Giant Faraday Rotation through Ultrasmall Fe0n Clusters in Superparamagnetic FeO-SiO2 Vitreous Films

Abstract

Magnetooptical (MO) glasses and, in particular, Faraday rotators are becoming key components in lasers and optical information processing, light switching, coding, filtering, and sensing. The common design of such Faraday rotator materials follows a simple path: high Faraday rotation is achieved by maximizing the concentration of paramagnetic ion species in a given matrix material. However, this approach has reached its limits in terms of MO performance; hence, glass-based materials can presently not be used efficiently in thin film MO applications. Here, a novel strategy which overcomes this limitation is demonstrated. Using vitreous films of xFeO·(100 − x)SiO2, unusually large Faraday rotation has been obtained, beating the performance of any other glassy material by up to two orders of magnitude. It is shown that this is due to the incorporation of small, ferromagnetic clusters of atomic iron which are generated in line during laser deposition and rapid condensation of the thin film, generating superparamagnetism. The size of these clusters underbids the present record of metallic Fe incorporation and experimental verification in glass matrices.

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Giant Faraday Rotation through Ultrasmall Fe0n Clusters in
Superparamagnetic FeO-SiO2 Vitreous Films
Yuko Nakatsuka, Kilian Pollok, Torsten Wieduwilt, Falko Langenhorst,
Markus A. Schmidt, Koji Fujita, Shunsuke Murai, Katsuhisa Tanaka,*
and Lothar Wondraczek*
Y. Nakatsuka, Prof. K. Fujita, Prof. S. Murai,
Prof. K. Tanaka
Department of Material Chemistry
Graduate School of Engineering
Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
E-mail: tanaka@dipole7.kuic.kyoto-u.ac.jp
Y. Nakatsuka, Prof. M. A. Schmidt, Prof. L. Wondraczek
Otto Schott Institute of Materials Research
University of Jena
Fraunhoferstr. 6, 07743 Jena, Germany
E-mail: lothar.wondraczek@uni-jena.de
Dr. K. Pollok, Prof. F. Langenhorst
Institute of Geosciences
University of Jena
Carl-Zeiss-Promenade 10, 07745 Jena, Germany
T. Wieduwilt, Prof. M. A. Schmidt
Leibnitz Institute of Photonic Technology
Albert-Einstein-Str. 9, 07745 Jena, Germany
DOI: 10.1002/advs.201600299
technology.[1] Here, the Faraday effect is
one of the most widely employed magne-
tooptical effects (schematically shown in
Figure 1a): when linearly polarized light
passes through a transparent longitudi-
nally magnetized material, the plane of
polarization rotates as a function of the
amplitude of the externally applied mag-
netic field. Since the Faraday effect is an
optically nonreciprocal phenomenon, it is
used to construct optical isolators, sensors,
diodes, and switches. In many of these
applications, Y3Fe5O12 (YIG),[2] Bi3Fe5O12
(BIG),[3] and Tb3Ga5O12[4] crystals are the
present benchmark materials because of
their large Faraday effect, especially in the
infrared spectral region. Together with
high optical transparency, this provides
efficient Faraday rotation at low optical
loss.
