as a nucleation site for Î±-Fe2O3

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Specific domain pattern of #-Fe2O3 thin films grown on YSZ (100) as a nucleation site for #-Fe2O3 Vu Quoc Viet, Salawu Yusuff Adeyemi, WonHyuk Shon, Jong Soo Rhyee, Nam-Suk Lee, and Kim Heon-Jung Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00338 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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Crystal Growth & Design

Specific domain pattern of ε-Fe2O3 thin films grown on YSZ (100) as a nucleation site for α-Fe2O3 Vu Quoc Viet1, Salawu Yusuff Adeyemi1, Won Huyk Son 2, Jong-Soo Rhyee 2, Nam-Suk Lee 3,* and Heon-Jung Kim1,4,* 1 2

Department of Physics, College of natural Science, Daegu University, Gyeongbuk 38453, Republic of Korea.

Department of Applied Physics & KHU-KIST Department of Converging Science and Technology, Kyung Hee University, Yong-In, 17104, Republic of Korea 3

National Institute for Nanomaterials Technology (NINT), Pohang University of Science and Technology (POSTECH), Pohang, 37673, Republic of Korea

4

Department of Materials-Energy Science and Engineering, College of Engineering, Daegu University, Gyeongbuk 38453, Republic of Korea.

Abstract This study examined the phase competition and critical thickness of ε-Fe2O3 in epitaxial εFe2O3 thin films grown on a (100)-oriented yttrium-stabilized zirconia (YSZ) substrate using a pulsed laser deposition (PLD) technique. The maximum critical thickness was found to be ~20 nm for a single phase of ε-Fe2O3 at the optimal laser spot size without impurities. Above this critical thickness, (00l)-oriented α-Fe2O3 begins to appear along with ε-Fe2O3, eventually dominating in the thicker samples. These results suggest that the stability of ε-Fe2O3 films are influenced by the appearance of a specific domain pattern, which becomes a favorable nucleation site for α-Fe2O3. The ε-Fe2O3 films grown on the YSZ (100) substrate have a larger saturation moment than ε-Fe2O3 grown on SrTiO3 (111) and its coercive field approaches 1/4 of the maximum coercive field of ε-Fe2O3 nanoparticles. *

[email protected]

*

[email protected]

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I. Introduction Magnetoelectric multiferroics are being studied intensively because of their rich physics and potential applications as multifunctional devices [1,2]. Currently, the development of magnetoelectric memory using these materials is limited by the low transition temperature. With the exception of ferroelectric antiferromagnetic BiFeO3, most magnetoelectric multiferroic materials show multiferroism at very low temperatures [3,4]. Therefore, it is essential to search for new multiferroic materials demonstrating a multiferroic effect with significant magnetoelectric coupling near or above room temperature to realize their technological potential. ε-Fe2O3, which is a metastable intermediate between maghemite γ-Fe2O3 and hematite αFe2O3, is an interesting candidate in this respect. Indeed, this material is stabilized as nanoparticles via the γ-Fe2O3 ε-Fe2O3 α-Fe2O3 phase transformation pathway only when the grains are not agglomerated or they are in a supporting medium. On the other hand, γ-Fe2O3 transforms directly to α-Fe2O3 when grains are highly agglomerated [5,6]. ε-Fe2O3 is the first multiferroic system at room temperature with a single active ion [7,8], belonging to a non-centrosymmetric structure with the Pna21 space group [9,10]. Indeed, ε-Fe2O3 is isostructural with the polar ferrimagnet, GaFeO3 (GFO) [11]. Compared to GFO, ε-Fe2O3 has a larger coercive field, probably because of the weaker disorder effect [11–13]. Compared to other magnetoelectric oxides with multi-elements, ε-Fe2O3 has an advantage because it is a single-element, single-valent oxide with a very high Curie temperature (~ 510 K). In addition, its high coercive field is promising. Ohkishi reported the highest coercive field of 20 kOe and its strong dependence on the nanoparticle size [13]. The strong coupling of its magnetic and dielectric properties was also demonstrated [14]. Up to now, there are only two substrates onto which ε-Fe2O3 thin films can be deposited directly: SrTiO3 (STO) (111) [15] and yttrium-stabilized zirconia (YSZ) (100) [16]. Although ε-Fe2O3 thin films have been stabilized successfully on those substrates, their saturation magnetization and coercive field were lower than those of ε-Fe2O3 nanoparticles. One possible way to improve the quality of ε-Fe2O3 thin films is to use a buffer layer. As reported previously [17], GFO acts as a buffer layer because it is isostructural with ε-Fe2O3. Moreover, it can be grown easily on several substrates [17]. Therefore, this approach extends the growth regions and substrate types for ε-Fe2O3 thin films. One interesting observation is the increase


