Studying the Crucial Physical Characteristics Related to Surface Roughness and Magnetic Domain Structure in CoFeSm Thin Films

Abstract

This study investigated the effects of varying film thicknesses and annealing temperatures on the surface roughness and magnetic domain structure of CoFeSm thin films. The results revealed that as the film thickness increased, both the crystalline size and surface roughness decreased, leading to a reduction in coercivity (Hc) and improved magnetic contrast performance. Energy-dispersive X-ray spectroscopy (EDS) analysis confirmed the presence of cobalt (Co), iron (Fe), and samarium (Sm) within the thin films. Notably, the 40 nm Co40Fe40Sm20 thin film annealed at 200 °C exhibited lower sheet resistance (Rs) and resistivity (ρ), indicating higher conductivity and a relatively higher maximum magnetic susceptibility (χac) at 50 Hz. These findings suggest that these films are well suited for low-frequency magnetic components due to their increased spin sensitivity. The 40 nm Co40Fe40Sm20 thin film, subjected to annealing at 200 °C, displayed a distinct stripe domain structure characterized by prominently contrasting dark and bright patterns. It exhibited the lowest Hc and the highest saturation magnetization (Ms), leading to a significant improvement in their soft magnetic properties. It is proposed that the surface roughness of the CoFeSm thin films plays a crucial role in shaping the magnetic properties of these thin magnetic films.

Full text

Citation: Fern, C.-L.; Liu, W.-J.;
Chang, Y.-H.; Chiang, C.-C.; Lai, J.-X.;
Chen, Y.-T.; Chen, W.-G.; Wu, T.-H.;
Lin, S.-H.; Lin, K.-W. Studying the
Crucial Physical Characteristics
Related to Surface Roughness and
Magnetic Domain Structure in
CoFeSm Thin Films. Coatings 2023,
13, 1961. https://doi.org/10.3390/
coatings13111961
Academic Editors: Daniela Predoi
and Torsten Brezesinski
Received: 7 October 2023
Revised: 2 November 2023
Accepted: 15 November 2023
Published: 17 November 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
coatings
Article
Studying the Crucial Physical Characteristics Related to Surface
Roughness and Magnetic Domain Structure in CoFeSm
Thin Films
Chi-Lon Fern 1, Wen-Jen Liu 2, Yung-Huang Chang 3, Chia-Chin Chiang 4, Jian-Xin Lai 5, Yuan-Tsung Chen 5, * ,
Wei-Guan Chen 5, Te-Ho Wu 5, Shih-Hung Lin 6and Ko-Wei Lin 1
1
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan;
fengcl@yuntech.edu.tw (C.-L.F.); kwlin@dragon.nchu.edu.tw (K.-W.L.)
2Department of Materials Science and Engineering, I-Shou University, Kaohsiung 84001, Taiwan;
jurgen@isu.edu.tw
3Bachelor Program in Industrial Technology, National Yunlin University of Science and Technology,
123 University Road, Section 3, Douliou, Yunlin 64002, Taiwan; changyhu@yuntech.edu.tw
4Department of Mechanical Engineering, National Kaohsiung University of Science and Technology,
Kaohsiung 80778, Taiwan; ccchiang@nkust.edu.tw
5Graduate School of Materials Science, National Yunlin University of Science and Technology,
123 University Road, Section 3, Douliou, Yunlin 64002, Taiwan; jianxin19971124@hotmail.com (J.-X.L.);
m11147003@gemail.yuntech.edu.tw (W.-G.C.); wuth@yuntech.edu.tw (T.-H.W.)
6Department of Electronic Engineering, National Yunlin University of Science and Technology,
123 University Road, Section 3, Douliou, Yunlin 64002, Taiwan; isshokenmei@yuntech.edu.tw
*Correspondence: ytchen@yuntech.edu.tw; Tel.: +886-5-534-2601
Abstract:
This study investigated the effects of varying film thicknesses and annealing temperatures
on the surface roughness and magnetic domain structure of CoFeSm thin films. The results revealed
that as the film thickness increased, both the crystalline size and surface roughness decreased, leading
to a reduction in coercivity (H
c
) and improved magnetic contrast performance. Energy-dispersive
X-ray spectroscopy (EDS) analysis confirmed the presence of cobalt (Co), iron (Fe), and samarium
(Sm) within the thin films. Notably, the 40 nm Co
40
Fe
40
Sm
20
thin film annealed at 200
◦
C exhibited
lower sheet resistance (R
s
) and resistivity (
ρ
), indicating higher conductivity and a relatively higher
maximum magnetic susceptibility (
χac
) at 50 Hz. These findings suggest that these films are well
suited for low-frequency magnetic components due to their increased spin sensitivity. The 40 nm
Co
40
Fe
40
Sm
20
thin film, subjected to annealing at 200
◦
C, displayed a distinct stripe domain structure
characterized by prominently contrasting dark and bright patterns. It exhibited the lowest H
c
and the
highest saturation magnetization (M
s
), leading to a significant improvement in their soft magnetic
properties. It is proposed that the surface roughness of the CoFeSm thin films plays a crucial role in
shaping the magnetic properties of these thin magnetic films.
Keywords:
CoFeSm thin films; surface roughness; magnetic domain structure; electrical properties;
soft magnetic properties
1. Introduction
Spin-transfer torque magnetic random access memory (STT-MRAM) is hailed as the
future of reliable magnetic random access memory (MRAM) technology, primarily due to
its exceptional thermal stability, non-volatility, low write power consumption, and efficient
low-current, low-frequency switching capabilities. The power consumption and overall
performance of magnetic tunnel junctions (MTJs) are intricately intertwined with the com-
position, structure, and fabrication techniques employed for the ferromagnetic (FM) layers.
Achieving cost-effective magnetization reversal hinges on the selection of soft magnetic
materials featuring high saturation magnetization (M
s
), elevated Curie temperature (T
c
),
Coatings 2023,13, 1961. https://doi.org/10.3390/coatings13111961 https://www.mdpi.com/journal/coatings

