Formation of magnetically-oriented barium hexaferrite films by aerosol deposition

Accepted Manuscript Formation of Magnetically-Oriented Barium Hexaferrite films by Aerosol Deposition Scooter D. Johnson, Dong-Soo Park, Syed B. Qadri, Edward P. Gorzkowski PII: DOI: Reference:

S0304-8853(18)33504-2 https://doi.org/10.1016/j.jmmm.2019.02.025 MAGMA 64944

To appear in:

Journal of Magnetism and Magnetic Materials

Please cite this article as: S.D. Johnson, D-S. Park, S.B. Qadri, E.P. Gorzkowski, Formation of MagneticallyOriented Barium Hexaferrite films by Aerosol Deposition, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.02.025

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Formation of Magnetically-Oriented Barium Hexaferrite films by Aerosol Deposition Scooter D. Johnsona,∗, Dong-Soo Parkb , Syed B. Qadria , Edward P. Gorzkowskia a

b

U.S. Naval Research Laboratory, Washington, D.C. Korean Institute of Materials Science, Changwon, Korea

Abstract We have deposited polycrystalline films of barium hexaferrite utilizing aerosol deposition at room temperature. Magnetic orientation was achieved by using a 4 kOe permanent magnet mounted in close proximity to the substrate during deposition. We find that these films show crystallographic orientation and magnetic anisotropy out of plane corresponding to the {0 0 2l } diffraction planes. To further explore the film properties we performed sintering treatments up to 1000◦ C and found that the magnetic properties are further improved. For these films the magnetic saturation is 3683 G with a squareness value of 0.82. The magnetic and crystallographic texturing indicate 29% grain orientation with a coercivity of 4.6 kOe. These values indicate that the films are of good quality for potential integration into microwave systems that require high values of saturation magnetization, squareness, and coercivity with a good degree of self-bias. Keywords: ceramics, self-bias, microwave, aerosol deposition, barium hexaferrite, circulator 1. Introduction Barium hexaferrite (BaFe12 O19 , BaM) is an important material for microwave circuitry, such as circulators due to its high magnetic anisotropy Ha = 1352 kA/m (17 kOe) , large magnetic saturation 4πMs = 4.8 kG (72 emu/g), and large theoretical coercivity Hc = 594 kA/m (7.5 kOe), but due ∗

Corresponding author. E-mail: [email protected]

Preprint submitted to Journal of Magnetism and Magnetic Materials

February 4, 2019

to its high melting temperature of 1611◦ C integration into standard CMOS processing remains a challenge[1, 2]. There have been several studies focused on creating magnetically oriented BaM with the magnetic easy axis oriented out-of-plane. Liquid phase epitaxy has been used to produce thick single-crystal films with good saturation magnetization of 4.4 kG and out-of-plane orientation along the {0 0 2l } planes, however coercivity is generally very low (. 10 Oe) owing to the single-crystal nature of the film[3, 4]. Therefore, several efforts have involved attempting to produce high-quality quasi-single crystal materials. Modified liquid phase epitaxy or liquid phase reflow technique has been used to produce 350-µm-thick highly oriented quasi-single crystal BaM samples, but with Hc = 102 Oe and a diminished 4πMs ≈ 2 kG [5]. A solid-state reaction process at temperatures of 1300◦ C– 1400◦ C produced high-quality quasi-single-crystal samples with good saturation magnetization of about 4.48 kG and out-of-plane orientation along the {0 0 2l} planes[6]. In all these techniques high temperatures > 800◦ C were required to grow the films and the coercivity was very low Hc < 102 Oe due to the single-crystal nature. One route to lower temperature fabrication and larger values of coercivity is to fabricate polycrystalline samples. Along this direction there have been several efforts to form oriented polycrystalline materials. These techniques generally involve attempting physical rotation and orientation of the BaM hexagonal platelets by forming the bulk pellet in the presence of a magnetic field[7]. For example, thick films of 100 to 500 µm, large coercivity and saturation magnetization values were achieved by using a screen printing technique in the presence of an 8 kOe biasing field, however the films needed to be hot pressed to adequately densify the films and to burn out the binder. The resulting samples achieved good values of 4πMs = 4 kG and Hc = 1935 Oe[8]. A similar technique involved simply pressing the pellets after shaking the powder in the presence of a magnetic field to align the loose powder. The loosly-packed and magnetically-oriented powder was then pressed and sintered at 1300◦ C to densify the pellets. The results produced pucks with good saturation magnetization of 71 emu/g and texturing [9]. A review of these and similar techniques can be found in a review by Harris et al. [1]. In this study, we use aerosol deposition (AD) as another route to explore creating magnetically textured films of BaM. AD is a film deposition 2