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in saturation magnetization in GFO-buffered ε-Fe2O3 films grown on a YSZ(100) substrate compared to the GFO-buffered ε-Fe2O3 film on STO(111) [17]. This increase was attributed to unique anisotropic strain that the YSZ(100) substrate makes on a ε-Fe2O3 film. Indeed, the large orbital moment and high coercive field in ε-Fe2O3 result from the same off-centering of the Fe3+ ion. Therefore, the increase in saturation magnetization in the GFO-buffered ε-Fe2O3 film grown on a YSZ(100) substrate suggests a greater degree of off-centering in this sample. Furthermore, as the saturation magnetization and the coercive field of a ε-Fe2O3 thin film do not reach the maximum values of ε-Fe2O3 nanoparticles [17], the properties of ε-Fe2O3 thin films, such as the structure, can still be optimized through anisotropic strain so that the saturation magnetization and coercive field are maximized. This paper reports the systematic growth of ε-Fe2O3 films on a YSZ (100) substrate by optimizing the laser spot size in a pulsed laser deposition (PLD) technique. The maximum critical thickness was found to be ~20 nm for a single phase of ε-Fe2O3 at the optimal laser spot size without impurities. Above this critical thickness, (00l)-oriented α-Fe2O3 begins to appear along with ε-Fe2O3, eventually dominating the thicker samples. The stability of the εFe2O3 films are influenced by the appearance of a specific domain pattern, which becomes a nucleation site for α-Fe2O3. Because of the optimization of the laser spot size, the film on the YSZ (100) substrate has a high coercive field of 5 kOe and a saturation moment of ~0.4 µB/Fe3+, which is 1/4 of the maximum coercive field of ε-Fe2O3 nanoparticles and larger than ~0.2 µB/Fe3+ of a film grown on the STO (111) substrate.

II. Experiment ε-Fe2O3 thin films with different thicknesses were grown on a YSZ (100) substrate. A Kr excimer laser with a 248 nm wavelength was used at a 2 Hz repetition rate and focused at 1.8 J/cm2. A polycrystalline α-Fe2O3 was used as the target and placed 5 cm from the substrate. The growth conditions were those used in Ref. [15]. The substrate temperature was kept at 800°C. The oxygen pressure was maintained at 3×10-3 torr during growth and increased to 30 torr once the substrate temperature reached 600 °C upon cooling. The crystal structure of the films was determined by X-ray diffraction (XRD) using a Rigaku diffractometer with Cu K radiation (λ=1.54 Å). The thickness, microstructure, and atomic structure were examined by high-resolution transmission electron microscopy (HRTEM, Cs-corrected STEM, JEOL



JEM-2100F) at 200 kV. The chemical composition of the film was analyzed by energy dispersive X-ray spectroscopy (EDS) elemental mapping. The magnetic properties of the selected films were studied by measuring the DC magnetization using a vibrating sample magnetometer (VSM) in the physical property measurement system (PPMS, Quantum Design, Inc.)

III. Results and discussion Because the laser beam is the most important parameter in PLD, its influence was first investigated to optimize it. The spot size of the incident laser beam was changed systematically while fixing the laser energy. In each spot size, several samples were grown for different deposition periods. The absence or presence of impurity α-Fe2O3 phase in each sample was determined by XRD, as shown in the inset of Fig. 1. The (006) reflection of the α-Fe2O3 structure appears near the (004) reflection of the ε-Fe2O3 structure. Fig. 1 shows the thickness-dependent phase boundary of pure ε-Fe2O3 thin films in three different spot sizes. In the medium spot size, the critical thickness was largest with 20 nm while it was approximately 15 nm in the small and large spot sizes. In each spot size, above this critical thickness, α-Fe2O3 begins to appear. The spot size determines the growth or deposition rate directly because it is related to the amount of ablated material. Therefore, Fig. 1 shows that the growth rate is a relevant parameter to the stability of ε-Fe2O3 thin films and the formation of an impurity phase. Figure 2(a) shows a θ-2θ scan of the 20 nm-thick ε-Fe2O3 thin film grown on a YSZ (100) substrate at a medium spot size. This film is a single phase with no impurities. Several peaks corresponding to the (002), (004), (006), (008), and (0010) reflections of the ε-Fe2O3 structure were observed. The θ-2θ data show the out-of-plane epitaxial relationship of ε{001}ε//YSZ-{001} between the ε-Fe2O3 film and YSZ substrate, which agrees with previous reports [16,17]. As ε-Fe2O3 is orthorhombic while YSZ is cubic, it is important to understand how the ε-Fe2O3 thin film is grown on a YSZ (100) substrate. A pole figure around the (013) reflection of the ε-Fe2O3 layer was measured to obtain the in-plane texture of ε-Fe2O3 on YSZ (100), as shown in Fig. 2(b). The pole figure data can be understood by the match of the inplane lattice constant of YSZ (100) and ε-Fe2O3 unit cells. In the case of ε-Fe2O3 films on the YSZ (100) substrate, two distinct types of in-plane epitaxial match arise between the ε-Fe2O3