Coatings 2023,13, 1961 2 of 14
low coercivity (H
c
), high magnetic permeability (
µ
), and minimal magnetostriction (
λ
) for
the FM layer [1,2].
A memory cell for MRAM is an integration of a MTJ and a complementary metal–oxide
semiconductor (CMOS) pass transistor. An MTJ is composed of an ultra-thin insulating
layer and a tunnel barrier, situated between two FM metal layers [
3
]. Within the realm
of FM materials, cobalt (Co)–iron (Fe) alloys stand out due to their high saturation mag-
netization (M
s
), making them ideal for thin films used in read/write heads. However,
it is worth noting that these alloys suffer from excessive coercivity (H
c
) and insufficient
corrosion resistance. The incorporation of additional elements into the CoFe alloy has the
potential to enhance its soft magnetic properties and bolster its resistance to corrosion [
4
,
5
].
According to earlier studies, adding 1.7%–2.1% of vanadium (V) to CoFe alloys can provide
a composition with significantly improved strength and satisfactory magnetic properties
simultaneously [
6
]. The addition of rare earth (RE) elements in CoFe alloys is being inves-
tigated due to their electronic interaction. The interplay between the 4f-electrons of RE
elements and the 3d-electrons of transition metals (TMs) through spin-orbital coupling
significantly influences the electrical and magnetic characteristics of these materials [
7
].
Moreover, Sm
3+
possesses five unpaired 4f electrons, potentially leading to enhancements
in electrical and optical features, surface roughness, and dielectric properties [
8
]. Rakesh
et al. investigated praseodymium (Pr)-, lanthanum (La)-, and Sm-substituted nickel–zinc
(Ni-Zn) ferrite, resulting in the lattice deformation of the crystal with decreased particle size,
coercivity, and magnetic exchange [
9
]. Furthermore, CoFe alloys with an equal composition
of 50% Co and 50% Fe demonstrate remarkably high susceptibility [
10
]. Therefore, Sm
was chosen as the alloying element to be added, and for this study, a CoFeSm alloy with a
composition of 40 at. %, 40 at. %, and 20 at. % was selected.
As a third element is introduced into the parent phase, the impact of stress- and
strain-induced magnetic anisotropy on material properties becomes increasingly signifi-
cant. Stress–strain-induced magnetic anisotropy plays a pivotal role in determining the
coercivity and practical suitability of thin films [
11
–
13
]. The findings unveil a robust and
direct correlation between stress-induced magnetic anisotropy and coercivity in thin films.
Under diverse stress–strain conditions, distinct alterations in the magnetic properties of the
films become evident. Notably, an increase in applied stress leads to a marked enhance-
ment of induced magnetic anisotropy, consequently resulting in a corresponding rise in
coercivity [
14
,
15
]. This discovery emphasizes the central role of stress-induced magnetic
anisotropy in governing the coercivity of thin films. For the sake of comprehensive analysis,
we offer data on material parameters in both bulk and thin film states. The observed
connection between stress-induced magnetic anisotropy and coercivity underscores the
significance of mechanical stress in dictating the magnetization behavior of thin films. The
application of stress induces alterations in the crystallographic orientation of magnetic
domains, resulting in anisotropic characteristics. This, in turn, influences coercivity, making
it more challenging for magnetic domains to reorient themselves in the presence of an
external magnetic field.
In this study, Co
40
Fe
40
Sm
20
films of varying thicknesses and annealing tempera-
tures were deposited on Si(100) substrates using a direct current (DC) sputtering sys-
tem. The study aimed to assess how surface roughness affects the magnetic properties of
Co
40
Fe
40
Sm
20
films, particularly in low-frequency applications. These films are known for
their exceptional magnetic attributes and versatility, especially in low-frequency magnetic
components. Our research sought to determine their potential in low-frequency applica-
tions and explore their low-frequency magnetic characteristics. Our experiments revealed
that CoFeSm films excel in low-frequency settings, making them ideal for low-frequency
magnetic components. They exhibit high low-frequency magnetic induction, with peak
performance in the 50 Hz to 100 Hz range. Additionally, this work also observed variations
in the film’s magnetic domain structure and hysteresis loop under different thicknesses
and annealing conditions, which is vital for optimizing their performance. This study

Coatings 2023,13, 1961 3 of 14
offers valuable insights for utilizing these films in low-frequency magnetic applications
and serves as a valuable reference for future research and development.
2. Materials and Methods
Thin films of CoFeSm with varying film thicknesses from 10 nm to 50 nm were
produced on Si(100) substrates using a DC sputtering system at room temperature (RT).
The pre-cleaning steps for the Si(100) substrate are to use alcohol and then acetone for
cleaning, vibrate and clean in a ultrasonic wave vibrator, and finally blow dry with nitrogen
(N
2
) and put it into a sputtering vacuum chamber. The composition of the alloy target
of CoFeSm was 40 at. % Co, 40 at. % Fe, and 20 at. % Sm. The power density was
1.65 W/cm
2
, and the deposition rate was 1.2 nm/min. The chamber base pressure was
kept at 3.0
×
10
−4
mTorr before sputtering. During sputtering, the flowing rate of argon
(Ar) gas was kept at 20 sccm, the deposition power was set as 50 W, and the sputtering
pressure was maintained at 3.5
×
10
−2
mTorr. After deposition, Co
40
Fe
40
Sm
20
thin films
were subjected to an annealing process and annealed at various temperatures up to
300 ◦C
for 1 h with a fixed heating rate of 50
◦
C/min. The investigations were carried out on
as-deposited films and after various annealing treatments at 100
◦
C, 200
◦
C, and
300 ◦C
for 1 h in an Ar environment. X-ray diffraction pattern (XRD) was used to identify the
crystal structure of Co
40
Fe
40
Sm
20
thin films. Energy dispersive X-ray spectroscopy (EDS,
JEOL, Tokyo, Japan) was used to analyze the elemental composition. An atomic force
microscope (AFM, NanoMagnetics Instruments, Ankara, Turkey, ezAFM) was used to
investigate the surface roughness of Co
40
Fe
40
Sm
20
thin films. The AFM was utilized in non-
contact mode, with three scanning repetitions performed at RT to ensure accurate average
area assessment. Surface roughness, quantified by the arithmetic mean deviation (Ra),
was determined using a scanning size of 2.5
µ
m
×
2.5
µ
m. Electrical characteristics were
analyzed employing a four-point probe measurement setup (Sadhudesign, Hsinchu City,
Taiwan). Magnetic characteristics, including low-frequency alternating-current magnetic
susceptibility (
χac
), magnetic domain behavior, and hysteresis loops, were assessed using a
MagQu
χac
Quan II analyzer (MagQu, New Taipei City, Taiwan), magnetic force microscopy
(MFM, NanoMagnetics Instruments, ezAFM), and an alternating gradient magnetometer
(AGM, PMC, MicroMagTM 2900, Westerville, OH, USA), respectively. During the process
of acquiring comprehensive measurements, each collected data point is determined by
averaging three times.
3. Results
3.1. Structure Property and Grain Size Distribution
The XRD patterns of the as-deposited and annealed Co
40
Fe
40
Sm
20
thin films are il-
lustrated in Figure 1. Distinct peaks in the X-ray diffraction (XRD) patterns are evident
at specific diffraction angles (2
θ
) of 47.7
◦
, 54.6
◦
, and 56.4
◦
, corresponding to the crystal-
lographic planes of Co (0002), Co
2
O
3
(422), and Co
2
O
3
(511) [
16
,
17
]. Guojian Li et al.
fabricated Co films on Si(100) substrate and observed the layer of Co films, SiO
2
, and
Si by transmission electron microscopy (TEM), with SiO
2
being the nature oxide for the
Si substrate surface [
18
]. Hence, the emergence of the oxidation peak can be ascribed to
the interaction of oxygen with the metal, leading to the partial oxidation of the CoFeSm
thin films.
The average crystalline sizes (D) for the as-deposited and annealed Co
40
Fe
40
Sm
20
thin films were calculated using the Debye–Scherrer equation, as depicted in the equation
below [19]:
D=
0.9λ
βcos θhkl
where
λ
is the wavelength of the X-ray (
λ
= 0.154056 nm),
β
is the full width at half
maximum (FWHM), and θhkl is the diffraction angle of the crystal plane (hkl).

