Effective optical faraday rotations of semiconductor EuS nanocrystals with paramagnetic transition-metal ions

Article abstract of DOI:10.1021/ja3106253

Novel EuS nanocrystals containing paramagnetic Mn(II), Co(II), or Fe(II) ions have been reported as advanced semiconductor materials with effective optical rotation under a magnetic field, Faraday rotation. EuS nanocrystals with transition-metal ions, EuS:M nanocrystals, were prepared by the reduction of the Eu(III) dithiocarbamate complex tetraphenylphosphonium tetrakis(diethyldithiocarbamate)europium(III) with transition-metal complexes at 300 C. The EuS:M nanocrystals thus prepared were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectroanalysis (ICP-AES), and a superconducting quantum interference device (SQUID) magnetometer. Enhanced Faraday rotations of the EuS:M nanocrystals were observed around 550 nm, and their enhanced spin polarization was estimated using electron paramagnetic resonance (EPR) measurements. In this report, the magneto-optical relationship between the Faraday rotation efficiency and spin polarization is discussed.

Full text of DOI:10.1021/ja3106253

Article
pubs.acs.org/JACS
Effective Optical Faraday Rotations of Semiconductor EuS
Nanocrystals with Paramagnetic Transition-Metal Ions
,
†
†
†
‡
‡
Yasuchika Hasegawa,* Masashi Maeda, Takayuki Nakanishi, Yoshihiro Doi, Yukio Hinatsu,
§
§
†
†
Koji Fujita, Katsuhisa Tanaka, Hitoshi Koizumi, and Koji Fushimi
†
‡
Faculty of Engineering and Faculty of Science, Hokkaido University, N13 W8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
§Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
S Supporting Information
*
ABSTRACT: Novel EuS nanocrystals containing paramagnetic Mn(II), Co(II), or
Fe(II) ions have been reported as advanced semiconductor materials with effective
optical rotation under a magnetic field, Faraday rotation. EuS nanocrystals with
transition-metal ions, EuS:M nanocrystals, were prepared by the reduction of the
Eu(III) dithiocarbamate complex tetraphenylphosphonium tetrakis-
(
diethyldithiocarbamate)europium(III) with transition-metal complexes at 300 °C.
The EuS:M nanocrystals thus prepared were characterized using X-ray diffraction
XRD), transmission electron microscopy (TEM), inductively coupled plasma atomic
(
emission spectroanalysis (ICP-AES), and a superconducting quantum interference
device (SQUID) magnetometer. Enhanced Faraday rotations of the EuS:M
nanocrystals were observed around 550 nm, and their enhanced spin polarization was estimated using electron paramagnetic
resonance (EPR) measurements. In this report, the magneto-optical relationship between the Faraday rotation efficiency and spin
polarization is discussed.
16−35
INTRODUCTION
ties have been explored.
In particular, EuS nanocrystals are
■
distinctly characterized by their ferromagnetic properties and
characteristic Faraday effect in the visible region. EuS
nanocrystals are expected to be useful in applications such as
advanced optical isolators and spintronic devices using green
and red laser light.
The optical Faraday effect causes rotation of the plane of
polarized light, which is linearly proportional to the component
of the magnetic field in the direction of propagation. This
characteristic Faraday effect is important for the construction o1f
optical isolators for fiber-optic telecommunication systems.
Various types of magneto-optical materials showing the Faraday
The magnitude of the Faraday rotation angle is directly
linked to the magnetic behavior and related to spin polarization
in the ground and excited states of magnetic materials. Effective
Faraday rotation in EuS nanocrystals requires that the
characteristic spin structure and spin configuration of EuS
nanocrystals be improved by the addition of extra metal ions.
Moruzzi and Gambino showed that the magnetic specific heat
of Gd ion doped EuS bulk crystal is shifted to a higher
2
−13
effect have been reported over the past few decades.
In
particular, the preparation of semiconductor nanocrystals with
magnetic dopants has been investigated for the development of
optical isolator and magneto-optical devices, and stochastic
control for a number of magnetic dopants in II−VI or III−V
semiconductor nanocrystals has been investigated. A semi-
conductor nanocrystal with magnetic dopants is a promising
material for future magneto-optical devices such as an optical
isolator.
One of the most important series of intrinsic semiconductors
with magnetic properties is the europium chalcogenide family,
EuX (X = O, S, Se, Te). The characteristic Faraday effect of
the semiconductor EuX comes from localized 4f electrons
distributed between the conduction band (5d orbitals of
3
6,37
temperature.
Stoll and Jin also studied enhancement of the
32,33
Curie temperature of nanosized Gd ion doped EuS.
On the
basis of these previous reports, the Gd ion can be regarded as
an effective dopant for enhancement of the magnetic properties
of EuS crystals. Remarkable magnetic polarona based on the
14
4
f−5d exchange interaction of EuO bulk crystals with Gd ions
38
have been also reported. The Gd ion is one of the key
materials for enhancement of magnetic properties in EuX
crystals.