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and YSZ (100) unit cells [16,17]. The first epitaxial match is the alignment of aε with aYSZ, as shown in the lower part of Fig. 2(c). This leads to a tensile strain of 0.58 %. The second match is given by the upper part of Fig. 2(c). Here, the diagonal of the rectangle formed by the and axes of the ε-Fe2O3 unit cell is approximately twice the lattice constant of YSZ, aYSZ ( + ≈ 2aYSZ). In this case, the tensile strain was calculated to be 0.97 %. Fig 2(c) presents a schematic diagram of the ε-Fe2O3 domain structure on the YSZ (001) substrate. The angle between the direction of the lattice parameter and that of aYSZ is α = arcos ( / + ) ≈ 30 . This means there are six domains subtended by 30°. A similar domain structure was reported in the GFO-buffered ε-Fe2O3 films grown on a YSZ (001) substrate [17]. In the pole figure, there are four more intense diffraction peaks. This is because domains with smaller strain, whose axis is aligned with aYSZ, are more populated. A similar phenomenon occurred in the GFO-buffered ε-Fe2O3 film on the YSZ (100) substrate. Of the six domains, two domains with the smallest lattice mismatch are illustrated by a bold solid line in the lower part of Fig. 2(c). According to previous studies of ε-Fe2O3 nanoparticles, there are certain factors that determine the stability and size of ε-Fe2O3 nanoparticles, i.e., the chemical potential and surface energy [12]. In particular, the relationship between the free energy and nanoparticle size showed that ε-Fe2O3 can exist only when its size falls within a certain range [19]. Indeed, this is a result of competition between the large positive chemical potential and negative contribution of the surface energy. The optimal size of the ε-Fe2O3 nanoparticle is determined by this competition. Without a surface energy contribution, ε-Fe2O3 would be completely unstable. In contrast to nanoparticles, in addition to thermodynamic factors, other aspects are expected to play a critical role in the stability and formation of epitaxial ε-Fe2O3 thin films. On the other hand, the other factors and their roles have not been well explored up to now. Strain might play some role in a ε-Fe2O3 thin film. The elastic energy, in which strain might be considered to contribute, does not reduce the total energy. For different reasons, a ε-Fe2O3 thin film is stabilized only on STO(111) and YSZ(100). Even in these substrates, the surface energy between the domain boundaries is a key ingredient to stabilize a ε-Fe2O3 thin film. Related with this, a previous study proposed a growth mechanism of ε-Fe2O3 nanoparticles and thin films [16]. According to this study, there is a tendency that nanoparticles grow anisotropically along the [100] direction because of the high (100) surface energy of ε-Fe2O3.



In the case of ε-Fe2O3 thin films, domain patterns and the resulting domain boundaries reduce this surface energy. This “domain epitaxy” aids in the formation of ε-Fe2O3 thin films. According to the inset in Fig. 1, the (001) plane of the α-Fe2O3 structure is grown on top of the ε-Fe2O3 phase. As illustrated schematically in Fig. 2(d), there is an edge point where two ε-Fe2O3 domains meet. The angle of this edge is 120°. Note that α-Fe2O3 is rhombohedral with parameters, aα = bα = 5.03 Å, cα = 13.74 Å, and γ = 120o, whereas ε-Fe2O3 is orthorhombic with aε= 5.08 Å, bε=8.78 Å, and cε=9.47 Å. Therefore, the aforementioned edge point can be a good nucleation site for the growth of the (001) plane of the α-Fe2O3 structure. First, this can satisfy the γ angle of the rhombohedral structure, becoming an interface between ε-Fe2O3 and α-Fe2O3 if their height is the same. Second, there is relatively low tensile strain (~0.97 %) between ε-Fe2O3 and α-Fe2O3 when aε and aα are aligned. If these nucleation sites are abundant, they will effectively prevent the formation of ε-Fe2O3. This is an important factor to consider in the stability and formation of ε-Fe2O3 thin films, in addition to thermodynamic aspects. The results in Fig. 1, particularly in a small spot size, can be understood qualitatively based on this picture. In general, a low deposition rate results kinetically in the growth of many small grains. This suggests that there are more edge sites discussed above than the other cases. This limits the critical thickness of the ε-Fe2O3 film and may be an important reason why the critical thickness is reduced in a small spot size than in a medium spot size. As the critical thickness decreases in the large spot size, there must be other kinetic factors that limit the critical thickness. This is expected to be related to mass transport because it is limited by the high growth rate in this region. If the effect of nucleation sites is absent, the critical thickness will be determined solely by thermodynamic factors. In the medium spot size, however, this critical thickness is still much larger than the expected grain size of the nanoparticles at the substrate temperature (800 °C) [20]. In fact, at 800 °C, ε-Fe2O3 is not so stable compared to γ-Fe2O3, which suggests that the effective temperature is higher than 800 °C. In PLD synthesis, the additional energy is supplied by the energetic laser beam. The grain size is increased to approximately 10 nm at 1200 °C in the case of nanoparticles [20]. Hence, 1200 °C is considered to be an effective temperature in the medium spot size. The effective temperature will be even higher if the effect of the nucleation sites is considered. As the X-ray measurements only confirmed the epitaxial relationship and crystallinity of the