Coatings 2023,13, 1961 4 of 14
Coatings 2023, 13, x FOR PEER REVIEW 4 of 15
Figure 1. XRD paerns of Co40Fe40Sm20 (10–50 nm) thin films with as-deposited and annealing
temperatures (a) RT, (b) 100 °C, (c) 200 °C, and (d) 300 °C.
The average crystalline sizes (D) for the as-deposited and annealed Co40Fe40Sm20
thin films were calculated using the Debye–Scherrer equation, as depicted in the equa-
tion below [19]:
𝐷= 0.9𝜆
𝛽 cos 𝜃
where 𝜆 is the wavelength of the X-ray (𝜆= 0.154056 nm), 𝛽 is the full width at half
maximum (FWHM), and 𝜃 is the diffraction angle of the crystal plane (ℎ𝑘𝑙).
Figure 2 displays the average crystalline size of both the as-deposited and annealed
Co40Fe40Sm20 thin films. As the film thickness expanded from 10 nm to 50 nm and the
annealing temperature rose up to 300 °C, the crystalline size of the Co40Fe40Sm20 thin
films notably decreased. This reduction in size resulted in the diminishment of laice
spacing in the crystallographic planes, causing compressive stress within the thin films.
Figure 1.
XRD patterns of Co
40
Fe
40
Sm
20
(10–50 nm) thin films with as-deposited and annealing
temperatures (a) RT, (b) 100 ◦C, (c) 200 ◦C, and (d) 300 ◦C.
Figure 2displays the average crystalline size of both the as-deposited and annealed
Co
40
Fe
40
Sm
20
thin films. As the film thickness expanded from 10 nm to 50 nm and the
annealing temperature rose up to 300
◦
C, the crystalline size of the Co
40
Fe
40
Sm
20
thin films
notably decreased. This reduction in size resulted in the diminishment of lattice spacing in
the crystallographic planes, causing compressive stress within the thin films. Moreover, the
decrease in crystalline size with the escalation of film thickness and annealing temperatures
can be linked to a higher rate of nucleation [
20
,
21
]. The Scherrer–Debye equation stands as
a fundamental tool within the realm of crystallography, serving the purpose of ascertaining
the average size of crystal grains within a material. This mathematical equation establishes
a clear link between a set of parameters obtained from X-ray diffraction experiments and
the dimensions of crystalline regions within the material. The connection between the
Scherrer–Debye equation and experimental data is of utmost importance when it comes to
characterizing the structural attributes of materials. This equation effectively associates
the dimensions of crystalline regions, often referred to as crystallites, present in a material

Coatings 2023,13, 1961 5 of 14
with the observations made in X-ray diffraction patterns. The width of the diffraction peak,
quantified as the FWHM, serves as an inverse indicator of crystallite size—a broader peak
corresponds to smaller crystallites, and vice versa. This interplay between the Scherrer–
Debye equation and experimental data entails the collection of diffraction data, typically
in the form of X-ray diffraction patterns, followed by the application of the equation to
determine the average size of crystallites. The strength of this correlation lies in its capability
to extract structural information from a material’s diffraction pattern, thereby facilitating
the thorough characterization of its crystalline properties.
Coatings 2023, 13, x FOR PEER REVIEW 5 of 15
Moreover, the decrease in crystalline size with the escalation of film thickness and an-
nealing temperatures can be linked to a higher rate of nucleation [20,21]. The Scherrer–
Debye equation stands as a fundamental tool within the realm of crystallography, serv-
ing the purpose of ascertaining the average size of crystal grains within a material. This
mathematical equation establishes a clear link between a set of parameters obtained from
X-ray diffraction experiments and the dimensions of crystalline regions within the mate-
rial. The connection between the Scherrer–Debye equation and experimental data is of
utmost importance when it comes to characterizing the structural aributes of materials.
This equation effectively associates the dimensions of crystalline regions, often referred
to as crystallites, present in a material with the observations made in X-ray diffraction
paerns. The width of the diffraction peak, quantified as the FWHM, serves as an inverse
indicator of crystallite size—a broader peak corresponds to smaller crystallites, and vice
versa. This interplay between the Scherrer–Debye equation and experimental data entails
the collection of diffraction data, typically in the form of X-ray diffraction paerns, fol-
lowed by the application of the equation to determine the average size of crystallites. The
strength of this correlation lies in its capability to extract structural information from a
material’s diffraction paern, thereby facilitating the thorough characterization of its
crystalline properties.
Figure 2. Average grain size of as-deposited and annealed Co40Fe40Sm20 thin films with Co (0002)
diffraction peak.
3.2. Composition Analysis
Figure 3 shows the paern of energy-dispersive X-ray spectroscopy (EDS) ele-
mental analysis for the as-deposited Co40Fe40Sm20 thin films. The EDS elemental analysis
confirmed the existence of Co, Fe, and Sm in the thin films. However, it was observed that
the actual Co, Fe, and Sm contents did not precisely match the nominal stoichiometry of
40 at. %, 40 at. %, and 20 at. %. Nonetheless, the variation in atom content observed dur-
ing film growth can be aributed to material loss incurred during the spuering tech-
nique’s transport from the target to the substrate, possibly influenced by the impact of
argon ion bombardment and spuering gun angle [22–24].
Figure 2.
Average grain size of as-deposited and annealed Co
40
Fe
40
Sm
20
thin films with Co (0002)
diffraction peak.
3.2. Composition Analysis
Figure 3shows the pattern of energy-dispersive X-ray spectroscopy (EDS) elemental
analysis for the as-deposited Co
40
Fe
40
Sm
20
thin films. The EDS elemental analysis con-
firmed the existence of Co, Fe, and Sm in the thin films. However, it was observed that
the actual Co, Fe, and Sm contents did not precisely match the nominal stoichiometry of
40 at. %, 40 at. %, and 20 at. %. Nonetheless, the variation in atom content observed during
film growth can be attributed to material loss incurred during the sputtering technique’s
transport from the target to the substrate, possibly influenced by the impact of argon ion
bombardment and sputtering gun angle [22–24].
3.3. Surface Morphology and Roughness
Figure 4illustrates the Ra values of both the as-deposited and annealed Co
40
Fe
40
Sm
20
thin films. For this investigation, AFM images of the as-deposited and annealed Co
40
Fe
40
Sm
20
thin films were scanned across an area measuring 2.5
×
2.5
µ
m
2
. The results indicate that Ra
decreased as the thickness increased. The AFM images of as-deposited and annealed
50 nm
Co
40
Fe
40
Sm
20
thin films are displayed in Figure 5. Hence, a 50 nm thick Co
40
Fe
40
Sm
20
thin film annealed at 300
◦
C exhibited a smoother surface. This reduction in roughness
can be attributed to the minimization of compressive strain and the smoothening effect
caused by surface diffusion. As the annealing temperature rises, the increased energy of
the atoms allows for faster migration on the substrate surface, enhancing the mobility of
surface atoms and resulting in a more uniform and smoother surface. The decrease in Ra is
linked to the crystalline agglomeration of the Co40Fe40 Sm20 thin films [25,26].

