Structure and magnetic properties of pulsed laser deposited SrFe12O19 thin films on SrTiO3 (100) and (111) substrates

Accepted Manuscript Structure and magnetic properties of pulsed laser deposited SrFe12O19 thin films on SrTiO3 (100) and (111) substrates Dong Hun Kim, Seung Ho Han, Young-Min Kang, Daejin Yang, Caroline A. Ross PII:

S0925-8388(16)32780-3

DOI:

10.1016/j.jallcom.2016.09.046

Reference:

JALCOM 38880

To appear in:

Journal of Alloys and Compounds

Received Date: 8 June 2016 Revised Date:

25 August 2016

Accepted Date: 4 September 2016

Please cite this article as: D.H. Kim, S.H. Han, Y.-M. Kang, D. Yang, C.A. Ross, Structure and magnetic properties of pulsed laser deposited SrFe12O19 thin films on SrTiO3 (100) and (111) substrates, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.09.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Structure and magnetic properties of pulsed laser deposited SrFe12O19 thin

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films on SrTiO3 (100) and (111) substrates

Dong Hun Kim1,5,*, Seung Ho Han2, Young-Min Kang3, Daejin Yang4, and Caroline A. Ross5

Department of Materials Science and Engineering, Myongji University, Yongin, Republic of Korea

2

Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Seongnam,

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1

Republic of Korea

Department of Materials Science and Engineering, Korea National University of Transportation,

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3

Chungju, Republic of Korea 4

Samsung Electronics, 466-712 Yongin, Republic of Korea

5

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge,

Massachusetts, USA

Author to whom correspondence should be addressed. Electronic mail: [email protected]

Abstract

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*)

Epitaxial SrFe12O19 films were prepared on SrTiO3 (001) and (111) substrates by pulsed laser deposition and annealed in air at high temperature. As-deposited and annealed SrFe12O19 thin films were characterized using scanning electron microscopy, x-ray diffraction, and vibrating sample magnetometry.

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Films annealed at 800 ˚C and 1000 °C on (111) substrates showed an epitaxial growth whereas films on (001) substrates grew with both in-plane and out-of-plane c-axis. The magnetic hysteresis loops of

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annealed films on (111) substrates exhibited a strong magnetocrystalline anisotropy with out-of-plane easy axis originating from epitaxial growth while the films on (001) substrates were almost isotropic.

Keywords : SrFe12O19 thin film, pulsed laser deposition, M-type hexaferrite, magnetic anisotropy

Introduction M-type strontium or barium hexagonal ferrite with the magnetoplumbite structure, SrFe12O19 (SrM) or BaFe12O19 (BaM), has attracted great interest for permanent magnets and high density magnetic or 1

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magnetooptical recording devices due to its large uniaxial anisotropy.[1-6] Furthermore, SrM has excellent chemical stability, mechanical hardness, and corrosion resistance. The crystal structure of SrM consists of alternating stacks of spinel (S=Fe6O8) and hexagonal (R=SrFe6O11) blocks in the form of RSR*S*, where * denotes a 180° rotation around the hexagonal c-axis[7] The saturation magnetization, MS for single crystal

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SrM is 377 emu/cm3 (74 emu/g) and the magnetocrystalline anisotropy constant KU is 3.5 × 106 erg/cm3 with easy axis along the c-axis.[1]

To utilize this material in thin film devices, it is necessary to control the direction of the magnetic easy axis, e.g. normal or parallel to the substrate. This can be done by orienting the c-axis by epitaxial

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growth on a substrate. To date, amorphous, polycrystalline, or highly oriented SrM thin films have been grown on various substrates such as glass, metals, or single crystal sapphire, silicon, SrTiO3 (STO) or Gd3Ga5O12 substrates using pulsed laser deposition (PLD), radio frequency sputtering, electron beam

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deposition, or chemical solution deposition.[3, 8-14] Post-deposition annealing is commonly used to obtain well-crystallized films with high anisotropy and bulk-like properties, but this requires annealing temperatures of 900 °C or more, substrates.

[3]

[12, 15-16]

which can lead to interdiffusion between the film and certain

Although there have been several reports on epitaxial or highly oriented SrM[3, 8-9,12] and

BaM films,[15, 17-23] most work has used sapphire substrates and there are few reports on the relation between magnetic properties and the film microstructure. It has been reported that epitaxial SrM films

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have been grown on STO (111) substrates both by chemical solution deposition3 and by PLD12 as a result of the lattice match and low chemical reactivity between the film and substrate. Here, we report on the thin film growth of SrM on STO using PLD. (111) oriented STO substrates were used with a six-fold symmetry to match the basal plane of SrM, promoting epitaxial growth. The

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lattice misfit on the SrM basal plane/STO (111) plane between STO [110] and SrM [1010] is 6.6%, with the SrM film having the larger lattice parameter. SrM was also grown on STO (001) for comparison. The

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magnetic properties of as-grown films were measured then the films were annealed in a furnace in ambient atmosphere to improve the crystallinity and the magnetic anisotropy.