2−
Eu(II)) and the valence band (p orbitals of chalcogenides, O ,
2
−
2−
2− 15
S , Se , or Te ). The 4f−5d electronic transition and spin
configuration of EuX crystals lead to a large Faraday effect,
which makes these materials promising candidates for an
Generally, the trivalent gadolinium cation, Gd(III) ion, is
used as the source for doping Gd ions in EuS crystals. The
standard reduction potential of the Gd(III)/Gd(II) equilibrium
16−23
effective optical isolator.
in EuX crystals is markedly enhanced in nanoscale structures
The characteristic Faraday effect
39
in water has been reported to be E° = −2.40 V (SCE). The
due to quantum confinement effects on the electronic excited
1
6−23
states.
Nanoscale EuX crystals, or EuX nanocrystals, have
Received: October 29, 2012
Published: January 23, 2013
been synthesized, and their resulting magneto-optical proper-
©
2013 American Chemical Society
2659
dx.doi.org/10.1021/ja3106253 | J. Am. Chem. Soc. 2013, 135, 2659−2666

Journal of the American Chemical Society
Article
reduction potential is located in the less noble position in
superconducting quantum interference device (SQUID)
magnetometer. Enhanced Faraday rotations in the EuS:M
nanocrystals were observed around 550 nm. We also
successfully estimated enhanced spin polarization using
electron paramagnetic resonance (EPR) measurements.
In this study, we suggest that a large Faraday rotation
efficiency is based on spin polarization related to the
characteristic coercive field, although the magnetic moment
and Curie point of the magnetic semiconductor have been
previous areas of focus. Novel EuS:M nanocrystals with
enhanced Faraday rotation efficiency are demonstrated for
the first time.
comparison to the reduction potential of Eu(III)/Eu(II) (E° =
3
9
−
0.43 V; SCE). The less noble reduction potential of
Gd(III)/Gd(II) indicates that the divalent Gd(II) ion is
unstable at room temperature in air. The chemical reaction of
the stable trivalent Gd(III) ion with S leads to the formation
2
−
of various gadolinium sulfides, such as orthorhombic Gd S ,
2
3
hexagonal Gd S , and tetragonal GdS composed of Gd(III)
3
4
1.8
ions. Park and co-workers also recently reported GdS
40
nanocrystals composed of trivalent Gd(III) ions. Gd(III) is
a type of lanthanide ion, with a coordination number known to
vary from 6 to 12 depending on the nature of the ligating
41−43
element, such as oxygen, sulfur, and nitrogen.
On the basis
of electrochemical analysis, synthesis experiments, and
coordination structures, trivalent Gd(III) ions with a polyhedral
coordination structure may be contained in EuS cubic
nanocrystals. The trivalent Gd(III) ions in the vacancy of the
divalent Eu(II) ion provide unstable charge and structural
strains in the EuS cubic lattice. These strains in semiconductor
nanocrystals directly promote the decrease of the magnetic
moment and coercive field. Instead of the trivalent Gd(III) ion,
a stable divalent metal ion with a six-coordinated structure and
paramagnetic properties is expected to be a promising dopant
for the enhancement of magnetic properties and for obtaining
the Faraday effect in EuS nanocrystals.
EXPERIMENTAL SECTION
■
Materials. Europium(III) chloride hexahydrate (EuCl ·6H O),
manganese chloride tetrahydrate (MnCl ·4H O), iron chloride
3
2
2
2
tetrahydrate (FeCl ·4H O), and cobalt chloride tetrahydrate
2
2
(
CoCl ·4H O) were purchased from Kanto Chemical Co., Inc.
2 2
Sodium N,N-diethyldithiocarbamate trihydrate (Na(S CNEt )·3H O)
2
2
2
and toluene were purchased from Kanto Chemical Co., Inc.
Tetraphenylphosphonium bromide (BrPPh ) was purchased from
4
Wako Pure Chemical Industries, Ltd. Oleylamine was obtained from
Tokyo Chemical Industry Co., Ltd. All other chemicals and solvents
were reagent grade and were used without further purification.
1
Apparatus. H NMR data were measured using a JEOL JNM-
1
EX270 (270 MHz) instrument. H NMR chemical shifts were
Here, we focus on a stable divalent transition-metal ion with
paramagnetic properties as an ideal dopant for effective Faraday
rotation in EuS nanocrystals. Among the transition-metal ions,
paramagnetic Mn(II), Co(II), and Fe(II) ions are well-known
determined using tetramethylsilane (TMS) as an internal standard.
Infrared spectra were recorded on a JASCO FT/IR-350 spectrometer.
Elemental analyses were performed with a Yanaco MT-6 CHN corder.
XRD spectra were characterized by a Rigaku RINT 2200 X-ray
diffractometer. High-resolution images of the EuS nanocrystals were
obtained with a JEOL 2010 FASTEM (200 kV) TEM. Quantitative
elemental analysis was performed with ICP-AES. Magnetic measure-
ments were carried out using a Quantum Design MPMS SQUID
system. Electron paramagnetic resonance spectra of EuS:M nano-
crystals were measured using a JEOL JES-TE200 ESR spectrometer.