Coatings 2023,13, 1961 6 of 14
Coatings 2023, 13, x FOR PEER REVIEW 6 of 15
Figure 3. EDS element analysis of as-deposited Co40Fe40Sm20 (40 nm) thin films.
3.3. Surface Morphology and Roughness
Figure 4 illustrates the Ra values of both the as-deposited and annealed Co40Fe40Sm20
thin films. For this investigation, AFM images of the as-deposited and annealed
Co40Fe40Sm20 thin films were scanned across an area measuring 2.5 × 2.5 µm2. The results
indicate that Ra decreased as the thickness increased. The AFM images of as-deposited
and annealed 50 nm Co40Fe40Sm20 thin films are displayed in Figure 5. Hence, a 50 nm
thick Co40Fe40Sm20 thin film annealed at 300 °C exhibited a smoother surface. This reduc-
tion in roughness can be aributed to the minimization of compressive strain and the
smoothening effect caused by surface diffusion. As the annealing temperature rises, the
increased energy of the atoms allows for faster migration on the substrate surface, en-
hancing the mobility of surface atoms and resulting in a more uniform and smoother
surface. The decrease in Ra is linked to the crystalline agglomeration of the Co40Fe40Sm20
thin films [25,26].
Figure 4. Surface roughness of as-deposited and annealed Co40Fe40Sm20 (10–50 nm) thin films.
Figure 3. EDS element analysis of as-deposited Co40 Fe40Sm20 (40 nm) thin films.
Coatings 2023, 13, x FOR PEER REVIEW 6 of 15
Figure 3. EDS element analysis of as-deposited Co40Fe40Sm20 (40 nm) thin films.
3.3. Surface Morphology and Roughness
Figure 4 illustrates the Ra values of both the as-deposited and annealed Co40Fe40Sm20
thin films. For this investigation, AFM images of the as-deposited and annealed
Co40Fe40Sm20 thin films were scanned across an area measuring 2.5 × 2.5 µm2. The results
indicate that Ra decreased as the thickness increased. The AFM images of as-deposited
and annealed 50 nm Co40Fe40Sm20 thin films are displayed in Figure 5. Hence, a 50 nm
thick Co40Fe40Sm20 thin film annealed at 300 °C exhibited a smoother surface. This reduc-
tion in roughness can be aributed to the minimization of compressive strain and the
smoothening effect caused by surface diffusion. As the annealing temperature rises, the
increased energy of the atoms allows for faster migration on the substrate surface, en-
hancing the mobility of surface atoms and resulting in a more uniform and smoother
surface. The decrease in Ra is linked to the crystalline agglomeration of the Co40Fe40Sm20
thin films [25,26].
Figure 4. Surface roughness of as-deposited and annealed Co40Fe40Sm20 (10–50 nm) thin films.
Figure 4. Surface roughness of as-deposited and annealed Co40Fe40Sm20 (10–50 nm) thin films.
3.4. Electrical Characteristics
In Figure 6a,b, the sheet resistance and resistivity of both as-deposited and annealed
Co
40
Fe
40
Sm
20
thin films are depicted. To determine the sheet resistance and resistivity
values, a four-point probe instrument was used, applying a current (I) of 0.1 mA and a
voltage (V) of 5 V. A drastic decreased change was found in sheet resistance and resistivity
initially, but then saturated with increasing thickness. The sheet resistance and resistivity
exhibited a decrease as the film thickness increased from 10 nm to 50 nm, but they decreased
when the annealing temperature reached 200
◦
C, with a slight increase observed at 300
◦
C.
Consequently, the annealed Co
40
Fe
40
Sm
20
thin films achieved their lowest sheet resistance
and resistivity values at 200
◦
C, measuring 0.14 k
Ω
/sq and 0.058
×
10
−2Ω
-cm, respectively.