technique that produces very thick dense polycrystalline films at room temperature and shows promise for integration of ferrite materials into current semiconductor processes. The process was developed in the late 1990s by Jun Akedo et al.[10] building on earlier work in the 1980s[11, 12]. The main features of AD film growth are that it can produce micrometers-thick films that are dense (90 – 95% of theoretical density) from a solid powder precursor at room temperature[13, 14]. This allows integration of high-melting temperature materials into temperature-sensitive structures[15]. The essence of the deposition process is that a solid crystallite of about 0.5 µm in size is accelerated to about 300 m/s toward a substrate held in medium vacuum (1 – 15 Torr, typically). Upon impact, the particle fractures and plastically deforms and bonds to the substrate. Continued impact of subsequent particles further fractures the particle grains to 10 – 200 nm in size while continuing to promote densification of the film. Reviews of current AD technology can be found in the literature[16, 17]. The room temperature deposition characteristic of the AD process lends itself naturally for integrating ferrites such as BaM into microwave circuitry. Furthermore, the high-density and thick film capabilities could provide good magnet strength and desirable form factor. There have been several studies utilizing AD to deposit ferromagnetic materials, such as yttrium iron garnet[13, 18, 19], Ti-doped barium hexaferrite[20], Fe/Ni-Zn-Cu[21], Nd-Fe-B[22], Sm-Fe-N[23], Be-Fe-O[24] , Ni-Zn-Fe-O[25, 15], Ni-Zn-Cu[26, 14]. Films of Sm-Fe-N were grown in the presence of a ∼ 0.2 kOe permanent magentic field[27] to create a small degree of magnetic orientation. In this study, we adopt a similar technique to Sugimoto et al. to deposit BaM both under a standard non-magnetic bias (NMB) condition and under magnetic bias (MB) of 4 kOe oriented perpendicular to the film surface. We explore the degree to which these films of BaM can be oriented by comparing the scanning electron microscopy (SEM), x-ray diffraction (XRD) and vibrating sample magnetometry (VSM) results of films produced under MB and NMB conditions. 2. Materials & Methods Barium hexaferrite (BaM) powder was purchased from Trans-Tech, Inc., Adamstown, MD, US with a specified average particle size of 0.5 µm. The BaM powder was heat treated at 1000◦ C for 2 hours then sieved to obtain agglomerate sizes of 106 µm or less. Deposition was performed onto 3

a-plane sapphire using a custom AD system at the Korean Institute of Materials Science (KIMS), Changwon, Gyeongnam, S. Korea using medical-grade dried air as the carrier gas. The system employed a rectangular convergingdiverging nozzle of dimensions 0.4 mm by 35 mm set to a standoff distance of 10 mm. The deposition system and additional details are described in the literature[28, 29]. For deposition in the presence of a magnetic field three

Figure 1: A drawing of the magnet assembly, nozzle, and sample substrate used for deposition in the presence of a magnetic field. magnetic field values are shown at various points between the substrate and nozzle.