Experiment

A polycrystalline SrFe12O19 (magnetoplumbite) target was prepared by a conventional oxide sintering process. A mixture of SrCO3 (purity, 99.99 %) and Fe2O3 (purity, 99.5 %) powders were weighed, wet-ball milled, dried, and calcined at 1200 °C for 3 hours and then sintered at 1300 °C with a heating rate of 5 °C/min after ball milling, drying, and pelleting. 2

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SrM thin films were grown on single crystal STO (strontium titanate, JCPDS # 84-0444, abulk, STO = 3.905 Å) (001) and (111) substrates by PLD using a KrF excimer laser with 248 nm wavelength. Prior to growth the STO substrates were cleaned in trichloroethylene, acetone, and isopropyl alcohol sequentially

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for 10 minutes, rinsed with deionized water in an ultrasonicator and then finally dried in a flow of nitrogen gas.

The target was ablated with a fluence of 2.6 J/cm2 and a repetition rate of 10 Hz in 5 mTorr of oxygen after reaching a base pressure of 2 х 10-6 Torr, with a 6 cm substrate-target distance. During the pump-down and deposition, the substrate temperature was maintained at 650 °C and the sample was

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cooled to room temperature in the deposition atmosphere. The typical thicknesses of films measured with a surface profilometer (Tencor P-16) were 60 ~ 70 nm. Deposited films were annealed at 800 to 1000 °C

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in a tube furnace for one hour under an air atmosphere with a heating and cooling rate of 5 °C/ min. Structure and out-of-plane lattice parameters of the calcined powder and the films were investigated by X-ray diffraction (XRD, PANalytical X’Pert Pro) using a Cu source of 1.5406 Å wavelength. The microstructure of the calcined powder and sintered target and surface morphology of films were observed with scanning electron microscopy (SEM, Helios Nanolab 600). The SrM powder and pellet were coated with 3 nm thick gold-palladium by a sputter coater before being introduced into the SEM chamber. The

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sintered target was thermally etched at 1200 °C for 20 minutes to aid in visualizing the grain boundaries. Magnetization curves were measured at room temperature in a vibrating sample magnetometer (VSM, ADE model 1660) with a magnetic field range from -10 kOe to 10 kOe. The magnetic field was applied parallel and perpendicular to the substrate normal direction corresponding to the out-of-plane and

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in-plane directions, respectively. The hysteresis loop of the calcined powder was obtained after wrapping it with non-magnetic parafilm (plastic paraffin film, Bemis). Magnetic signals from the quartz holder, double side tape, parafilm, and substrate were subtracted and the magnetization was normalized by the net

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volume of the films or by the weight of the powder. The preparation conditions and properties of the films are shown in Table 1.

Results and Discussion

The wide angle XRD pattern of the SrM powder calcined at 1200 °C for 3 hours in air, Figure S1, reveals a hexagonal structure consisting of single phase SrM without additional reflections. Figure 1 (a) is a magnified view of the XRD scan of the calcined powder indicating polycrystalline material. All peaks are well indexed to the hexagonal SrFe12O19 (JCPDS # 33-1340) which has bulk lattice parameters of abulk, 3

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SrM

= 5.886 and cbulk, SrM = 23.037 Å. SrM phase formation by solid state reaction at 1200 °C is consistent

with previous reports.[10, 24] Figure 1 (b) is an SEM image of SrM powder calcined at 1200 °C showing facetted hexagonal

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plate-shaped grains of the range of ~ 1 µm diameter with random orientations. The grains became larger upon sintering at a higher temperature of 1300 °C as shown in Figure 1 (c). The larger flat planes and the  0} planes respectively. elongated rectangular planes correspond to the basal (0001) and prismatic {101 The magnetic hysteresis loop of SrM powder calcined at 1200 °C measured by VSM at room temperature is shown in Figure 1 (d). The maximum magnetic field in the VSM of 10 kOe is not

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sufficient to saturate the magnetic SrM powder, therefore the hysteresis loop is a minor loop and the magnetization value (60 emu/g) is below that of bulk SrM. The SrM powder exhibited a coercivity of