However, due to high optical loss,[2,5]
neither YIG nor BIG single-crystals are suitable for applications
in wavelength regimes other than the infrared, in particular, in
the visible (Vis) or even UV spectral ranges. In addition, their
use is restricted to a relatively small number of substrate mate-
rials,[6,7] and also possibilities for implementation with fiber
optical devices are rather limited.[8] These issues present very
strict limitations which can only be overcome with alterna-
tive material solutions. For one, besides telecommunication,
spectral ranges beyond the infrared play a very important role,
e.g., in magnetooptical coding or for oscillation stabilization in
blue lasers,[9] Furthermore, combination of a MO device with
oxide substrates such as vitreous silica or others, e.g., through
fiber splicing or direct deposition, is key to almost any specific
application in the above areas. Crystallinity of the MO mate-
rial is a major drawback in this context. Other materials have
therefore been proposed and applied, such as rare-earth (Eu2+,
Tb3+) containing glasses, where the paramagnetic rare-earth
ion is supposed to ensure high magnetooptical activity.[10–13]
The development of such glasses has been following a simple
strategy, aiming for incorporation of an as high as possible frac-
tion of the paramagnetic ion species without triggering destruc-
tive effects such as clustering, phase separation, or material
crystallization.[14,15] The joint basis of these approaches is that
glasses provide extremely high optical homogeneity, very high
surface quality and, in most cases,[16] structural isotropy which
Magnetooptical (MO) glasses and, in particular, Faraday rotators are becoming
key components in lasers and optical information processing, light switching,
coding, filtering, and sensing. The common design of such Faraday rotator mate-
rials follows a simple path: high Faraday rotation is achieved by maximizing the
concentration of paramagnetic ion species in a given matrix material. However,
this approach has reached its limits in terms of MO performance; hence, glass-
based materials can presently not be used efficiently in thin film MO applica-
tions. Here, a novel strategy which overcomes this limitation is demonstrated.
Using vitreous films of xFeO·(100 − x)SiO2, unusually large Faraday rotation
has been obtained, beating the performance of any other glassy material by up
to two orders of magnitude. It is shown that this is due to the incorporation of
small, ferromagnetic clusters of atomic iron which are generated in line during
laser deposition and rapid condensation of the thin film, generating superpara-
magnetism. The size of these clusters underbids the present record of metallic
Fe incorporation and experimental verification in glass matrices.
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.
1. Introduction
Magnetooptical (MO) effects play a key role in optical infor-
mation processing, data coding and, more generally, laser
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simplifies implementation and operation of the respective
optical devices. In addition, compared to crystalline candidates,
glassy materials provide the technological advantage of uni-
versal process ability which enables forming into a very broad
variety of shapes, including the ability for coating onto virtually
any kind of substrate material. Another significant advantage
over their crystalline counterparts is that their physical proper-
ties can usually be tuned continuously through changing the
composition. However, the absence of structural order leads to
an inherent downside, i.e., a usually significantly lower rota-
tion angle per unit sample thickness as compared to crystal-
line materials. This has led to the present limit in the achiev-
able Verdet constant of glasses of roughly −120 rad T−1 m−1 at
wavelengths around 630 nm.[14] In most cases, such low rota-
tion efficiency requires that glasses are used in bulk form in
order to obtain a total rotation angle which is comparable to
that of the crystalline benchmark materials. This, on the other
side, stands in sharp contrast to the recently increasing demand
for more compact integrated device architectures. Thus, new
routes toward notably enhanced Faraday rotation in vitreous
matrices are highly desired, and some approaches have been
proposed in this context. For example, the use of magneto-
photonic crystals,[17] combination with surface plasmon reso-
nance,[18–21] or inclusion of magnetic nanoparticles into glass
matrices[22,23] have been considered recently. However, none of
these approaches targets the chemistry of the MO material as
such. Rather, they are all usually associated with significantly
more complicated processing and also significantly higher
optical loss, making widespread application rather unlikely.
The present approach provides an alternative which is over-
coming the noted limitations, enabling record Faraday rotation
in a vitreous material and, hence, thin-film application. This is
achieved by generating ferromagnetic clusters of elemental iron
which are, during a rapid reactive gas-phase condensation pro-
cess, incorporated into a glassy matrix of iron silicate to form a
homogeneous, superparamagnetic layer of FeO-SiO2:0
n
Fe
. This
route can be generalized as a design strategy for super-efficient
vitreous Faraday rotators.