permanent rectangular magnets were mounted to the sample stage as shown in Figure 1. Magnetic field values between the magnet face and nozzle were mapped using a Lakeshore hall probe and gaussmeter. Films were deposited onto several a-plane sapphire substrate sizes simultaneously for sintering and characterization studies. A substrate size of 3 x 3 mm2 was chosen for VSM measurements. Substrates measuring 10 x 10 mm2 were used for XRD. Selected films of both sizes were sintered at 700◦ C, 900◦ C, 1000◦ C, and 1100◦ C, 4

but it was found that the 1100◦ C films delaminated and are not included in this study. Film mass was determined by weighing the substrate before and after deposition. Film thickness was determined by using a stylus profilometer at different locations on the film edge and the average taken. All of the samples used in this study had a film thickness between 3 – 4 µm. Film top-surface imaging was performed using a LEO Supra 55 SEM. The results shown were taken at a magnification of 60 kX, aperture size of 30 µm, and at an operating voltage of 3 kV in the inlens setting. Crystallographic data was acquired using a Rigaku SmartLab X-Ray Diffractometer with a Cu Kα wavelength of 1.540593 ˚ A. An offset of 4◦ was set to avoid substrate reflections. For texturing analysis, scans without offset were implemented between 2θ = 20◦ − 36◦ and 2θ = 52◦ − 61◦ to avoid the substrate reflections. Crystallite size and crystallographic texturing was determined by Reitveld refinement using Jade Software. Values were also checked by applying the well-known Debye-Scherrer formula to the (1 0 7) peak and was found to be in good agreement with the Reitveld refinement. Magnetic hysteresis curves were taken with a MicroSense vibrating sample magnetometer with a 2-T GMW model 3473-70 magnet. Magnetization was calculated using the sample mass and density of 95% of 5.28 g/cm3 . All VSM data are shown corrected for demagnetization effects. 3. Results Figure 2 shows top surface SEM images of four samples. As can be seen in the images, all the film surfaces show significant particle fracture, compaction, and deformation. Figure 2a is an image of an as-deposited film formed under NMB. The grains in this image are present as individual particles compacted or as larger regions of plastically deformed particles. Upon exposure to the 1000◦ C sinter the grains can be seen to coalesce and grow as shown in Figure 2b. Figures 2c and 2d show films as-deposited and after 1000◦ C sinter, respectively formed under MB. These films show qualitatively the same morphology as those deposited under NMB. Figure 3 shows the stacked XRD spectrum for each sample in this study with the PDF phase card #04-002-2503 for barium iron oxide shown at the bottom. The spectra are all scaled by the peak height of the (1 0 7) 0 kOe 1000◦ C peak, the largest in the stack. The bottom four spectra were obtained from films as-deposited and after post-deposition sintering deposited under NMB (0 kOe). The top four spectra were obtained from films as-deposited 5

Figure 2: SEM images of the top surface of films grown under non-magnetic bias (NMB) and magnetic bias (MB) as-deposited and after 1000◦ C sinter. The images were taken under the same SEM settings and magnification. The scale bar applies to all images.

6

and after post-deposition sintering deposited under MB (4 kOe). All of the spectra acquired matched well to the phase card and no additional peaks or phases were present. As can be seen in the plots, post-deposition sintering narrow and strengthen the peak signals due to increased grain growth for both NMB and MB conditions. We find from Rietveld refinement that the crystallite size of each film increases with sintering temperature (as expected), but shows no systematic change due the presence of the magnetic field during deposition. For the NMB and MB, respectively the crystal sizes are: no sinter; 122 ˚ A and 100 ˚ A, 700◦ C; 128 ˚ A and 146 ˚ A, 900◦ C; 184 ˚ A and 209 ˚ A, ◦ 1000 C; 261 ˚ A and 234 ˚ A. The prominence of the (1 0 7) peak compared to the (1 1 4) peak in the data further motivates an exploration into crystallographic texturing in these films. We fit all the spectra to the phase card adjusted for different degrees of texturing along the {0 0 2l} based on the March-Dollase method [30, 31]. We find that the March-Dollase factor for all the films are r = 0.6 with 1 being completely random and 0 being perfectly aligned. The percentage of oriented grains in the film is related to the March-Dollase factor [32] as s (1 − r)3 η = 100% . (1) (1 − r3 ) This gives the percentage of oriented grains η = 29% in these films. Figure 4 is a plot of the magnetic hysteresis curves for films oriented perpendicular to the VSM field for four representative samples. The green and red curves show as-deposited films under NMB (green) and MB (red) conditions. As can be seen by comparing these two curves the magnetic field has a clear affect on the magnetic saturation 4πMs and remanence 4πMr . In comparing the relative change between NMB to MB in these as-deposited samples we find a 20% improvement in 4πMs and a 38% improvement in 4πMr . In both NMB and MB the coercivity Hc = 2.7 kOe is unchanged. Postdeposition sintering improves the film properties progressively further. The temperature treatment of 1000◦ C shows the best results and is also shown in Figure 4. As seen in the Figure the increase in saturation magnetization and remanence in both NMB (blue) and MB (black) is much increased compared to the as-deposited films. The relative change between these 1000◦ C sintered samples are 24% improvement in 4πMs and 25% improvement in 4πMr . The coercivity for the 1000◦ C films are 4.7 kOe. We compare the ratio of remanence to magnetic saturation to obtain the squareness SQ = Mr /Ms = 0.83 7