Synthesis of Tetraphenylphosphonium Tetrakis-
44
as magnetic dopant ions in semiconductors. These transition-
metal ions show stable six-coordinated structures in the solid
crystal lattice. The paramagnetic Mn(II), Co(II), and Fe(II)
ions could be the most suitable as dopants for improving the
Faraday effect in EuS nanocrystals. In this study, we report
novel EuS nanocrystals with paramagnetic transition-metal ions
with divalent cations, EuS:M nanocrystals. The EuS:M
nanocrystals were prepared by the reduction of the Eu(III)
dithiocarbamate complex tetraphenylphosphonium tetrakis-
(diethyldithiocarbamate)europium(III), (PPh
solution of sodium dithicarbamate trihydrate (Na(S CNEt )·3H O; 14
g) in 30 mL of methanol was added to europium chloride hexahydrate
)[Eu(S CNEt ) ]. A
2 2 4
4
2
2
2
(
EuCl ·6H O; 5.6 g) dissolved in 30 mL of methanol with stirring and
3 2
(
(
diethyldithiocarbamate)europium(III) ((PPh )[Eu-
4
reacted for 3 h. After the reaction mixture was filtered, a solution of
S CNEt ) ]) with Mn(II), Co(II), and Fe(II) dithiocarbamate
2
2 4
BrPPh (6.4 g) in 30 mL of methanol was added to the filtered
4
complexes ([Mn(S CNEt ) ], [Co(S CNEt ) ], and [Fe-
2
2
2
2
2 2
solution and stirred for 9 h. The resulting precipitate was separated by
filtration and washed two times with ethanol. Orange powder. Yield:
30%. FT-IR spectrum (KBr): 2960 (C−H), 1485−1482 (C−N), 1442
(
S CNEt ) ]), as shown in Figure 1. The prepared EuS
2 2 2
nanocrystals with paramagnetic Mn(II), Co(II), and Fe(II)
ions, EuS:M, were characterized by X-ray diffraction (XRD),
transmission electron microscopy (TEM), inductively coupled
plasma atomic emission spectroanalysis (ICP-AES), and a
−1 1
(P-phenyl), 1113 (C−S) cm . H NMR spectrum (CD Cl): δ 7.91
3
(4H, t, J = 6.3 Hz, [Eu(S
CNEt
) ] (P(C H ) (p)), 7.70 (16H, m,
2 4 6 5 4
2
[
[
(
4
Eu(S CNEt ) ] (P(C H ) (o, m)), 3.17 (8H, q, J = 7 Hz,
2
2
4
6
5 4
Eu(S CNCH CH ) ] (PPh )). 1.61 (12H, q, J = 7 Hz, [Eu-
2
2
3
4
4
S CNCH CH ) ] (PPh )). Anal. Calcd for C H EuN PS : C,
2
2
3
4
4
44 60
4
8
8.79; H, 5.55; N, 5.06. Found: C, 48.53, H, 5.93, N, 5.15.
Synthesis of Bis(diethyldithiocarbamate)manganese, [Mn-
S CNEt ) ]. A solution of sodium dithicarbamate trihydrate (Na-
(
2
2 3
(
S CNEt )·3H O; 2.5 g) in 10 mL of ethanol was added to manganese
2 2 2
chloride tetrahydrate (MnCl ·4H O; 0.95 g) dissolved in 10 mL of
2
2
ethanol with stirring and reacted for 1 h. The resulting precipitate was
separated by filtration and washed two times with ethanol. Deep-
brown powder. Yield: 79%. FT-IR spectrum (KBr): 2974 (C−H),
−
1 1
1
485 (C−N), 995 (C−S) cm . H NMR spectrum (CD Cl): δ 4.02
3
(
(
6
8H, br, [Mn(S CNCH CH ) ]). 1.51 and 1.31 (12H, br, [Mn-
2
2
3 2
S CNCH CH ) ]). Anal. Calcd for C H MnN S : C, 36.05; H,
.12; N, 8.41. Found: C, 36.63, H, 6.12, N, 8.44.
Synthesis of Bis(diethyldithiocarbamate)iron, [Fe(S CNEt ) ].
2
2
3
2
10 20
2 4
2
2 3
The Fe(II) dithiocarbamate complex was obtained using the same
method as for the Mn(II) dithiocarbamate complex. Black powder.
Yield: 75%. FT-IR spectrum (KBr): 2976 (C−H), 1491 (C−N), 990
−
1
1
Figure 1. Preparation schemes of EuS and EuS:M nanocrystals.
(C−S) cm . H NMR spectrum (CD Cl): δ 4.04 (8H, br,
3
2
660
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Journal of the American Chemical Society
Article
[
(
Fe(S CNCH CH ) ]). 1.48 and 1.38 (12H, br, [Fe-
The 2θ diffraction peaks of the XRD measurements were very
similar to those of the corresponding (111), (200), (220),
2
2
3 2
S CNCH CH ) ]). Anal. Calcd for C H FeN S : C, 35.99; H,
2
2
3
2
10 20
2
4
6
.04; N, 8.39. Found: C, 35.80, H, 5.95, N, 8.39.
