL/mS
for comparable particle sizes. The orbital to spin ratio
mL/mS of Co atoms in our Co–Pt NPs is 0.11, which is
close to the value found for particles of 2.0 nm with com-
position close to Co3Pt [12].
The XMCD spectra measured at the Pt L2,3 edges are
shown in Fig. 7 and the data extracted from them are in-
cluded in Table 1. The non-zero values measured reflect
the polarization of the Pt by the magnetic Co. The values
found are in reasonable agreement with those found for
CoPt3, again [8]. The induced moment on Pt is one order
of magnitude larger than the moments induced on the Cu,
Ag or Au capping cases [2]. Obviously, the reason for this
difference is the proximity of Pt, with the 5d9 band, to
fulfill the Slater criterion for ferromagnetism, instead of
the nearly filled noble metal nd10 bands (n = 3, 4 and 5 for
Cu, Ag or Au).
Conclusion
The presence of PMA has been detected in all
(Al2O3/Co/Pt)N multilayers, independently of being N = 1
or 25. For the tCo = 0.7 nm samples the switching field of
one layer is μ0HS ≈ 0.5 T and the maximum coercive field
measured at low temperatures is μ0HC = 0.52 T.
The Pt capping by sputtering on the Co particles pre-
formed on Al2O3 gives rise to a quite different behavior
than when the same particles are capped with noble metals.
The magnetic behavior is that of ferromagnetic thin films
below TC = 365 K (when tCo = 0.7 nm), instead of blocked
superparamagnetic behavior. There is evidence that there
are two different contributions, a soft and a hard magnetic
component, the latter with a Curie temperature of circa
200 K. From Co K-edge XAS measurements it was proven
that there has been Co–Pt alloying in the capping process
[3], since the XANES spectra differ markedly from the
metallic Co one, and are very similar to that of CoxPt1–x
[13].
Thus, CoxPt1–x alloys are present in all the samples
studied. These alloys are known to have a variety of mag-
netic properties depending on their composition and
crystallinity [14,15]. For example, the crystallographically
ordered alloy CoPt3 has a Curie temperature of TC =
= 290 K. A departure from the exact composition or lack
of perfect crystallinity would reduce it to TC ≈ 200 K,
which is the temperature of disappearance of coercivity in
our samples. Therefore, one may identify the PMA appear-
ing on this material as due to the formation of CoxPt1–x
alloy with short range ordering in the L10 structure. When
annealed, the induced disorder destroys these crystallites
and the coercivity disappears. The XMCD data also point
towards the same explanation, by comparison to measure-
ments on different Co–Pt systems. Thus, a CoxPt1–x alloy
is probably surrounding the Co rich core, in view of the
particulate morphology of the Co–Pt layers observed by
TEM. The Co rich core of the sample would be responsible
for the soft magnetic component in the magnetic hyste-
resis.
The peak observed in the M(T) curve at Tf = 365 K
(Fig. 2) could be related to the magnetic coupling of the
particles which form in each layer a bidimensional array of
ferromagnetic particles. As it was described earlier, Tf de-
pends on the average particle size, and above Tf the mag-
netic behavior is superparamagnetic. Therefore, the overall
picture describing the collective behavior of these multi-
layers is that of CoxPt1–x alloyed particles, embedded in
the non-reacted Pt, strongly coupled via the polarized Pt,
via dipolar or RKKY interaction. Then, the short range
order within the grains would create the anisotropy in the
CoxPt1–x alloy which would give rise to the PMA.
Moreover, the sample preparation may play an im-
portant role, as demonstrated from comparison with results
of Co–Pt NPs systems prepared by different chemical and
physical methods [10–12].
It can also be concluded from this work that the search
for Co–Pt hybridization surface effects that was the leit
motif of this work, might be present but are overcome by
the CoxPt1–x alloying in the magnetic properties.