2. Results and Discussion
2.1. Magnetooptical Properties and Faraday Rotation
of FeO-SiO2 Films
The magnitude of magnetooptical activity of a material is
often expressed through the Verdet constant V, which pro-
vides a straightforward measure for the Faraday rotation angle
θ
as a function of the applied magnetic field with strength B,
and the geometrical path length d of linearly polarized light
passing through the rotator material,
θ
= VBd. This sim-
plistic formalism, however, does not apply to ferromagnetic
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Figure 1. Prominent Faraday rotation in vitreous iron silicate layers. a) Principle of the Faraday effect: Rotation of the polarization state of a linearly
polarized electromagnetic wave passing through a film experiencing a longitudinal static magnetic field (blue: electromagnetic wave, green arrows:
polarization vector, yellow: film, red arrow: direction of magnetic field). In (b) the specific rotation angle in vitreous layers FeO-SiO2 is shown as a func-
tion of wavelength at a constant field of 1.5 T for different molar fractions of FeO (labels), revealing very high rotation for FeO exceeding 38.2 mol%. The
corresponding spectrum of optical attenuation is shown in the inset. (c) depicts the field dependence of the rotation angle at a wavelength of 400 nm,
highlighting the field regimes of linear dependence for which an effective Verdet constant was calculated (black dashed line, see text for details). The
magnetization data shown in (d) are replotted for reference from Ref. [25]. Here, magnetization is shown for a DC field of 0.01 T, for field cooled (FC)
and zero field cooled (ZFC) specimen. The inset shows the frequency dependence of the temperature of freezing, Tf, reduced over the extrapolated
freezing temperature for infinite relaxation time, Tc.

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materials in which magnetic saturation is approached with
increasing magnetic field strength, and
θ
becomes independent
on field strength. The Verdet constant is therefore often used
only as a comparative estimate for MO performance. For
the present films of xFeO·(100 − x)SiO2, the specific rota-
tion angles are shown directly as a function of wavelength for
a magnetic field of 15 kOe in Figure 1b. The spectral optical
attenuation is shown for reference in the inset of Figure 1b.
For a nominal iron oxide fraction of x = 67.5, 54.8, 38.2, and
14.8, at a wavelength of 633 nm, we obtain rotation values
of 4110, 1400, 910, and −680 rad m−1, respectively. Espe-
cially for the higher contents of FeO, these absolute values
exceed the previous benchmarks of Eu2+ or Tb3+-containing
glasses by up to two orders of magnitude. For example, the
Verdet constant of 58.0EuO·12.0Al2O3·20.0B2O3·10.0SiO2
is about −300 rad T−1 m−1 at 633 nm,[11] and that of
17GeO2·23B2O3·32Al2O3·28Tb2O3 is −120 rad T−1 m−1.[14]
Instead, similarly high values have presently been known only
for ferrimagnetic crystals such as YIG. When the composi-
tion of the films is shifted to the silica-rich side, the value of V
rapidly decreases toward that of similar iron oxide containing
paramagnetic glasses. The surprisingly high Faraday rotation is
therefore clearly related to the presence of highly paramagnetic
ferrous iron, but its full magnitude cannot be explained on
the basis of Fe2+ alone. That is, ferrous phosphate glasses have
previously been reported with Verdet constants of around
−60 rad T−1 m−1 at a wavelength of 405 nm,[24] i.e., two orders
of magnitude below the values of the present observation. The
field dependence of Faraday rotation is shown in Figure 1c.
Up to a field strength of about 0.4 T (≈4 kOe), a roughly linear
scaling is observed for all investigated materials. This allows
for calculation of an effective Verdet constant within this field
regime which can be used for comparative purposes (Table 1).
At field strength higher than Bsat indicated in Table 1, Faraday
saturation sets-in, in accordance with magnetization data of
similar materials.[25]
As the magnitude of the Faraday effect in the present mate-
rials can clearly not be traced to Fe2+ species, magnetization
data are reinspected more closely (Figure 1d). Comparison of
magnetization as a function of temperature for field cooled and
zero field cooled samples reveals a spin-melting process in the
temperature regime of ≈16–32 K, depending on sample compo-
sition and observation time, and significantly reduced magneti-
zation at higher temperature. This is indicative for superspin
glass freezing, caused by diverging timescales of observation
and Néel relaxation of spins in small clusters with intercluster
interaction. The frequency dependence of this relaxation pro-
cess, shown in the inset of Figure 1d as the function of relaxa-
tion time versus the reduced freezing temperature follows a
scaling law with the critical exponent of z
ν
= 10.3 for x = 67.5,
confirming the superspin glass transition. In the previous work,
the temperature dependence of the inverse magnetizations was
analyzed by Curie–Weiss law, and the effective number of Bohr
magneton, MB, of 28, 34, and 29 was obtained for x = 38.2, 54.8,
and 67.5,[25] respectively. These values are much larger than the
theoretical spin-only value of Fe2+, 4.9, which lets anticipate
that each cluster has ferromagnetic ordering. As an interme-
diate conclusion at this point, it is therefore assumed that the
extreme degree of Faraday rotation is related to the presence of
small ferromagnetic cluster species.