Synthesis of Bis(diethyldithiocarbamate)cobalt, [Co-
S CNEt ) ]. The Co(II) dithiocarbamate complex was obtained
(
222), (400), (420), and (422) signals of NaCl type EuS. The
slight shifts in the diffraction peaks are due to lattice mismatch
of Eu−S by doping transition-metal ions as an impurity. We
estimated the lattice constants of the obtained EuS nanocrystals
with transition-metal ions using the diffraction peaks of the
corresponding diffraction (200) peak in the EuS crystal lattice.
The percentage of lattice mismatch is calculated by
(
2
2 2
using the same method as for the Mn(II) dithiocarbamate complex.
Green powder. Yield: 75%. FT-IR spectrum (KBr): 2977 (C−H),
1
and 3.60 (8H, br, [Co(S CNCH CH ) ]). 1.55 and 1.25 (12H, br,
[
−1 1
492 (C−N), 1002 (C−S) cm . H NMR spectrum (CD Cl): δ 3.81
3
2
2
3 2
Fe(S CNCH CH ) ]). Anal. Calcd for C H CoN S : C, 33.79; H,
2 2 3 2 10 20 2 4
5
.67; N, 7.88. Found: C, 33.63, H, 5.67, N, 7.79.
Synthesis of EuS Nanocrystals with Transition-Metal Ions.
Under an N2 atmosphere, (PPh )[Eu(S CNEt ) ] (0.54 g) and
matrix
− dmatrix with dopant)/dmatrix
lattice mismatch = 100 × (d
(1)
4
2
2 4
[
Mn(S CNEt ) ] (0.024 g) were dissolved into oleylamine (4.86 g),
where dmatrix and dmatrix with dopant are lattice constants of the
matrix and the matrix with a dopant, respectively. The lattice
constants and mismatch percentages of EuS nanocrystals with
manganese, cobalt, and iron ions, EuS:Mn, EuS:Fe, and EuS:Co
nanocrystals, are summarized in Table 1. The lattice mismatch
2
2 3
and the mixture was heated at 140 °C and stirred for 10 min. After the
reaction solvent was heated to 300 °C and stirred for 6 h, the obtained
purple liquid was centrifuged at 5000 rpm for 10 min. Prepared
EuS:Mn nanocrystals were added to 8 mL of toluene and centrifuged
at 13000 rpm for 15 min, and a clear purple liquid was obtained by
elimination of the deposit. EuS:Fe and EuS:Co nanocrystals were
prepared using the same method as for the EuS:Mn nanocrystals.
Preparation of Polymer Thin Films Containing EuS Nano-
particles and Faraday Rotation Measurements. The obtained
EuS nanocrystals with transition-metal ions (2 mg) were added to a
chloroform solution (8 mL) of poly(methyl methacrylate) (PMMA;
45
Table 1. Lattice Constants, Lattice Mismatch, Crystal Sizes,
and Dopant Percentages of EuS, EuS:Mn, EuS:Fe, and
EuS:Co Nanocrystals
cryst size
from
dopant
ions from
ICP-AES
(%)
2
.0 g) and dispersed well under ultrasonic treatment, giving a colloidal
lattice
lattice
crystal grain
a
constant mismatch
size from
TEM
suspension. PMMA thin films were prepared on a glass substrate from
the colloidal suspension via a casting method for the Faraday rotation
measurements.
The Faraday effect measurements were performed using a
measurement system for Faraday and Kerr effects (JASCO, Model
K-250). A Xe lamp was used as the light source. The external magnetic
field was 15000 Oe.
b
sample
(Å)
(%)
XRD (nm)
(nm)
EuS
0.593
0.579
0.588
0.586
0.00
2.62
0.843
1.18
19.9
19.6
20.3
27.2
26.6
27.4
26.2
0.0
30
EuS:Mn
EuS:Fe
EuS:Co
4.7
17
19.4
a
b
The lattice mismatch is estimated by eq 1. The crystal grain size is
given by calculations using Scherrer’s equation.
RESULTS AND DISCUSSION
■
of EuS:Mn nanocrystals is larger than those of EuS:Co and
EuS:Fe. Generally, lattice mismatch of a semiconductor with
dopants is related to the amount of dopant ions. The lattice
mismatch analysis indicates that the amount of Mn(II) ions in
the EuS:Mn crystal grain is greater than those of Fe(II) in
EuS:Fe and Co(II) in the EuS:Co crystal grain. The crystal
grain size of EuS, EuS:Mn, EuS:Co, and EuS:Fe calculated
using the Scherrer equation from the corresponding (200)
diffraction peak was found to be 19.9, 19.6, 20.3, and 19.4 nm,
respectively (Table 1).