Fig. 7. XMCD spectra at the Pt L2,3 edges for a trilayer of
(Al2O3/Co/Pt) N = 25, and tCo = 0.7 nm, tPt = 1.5 nm measured at
7 K and 1 T.
11560 11600 13280 13320
–0.06
–0.04
–0.02
0
0.02
0.04
7 K, 1 T
–0.6
–0.4
–0.2
0
0.2
0.4
qp
X
M
C
D
n
or
m
al
iz
ed
Photon energy, eV
X
M
C
D
in
te
gr
al
Pt L2,3
L2
L3
Perpendicular magnetic anisotropy in Co–Pt granular multilayers
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 9 1057
Acknowledgment
This work was supported by the MECOM under Grant
MAT11/23791 and Project IMANA from the DGA, in
Spain. A.I.F. acknowledges a CSIC JAE2008-Predoc grant.
The experiment HE3136 at the ESRF is acknowledged.
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|
| id | nasplib_isofts_kiev_ua-123456789-117613 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0132-6414 |
| language | English |
| last_indexed | 2025-12-07T15:38:27Z |
| publishDate | 2012 |
| publisher | Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| record_format | dspace |
| spelling | Bartolomé, J. Figueroa, A.I. García, L.M. Bartolomé, F. Ruiz, L. González-Calbet, J.M. Petroff, F. Deranlot, C. Wilhelm, F. Rogalev, A. Brookes, N. 2017-05-25T17:02:13Z 2017-05-25T17:02:13Z 2012 Perpendicular magnetic anisotropy in Co–Pt granular multilayers / J. Bartolomé, A.I. Figueroa, L.M. García, F. Bartolomé, L. Ruiz, J.M. González-Calbet, F. Petroff, C. Deranlot, F. Wilhelm, A. Rogalev, N. Brookes // Физика низких температур. — 2012. — Т. 38, № 9. — С. 1053-1057. — Бібліогр.: 15 назв. — англ. 0132-6414 PACS: 75.30.–m, 75.70.–i https://nasplib.isofts.kiev.ua/handle/123456789/117613 Magnetization hysteresis curves have been measured on Co granular multilayers, (Al₂O₃/Co/Pt)N (N = 1 and 25), with the applied magnetic field parallel and perpendicular to the substrate plane. In all samples perpendicular magnetic anisotropy was observed. For Co particles with average diameter 3 nm, the coercive field at low temperature is μ₀HC = 0.5 T. HC decreases for increasing temperature and disappears at ≈200 K. A soft magnetic component is also present in all samples up to the freezing temperature Tf = 365 K. Co and Pt XMCD measurements at the L2,3 edges were performed, yielding to the orbital mL and spin mS contributions to the total magnetic moment of the system. These results, in addition to XANES ones, indicate the presence of CoxPt₁₋x alloy. Particles conformed of CoPt alloy, embedded in Pt and coupled magnetically by dipolar or RKKY interaction, may explain the phenomenology observed in these systems. This work was supported by the MECOM under Grant MAT11/23791 and Project IMANA from the DGA, in Spain. A.I.F. acknowledges a CSIC JAE2008-Predoc grant. The experiment HE3136 at the ESRF is acknowledged. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур К 80-летию Виктора Валентиновича Еременко Perpendicular magnetic anisotropy in Co–Pt granular multilayers Article published earlier |
| spellingShingle | Perpendicular magnetic anisotropy in Co–Pt granular multilayers Bartolomé, J. Figueroa, A.I. García, L.M. Bartolomé, F. Ruiz, L. González-Calbet, J.M. Petroff, F. Deranlot, C. Wilhelm, F. Rogalev, A. Brookes, N. К 80-летию Виктора Валентиновича Еременко |
| title | Perpendicular magnetic anisotropy in Co–Pt granular multilayers |
| title_full | Perpendicular magnetic anisotropy in Co–Pt granular multilayers |
| title_fullStr | Perpendicular magnetic anisotropy in Co–Pt granular multilayers |
| title_full_unstemmed | Perpendicular magnetic anisotropy in Co–Pt granular multilayers |
| title_short | Perpendicular magnetic anisotropy in Co–Pt granular multilayers |
| title_sort | perpendicular magnetic anisotropy in co–pt granular multilayers |
| topic | К 80-летию Виктора Валентиновича Еременко |
| topic_facet | К 80-летию Виктора Валентиновича Еременко |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/117613 |
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