The first observation of the magnetic circular dichroism in EuS nanocrystals

Article abstract of DOI:10.1016/j.jallcom.2005.04.055

EuS nanocrystals were prepared by the liquid ammonia method. The size effect of EuS nanocrystals was observed in the magnetic measurements, the absorption spectra and the MCD spectra.

Full text of DOI:10.1016/j.jallcom.2005.04.055

Journal of Alloys and Compounds 408–412 (2006) 203–206
The first observation of the magnetic circular dichroism
in EuS nanocrystals
a
a
a
Yasunori Tsukahara , Tomoharu Kataoka , Yasuchika Hasegawa ,
b
∗
Sumio Kaizaki , Yuji Wada
a
Material of Life Science, Graduate School of Engineering, Osaka University, Yamadaoka Suita, Osaka 565-0871, Japan
b
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
Available online 20 June 2005
Abstract
EuS nanocrystals were prepared by the liquid ammonia method. The size effect of EuS nanocrystals was observed in the magnetic
measurements, the absorption spectra and the MCD spectra.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Nanostructure; Semiconductor; Magnetic measurement
1
. Introduction
In this work, the spectroscopic and magnetic properties of
EuS nanocrystals have been examined systematically (size:
18, 29 and 36 nm). We report the characterization, the mag-
netic properties, the absorption spectra and the MCD analysis
of the EuS nanocrystals.
There has been significant interest in the ferromagnetism
of europium chalcogenides from the viewpoint of the ideal
Heisenberg model. The chalcogenides (EuO and EuS) have
4
f orbitals that exist as the degeneracy levels between the con-
duction band (5d orbitals of Eu(II)) and the valence band (2p
orbitals of O2 or 3p orbitals of S ) [1]. The spin moments
−
2−
2. Experiment
of Eu(II) ions come from the 4f electrons [2,3].
We reported the preparation of EuS nanocrystals (aver-
age size of 20–40 nm) by use of liquid ammonia [4]. It is
knownthatcrystalsizeaffectsopto-magneticproperties[5–7]
of europium chalcogenides. However, the size contribution
to the opto-magnetic properties has not been clear yet. We
propose that the spectral analysis of EuS nanocrystals in
magnetic field is directly connected to understanding of the
properties depending on their sizes.
Here, we have attempted to observe the absorption spec-
tra of EuS nanocrystals under magnetic field using magnetic
circular dichroism (MCD). MCD is a useful tool for infor-
mation on the excited states connected with the ground states
through the magnetic interactions [8].
2
.1. Materials
Europium metal (Eu 99.9%) was obtained from Sterm
Chemicals. Hydrogen sulfide (pure H2S) and ammonia
NH3 99.9%) were purchased from Sumitomo-seika and
Iwatani-neriki, respectively. The standard europium solution
1000 ppm) and sodium sulfate anhydrous (Na2SO4 99.0%)
(
(
werepurchasedfromWako. Allofthechemicalswerereagent
grade and were used as received.
2.2. Preparation of EuS nanocrystals
Europium metal (0.3 g) was added to liquid ammonia
◦
(
50 mL, −78 C) in a reaction flask. The color of the solution
∗
turned into deep blue with the dissolution of europium. H S
2
Corresponding author.
E-mail address: ywada@mls.eng.osaka-u.ac.jp (Y. Wada).
gas was introduced into the solution through bubbling until
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.04.055

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Y. Tsukahara et al. / Journal of Alloys and Compounds 408–412 (2006) 203–206
Fig. 1. (a) TEM images and (b) XRD spectra of the EuS nanocrystals with
8 nm particle size.
1
the color of the solution changed to yellow. Liquid ammo-
nia was removed by evaporation at room temperature in 1.5 h.
The resulting product was purple-black powder. The samples
with various sizes were prepared by controlling evaporation
rate of liquid ammonia.
Fig. 2. (a and b) Correlation between magnetization (M) and temperature
(T) under magnetic field of 100 Gauss with 18 nm (ꢀ), 29 nm (ꢁ) and 36 nm
(ꢀ) particle size; (c) correlation between magnetization (M) and magnetic
field (H) at 10 K, of the EuS nanocrystals.
2
.3. Apparatus
The XRD spectrum of the sample is shown in Fig. 1b. The
◦
◦
◦
◦
◦
◦
diffraction peaks 2θ = 25.9 , 30.0 , 43.0 , 50.8 , 53.4 , 62.3 ,
◦
◦
X-ray diffraction patterns were recorded on a Rigaku X-
70.9 and 79.0 of the sample were assigned to the (1 1 1),
(2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 0) and (4 2 2) planes
of NaCl type of EuS. The crystal sizes of the samples were
calculated by the Scherer equation from the X-ray diffraction
(XRD) spectra, and were found to be 18, 29 and 36 nm.
ray Diffractrometer Multiflex using monochrometer Cu K␣
radiation. The measurement conditions were 40 kW/40 mA,
scan speed 3 /min, sampling width 0.1 , in air at room tem-
perature. Transmission electron microscopy (TEM) images
were obtained with a Hitachi H-9000 TEM equipped with a
◦
◦
◦
tilting device (± 10 ) and operating at 300 kV (CS = 0.9 nm)
3
.2. Magnetic properties
UV–vis absorption spectra were recorded on a Perkin-
Elmer Lambda 19 spectrometer at room temperature. MCD
spectra were measured on a JASCO J-720W spectrolarimeter.
The Faraday effect spectra were measured on JASCO Model
K-250. Magnetic susceptibility data were obtained between
Temperaturedependenciesofthemagnetizationareshown
in Fig. 2a. On lowering the temperature, the magnetiza-
tion products became larger according to the Curie law
(
300–15 K), and the Curie points of the samples (16 K) were
2
and 300 K by using a SQUID susceptometer (MPMS-5S,
the same order as that of bulk EuS as shown in Fig. 2b.
However, we observed that the magnetic moments of 18 nm
EuS nanocrystals was smaller than those of corresponding
Quantum Design).
29 and 36 nm EuS. The correlation between magnetization
3
. Results and discussion
(M) and magnetic field (H) at 10 K in the EuS nanocrys-
tals is also shown in Fig. 2c. The saturation magnetization
of the EuS nanocrystals is increased with increasing the
size.
3
.1. Characterization of EuS nanocrystal
The TEM image of EuS is shown in Fig. 1a. In the TEM
images, we found square-shaped crystals of EuS.
We proposed a mechanism of the magnetization supported
by a paper [2] reporting that the magnetization of Eu chalco-