Figure 3 shows TEM images of EuS, EuS:Mn, EuS:Co, and
EuS:Fe nanocrystals. Rugged nanocrystals of EuS:Mn, EuS:Co,
and EuS:Fe were observed, although the EuS nanocrystals
showed a smooth surface with clear facets. The average sizes of
the EuS, EuS:Mn, EuS:Co, and EuS:Fe nanocrystals were
estimated from the TEM images to be 27.2, 26.2, 27.4, and 26.6
nm, respectively (see Table 1). The average sizes are larger than
crystal grain sizes calculated from Scherrer’s equation. These
size estimations indicate that the EuS nanocrystals with
transition-metal ions are covered with an amorphous inorganic
layer with a ragged surface.
Preparation of EuS Nanocrystals with Transition-
Metal Ions. The EuS nanocrystals composed of Eu(II) ions
were prepared by thermal reduction of the single-source
precursor (PPh )[Eu(S CNEt ) ] at controlled reaction times
4
2
2 4
2
2,23
and temperatures.
sulfide nanocrystals, M S, were obtained by the decomposition
We also confirmed that transition-metal
II
of the divalent transition-metal precursor, [M(S CNEt ) ], at
2
2 2
300 °C for 6 h (see the Supporting Information: preparation,
Figures S1 and S2). The transition-metal precursors are
promising chemical sources for the preparation of EuS
nanocrystals with transition-metal ions. We attempted to
synthesize these using an oleylamine solution containing
(
PPh )[Eu(S CNEt ) ] and [M(S CNEt ) ] at 300 °C for 6
4 2 2 4 2 2 2
h. XRD spectra of the prepared powders are shown in Figure 2.
To determine the amount of transition-metal ions in the EuS
nanocrystals, ICP-AES of EuS:Mn, EuS:Co, and EuS:Fe was
carried out in nitric acid, for dissolution of the nanocrystals.
The percentages of transition-metal ions in the EuS obtained
using ICP-AES are summarized in Table 1. The EuS:Mn
nanocrystals contain a large amount of elemental Mn (30%),
larger than the fraction in the EuS:Co (17%) and EuS:Fe
(
4.7%) nanocrystals. In the EuS nanocrystals with transition-
Figure 2. XRD profiles of (a) EuS, (b) EuS:Fe, (c) EuS:Co, and (d)
EuS:M nanocrystals.
metal ions, the Eu(II) ion is partially replaced by divalent
transition-metal ions, M(II), resulting in the formation of M −
II
2
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Journal of the American Chemical Society
Article
Figure 3. TEM images and size distribution histograms (inset) of (a) EuS, (b) EuS:Mn, (c) EuS:Co, and (d) EuS:Fe nanocrystals.
II
S bonds. The crystal lattice composed of Mn −S bonds yields
the single manganese sulfide MnS with an NaCl type cubic
II
structure. In contrast, the crystal lattice containing Co −S
bonds yields various types of cobalt sulfides with cubic and
hexagonal structures, such as Co S , CoS , Co S, Co S ,
3
4
1.035
1‑x
4 3
and β-CoS1 crystals. The crystal lattice constructed by the
.097
II
Fe −S bonds also exhibits three types of iron sulfides, FeS, with
hexagonal, monoclinic, and tetragonal structures. We consider
that the large amount of Mn(II) ions in the EuS:Mn
nanocrystals might be due to the stable formation of single
II
and uniform Mn −S bonds in the cubic EuS lattice, formed
homogeneously.
Magnetic Properties. The magnetic properties of EuS,
EuS:Mn, EuS:Co, and EuS:Fe nanocrystals were determined
from SQUID measurements (Figure 4 and Table 2). The EuS
nanocrystals turned into the ferromagnetic phase at 17 K,
consistent with the Curie point of the bulk EuS single crystal.
The magnetic moment per Eu(II) ion at 5 K was 2.9 μ , smaller
B
8
than the expected value of 7.0 μ for the S state of Eu(II) at
B
7/2
0
K. In the range 10−100 nm, ferromagnetic nanocrystals are
Figure 4. Magnetic field dependences of EuS (a and b, black dotted
lines) and EuS:Mn nanocrystals (b, red bold lines).
usually composed of a single domain structure. The scale of 20
nm is probably effective in creating a single domain structure in
the EuS nanocrystals. However, the complicated morphology of
the EuS nanocrystals was considered to affect the magnetic
moment, because the disordered arrangement of tiny crystals
on the surface of the nanocrystals possibly decreases the
exchange interaction between Eu(II) ions.