2.2. X-Ray Photoelectron Spectroscopy (XPS) and X-Ray
Absorption Near Edge Structure (XANES) Analyses of Iron
Valence State
XPS and XANES analyses were employed as complementary
methods to elucidate the atomic state in which the iron species
is incorporated into the present material and, thus, to provide
further evidence for the chemical origin of the magnitude of
Faraday rotation. This assumes that iron is the key element for
producing the observed increase in MO performance. An exem-
plary XPS spectrum is shown for the highest iron content of
x = 67.5 in Figure 2a, comparing surface and bulk state of the
material by examining the thin film in its pristine state and
during sputtering. For the nonsputtered state, the binding ener-
gies of Fe 2p3/2 and Fe 2p1/2 and the satellite of Fe 2p3/2 are
708.9 and 722.5, and 713.3 eV, respectively. For reference, the
Fe 2p3/2 band position of analytical grade Fe, FeO, and Fe2O3,
respectively, was determined at 706.8, 709.8, and 711.2 eV.[26] In
the present case, we find that the binding energy of Fe 2p3/2
of the sample with x = 67.5 exhibits a best match with FeO,
indicating that iron is present in its divalent state, Fe2+. Upon
sputtering, there is a slight shoulder band located at ≈705.8 eV
which increases in intensity with increasing sputtering depth
(i.e., increasing number of sputtering cycles, where for pure
SiO2, a sputtering rate of 0.95 nm per sputtering cycle was deter-
mined). This latter band is assigned to 2p3/2 in metallic iron,
Fe0. The further band located at ≈718.6 eV which also increases
in intensity during sputtering is attributed to 2p1/2 in Fe0 (with
a reference position of 719.8 eV in metallic iron[26]). These
observations provide evidence for the presence of metallic iron
in the film. At this point, it can however not be fully excluded
that this is a result of Fe2+ reduction induced by the ion sput-
tering process. Therefore, independent verification is obtained
from XANES analyses (Figure 2b). For the references of Fe2O3,
FeO, and Fe, respectively, we find an absorption edge of 7125.8,
7121.4, and 7110.6 eV. The absorption edges of the thin films
are found at 7118.1, 7119.2, 7119.2, and 7119.2 eV for x = 14.8,
38.2, 54.8, and 67.5, i.e., best matching that of FeO and par-
tially overlain by contributions from Fe0. They are also close
to the absorption edge of crystalline iron orthosilicate with
x = 67.5, fayalite, at 7120.5 eV.[27] The pre-edge centroid posi-
tion is 7111.74, 7111.67, 7111.65, and 7111.35 eV for x = 14.8,
38.2, 54.8, and 67.5, respectively. These values are in the lower
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Table 1. Real part n and imaginary part k of the refractive index, effective
Verdet constant at 400 nm, Vef, and saturation onset Bsat of the studied
films. The imaginary parts of the refractive indices have been determined
by two independent methods, i.e., through ellipsometry (kel) and from
the optical absorption spectra (kabs =
α
abs·
λ
/(2
π
)).