Y. Tsukahara et al. / Journal of Alloys and Compounds 408–412 (2006) 203–206
205
Fig. 4. (a) Correlation between the crystal-size and the peak top of the 4f–5d
transitions of MCD spectra with 18, 29 and 36 nm particle size in DMF at
room temperature. (b) Correlation between the particle size (ꢀ) and the
MCD peak top intensity (ꢀ).
eg) electron transition of Eu(II). In Fig. 3b, the peak
top wavelength of the absorption of the EuS nanocrys-
tals shifted to shorter wavelength with decreasing the
sizes.
Fig. 3. (a) Absorption spectra of EuS nanocrystals (18, 23, 29 and 36 nm
particle size) at room temperature. (b) Correlation between the crystal-size
and the peak top of the 4f–5d transitions.
The MCD spectra with various particle sizes (18, 29 and
6 nm) of EuS in DMF at room temperature are shown in
genides crystals with fcc structure depends on the distance
between the two Eu ions. The ratio of the nearest neigh-
bor Eu(II) to the next nearest one (i/j) should be related to
the average distance between the Eu cations. According to
the rocksalt (fcc) structure of EuS, each Eu(II) ion has 12
nearest neighbor ions (i: ferromagnetic interaction) and 6
next-nearest neighbor ions (j: antiferromagnetic interaction).
Both positive and negative exchange interaction are present.
A positive (ferromagnetic) interaction occurs between the
nearest neighbors and a negative (antiferromagnetic) interac-
tion does between the next-nearest neighbors. Smaller crys-
tals size leads to decrease of i/j, resulting in smaller magnetic
moment.
3
Fig. 4a. The shape of the MCD spectra was the typical type.
All of EuS exhibited the same MCD pattern structures, and
the positive and negative MCD peaks around 500–600 nm of
EuS were assigned to 4f–5d transitions.
The crystal-size dependences of the intensities and wave-
length at the top of the MCD spectra of the EuS nanocrystals
are shown in Fig. 4b. The MCD intensity was normalized by
the oscillator strength in order to characterize opto-magnetic
properties of EuS. In Fig. 4b, the peak top wavelength of the
MCD of the EuS nanocrystals shifted to shorter wavelength
with decreasing the sizes. In contrast, the intensities of the
MCD of the EuS increased with the decreasing the crystal
size (Fig. 4b).
3
.3. UV–vis absorption and MCD spectra analysis
Smaller particle size leads to increasing of the “terminal-
Eu(II)” on the crystal surface. Physical properties of the
“terminal-Eu(II)” on the crystal surface should be much dif-
ferentfromthatofthe“inner-Eu(II)”inthecrystals. Theopto-
The absorption spectra with various particle sizes (18,
9 and 36 nm) of EuS at room temperature are shown in
2
2+
Fig. 3a. The absorption spectra around 650 nm of the EuS
magnetic effect of the surface Eu ions might be stronger
nanocrysatals originates from 4f7( S7/2) → f ( FJ)5d(t2g,
8
4
7
than that of the inner Eu ions.
2+
6

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4
. Conclusions
Wehaveobservedthatthemagneticmomentsandthemag-
Acknowledgement
This research is supported by New Energy Industrial Tech-
nology Development Organization (NEDO).
netization of EuS nanocrystals decreased with decreasing the
crystal-size.
Dependence of the MCD intensity on the crystal-size
has been observed for the first time for EuS. Decreasing
the size of the EuS led to the enhancement of the MCD
intensity. These behaviors resulted from the physical prop-
erties of the “terminal-Eu(II)” on the crystal surface should
be much different from those of the “inner-Eu(II)” in the
crystals.
The size dependences of magnetization were different
from that of the MCD intensity. Further studies on the VT-
MCD and synthesis of the EuS less than 15 nm are in progress
in our laboratory.
References
[1] Y. Oka, J. Magn. Soc. Jpn. 17 (1993) 869.
[2] U. Enz, J.F. Fast, S. van Houten, J. Sumit, Philip Res. Rep. 17 (1962)
451.
[3] G. Bush, J. Appl. Phys. 33 (1967) 1386.
[4] S. Thonhgchan, Y. Hasegawa, Y. Wada, S. Yanagida, J. Phys. Chem.
B 107 (2003) 2193.
[
[
5] J.H. Greiner, G. Fan, J. Appl. Phys. Lett. 9 (1966) 27.
6] N. Miura, M. Kawanishi, H. Matsumoto, K. Hirao, J. Appl. Phys. 38
(1999) 1291.
[7] F. Stern, in: F. Seitz, D. Turnbull (Eds.), Solid State Physics, vol. 15,
The size-controlled EuS nanocrystals would be a promis-
ing material for opto-magnetic device in the future.
Academic Press, 1963, p. 299.
[8] Y. Tsukahara, H. Nakata, S. Kaizaki, Inorg. Chem. 43 (2004) 4385.