The Curie points and magnetic moments per Eu(II) ion at 5
K of the EuS:M nanocrystals are smaller than those of the EuS
nanocrystals. The smaller Curie points and magnetic moments
are due to the complicated morphology on the surface of the
nanocrystals. The presence of disordered tiny crystals and
2
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Journal of the American Chemical Society
Article
Table 2. Magnetic Properties of EuS, EuS:Mn, EuS:Fe, and
EuS:Co Nanocrystals
a
Curie temp
(K)
magnetic moment per Eu(II)
coercive field
a
sample
ion (μB)
(Oe)
EuS
17.5
14.0
15.0
15.0
2.9
25
70
40
30
EuS:Mn
EuS:Fe
EuS:Co
1.3
0.86
0.80
a
The magnetic moment per Eu(II) ion and coercive field are
estimated under 5 K.
oxidized Eu(III) ions on the EuS:M surface might contribute to
a decrease in their magnetic moments. The Curie points may
also be affected by a lattice mismatch of EuS nanocrystals with
transition-metal ions. These results indicate that impurities,
such as a transition-metal ion, led to a decrease in the intrinsic
magnetic moment of the EuS crystal lattice.
Figure 5. (a) Absorption and (b) Faraday rotation spectra of polymer
thin film EuS:Mn (red line), EuS:Co (green line), and EuS:Fe
nanocrystals (black line). The concentration of nanocrystals in
polymer films was 1 wt %. The thicknesses of thin films using a
laser scanning microscope of PMMA containing EuS, EuS:Mn,
EuS:Fe, and EuS:Co nanocrystals are found to be 87, 84, 84, 84,
and 84 μm, respectively.
In contrast, we observed that the coercive field of the EuS
nanocrystals is enhanced by the addition of transition-metal
ions in the EuS crystal lattice. In particular, the coercive field of
the EuS:Mn nanocrystals is approximately 3 times larger than
that of the EuS nanocrystals. In the case of semiconductor
nanocrystals with transition-metal ions, the coercive field is
strongly dependent on the ferromagnetic dipole interaction
between metal ions. Chaudret et al. have reported that highly
ordered assembling of cube-shaped magnetic nanocrystals
indicates a strong ferromagnetic dipole interaction between
Table 3. Faraday Rotation Wavelengths and Verdet
Constants of PMMA Thin Films Containing EuS, EuS:Mn,
EuS:Fe, and EuS:Co Nanocrystals
absorption
4
6
the nanocrystals. We consider that the larger coercive field of
the EuS nanocrystals with transition-metal ions might be due to
a strong magnetic dipole interaction between Eu(II) and the
transition-metal ions. A strong magnetic dipole interaction is
directly linked to enhancement of spin polarization in the
ground and excited states of the EuS nanocrystals. EuS
nanocrystals with large coercive field, EuS:Mn and EuS:Fe,
are expected to exhibit large Faraday rotations under a magnetic
field.
wavelength of positive peak MFM constant at line width
4
f−5d
wavelength of
Faraday
positive peak
of EPR
spectra,
a
transition band
(nm)
wavelength
−6
sample
rotation (nm) (×10 deg Oe) ΔH (mT)
EuS
504
501
485
480
599
584
572
568
3.9
6.1
4.2
0.9
96.9
75.8
86.3
89.9
EuS:Mn
EuS:Fe
EuS:Co
a
MFM constant: absorption-normalized Veldet constant. MFM
constant is calculated by eq 3. Thickness of thin films using laser
scanning microscope of PMMA containing EuS, EuS:Mn, EuS:Fe,
EuS:Co nanocrystals are found to be 87, 84, 84, 84, and 84 μm,
respectively.
Magneto-optical Properties. Absorption spectra of
EuS:M nanocrystals in toluene are shown in Figure 5a. The
characteristic absorption band of EuS is observed at around 550
4
8
4
7
nm and can be assigned to the 4f−5d ( f ( S )− f ( F )5d-
7
7/2
6
J
(
t ,e )) transition bands. The energy gap band of the EuS
2g g
19
compounds, respectively. These transition bands also indicate
the formation of EuS nanocrystals. According to the absorption
spectrum of EuS:M nanocrystals, we observed that the
wavelengths of the 4f−5d transition bands are slightly blue-
shifted, in comparison with the EuS nanocrystals (Figure 5a).
The absorption wavelength of the 4f−5d transition bands is
summarized in Table 3. The crystal grain sizes of EuS:Mn,
EuS:Fe, and EuS:Co nanocrystals using Scherrer’s equation are
very similar to those of the prepared EuS nanocrystals (Table
EuS nanocrystals, which is related to the absorption spectra
(
Table 3). The Verdet constant, which indicates the strength of
the Faraday rotation, is calculated using eq 2:
V = θ/Hl
(2)
where θ, H, and l are the Faraday rotation angle (degrees), the
external magnetic field, and the thickness of the thin film (cm),
respectively. The Faraday rotation angle is dependent on the
concentration of the Eu(II) ion with magnetic spins. The
amount of characteristic Eu(II) ions in the EuS nanocrystals
with transition-metal ions is related to the absorption
coefficient of the 4f−5d electronic transition. In order to
estimate the Faraday rotation efficiency of the nanocrystals, we
consider that the Verdet constant should be normalized by the
photon-absorption ability of the nanocrystals, which contains
information on the absorption coefficient ε, concentration c,
and optical pass length l. The magneto-optical figure of merit
1
). The characteristic blue shift of the 4f−5d transition bands of
EuS:M nanocrystals might be due to the effect of Zeeman
splitting in the excited state of EuS with paramagnetic
transition-metal ions.