Coatings 2023,13, 1961 7 of 14
The phenomenon of surface scattering becomes significant when the film thickness is
comparable to the mean free path of electrons, resulting in scattering from the film’s surface,
and this effect diminishes as the film thickness increases, thereby boosting conductivity [
27
].
Coatings 2023, 13, x FOR PEER REVIEW 7 of 15
Figure 5. AFM images of 50 nm Co40Fe40Sm20 thin films with different annealing temperatures: (a)
RT, (b) 100 °C, (c) 200 °C, and (d) 300 °C.
3.4. Electrical Characteristics
In Figure 6a,b, the sheet resistance and resistivity of both as-deposited and annealed
Co40Fe40Sm20 thin films are depicted. To determine the sheet resistance and resistivity
values, a four-point probe instrument was used, applying a current (I) of 0.1 mA and a
voltage (V) of 5 V. A drastic decreased change was found in sheet resistance and resistiv-
ity initially, but then saturated with increasing thickness. The sheet resistance and resis-
tivity exhibited a decrease as the film thickness increased from 10 nm to 50 nm, but they
decreased when the annealing temperature reached 200 °C, with a slight increase ob-
served at 300 °C. Consequently, the annealed Co40Fe40Sm20 thin films achieved their low-
est sheet resistance and resistivity values at 200 °C, measuring 0.14 kΩ/sq and 0.058 × 10−2
Ω-cm, respectively. The phenomenon of surface scaering becomes significant when the
film thickness is comparable to the mean free path of electrons, resulting in scaering
from the film’s surface, and this effect diminishes as the film thickness increases, thereby
boosting conductivity [27].
Figure 5.
AFM images of 50 nm Co
40
Fe
40
Sm
20
thin films with different annealing temperatures:
(a) RT, (b) 100 ◦C, (c) 200 ◦C, and (d) 300 ◦C.
Consequently, an increase in film thickness leads to a reduction in the sheet resistance
and resistivity of Co40Fe40Sm20 thin films.
3.5. Magnetic Properties
3.5.1. Magnetic Susceptibility
Figure 7a,b show the maximum
χac
values and optimal resonance frequency of as-
deposited and annealed Co
40
Fe
40
Sm
20
thin films. In Figure 7a, the maximum
χac
demon-
strates an increase with greater thickness and higher annealing temperatures. However,
there was a decrease noticed at annealed 40 nm and 50 nm and 300
◦
C, which is likely
the result of an intensified thermal disturbance effect [
28
]. The maximum
χac
values for
the as-deposited, 100
◦
C-annealed, and 200
◦
C-annealed Co
40
Fe
40
Sm
20
thin films were
higher at 50 nm, measuring 0.11, 0.12, and 0.13, respectively. In contrast, the maximum
χac
value for the 300
◦
C-annealed Co
40
Fe
40
Sm
20
thin film was higher at 30 nm, amounting
to 0.13. Consequently, the 50 nm film annealed at 200
◦
C and the 30 nm film annealed
at 300
◦
C exhibited the highest maximum
χac
values. Prior research has suggested that
higher maximum
χac
values correspond to reduced motion of the free magnetic domain and
heightened spin sensitivity [
29
]. The maximum
χac
values for both the as-deposited and

Coatings 2023,13, 1961 8 of 14
annealed Co
40
Fe
40
Sm
20
thin films peaked within the frequency range of 50 Hz to 100 Hz.
The optimal resonance frequency (f
res
) was detected by an
χac
analyzer, which means the
frequency of the maximum
χac
. This suggests that Co
40
Fe
40
Sm
20
thin films are well suited
for applications in low-frequency magnetic devices such as transformers, spin valves, and
magnetic recording mediums [
30
]. The summary of the D, Ra, Rs,
ρ
, maximum
χac
, and
optimal resonance frequency of Co
40
Fe
40
Sm
20
thin films subjected to various annealing
temperatures is provided in Table 1.
Coatings 2023, 13, x FOR PEER REVIEW 8 of 15
(a) (b)
Figure 6. (a) Sheet resistance and (b) resistivity of as-deposited and annealed Co40Fe40Sm20 (10–50
nm) thin films.
Consequently, an increase in film thickness leads to a reduction in the sheet re-
sistance and resistivity of Co40Fe40Sm20 thin films.
3.5. Magnetic Properties
3.5.1. Magnetic Susceptibility
Figure 7a,b show the maximum χac values and optimal resonance frequency of
as-deposited and annealed Co40Fe40Sm20 thin films. In Figure 7a, the maximum χac
demonstrates an increase with greater thickness and higher annealing temperatures.
However, there was a decrease noticed at annealed 40 nm and 50 nm and 300 °C, which is
likely the result of an intensified thermal disturbance effect [28]. The maximum χac values
for the as-deposited, 100 °C-annealed, and 200 °C-annealed Co40Fe40Sm20 thin films were
higher at 50 nm, measuring 0.11, 0.12, and 0.13, respectively. In contrast, the maximum
χac value for the 300 °C-annealed Co40Fe40Sm20 thin film was higher at 30 nm, amounting
to 0.13. Consequently, the 50 nm film annealed at 200 °C and the 30 nm film annealed at
300 °C exhibited the highest maximum χac values. Prior research has suggested that
higher maximum χac values correspond to reduced motion of the free magnetic domain
and heightened spin sensitivity [29]. The maximum χac values for both the as-deposited
and annealed Co40Fe40Sm20 thin films peaked within the frequency range of 50 Hz to 100
Hz. The optimal resonance frequency (fres) was detected by an χac analyzer, which means
the frequency of the maximum χac. This suggests that Co40Fe40Sm20 thin films are well
suited for applications in low-frequency magnetic devices such as transformers, spin
valves, and magnetic recording mediums [30]. The summary of the D, Ra, Rs, ρ, maxi-
mum χac, and optimal resonance frequency of Co40Fe40Sm20 thin films subjected to various
annealing temperatures is provided in Table 1.
Figure 6.
(
a
) Sheet resistance and (
b
) resistivity of as-deposited and annealed Co
40
Fe
40
Sm
20
(10–50 nm) thin films.
Coatings 2023, 13, x FOR PEER REVIEW 9 of 15
(a) (b)
Figure 7. (a) Maximum χac values and (b) optimal resonance frequency of as-deposited and an-
nealed Co40Fe40Sm20 (10–50 nm) thin films.
Table 1. Crystalline size (𝐷), surface roughness (Ra), sheet resistance (Rs), resistivity (ρ), maximum
χac, and optimal resonance frequency (fres) of Co40Fe40Sm20 (10–50 nm) thin films with different an-
nealing temperatures.
Ta
(°C)
Thickness
(nm) 𝑫 (nm) Ra (nm) Rs
(kΩ/sq)
ρ
(×10−2 Ω-cm)
Maximum
χac
(a.u.)
Optimal
Resonance
Frequency
(Hz)
RT
10 68.96 6.97 543.9 54.4 0.010 50
20 67.13 6.58 87.90 17.6 0.027 100
30 61.76 6.50 2.53 0.78 0.036 50
40 56.95 6.44 0.29 0.10 0.072 50
50 49.50 6.31 0.24 0.12 0.110 50
100
10 66.47 6.88 543.9 54.4 0.025 50
20 61.10 6.53 39.36 7.87 0.034 100
30 57.94 6.44 1.97 0.59 0.050 50
40 55.79 6.39 0.28 0.11 0.075 50
50 48.44 6.28 0.17 0.086 0.120 50
200
10 65.32 6.68 543.9 54.4 0.037 50
20 57.10 6.37 25.03 5.00 0.052 50
30 53.15 6.35 0.64 0.19 0.070 50
40 53.08 6.29 0.14 0.058 0.083 50
50 47.85 6.21 0.14 0.071 0.130 50
300
10 60.89 6.57 543.9 54.4 0.071 50
20 54.62 6.32 28.05 5.61 0.110 50
30 47.23 6.29 0.67 0.20 0.130 50
40 46.46 6.26 0.22 0.086 0.120 50
50 41.21 6.17 0.17 0.086 0.100 50
Figure 7.
(
a
) Maximum
χac
values and (
b
) optimal resonance frequency of as-deposited and annealed
Co40Fe40 Sm20 (10–50 nm) thin films.