Sample n kel kabs Vef
[deg (µmT)−1]
Bsat
[T]
x = 14.8 1.647 0.042 0.037 −0.122 1.5
x = 38.2 1.743 0.064 0.075 0.360 0.8
x = 54.8 1.88 0.097 0.096 1.090 0.5
x = 67.5 2.00 0.131 0.122 2.417 0.4

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energy side compared to those of Fe2+-bearing minerals such
as staurolite (7112.09 eV), grandidierite (7112.07 eV), siderite
(7112.04 eV), fayalite (7112.09 eV), and Fe2+-containing alkali
silicate glasses[28] (Figure 2c). The sum of integrated band areas
for x = 14.8 is close to that of staurolite, where divalent iron
ions occupy tetrahedral lattice sites. This value decreases with
increasing iron fraction x, providing evidence for an increase
in iron coordination number (with fivefold coordination VFe2+,
in the grandidierite mineral reference). In a first consideration,
this observation is in very good agreement with XPS data in that
iron is predominantly present in its divalent form, Fe2+. Addi-
tional evidence, however, is provided for the further presence
of Fe0 or metallic iron at increasing extent with increasing iron
content. However, as with XPS, this evidence remains ambig-
uous, here due to eventual photoreduction by high-energy X-ray
irradiation.[29]
2.3. Direct Observation of Iron Clusters
For final evidence of the presence, size and number density of
metallic iron particles or clusters, analytical transmission elec-
tron microscopy (TEM) was performed. Exemplary TEM images
(obtained on sample x
=
67.5) are shown in Figure 3. A general
overview of an ion-thinned specimen is provided in Figure 3a,
showing a cross-section of the as-deposited iron silicate thin
film on the silica substrate, together with layers of carbon and
platinum that were deposited during focused-ion beam (FIB)
sample preparation. The darker appearance of the as-deposited
film is a result of the higher electron absorption cross section
of the iron silicate as compared to the silica substrate. Selected
area electron diffraction (SAED) on silica substrate and on the
iron silicate thin film shows distinct differences, where the film
exhibits a more complex diffraction pattern as compared to the
glass substrate (which yields only the broad halo of a typical
amorphous material (Figure 3b,c). In particular, the SAED pat-
tern taken on the film exhibits an additional, broad ring which
reflects ordering at an atomic distance of ≈2 Å, consistent with
the lattice spacing of the (110) plane of metallic iron. However,
as this spacing is also close to the Fe–O distance in typical
iron-bearing silicate glasses and does therefore not allow for
final assignment to metallic iron. More detailed inspection by
bright- and composite dark-field (DF) imaging (Figure 3d,e)
clearly points to an inhomogeneous film structure as compared
to the substrate. In the bright-field (BF) image, this is not yet
unambiguous evidence for the presence of cluster or particle
species because similar fluctuations could also be generated by,
e.g., variations in sample thickness. However, the very same
fluctuations are also visible in the dark field image taken with
electrons from the diffraction ring at 2 Å. This excludes thick-
ness fluctuations. Instead, iron nanoparticles which are in
diffraction condition in one of the underlying DF images are
displayed as bright speckles in Figure 3e. The size (diameter) of
these clusters is in the order of 2–3 nm. A closeup of the micro-
structure is subsequently obtained byhigh-resolution (HR)
TEM (Figure 3f,g). Imaging of the tiny Fe particles is compli-
cated by the fact that they are visualized in projection together
with the over- and underlying glass. Therefore, the images rep-
resent a superposition of information from particles and glass.
Despite this, several of the 1–3 nm Fe particles could be directly
imaged. Here, an individual particle with a size of about 2 nm
was selected for SAED and high-resolution imaging, indicating
a regular lattice spacing of 2.04 Å and a textured diffraction pat-
tern with distinct diffraction spots along the Fe(110) diffraction
ring (Figure 3h). Together with the independent confirmations
by XPS and XANES, these observations provide clear evidence
for the presence of metallic iron in the pulsed laser deposition
(PLD)-deposited layers of FeO-SiO2.
Effective refractive indices ñef = n + ik as obtained from ellip-
sometric analyses of the films are shown in Table 1, where ñ
is the complex refractive index, n is its real part, and k is the
extinction coefficient. The observed values are larger than
expected for a typical oxide glass, i.e., n = 1.509 and k = 8 × 10−8
for SiO2 glass.[30] We however believe that the presented index
values are correct since the ellipsometric measurements yields
an imaginary of the refractive index kel which correspond to
those obtained from the absorption measurements, kabs (inset
of Figure 1b). The apparent values of k exceed those of oxide
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Figure 2. Spectroscopic analyses of iron precipitation in vitreous FeO-SiO2 layers. a) Exemplary XPS data for a sample with FeO content of 67.5 mol%
before sputtering and with increasing sputtering depth, where one sputtering cycle (labels) corresponds to ≈1 nm of sputtering depth. (b) presents
XANES spectra, together with reference scans on metallic Fe, FeO, and Fe2O3, evidencing the presence of Fe2+ and Fe0, and an average coordination
change from IVFe2+ to VFe2+. (c) summarizes the sums of integrated band areas in comparison to mineral data from Ref. [28].