The wavelength dependence of the Faraday rotation angle
was measured for PMMA (poly(methyl methacrylate)) thin
films containing EuS, EuS:Mn, EuS:Fe, or EuS:Co (Figure 5b
and Table 3). The Faraday spectrum at room temperature with
1
5000 Oe had clear positive and negative peaks, which could
have contributed to the 4f−5d transitions of the EuS
nanocrystals. The wavelength of the positive peaks for the
EuS:M nanocrystals is blue-shifted in comparison to that of the
(
MFM) can be rewritten as
MFM = θ/(H × A)
(3)
2
663
dx.doi.org/10.1021/ja3106253 | J. Am. Chem. Soc. 2013, 135, 2659−2666

Journal of the American Chemical Society
Article
where A is the absorbance of the PMMA film containing EuS
and EuS:M(II) nanocrystals. The refractive index of PMMA is
in Figure 6. The line widths ΔH of the EPR spectra are
summarized in Table 3. The EPR spectra provide the typical
1
2
.48, which is much smaller than that of EuS materials (n =
.43). Circular birefringence of PMMA as a matrix is also
47
much smaller than Faraday rotation of EuS nanocrystals (see
the Supporting Information, Figure S3). The double refraction
in the PMMA thin films does not affect the Faraday rotations of
EuS nanocrystals in the measurement of circular birefringence.
The calculated MFM constants are summarized in Table 3. The
MFM constants of EuS:Mn and EuS:Fe are larger than those of
the EuS nanocrystals. The enhanced MFM constants of
EuS:Mn and EuS:Fe nanocrystals in the PMMA thin films
may be caused by the their strong ferromagnetic interactions
related to their large coercive field. In particular, the MFM
constant of the EuS:Mn nanocrystals with a large coercive field
is the largest in the EuS nanocrystals with transition-metal ions.
In contrast, we also observed a decrease in the Faraday rotation
angle of the EuS:Co nanocrystals. Amplification of the coercive
field of the EuS:Co nanocrystals is smaller than that of the
EuS:Fe nanocrystals, although the percentage of Co(II) ions
from ICP-AES is much larger than that of Fe(II) ions in the
EuS nanocrystals. Here, we propose that the Faraday rotation
of the EuS:M nanocrystals can be directly linked to the coercive
field, which leads to enhancement of ferromagnetic exchange
interaction and spin polarization in the ground and excited
states of the EuS nanocrystals.
Figure 6. Normal and differential EPR spectra of EuS (a, black line),
EuS:Co (b, green line), EuS:Fe (c, orange), and EuS:Mn nanocrystals
(d, red line).
spin configuration of Eu(II) ions in the EuS crystal. ΔH
calculated using the EPR spectrum of the EuS:Mn nanocrystals
is found to be 75.8 mT, which is the smallest value obtained in
our experiments. The ferromagnetic exchange interaction in the
EuS:Mn nanocrystals is much stronger than that in the EuS
nanocrystals. For the first time, we successfully observed the
intrinsic driving force, the ferromagnetic exchange interaction,
for understanding the Faraday effect. On the other hand, the
EPR measurement of EuS:Co gives a large ΔH value of 89.9
mT.
According to the spin polarization on the magneto-optical
effect, Wachter and co-workers previously suggested their skew
48
scattering model. The theoretical model is based on the
asymmetric spin−orbit interaction, which is estimated using
conductivity considering an electric field and assuming a
relaxation lifetime approximation. The relaxation lifetime for
spin−orbit asymmetric scattering is one of the key factors in the
Generally, the magnetic dipole interaction H is directly
d
linked to the distance between magnetic ions. The distance
between Eu(II) ions in EuS:Mn using XRD analysis is longer
than in EuS:Co. However, the line width of the EPR signal of
EuS:Mn with a smaller magnetic dipole interaction, ΔH of
EuS:Mn, is narrow. The small magnetic dipole interaction by
parameters of magneto-optical effect, off-diagonal elements in
2
51
the optical conductivity tensor σ . We consider that the
doping of Mn(II) ions is supported by a previous study. We
xy
asymmetric spin−orbit interaction in the past theoretical model
might be linked to our experimental data and related to the spin
polarization in EuS:M nanocrystals. On the other hand, Erskine
and Stern have focused on the minority spin electron in
magnetic materials. Minority spin transitions of the magnetic
materials produce the first negative peak and second positive
peak in magneto-optical rotation spectra. At the present time,
measurement and calculation of the spin polarization or
minority spin electron in optical transition is not clear in our
present EuS:M nanocrystals. However, asymmetric spin−orbit
interaction and spin polarization of EuS:M in the optical
think that the magnetic dipole interaction H is not subject to
d
the influence of ferromagnetic interaction, directly. A magnetic
material with a narrower ΔH value provides a larger
4
5
ferromagnetic exchange interaction. The Mn(II) ion
promotes effective enhancement of the ferromagnetic exchange
interaction in the EuS nanocrystals. The ferromagnetic
exchange interaction in the EuS nanocrystals is dependent on
the species of transition-metal ion used as the dopant. The
small Faraday rotation angle of the EuS:Co nanocrystals might
be not only due to small enhancement of the ferromagnetic
exchange interaction but also due to the decrease of the
intrinsic magnetic moment of the EuS crystal lattice as an
impurity. From these results, we conclude that the magneto-
optical effect, the Faraday rotation, is affected by the
ferromagnetic exchange interaction related to the coercive
field of magnetic materials.