Coatings 2023,13, 1961 9 of 14
Table 1.
Crystalline size (
D
), surface roughness (R
a
), sheet resistance (R
s
), resistivity (
ρ
), maximum
χac
, and optimal resonance frequency (f
res
) of Co
40
Fe
40
Sm
20
(10–50 nm) thin films with different
annealing temperatures.
Ta(◦C) Thickness
(nm) D(nm) Ra
(nm) Rs(kΩ/sq) ρ
(×10−2Ω-cm)
Maximum χac
(a.u.)
Optimal
Resonance
Frequency (Hz)
RT
10 68.96 6.97 543.9 54.4 0.010 50
20 67.13 6.58 87.90 17.6 0.027 100
30 61.76 6.50 2.53 0.78 0.036 50
40 56.95 6.44 0.29 0.10 0.072 50
50 49.50 6.31 0.24 0.12 0.110 50
100
10 66.47 6.88 543.9 54.4 0.025 50
20 61.10 6.53 39.36 7.87 0.034 100
30 57.94 6.44 1.97 0.59 0.050 50
40 55.79 6.39 0.28 0.11 0.075 50
50 48.44 6.28 0.17 0.086 0.120 50
200
10 65.32 6.68 543.9 54.4 0.037 50
20 57.10 6.37 25.03 5.00 0.052 50
30 53.15 6.35 0.64 0.19 0.070 50
40 53.08 6.29 0.14 0.058 0.083 50
50 47.85 6.21 0.14 0.071 0.130 50
300
10 60.89 6.57 543.9 54.4 0.071 50
20 54.62 6.32 28.05 5.61 0.110 50
30 47.23 6.29 0.67 0.20 0.130 50
40 46.46 6.26 0.22 0.086 0.120 50
50 41.21 6.17 0.17 0.086 0.100 50
3.5.2. Magnetic Domain Structure
Figure 8(a1–a4) and Figure 8(b1–b4) display the MFM images of Co
40
Fe
40
Sm
20
thin
films, specifically those with 20 nm and 40 nm thicknesses, which were subjected to dif-
ferent annealing temperatures. During the course of this study, MFM images of both the
as-deposited and annealed Co
40
Fe
40
Sm
20
thin films were obtained over a scanning area
measuring 10
×
10
µ
m
2
. To prevent interference from AFM signals or Van der Waals
forces, a lift height of 130 nm was employed [
31
]. The MFM images presented variations
in contrast, with regions appearing bright, dark, and exhibiting intermediate contrast.
Figure 8(a1–a4) depict the MFM images of Co
40
Fe
40
Sm
20
thin films with a
20 nm
thickness
in both the as-deposited state and after annealing. At RT, a wave stripe domain structure is
evident, with the wave stripe shape growing larger upon annealing at
100 ◦C
. However, the
magnetic domain structure becomes less distinct when annealed at
200 ◦C
and 300
◦
C. This
observation explains the deterioration in the magnetic properties of 20 nm Co
40
Fe
40
Sm
20
thin films at annealing temperatures exceeding 100
◦
C. On the other hand, the domain
structure of the 40 nm Co40Fe40Sm20 thin film exhibits a particle-like pattern at RT. As the
annealing temperature increases to 100
◦
C, the domain size expands. When annealed at
200
◦
C, the 40 nm Co
40
Fe
40
Sm
20
thin film displays a stripe domain structure with more
pronounced dark and bright variations. However, at 300
◦
C, the magnetic properties deteri-
orate, resulting in ambiguous magnetic domain patterns. Consequently, magnetic contrast
increases with both increasing film thickness and higher annealing temperatures due to
the smoother surface. Additionally, a rougher film surface leads to a greater tip-to-sample
distance, causing a decrease in magnetostatic forces and magnetic contrast [
32
]. While rais-
ing the annealing temperature improved the magnetic characteristics of Co
40
Fe
40
Sm
20
thin
films, it also led to a substantial degradation at elevated annealing temperatures in films
with specific 20 nm and 40 nm thicknesses. They exhibited deteriorated magnetic proper-
ties at 200
◦
C and 300
◦
C annealing temperatures, respectively. The results demonstrated
that the magnetic properties of these films are significantly affected by changes in surface