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glasses by several orders of magnitude. Also this is a result of
the presence of metallic iron inclusions which intrinsically have
higher k values in accordance with the above observations.
3. Conclusion
In summary, we demonstrated record Faraday rotation efficiency in
PLD-derived vitreous films of FeO-SiO2, beating the performance
of conventional MO glasses by up to two orders of magnitude.
The origin of this is the incorporation of nano- and subnanoscopic
ferromagnetic clusters of Fe0n. The formation of these clusters is
facilitated by the rapid deposition kinetics occurring during the
PLD process and hindering further segregation and growth of pre-
cipitates. Due to only weak intercluster interaction, the assembly
of FeO-SiO2:Fe0n exhibits superparamagnetic behavior which is
the reason for the unusually strong Faraday effect at room temper-
ature. Superparamagnetic blocking and superspin glass formation
occurs in the temperature range of ≈16–32 K, leading to antifer-
romagnetic interaction between clusters at lower temperature. The
presence of metallic Fe was confirmed by XANES and XPS anal-
yses, and also on the complex refractive index of the layers. Using
SAED, and HR-TEM, for the iron orthosilicate thin film, we found
cluster sizes of about 1–3 nm.
Of particular interest is the high rotation efficiency of up to
4110 rad m−1 achieved in the visible spectral range, i.e., the opera-
tion regime of blue, green, and red lasers, and also of super-con-
tinuum light sources. Besides providing the major advantages of
superior performance over conventional glassy Faraday rotators,
compatibility with virtually any oxide material, optical fiber splicing
and fiber device integration, the present approach also presents a
general example for creating superparamagnetic vitreous layers
with MO functionality. This opens new routes in optical informa-
tion and laser processing, switching, coding, filtering, and sensing.
4. Experimental Section
Amorphous thin films with nominal compositions of xFeO·(100 − x)SiO2
(x = 10, 33, 50, 67 in mol%) were deposited on silica glass substrates by
PLD. The employed PLD targets were prepared via conventional solid-
state reactions as follows: Reagent-grade FeO and SiO2 were weighed to
obtain the prescribed composition, mixed thoroughly, and pelletized. The
pellets were sintered at 1000 °C for 10 h in Ar atmosphere for x = 67,[31]
and at 1000 °C for 6 h in N2 atmosphere for x = 10, 33, and 50, respectively.
Noteworthy, the choice of inert atmosphere, i.e., Ar or N2, does not affect the
crystalline phase of the resultant target material. In order to achieve optimal
chemical homogeneity of the targets, the procedure of pulverization and
resintering was repeated two times in total. The resultant pellets comprised
mixtures of Fe2SiO4, SiO2, Fe3O4, and Fe2O3 crystalline phases. For each
series of material deposition, a PLD target was placed at a distance of 3 cm
from the silica substrate. A KrF excimer laser operating at a wavelength
of
λ
= 248 nm and a pulse energy of 180 mJ (10 Hz) was used for target
vaporization. Substrates were kept at room temperature in vacuum with a
base pressure of 1 × 10−6 Pa. For each run, the deposition time was 60 min,
resulting in films with a thickness of 260, 360, 280, and 240 nm for x = 10,
33, 50, and 67, respectively, as determined with a scanning surface profiler
(KLA Tencor Alpha-Step IQ). With these data, the complex refractive index
was modeled from ellipsometric analyses. The chemical composition of the
as-deposited films was verified by Rutherford backscattering spectroscopy
using a 2.0 MeV He2+-beam. These analyses yielded actual film
compositions of 14.8FeO·85.2SiO2, 38.2FeO·61.8SiO2, 54.8FeO·45.2SiO2,
and 67.5FeO·32.5SiO2, for the nominals of 10FeO·90SiO2, 33FeO·67SiO2,
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Figure 3. Analytical transmission electron microscopy on vitreous FeO-SiO2. Taking the example of x = 67.5, an overview of sample preparation is
shown in (a), indicating the locations of further observations on a FIB-cut specimen. (b) and (c) provide SAED patterns of the deposit layer of FeO-SiO2
and of the substrate, respectively. Bright- and composite dark-field TEM images are given in (d) and (e) (see text for details), and a HR-TEM closeup
is shown in (f)–(h), clearly visualizing the presence of particles with a regular lattice spacing of ≈0.2 nm, and the corresponding SAED pattern with
several sharp reflections (exemplarily indicated with circles) originating from metallic iron.