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ε-Fe2O3 film grown on the YSZ (100) substrate, it is still necessary to examine its local structure by HRTEM. Fig. 3 shows cross-sectional scanning TEM (STEM) images and EDS elemental mapping analysis of an epitaxial ε-Fe2O3 film deposited on a YSZ (100) substrate at the medium spot size. The sharp interface can be observed clearly in the high angle annular dark field (HAADF) STEM image [Fig. 3(a)] between the ɛ-Fe2O3 film and YSZ substrate. The interface can be identified more clearly when HAADF-STEM images are coupled with the EDS map. The EDS elemental maps in Figs. 3(b)-(g) show the spatial distribution of Fe, Zr, Y, and O ions in each layer. The A, B, and C marks in Fig. 3(b)-(g) refer to air, ε-Fe2O3, and YSZ, respectively. In particular, these three layers are all distinguishable with very clear interfaces. These results are in good agreement with the θ-2θ scan discussed above, which distinguishes both the film and substrate. The symmetry of the ε-Fe2O3 layer was examined further by a fast Fourier transformation (FFT). The inset in Fig 3(a) shows the corresponding FFT image of ε-Fe2O3 film taken at the [3-10] zone axis. Magnetization measurements were taken of the ε-Fe2O3 films grown on the YSZ (100) and STO (111) substrates. Because ε-Fe2O3 is an anisotropic material that exhibits easy in-plane and hard out-of-plane magnetic properties [19], magnetization measurements were conducted with magnetic fields parallel to the substrate. Fig. 4 presents the magnetization curve as a function of the magnetic field M(H) measured at T = 300 K. These M(H) curves exhibit the typical shapes of a ferromagnetic or a ferrimagnetic material. Both samples have a coercive field of approximately 5 kOe. This value is highest among the ε-Fe2O3 films grown on YSZ (100) [16] and is comparable to the highest value of 8 kOe reported for ε-Fe2O3 films grown on STO (111) [15]. Furthermore, this value is the same as that measured in the a-axis-oriented ε-Fe2O3 nanoparticles with H applied along the a-direction [21]. In addition, the in-plane saturation magnetization of ε-Fe2O3 on the YSZ (100) substrate is ~ 0.4 µB/Fe3+, which is larger than the ~ 0.2 µB/Fe3+ of a ε-Fe2O3 film grown on a STO (111) substrate. This value, however, is still lower than the maximum saturation magnetization reported in ε-Fe2O3 nanoparticles [21]. This sample dependence of the saturation magnetization is intriguing and may be understood based on the magnetic structure of εFe2O3. In fact, there are four Fe sites in a ε-Fe2O3 unit cell: three at the octahedral sites and one at the tetrahedral site. Owing to the off-centering of Fe3+ ions, sublattice magnetization at the tetrahedral site is smaller. As the magnetism of ε-Fe2O3 is collinear ferrimagnetism, in which two of the sublattice magnetizations are antiparallel to the other two, the difference



between sublattice magnetization at the tetrahedral and octahedral sites is approximately the total magnetization of ε-Fe2O3. Therefore, the saturation magnetization of ε-Fe2O3 can change with the degree of off-centering of the Fe3+ ion. As the optimal spot size can give rise to structurally ideal films, probably with a greater degree of off-centering of the Fe3+ ion, optimization of the laser spot size leads to increased saturation magnetization in ε-Fe2O3 thin films, as observed experimentally.