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3650 ± 50 Oe and a remanent magnetization of ~ 38 emu/g. The magnetic hysteresis loop shape and magnetic parameters are in good agreement with a previous study on strontium hexaferrite produced by a solid state reaction method.[1, 5]

Figure 2 (a) and (b) shows top view SEM images of 70 nm thick SrM thin films grown by PLD on  10], [101] direction of the STO substrates are (001) and (111) STO substrates. The [100], [010] and [1

drawn in the figures. ‘Knitting’ patterns consisting of rectangular crystals growing along [110] and [110]

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of the STO (001) substrate are visible in the film (Fig. 2 (a)) with voids between them. The faceted rectangular planes correspond to the prismatic {1010} planes of SrM, shown in the schematic drawing of the epitaxial relationship of SrM on STO (001) (Fig. 2 (c)). The lattice parameters 4√2 abulk,

STO

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of SrM (abulk, SrM = 5.886 and cbulk, SrM = 23.037 Å) can be compared with √2 abulk, STO = 5.523 Å and = 22.092 Å indicating a large lattice mismatch of 6.57 and 4.28 % respectively. This

suggests that the SrM films would be under compressive in-plane strain, though strain relaxation is likely.

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The x-ray peak intensity was insufficient to obtain reciprocal space maps to measure in-plane lattice parameters of the SrM.

On the other hand, on the STO (111) substrate the SrM grew as faceted pillars with in-plane axes (Fig. 2(b)). Despite their facetted morphology, the films grown on both STO (001) and (111) substrates did not show clear SrM XRD peaks (Figure S2) implying a low crystal quality attributed to the low deposition temperature. The magnetic hysteresis loops of the as-deposited films grown on (001) and (111) STO substrates are shown in Figure 2 (d) and (e), respectively, measured up to 10 kOe. In the case of the SrM film on

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STO (001), the in-plane loop was measured along STO [100]. The in-plane and out-of-plane loops of this sample (Fig. 2 (d)) did not reach saturation at 10 kOe and the film appeared isotropic, with high coercivity as in a prior report.[1] For the SrM film on STO (111) substrate (Fig. 2 (e)), the coercive field is much

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lower (1000 - 1200 Oe for out-of-plane and 100 - 200 Oe for in-plane field direction) than that of the film on STO (001) substrate. Both as-deposited films have ~10 times lower magnetic moments compared to bulk SrM, attributed to the poor crystallinity.

The films were annealed in air to improve the crystallinity and the magnetic properties. Surface morphologies of films annealed at 800 °C for 1 hr are shown in Figure 3 (a) and (b). The sample grown

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on (001) STO shows a structure reminiscent of a knitting pattern consisting of rectangular crystals arranged along the [110] or [110] directions of the STO. This resembles the as-deposited SrM film but

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with larger grain size (Fig. 2(a)). The film grown on STO (111) substrate shown in Fig. 3 (b) revealed very flat surface morphology unlike the as-deposited film. Hexagonal shaped grains with a size of ~ 100 150 nm were observed, corresponding to the basal plane of SrM as drawn in Figure 3 (c). In addition, sparse bright nanorods with ~ 1 µm length growing in the [101] , [110], and [011] STO directions were detected in low magnification images. These likely represent a secondary phase such as another ferrite, e.g. maghemite (γ-Fe2O3, JCPDS #39-1346) which has a cubic spinel structure.

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Figure 4 (a) and (b) shows the θ-2θ XRD scans of SrM thin films on STO (001) and (111) substrate after 800 °C annealing. The annealed SrM film on STO (001) substrate exhibited peaks corresponding to (103), (006), (008), (205), and (206) planes. Although the film did not have a single preferred orientation, its diffraction pattern differed from that of the polycrystalline phase shown in Fig. 1 (a) and (b). The film

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consisted of grains with their c-axis along the [110] STO directions, but there was also a significant population of grains with c-axis out of plane. The multiple orientations in the film are attributed to tilting

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of the crystals originating from the large lattice mismatch. The formation of multiple crystal orientations including some with c-axis out of plane may relieve the strain in the film. The XRD pattern of the annealed film on (111) STO substrate shows only (00l) reflections (l = 6, 8, 10, 14) of the magnetoplumbite SrM phase indicating highly oriented film growth with the c-axis out of plane. The highly oriented growth on a six-fold symmetry substrate is consistent with previous results on sapphire and STO (111) substrate.[4, 7] The out-of-plane lattice parameter of the annealed SrM thin film on STO (111) was calculated from the geometry of a hexagonal structure (





   

=  





 +   ), where d

is the interplanar spacing and λ is the x-ray wavelength. The out-of-plane lattice parameter calculated from the (006), (008), and (0,0,14) peak was 23.052, 23.052, and 23.056 Å, respectively which exceeds 5

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the bulk lattice parameter, 23.037 Å. This suggests a tensile strain along the out-of-plane direction due to the in-plane compression derived from the lattice mismatch between the film and the substrate. Lattice parameters derived for the SrM on (001) STO were similarly larger than the bulk values.