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50FeO·50SiO2, and 67FeO·33SiO2, respectively. Magnetooptical properties
and, in particular, Faraday rotation were analyzed at room temperature on
a commercial measurement system for Faraday and Kerr effects (JASCO
K-250). For reference, also magnetic properties and the temperature-
dependence of magnetization in a 100 Oe external magnetic field
were examined, using a superconducting quantum interference device
magnetometer (Quantum Design MPMS-XL). In order to evaluate the
chemical state of iron, the authors carried-out XPS (ULVAC-PHI MT-5500)
at room temperature, simultaneously ion sputtering to obtain information
in surface and bulk properties of the films (see text for details). The binding
energy of the Fe 2p peak was calibrated with the O 1s peak at 529.9 eV.[26]
In parallel, the chemical state of iron was also examined by using XANES
measurements on the Fe K-edge at the BL01B1 beamline of SPring-8,
Hyogo, Japan. The intensity of the XANES spectra was normalized to
the edge-step height and energy after removing the background. The
pre-edge region for each spectrum was integrated to obtain the sums of
integrated area. Direct imaging of superparamagnetic iron clusters and
particles was performed by TEM using a FEI Tecnai G2 FEG operating at
200 kV. Cross-sections of the films were cut from as-deposited samples by
focused ion beam etching using a FEI Quanta3D FEG dual beam FIB-SEM
workstation. The FIB preparation involved the deposition of a protective
Pt layer on the film surface, the thinning with Ga+ ions at a high incident
angle for minimizing damage, and the sample transfer with an internal
micromanipulator. In the present report, the authors focused on analytical
TEM data which were obtained for the sample of 67.5FeO·32.5SiO2.
Additional TEM data for 54.8FeO·45.2SiO2 are provided in Figure S1
(Supporting Information). Transmission electron microscopic imaging was
done by conventional BF and DF techniques as well as by high-resolution
imaging. SAED was subsequently employed to detect metallic iron particles
in the film. The composite dark-field image was constructed from the
maximum gray value of eight DF images of the same area. The underlying
DF images were generated by using different parts of a quarter of the ≈2 Å
ring in the SAED pattern with the smallest objective aperture of 10 µm.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
XANES measurements were performed at the BL01B1 of the SPring-8
with the approval of the Japan Synchrotron Radiation Research Institute
(Nos. 2013A1691 and 2014B1128). This research was supported by a
Grant-in-Aid for Scientific Research (A) (No. 25249090) from the Ministry
of Education, Culture, Sports, Science and Technology of Japan. Y.N.
received a Grant-in-Aid for Japan Society for the Promoting of Science
(JSPS) Fellows (No. 15J07889) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan, and “Program for Advancing
Strategic International Networks to Accelerate the Circulation of Talented
Researchers (Brain Circulation)” of JSPS. L.W. and Y.N. gratefully
acknowledge further financial support from the Carl-Zeiss-Foundation.
F.L. thanks the Deutsche Forschungsgemeinschaft for funding the
FIB-TEM facilities via the Gottfried Wilhelm Leibniz programme
(LA 830/14-1). L.W. finally thanks the European Research Council (ERC)
for support through the ERC-Grant UTOPES, Grant No. 681652.
Received: August 9, 2016
Revised: October 26, 2016
Published online:
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