IV. Conclusion Metastable ɛ-Fe2O3 thin films were synthesized on YSZ (001) substrates using a pulsed laser deposition technique. This study focused on optimizing the laser spot size to improve the quality of the ɛ-Fe2O3 thin film and examine its influence on the critical thickness and phase stability. A critical thickness of ~ 20 nm was found at the optimal spot size. Above this critical thickness, (00l)-oriented α-Fe2O3 begins to emerge along with ε-Fe2O3, eventually dominating the thicker samples. XRD and TEM confirmed the particular domain distribution patterns and smooth interface, respectively. These results suggest that the stability of ε-Fe2O3 films is influenced by the appearance of a specific domain pattern, which becomes a nucleation site for α-Fe2O3. The ɛ-Fe2O3 thin film on a YSZ (001) substrate has not only a high coercive field, but also large saturation magnetization. This might be the result of laserspot-size optimization. As the high coercive field of ε-Fe2O3 nanoparticles has potential applications in non-volatile memory and permanent magnets, the present demonstration of the high coercive field and high saturation magnetization in these ɛ-Fe2O3 thin films will accelerate the application of this interesting material in a wide range of fields.

Acknowledgement This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2017R1A2B2002731).

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Spot size Fig. 1 Thickness dependence of phase stabilization in ɛ-Fe2O3 films on YSZ(100) substrates. In the medium spot size, (00l) ɛ-Fe2O3 begins to transform to (00l) α-Fe2O3 at a thickness of 20 nm. In small and large spot sizes, this change occurs at a smaller thickness (~15 nm). The inset shows XRD patterns of the films, where one phase evolves into 2 phases until α-Fe2O3 is dominant at the thick sample. S, M, and L mean 1.9 mm2, 3.2 mm2, 5.1 mm2, respectively.


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2θ θ(deg.)

b)

Fig. 2 (a) θ-2θ scan for a ɛ-Fe2O3 film grown on a YSZ (001) substrate. (b) Pole figure of the {013} plane of a c-oriented ɛ-Fe2O3 film. (c) Visualization of the domain structure on YSZ (001) substrates. Two domains of ɛ-Fe2O3 on the YSZ (001) surface. (d) two domains sharing a horizontal line (the upper figure) and in-plane shape of a rhombohedral α-Fe2O3 unit cell (the lower figure).



a)

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b)

c)

d)

d)

e)

g)

Fig. 3 High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image and EDS elemental mapping analysis for Fe(K), Zr(K), O(K) and Y(K) of an ɛ-Fe2O3 film on a YSZ(100) substrate. (a) The inset represents the fast Fourier transform (FFT) of the lattice-resolved image of the ɛ-Fe2O3 film, (b)-(g) EDS elemental mapping analysis of the ɛ-Fe2O3 film.


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ε− Fe2O3/YSZ

0.30

ε− Fe2O3/STO 0.15

M(µ µ B/Fe3+)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.00 -0.15 -0.30 -0.45 -20

-15

-10

-5

0

5

10

15

20

H(kOe)

Fig. 4 M(H) curve of ɛ-Fe2O3 (~20 nm) films grown on YSZ(100) and STO(111) substrates at T = 300 K.



“For Table of Contents Use Only”

Table of Content

Specific domain pattern of ε-Fe2O3 thin films grown on YSZ (100) as a nucleation site for α-Fe2O3 Vu Quoc Viet1, Salawu Yusuff Adeyemi1, Wonhyuk Son 2, Jong-Soo Rhyee 2, Nam-Suk Lee 3,* and Heon-Jung Kim1,4,* 1 2

Department of Physics, College of natural Science, Daegu University, Gyeongbuk 38453, Republic of Korea.

Department of Applied Physics & KHU-KIST Department of Converging Science and Technology, Kyung Hee University, Yong-In, 17104, Republic of Korea 3

National Institute for Nanomaterials Technology (NINT), Pohang University of Science and Technology (POSTECH), Pohang, 37673, Republic of Korea

4

Department of Materials-Energy Science and Engineering, College of Engineering, Daegu University, Gyeongbuk 38453, Republic of Korea.


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Synopsis: Plane view of ɛ-Fe2O3 and α-Fe2O3 unit cells on a YSZ(001) substrate oriented along the c-direction. Edge structure of two ɛ-Fe2O3 domains sharing a horizontal line is a possible nucleation site for α-Fe2O3. Formation of this structure limits stable growth of ɛFe2O3 on the YSZ(001)


as a nucleation site for Î±-Fe2O3

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