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Figure 4 (c) and (d) shows the room temperature magnetic hysteresis loops of the annealed SrM films. The SrM/STO (001) showed hard magnetic loops both in-plane (along STO [100]) and out-of-plane, resembling the as-deposited loops but with higher magnetization. The coercive fields were ~4600 and 5200 Oe for in-plane and out-of-plane hysteresis loops and the saturation magnetization was 235 emu/cm3 (± 6%) for both directions. The in-plane loop required less field to saturate (~7 kOe) and had higher

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remanence than the out-of-plane loop, indicating a net in-plane anisotropy.

In contrast, the magnetic hysteresis loops of annealed SrM/STO (111) exhibited magnetic

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anisotropy with the easy axis normal to the substrate. The 10 kOe field was insufficient to saturate the film in either direction. The out-of-plane loop had coercivity of ~1600 Oe and the magnetization was 330 emu/cm3 at 10 kOe. The out-of-plane magnetic easy axis is a result of the c-axis orientation. The anisotropy of the SrM includes contributions from the magnetocrystalline anisotropy, shape anisotropy and magnetoelastic anisotropy. The magnetocrystalline anisotropy is expected to be the largest contribution, leading to an out-of-plane easy axis for the SrM/STO (111) sample, but a more isotropic

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behavior for the SrM/STO (001) sample which consisted of crystals with both in-plane and out-of-plane c-axis orientation. The shape anisotropy promotes an in-plane easy axis but considering a thin film of SrF, the magnitude of the shape anisotropy is 4 times smaller than that of the magnetocrystalline anisotropy. Magnetostriction coefficients for polycrystalline SrF have been reported as negative, ~ -2 10-5 at room

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temperature and moderate magnetic fields.[25,26] Combined with the presumed in-plane compression and out-of-plane tensile strain this would favor an in-plane easy axis, but even for large stresses (order of GPa)

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the magnetoelastic anisotropy would be an order of magnitude lower than the magnetocrystalline anisotropy. Hexagonal crystals are described typically by four magnetostriction coefficients with different magnitude and even sign compared to the polycrystalline magnetostriction coefficient,[27] so the magnetoelastic anisotropy of single crystal and polycrystalline films will differ. Annealing at 1000 ˚C led to similar microstructures and magnetic properties of SrM films (Figure 5 and Figure S3) compared to those of films annealed at 800 ˚C. The top view SEM image of SrM/STO (001) annealed at 1000 °C, Fig. 5 (a), shows facetted crystals aligned along the [110] and [110] directions. A spread in the orientation of the crystals is evident which is consistent with the multiple orientations shown in the XRD scans in Fig. 4 (a) and Fig. S3 (a). The magnetic hysteresis was similar for the 6

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SrM/STO (001) annealed at 1000 ˚C and 800 ˚C, except that the coercivity was higher after the 1000 ˚C anneal, ~5200 Oe for an in-plane field and 5800 Oe for an out-of-plane field. The SrM/STO (111) sample annealed at 1000 °C revealed a flat surface as the plate-like grains

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merged. The elongated bright nanorods along the STO [101] , [110] and [011] were still present, seen in the low magnification SEM image at the inset of Fig. 5 (b), but no secondary phase was detected by XRD (Fig. S3 (b)). The XRD peak positions of the SrM films annealed at 1000 °C were almost the same as those of films annealed at 800 °C implying similar strain states.

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The out-of-plane magnetic hysteresis loop of SrM/STO (111) annealed at 1000 ˚C showed a lower field step with a higher field tail. It seems unlikely that this originates from switching of two magnetic phases, since the XRD showed only the presence of SrM. Instead it may be reflective of the reversal

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process, with nucleation and propagation of reverse domains at the lower field (~800 Oe) but a much higher field needed to fully eliminate the remaining domains to obtain saturation. Domain wall propagation may require a lower field for the 1000 ˚C annealed sample because the film has larger and more continuous grains compared to the 800 ˚C annealed film, which also showed a slow approach to

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saturation.

Table I. Structural and magnetic data for SrM films of different orientations and annealing temperature

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(substrate, annealing temp.)