Coatings 2023,13, 1961 10 of 14
characteristics and annealing conditions. Specifically, we observed that as the film thickness
increased, the magnetic contrast improved, primarily due to the smoother surface promot-
ing enhanced magnetostatic forces. On the other hand, rougher film surfaces increased the
tip-to-sample distance, resulting in reduced magnetostatic forces and magnetic contrast.
Additionally, our investigation revealed that the annealing process played a crucial role in
altering the magnetic domain structure. While moderate annealing temperatures led to
improved magnetic properties, such as larger and more distinct magnetic domains, higher
annealing temperatures had a detrimental effect, leading to ambiguous or deteriorated
magnetic domains. These findings underscore the importance of careful control over film
thickness and annealing parameters in optimizing the magnetic properties of thin films.
Further research may explore additional factors that could influence magnetic domain
structure and offer a more comprehensive understanding of the underlying mechanisms.
This knowledge is essential for the development and enhancement of thin film materials
for various technological applications, particularly in the field of magnetic devices and
data storage.
Coatings 2023, 13, x FOR PEER REVIEW 11 of 15
Figure 8. MFM images of 20 nm Co40Fe40Sm20 thin films with annealing temperatures of (a1) RT,
(a2) 100 °C, (a3) 200 °C, and (a4) 300 °C. 40 nm Co40Fe40Sm20 thin films with annealing tempera-
tures of (b1) RT, (b2) 100 °C, (b3) 200 °C, and (b4) 300 °C.
3.5.3. Hysteresis Loop
The Co40Fe40Sm20 thin films annealed at 200 °C with a thickness of 40 nm exhibited
reduced sheet resistance and resistivity, along with improved magnetic contrast. Con-
sequently, an in-plane hysteresis loop was investigated for the 40 nm Co40Fe40Sm20 thin
films using AGM. Figure 9a illustrates the in-plane hysteresis loop of these thin films, and
Figure 9b shows the plot of the coercivity and maximum saturation magnetization for
in-plane magnetized 40 nm Co40Fe40Sm20 thin films at various annealing temperatures.
Table 2 presents the values of Hc, Ms, and the remanence ratio (Mr/Ms) for the 40 nm thin
films at various annealing temperatures. The Hc values exhibit a decreasing trend as the
annealing temperature rises, likely due to the reduction in crystalline size within the 40
nm as the annealing temperature increases from RT to 300 °C. It is worth noting that
Co40Fe40Sm20 thin films with finer grain structures typically display lower coercivity and
adhere to a power law relationship, Hc ∝ D6 [5,35]. Conversely, the Ms of the 40 nm thin
films increases with higher annealing temperatures, up to 200 °C, after which it decreases
at 300 °C. Consequently, the 40 nm thin film annealed at 200 °C exhibits superior soft
magnetic properties and is well suited for applications in spintronics, micro-actuators,
magnetic memories, and storage devices. Furthermore, the 40 nm thick Co40Fe40Sm20 thin
film annealed at 200 °C displays a lower Mr/Ms ratio, requiring only a low demagnetiza-
tion field to return to its initial magnetization state [36].
Figure 8.
MFM images of 20 nm Co
40
Fe
40
Sm
20
thin films with annealing temperatures of (
a1
) RT,
(
a2
) 100
◦
C, (
a3
) 200
◦
C, and (
a4
) 300
◦
C. 40 nm Co
40
Fe
40
Sm
20
thin films with annealing temperatures
of (b1) RT, (b2) 100 ◦C, (b3) 200 ◦C, and (b4) 300 ◦C.
Surface roughness is a prevalent characteristic in various materials and significantly
influences the behavior of magnetic domains. Understanding how surface roughness
impacts the formation and behavior of magnetic domains is critical for optimizing the
performance of magnetic materials. This finding reveals the considerable influence of
surface roughness on the formation and properties of magnetic domains. Coarser surfaces
may lead to irregular distribution of magnetic domains, while smoother surfaces could
contribute to more orderly arrangements of magnetic domains [
33
,
34
]. This study highlights
the close relationship between surface roughness and magnetic domains and provides
valuable insights into how optimizing surface roughness can enable precise control of
magnetic domains.

Coatings 2023,13, 1961 11 of 14
3.5.3. Hysteresis Loop
The Co
40
Fe
40
Sm
20
thin films annealed at 200
◦
C with a thickness of 40 nm exhibited
reduced sheet resistance and resistivity, along with improved magnetic contrast. Conse-
quently, an in-plane hysteresis loop was investigated for the 40 nm Co
40
Fe
40
Sm
20
thin
films using AGM. Figure 9a illustrates the in-plane hysteresis loop of these thin films, and
Figure 9b shows the plot of the coercivity and maximum saturation magnetization for
in-plane magnetized 40 nm Co
40
Fe
40
Sm
20
thin films at various annealing temperatures.
Table 2presents the values of H
c
, M
s
, and the remanence ratio (M
r
/M
s
) for the 40 nm
thin films at various annealing temperatures. The H
c
values exhibit a decreasing trend as
the annealing temperature rises, likely due to the reduction in crystalline size within the
40 nm as the annealing temperature increases from RT to 300
◦
C. It is worth noting that
Co
40
Fe
40
Sm
20
thin films with finer grain structures typically display lower coercivity and
adhere to a power law relationship, Hc
∝
D
6
[
5
,
35
]. Conversely, the M
s
of the 40 nm thin
films increases with higher annealing temperatures, up to 200
◦
C, after which it decreases
at 300
◦
C. Consequently, the 40 nm thin film annealed at 200
◦
C exhibits superior soft
magnetic properties and is well suited for applications in spintronics, micro-actuators, mag-
netic memories, and storage devices. Furthermore, the 40 nm thick Co
40
Fe
40
Sm
20
thin film
annealed at 200
◦
C displays a lower M
r
/M
s
ratio, requiring only a low demagnetization
field to return to its initial magnetization state [36].
Coatings 2023, 13, x FOR PEER REVIEW 12 of 15
Tab l e 2. Coercivity (H
c
), saturation magnetization (M
s
), and remanence ratio (M
r
/M
s
) of
Co
40
Fe
40
Sm
20
(40 nm) thin films with different annealing temperatures (T
a
).
T
a
(°C) H
c
(kOe) M
s
(emu/cm
3
) M
r
/M
s
RT 0.330 830.29 0.47
100 0.290 1059.82 0.60
200 0.030 1119.44 0.17
300 0.031 988.19 0.44
Figure 9. (a) In-plane hysteresis loop and (b) variations in coercivity, saturation magnetization, and
remanence ratio for in-plane magnetization of deposited and annealed Co
40
Fe
40
Sm
20
(40 nm) thin
films.
Taking into account the entirety of this study, the presence of oxide impurities no-
tably impacts various physical properties of CoFeSm thin films. The exploration of how
cobalt oxide (Co
2
O
3
) influences the magnetic and electrical characteristics of antiferro-
magnetic materials has become a compelling focal point in the domains of materials sci-
ence and magnetism research. Cobalt oxide possesses the capacity to exert a multifacet-
ed influence on the properties of antiferromagnetic materials. When applied to CoFeSm
thin films, cobalt oxide demonstrates the potential to modulate exchange interactions
between magnetic moments and modify the materials’ magnetic anisotropy, thereby
influencing the orientation and stability of magnetic moments, and even decrease their
overall magnetization [37]. Furthermore, the introduction of cobalt oxide impurities can
induce substantial adjustments in the electrical conductivity of these materials. Conse-
quently, this leads to variations in resistivity, refinements in conductivity mechanisms,
and consequential alterations in the materials’ electrical responses. It is noteworthy that
the presence of oxides can obstruct the flow of electrons, resulting in electron scaering
effects and an increase in the material’s resistivity [38]. Recognizing the impact of cobalt
oxide on the magnetic and electrical aributes of these materials is essential for tailoring
their characteristics to meet specific criteria, especially in domains such as data storage,
sensor technology, and emerging technological applications.
Figure 9.
(
a
) In-plane hysteresis loop and (
b
) variations in coercivity, saturation magnetization,
and remanence ratio for in-plane magnetization of deposited and annealed Co
40
Fe
40
Sm
20
(40 nm)
thin films.
Table 2.
Coercivity (H
c
), saturation magnetization (M
s
), and remanence ratio (M
r
/M
s
) of
Co40Fe40 Sm20 (40 nm) thin films with different annealing temperatures (Ta).
Ta(◦C) Hc(kOe) Ms(emu/cm3)Mr/Ms
RT 0.330 830.29 0.47
100 0.290 1059.82 0.60
200 0.030 1119.44 0.17
300 0.031 988.19 0.44
Taking into account the entirety of this study, the presence of oxide impurities notably
impacts various physical properties of CoFeSm thin films. The exploration of how cobalt
oxide (Co
2
O
3
) influences the magnetic and electrical characteristics of antiferromagnetic
materials has become a compelling focal point in the domains of materials science and