Figure 3: Intensity spectra for NMB (bottom four) and MB films (top four) after sintering at 700◦ C, 900◦ C, and 1000◦ C. The bottom points are from PDF phase card # 04-002-2503 for barium iron oxide showing prominent peaks.

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for the 1000◦ C film and SQ = 0.64 for the as-deposited films under MB. The magnetic properties of these films, as well as, those of the intermediate sintering temperatures are summarized in Table 1.

Figure 4: Plot of magnetic hysteresis for films sintered at 1000◦ C and as-deposited under NMB (0 kOe) and MB (4 kOe) oriented perpendicular to the VSM field.

Figure 5 is a plot of the magnetic hysteresis curves for the same films in Figure 4 oriented parallel to the VSM field. Here, a similar trend can be seen. For the as-deposited films, we see improvement in the magnetization as it approaches saturation and a 21% improvement in 4πMr between depositing the films in a NMB or MB condition. In both NMB and MB the coercivity Hc = 2.7 kOe is unchanged. The 1000◦ C sintered films shown in blue (NMB) and black (MB) show marked improvement in overall properties of magnetization, remanence, and coercivity. The relative change between these 1000◦ C sintered samples is 15% improvement in 4πMr . Figure 6 shows VSM data for two as-deposited films deposited under MB (red) and NMB (black) oriented perpendicular (solid) and parallel (dashed) 9

Figure 5: Plot of magnetic hysteresis for films sintered at 1000◦ C and as-deposited under NMB (0 kOe) and MB (4 kOe) oriented parallel to the VSM field.

10

to the VSM field. In taking the remanence value as a measure of magnetic orientation, we see that in both MB and NMB conditions there is a clear out-of-plane orientational preference due to the increase in remanence value perpendicular compared to parallel. If we take the ratio Mrpara /Mrperp as a measure of the magnetic texturing we obtain values of 0.5 for the MB and 0.6 for the NMB condition. These values are relatively unchanged for the different sintering temperatures used, with all values falling between 0.5 to 0.6.

Figure 6: Plot of magnetic hysteresis curves for as-deposited films deposited under MB (red) and NMB (black) oriented perpendicular (solid) and parallel (dashed) to the VSM field.

Table 1 shows the values for saturation magnetization 4πMs , remanence 4πMr , coercive field Hc , and the relative change between in-field and no-field deposition. With the exception of the 900◦ C sintered film it is evident that there is a significant increase in both 4πMs and 4πMr when comparing films deposited in-field compared to no-field. 11

Table 1: Measured magnetic properties of films under non-magnetic bias (NMB) and magnetic bias (MB) and after sintering.