49
transition might promote enhancement of the optical
2
conductivity tensor σ and Faraday effects.
xy
Estimation of Ferromagnetic Exchange Interaction
using EPR Spectra. The ferromagnetic exchange interaction is
a key factor for understanding the optical Faraday rotation of
EuS:M nanocrystals. The ferromagnetic exchange interaction,
H , is given by
SUMMARY AND CONCLUSIONS
e
■
2
EuS nanocrystals with paramagnetic transition-metal ions,
EuS:M, were successfully prepared by the reaction of a Eu(III)
dithiocarbamate complex with Mn(II), Fe(II), and Co(II)
complexes. Their Faraday rotation angles depended on the
species of transition-metal ion used as a dopant. In particular, a
PMMA thin film containing EuS:Mn nanocrystals showed
effective Faraday rotation under a magnetic field. In this study,
we also suggested that the large Faraday rotation efficiency is
based on the ferromagnetic exchange interaction related to the
characteristic coercive field of EuS:Mn nanocrystals. The
ΔH1/2 = H /H
d
e
(4)
where H and ΔH are terms of magnetic dipole interaction
d
1/2
and fwhm (full width at half-maximum) of the EPR signal,
respectively. He can be estimated from the ΔH value
1/2
usingthe line width ΔH of observed EPR spectra (differential
5
0
form).
Thus, in order to estimate the ferromagnetic exchange
interaction of the EuS nanocrystals with transition-metal ions,
we carried out EPR measurements. The EPR spectra are shown
2
664
dx.doi.org/10.1021/ja3106253 | J. Am. Chem. Soc. 2013, 135, 2659−2666

Journal of the American Chemical Society
Article
intrinsic effect of the ferromagnetic exchange interaction on
magneto-optical properties is a new aspect for the development
of magnetic semiconductors, although the magnetic moment
and Curie point of a magnetic semiconductor have been
previously examined.
This study is significant in that it is directly related to
photophysical chemistry and future photofunctional materials
science research. EuS:M with enhanced magnetic and magneto-
optical properties is expected to be useful in applications such
as novel optical isolators and spintronic devices.
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ASSOCIATED CONTENT
Supporting Information
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Commun. 2005, 6038−6040.
■
*
S
(20) Hasegawa, Y.; Okada, Y.; Kataoka, T.; Sakata, T.; Mori, H.;
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(
AUTHOR INFORMATION
Corresponding Author
hasegaway@eng.hokudai.ac.jp.
■
(
1
2
(
7, 3114−3121. (b) Zhao, F.; Sun, H.; Gao, S.; Su, G. J. Mater. Chem.
005, 15, 4209−4214.
Notes
25) Mirkovic, T.; Hines, M. A.; Sreekumari, Nair P.; Scholes, G. D.
The authors declare no competing financial interest.
Chem. Mater. 2005, 17, 3451−3456.
(
26) Regulacio, M. D.; Bussmann, K.; Lewis, B.; Stoll, S. L. J. Am.
ACKNOWLEDGMENTS
Chem. Soc. 2006, 128, 11173−11179.
■
(27) Zhao, F.; Sun, H.-L.; Su, G.; Gao, S. Small 2006, 2, 244−248.
28) Huxter, V. M.; Mikovic, T.; Nair, P. S.; Scholes, G. D. Adv.
We are grateful to Mr. Akira Kawashima, Graduate School of
Engineering, Hokkaido University, for the Faraday rotation
measurements. This work was partly supported by Grants-in-
Aid for Scientific Research on Innovative Areas of “Emergent
chemistry of nano-scale molecular systems” and “New
Polymeric Materials Based on Element-Blocks (No. 2401)”
(
Mater. 2008, 20, 2439−2443.
(29) Regulacio, M. D.; Kar, S.; Zuniga, E.; Wang, G.; Dollahon, N. R.;
Yee, G. T.; Stoll, S. L. Chem. Mater. 2008, 20, 3368−3376.
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Timmons, A.; Trindade, T. J. Mater. Chem. 2008, 18, 4572−4578.
(31) Selinsky, R. S.; Keavney, D. J.; Bierman, M. J.; Jin, S. Appl. Phys.
Lett. 2009, 95, 202501.
(
24102012) of the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of Japan.
(
32) Kar, S.; Boncher, W. L.; Olszewski, D.; Dollahon, N.; Asdh, R.;
Stoll, S. L. J. Am. Chem. Soc. 2010, 132, 1360−13962.
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