Coatings 2023,13, 1961 12 of 14
magnetism research. Cobalt oxide possesses the capacity to exert a multifaceted influence
on the properties of antiferromagnetic materials. When applied to CoFeSm thin films,
cobalt oxide demonstrates the potential to modulate exchange interactions between mag-
netic moments and modify the materials’ magnetic anisotropy, thereby influencing the
orientation and stability of magnetic moments, and even decrease their overall magnetiza-
tion [
37
]. Furthermore, the introduction of cobalt oxide impurities can induce substantial
adjustments in the electrical conductivity of these materials. Consequently, this leads
to variations in resistivity, refinements in conductivity mechanisms, and consequential
alterations in the materials’ electrical responses. It is noteworthy that the presence of oxides
can obstruct the flow of electrons, resulting in electron scattering effects and an increase
in the material’s resistivity [
38
]. Recognizing the impact of cobalt oxide on the magnetic
and electrical attributes of these materials is essential for tailoring their characteristics to
meet specific criteria, especially in domains such as data storage, sensor technology, and
emerging technological applications.
4. Conclusions
Co
40
Fe
40
Sm
20
thin films were manufactured on Si(100) substrates utilizing a direct
current sputtering system and subsequently subjected to annealing at temperatures up to
300
◦
C. The investigation focused on exploring the interplay between surface roughness
and the magnetic domain structure of these thin films, varying film thicknesses, and an-
nealing temperatures. XRD analysis revealed distinct crystalline structures, notably Co
(0002), Co
2
O
3
(422), and Co
2
O
3
(511), observed at diffraction angles of 47.7
◦
, 54.6
◦
, and
56.4
◦
, respectively. EDS confirmed the presence of Co, Fe, and Sm atoms within the thin
films. Notably, both the crystalline size and surface roughness decreased with escalating
film thickness and higher annealing temperatures, contributing to a smoother surface. The
heightened maximum
χac
values denote smoother motion of free magnetic domains, sug-
gesting increased spin sensitivity. Moreover, these films displayed their highest maximum
χac
values at 50 Hz and 100 Hz, underscoring their suitability for applications in low-
frequency magnetic devices. MFM revealed a more distinct stripe-like domain structure
with heightened dark and bright contrasts in the 40 nm Co
40
Fe
40
Sm
20
thin film annealed
at 200
◦
C. This study has provided valuable insights into the influence of various factors,
including surface roughness and annealing temperature, on the magnetic domain structure
of thin films. The results demonstrated that the magnetic properties of these films are
significantly affected by changes in surface characteristics and annealing conditions. Specif-
ically, we observed that as the film thickness increased, the magnetic contrast improved,
primarily due to the smoother surface promoting enhanced magnetostatic forces. On the
other hand, rougher film surfaces increased the tip-to-sample distance, resulting in reduced
magnetostatic forces and magnetic contrast. Additionally, our investigation revealed that
the annealing process played a crucial role in altering the magnetic domain structure. While
moderate annealing temperatures led to improved magnetic properties, such as larger and
more distinct magnetic domains, higher annealing temperatures had a detrimental effect,
leading to ambiguous or deteriorated magnetic domains. The 40 nm Co
40
Fe
40
Sm
20
thin
film annealed at 200
◦
C displayed the H
c
and the highest M
s
, highlighting their enhanced
soft magnetic characteristics. Consequently, this study underscores the significant influence
of surface morphology on the modulation of magnetic properties within Co
40
Fe
40
Sm
20
thin films.
Author Contributions:
Conceptualization, C.-L.F., W.-J.L., Y.-H.C., Y.-T.C. and S.-H.L.; methodology,
C.-L.F., Y.-T.C., Y.-H.C., J.-X.L., W.-G.C. and K.-W.L.; validation and formal analysis, J.-X.L., Y.-T.C.
and W.-G.C.; investigation, C.-L.F., Y.-T.C. and W.-J.L.; resources, C.-C.C., T.-H.W. and K.-W.L.;
writing—original draft preparation, Y.-T.C.; writing—review and editing, C.-L.F., Y.-T.C. and W.-
J.L.; supervision, Y.-T.C. and Y.-H.C.; project administration, Y.-T.C., T.-H.W., and S.-H.L.; funding
acquisition, C.-L.F., Y.-H.C. and C.-C.C. All authors have read and agreed to the published version of
the manuscript.

Coatings 2023,13, 1961 13 of 14
Funding:
This work was supported by the National Science Council, under Grant Nos. MOST
110-2221-E-992-054 -MY3, MOST108-2221-E-224-015-MY3, MOST105-2112-M-224-001, and National
Yunlin University of Science and Technology, under Grant Nos. 111T01 and 113T01.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data are contained within the article.
Conflicts of Interest:
The authors declare that there is no conflict of interest regarding the publication
of this paper. The funders had no role in the design of the study; in the collection, analyses, or
interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
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