Morphology

(100) as deposited

(100) 800 °C anneal

Crystal orientation

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Deposition condition

Knitting pattern

Knitting pattern

Magnetic properties Coercive field (Oe) +- 50 Oe

SrM [0001] || STO [110]

In-plane

Out-of-plane

4000

4000

SrM [0001] || STO [110] SrM [0001] || STO [110]

Saturation magnetization (emu/cm3) Not saturated (> 28)

4600

5200

235

5200

5800

275

200

1100

Not saturated

SrM [0001] || STO [110] (100) 1000 °C anneal

Knitting pattern

SrM [0001] || STO [110] SrM [0001] || STO [110]

(111) as deposited

Facetted pillars

randomly oriented

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(> 25) (111) 800 °C anneal

Hexagonal plate

SrM [2110] || STO [110]

2200

1600

330

2200

800

Not saturated

(111) 1000 °C anneal

Hexagonal plate

SrM [2110] || STO [110]

(> 330)

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SrM [0001] || STO [111]

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SrM [0001] || STO [111]

Conclusions

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SrM thin films were grown on (001) and (111) oriented STO substrates by PLD at 650 ˚C in oxygen and annealed at 800 and 1000 °C in air. The as-deposited films exhibited low magnetization with poor crystallinity, whereas annealed films showed crystallites with faceted surfaces. Annealed films on STO (001) showed a microstructure resembling a knitting pattern consisting of rectangular grains grown along the diagonal directions of the cubic STO lattice, but XRD showed the presence of multiple crystal orientations including a population with c-axis out of plane. In contrast, annealed films on STO (111)

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were well textured with c-axis out of plane. The magnetic hysteresis loops showed that annealed films on STO (001) were almost isotropic whereas those on STO (111) had a strong out-of-plane anisotropy. The anisotropy is interpreted as the sum of shape, magnetoelastic and magnetocrystalline contributions with

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the magnetocrystalline term being dominant.

Acknowledgment

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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT & Future Planning, No. 2015R1C1A1A01051656) and by C-SPIN, a STARnet Center of SRC supported by MARCO and DARPA. Shared facilities of MIT’s Center for Materials Science and Engineering, NSF DMR1419807 were used.

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Figure Captions

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Figure 1. (a) XRD scan of SrM powder calcined at 1200 °C. SEM images of (b) Powder after calcining at 1200 °C and (c) Target after sintering at 1300 °C. (d) Magnetic hysteresis loop of SrM powder calcined

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at 1200 °C.

Figure 2. Top view SEM images of SrM thin film deposited at 5 mTorr oxygen pressure and 650 °C on (a) STO (001) and (b) STO (111) substrates. (c) Schematic illustrations of the epitaxial relationship of SrM grown on STO (001). In-plane and out-of-plane hysteresis loops of SrM thin film on (d) STO (001) and (e) STO (111) substrates within an applied magnetic field range of -10 kOe to 10 kOe. The in-plane direction of the SrM film on STO (001) corresponds to the STO [100] direction. The solid and dashed

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lines correspond to the unit cells of STO and SrM, respectively.

Figure 3. Top view SEM images of SrM thin films on (a) STO (001) and (b) STO (111) substrates after annealing at 800 °C. Inset in Fig. 3 (b) is a top view SEM image of the SrM film on STO (111) at lower

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magnification. (c) Schematic illustrations of the epitaxial relationship of SrM grown on STO (111) substrates. The solid and dashed lines correspond to the unit cells of STO and SrM, respectively.

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Figure 4. θ-2θ scans of SrM thin films on (a) STO (001) and (b) STO (111) substrates after thermal annealing at 800 °C for one hour. Room temperature in-plane and out-of-plane hysteresis loops of SrM films on (c) STO (001) and (d) STO (111) substrates after annealing at 800 °C. The in-plane direction of the SrM film on STO (001) corresponds to the STO [100] direction.

Figure 5. Top view SEM images and hysteresis loops of SrM thin films on STO (001) substrates ((a) and (c)) and STO (111) substrates ((b) and (d)) after annealing at 1000 °C. The in-plane direction of the SrM film on STO (001) corresponds to the STO [100] direction. The inset in (b) is a top view SEM image of the SrM thin film on STO (111) at low magnification.

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ACCEPTED MANUSCRIPT • SrFe12O19 thin films were grown on SrTiO3 substrates by pulsed laser deposition.

• Annealed SrFe12O19 films showed crystallites with faceted surfaces.

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• The as-deposited SrFe12O19 films exhibited low magnetization with poor crystallinity.

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• Annealed SrFe12O19 films on SrTiO3 (111) substrate had a strong magnetic anisotropy.