The paramagnetic contribution is associated with small grains of Fe, whereas the magnetic component is contributed by large iron grains.. The average grain size of BCC-Fe Scherrer formul
Trang 1Magnetic study of nanocrystalline iron particles in
alumina matrix
A Fnidikia,*, C Doriena, F Richommea, J Teilleta, D Lemarchanda, N.H Ducb,
J Ben Youssefc, H Le Gallc
a Groupe de Physique des Materiaux UMR CNRS 6634, Universit !e de Rouen, Site Universitaire du Madrillet, B.P 12,
76801 Saint-Etienne-Du-Rouvray Cedex, France
b Cryogenic Laboratory, Faculty of Physics, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
c Laboratoire de Magn !etisme de Bretagne, CNRS, B.P 809, 29285 Brest Cedex, France
Abstract
X-ray diffraction, M.ossbauer effect and magnetisation investigations have been performed on sputtered
Fe40(Al2O3)60thin films Ultrafine Fe particles of nanometer size in an amorphous Al2O3 matrix have been formed
by annealing in the temperature range from 100C to 500C Their particle sizes, however, show a rather wide distribution M.ossbauer spectra are constituted of both paramagnetic and ferromagnetic contributions The paramagnetic contribution is associated with small grains of Fe, whereas the magnetic component is contributed by large iron grains This assumption is supported by the ZFC- and FC-measurements, in which the ‘‘blocking’’ temperatures of 60 and 90 K were evidenced for as-deposited and 200C-annealed films, respectively
r2003 Elsevier Science B.V All rights reserved
PACS: 75.50.Tt, 75.75.+a
Keywords: Nanomagnetism; M ossbauer spectra; Superparamagnetism
1 Introduction
Metal/insulator granular films consisting of
nanometer sized ferromagnetic metals immersed
in an insulating medium have been intensively
studied in the last decade because of their
interesting giant magnetoresistance (GMR)
ef-fect—called tunnelling magnetoresistance (TMR)
[1,2] These materials exhibit also other novel
candidates for future technological applications Giant magnetic coercivity has been reported for the ferromagnetic transition metal (T=Fe,
references therein) The coercivity of these gran-ular systems is as large as about 60 mT at room temperature as well as about 250 mT at T ¼ 2 K
contribu-tion to giant magnetic coercivity was reported for
*Corresponding author Group de Physique des
Materiaux-UMR CNRS 6634, Universite de Rouen, Mont-Saint-Aignam
Cedex 76821, France Tel.: 67-65; fax:
+33-35-14-66-52.
E-mail address: abdeslem.fnidiki@univ-rouen.fr
(A Fnidiki).
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved.
doi:10.1016/S0304-8853(03)00064-7
Trang 2granular systems having uniformly nanometer
the presence of single domain regions is
respon-sible for the high value of the coercivity, where
magnetisation reversal takes places only by
rota-tion of saturarota-tion magnetisarota-tion vectors In Ref
the metastable hysteretic response is lost, was
found and superparamagnetic behaviour of
nano-crystalline iron particles in alumina matrix was
reported
In this paper, we focus our attention on the
nanocrys-talline iron crystallites evolved by annealing in the
are discussed in connection with structural and
magnetic data
2 Experimental
glass substrate at 300 K using a triode
RF-sputtering system The film thickness is 560 nm
The composition was analysed using energy
dispersive X-ray spectroscopy (EDX) After
de-positing, samples were annealed for 1 h in a
The microstructure of the sample was
investi-gated using a JEOL 200 FX electron microscope
operating at 200 kV
The structure of the samples was investigated by
X-ray diffraction using a cobalt anticathode
(lCo-Ka¼ 0:1790 nm) The grain size was
(FWHM) of the principal diffraction peaks using
the Scherrer relation
Various magnetic properties, such as magnetic
hysteresis loops, zero-field cooled (ZFC) and field
cooled (FC) magnetisation, were measured with a
SQUID magnetometer in magnetic fields up to
5 T, in the temperature range from 4.2 to 300 K
(CEMS) at room temperature were recorded using
a conventional spectrometer equipped with a
home-made helium–methane proportional
films were set perpendicular to the incident g-beam The spectra were fitted with a least-squares technique using a histogram method relative to discrete distributions, constraining the linewidth of each elementary spectrum to be the same Isomer shifts are given relative to BCC-Fe at 300 K
3 Experimental results and discussion
the experimental XRD patterns show three char-acteristic peaks of (1 1 0), (2 1 1) and (2 0 0)
indicating the amorphous structure of the alumina
to the (1 1 0) reflections, is displayed For the
is rather broad The average grain size of BCC-Fe
Scherrer formula for the width of the (1 1 0)
equals about 3 nm for the as-deposited sample The microstructure of the sputtered film was
reveals that the mean size of the BCC Fe grains
is 2 nm
no appreciable change in the XRD patterns After
peak becomes narrower and its intensity increases
increases up to about 6 nm Finally, it is worth-while to mention that, the (1 1 0) BCC-reflection
This indicates that the annealing causes not only the evolution of crystallites, but also the increase
of the Fe-concentration
as-deposited film, the CEM spectrum consists of a paramagnetic asymmetric doublet (with fraction
Trang 3Apar¼ 51%) and a magnetic broadened sextet
asymmetric doublet can be associated with the
contribution from iron grains with small sizes, and
also probably from Fe atoms diluted in the alumina matrix The broadened magnetic sextet
is associated with the contribution of iron crystal-lites with large grain sizes It was fitted with a wide
20 nm
Fig 2 Dark field electron micrograph (a) and corresponding electron diffraction pattern of the as-deposited thin film.
(a) (b) (c)
(d)
(e)
(110) Fe
Fig 1 X-ray diffraction patterns of the Fe40(Al2O3)60 thin films: (a) as-deposited film, (b) after annealing at 100 C, (c) 200 C, (d) 300 C and (e) 500 C.
Trang 4hyperfine field distribution PðBhfÞ in order to take
into account the different environments of the iron
fitted hyperfine field exhibits a rather wide
characterising the pure BCC-Fe is observed The
described in terms of the transient composition
and ferromagnetism At present, one can add to
this picture an effect of the wide distribution of Fe-grain sizes The CEM spectrum remains almost
Fig 3b) After annealing at 200C, the relative
while the corresponding paramagnetic contribu-tion decreases This process takes place strongly in
slowing down at further heat treatments At
found to be in the BCC-Fe phase As the annealing
0
1.00 1.02
1.00 1.02
1.00 1.02
1.00 1.02
1.00 1.02
0 10 20 30 40 50
0 10 20 30 40
0 10 20 30
0 10 20 30 40
0 10 20 30 40 50
Bhf
Velocity (mm/s)
Fig 3 M ossbauer spectra and hyperfine field distributions of the Fe40(Al2O3)60 thin films: (a) as-deposited film, (b) after annealing at
100 C, (c) 200 C, (d) 300 C and (e) 500 C.
Trang 5temperature increases, the main peak observed at
28 T on the hyperfine field distribution for the
as-deposited film is shifted toward the characteristic
value (33 T) of bulk BCC-Fe This is in agreement
with the increasing size of the Fe grains observed
in XRD
The remaining traces of the doublet in the
of BCC-Fe was not completed In accordance with
the XRD results, the increase of the relative
fraction of BCC-Fe phase is associated with the
increase of the grain size as well as of the
Fe-concentration in the grains with increasing
anneal-ing temperature, indicatanneal-ing that the Fe atoms
BCC-Fe phase
Fig 4 illustrates the ZFC and FC curves
measured in a magnetic field (of 3 mT) applied in
seen that the ZFC and FC magnetisation
bifur-cates at different temperatures in different
sam-ples These temperatures are regarded as the
experimentally measured blocking temperature
accordance with the increase of the Fe grain size
find the mean diameter of the Fe particles d ¼
In order to obtain better results for the d parameter, one must approach the real value of the magnetic anisotropy constant, possibly taking into account a surface contribution After
10 K Here, the observed ‘‘curvature’’ of the ZFC–
FC curves differs from the Langevin ‘‘curvature’’ generally observed for systems containing non-interacting particles It indicates probably the existence of a strong dipolar interaction between iron grains in the samples Indeed, in all cases, the ratio of the remanence to the saturation magneti-sation was found to be about 0.7 at T ¼ 5 K
Table 1
Hyperfine parameters for (Fe)40(Al2O3)60 granular films:
hyperfine field (/B hf S) and M ossbauer fractions (A)
T (K)
T (K)
T (K)
Fig 4 ZFC and FC curves of the (a) as-deposited film, (b) after annealing at 200 C, (c) 500 C.
Trang 6(Fig 5) The value of the saturation magnetisation
corresponds to 43% of Fe with an average
amount of Fe is in the BCC form The smaller
values obtained for the as-deposited and the
surface effect, in accordance with the small value
of the Fe grain size obtained in TEM and XRD
These effects of the surface can explain the low
value of the transition temperature of about 340 K observed on the ZFC–FC curve for the
corroborated by the fact that the transition temperature increases with increasing annealing
coercive field of the investigated sample The coercivity decreases with increasing annealing temperature, i.e with increasing Fe grain size It
is worthwhile, however, to mention that this behaviour is observed in a rather small grain size region
4 Concluding remarks M.ossbauer spectrometry shows the existence of
a broad distribution of iron grain sizes Ferro-magnetism is formed in large sized BCC-Fe grains The existence of small particles of Fe leads strong surface effects and magnetic disorder, evidenced
A simple simulation of the hysteresis loops and the ZFC- and FC-curves will be presented in a forthcoming publication
Acknowledgements This work is partly supported by the State Program for Natural Scientific Researches of Vietnam, within project 420.301
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(b)
-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
B (T)
-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
B (T)
(a)
-1.2
-0.8
-0.4
0
0.4
0.8
1.2
B (T)
(c)
Fig 5 Hysteresis loops measured at 5 K for the as-deposited
Fe40(Al2O3)60 thin films, the 200 C- and the 500 C-annealed
films.