Sinter Temp. (◦ C) Perpendicular Room Temp. 700 900 1000 Parallel Room Temp. 700 900 1000

4πMs (G) NMB MB

Rel. Chng. in 4πMs (%)

4πMr (G) NMB MB

Rel. Chng in 4πMr (%)

Hc (kOe) NMB MB

2019 2647 3104 2898

2448 3121 3129 3583

20 18 1 24

1141 1902 2478 2363

1574 2377 2571 2961

38 25 4 25

2.7 2.8 4.0 4.7

2.7 2.9 4.1 4.6

– – – –

– – – –

– – – –

685 1014 1461 1318

829 1196 1419 1514

21 18 -3 15

2.1 2.5 3.8 4.3

2.3 2.7 3.9 4.3

4. Discussion The SEM images show that the films formed in this study are typical for the AD process. The fact that the images appear qualitatively similar between MB and NMB, in spite of the XRD and VSM results, may suggest that the influence of the magnetic field is working alongside the fracture and densification process during particle impact. That is, the particles are not just simply rotated into alignment, but particles already deposited may continue to be oriented by the combination of applied field and high-energy bombardment of incoming particles; thereby influencing the bulk of the film to become oriented. One fact that may confound interpretation of the SEM images is that it is not evident what the direction of the magnetic easy axis in in these particles. In an ideal particle we might expect hexagonal particles, which would suggest the direction of the easy axis, but because the particles undergo extensive fracturing the true orientation of the particles can not be observed in the SEM images. The XRD analysis and VSM results both indicate texturing in the films due to the deposition process itself, i.e., under NMB condition. It appears that the degree of preferred orientation as measured by XRD is determined by the as-deposited films and does not change significantly with sintering 12

or application of magnetic field during deposition. It is possible that the AD process imparts sufficient energy into the film during deposition that preferred crystallographic orientation can be achieved by either some form of self-assembly or by influence of the substrate. The VSM data appears to reveal a stronger dependence between the various conditions used in this study. As seen in Table 1 all samples show a clear relative improvement in the remanence and saturation magnetization between the NMB and MB films. The improvement in both Ms and Mr for all the samples in this study indicates that the magnetic field is clearly improving the film properties in a way that the XRD is insensitive to. It may be that the MB condition is clustering grains together to form larger regions of oriented material that gives rise to larger domains and better magnetic properties. Since these domain clusters may be isolated between areas of randomly oriented grains it appears in the XRD as the same overall percentage of oriented grains as if the domains do not exist. From a magnetic standpoint, however, these larger regions have a strong influence on the saturation and remanence as seen in the data presented. The saturation is improved by sintering due to crystallite grain growth as seen by the XRD analysis, which permits larger magnetic domains to form. This also facilitates a larger coercivity in these films so that the properties of the films after 1000◦ C sinter show good overall magnetic properties along with a clear indication of self-bias. In an attempt to further quantify and compare the XRD and VSM data we take the remanence value as our measure of magnetic orientation. By taking the ratios of the remanence values for each film parallel to perpendicular we obtain a value of Mrpara /Mrperp between 0.5 and 0.6 for all the films. It may be interesting to note that this is very close to the obtained MarchDollase factor of 0.6. In other samples showing preferred orientation we have studied (data not shown) we also find agreement between Mrpara /Mrperp to the March-Dollase r-factor. It may be worth further study to correlate these factors for preferred orientation. In considering the low-temperature appeal of the AD process we focus on the films grown without sintering. It is evident that these film show good magnetic properties of coercivity and remanence as shown in Figure 6 that are further improved by MB condition. This suggests that there are factors present within the NMB condition that facilitates some amount of preferred orientation within the film. This could be due to the influence of the substrate or self-orientation of the resulting nano-grains within the 13

film during impact. When the magnetic field is implemented the inherent mechanism of deposition and orientation is further bolstered. Other routes to further improving the films without post-deposition sintering could entail using a larger magnetic field, different substrates that may influence the growth better, or diluting the particles with another material that facilitates post-impact rotation and alignment. We find the best film properties occur with the films deposited under MB conditions and after 1000◦ C sintering treatment. For these films 4πMs = 3.6 kG is approaching the reported value of 4.8 kG. The squareness value is Mr /Ms = 0.8 and magnetic texturing = Mrpara /Mrperp = 0.6 with a coercivity of 4.6 kOe. All of the films show crystallographic texturing with a MarchDollase factor of 0.6 (percentage of aligned grains η = 29%). These values indicate that the films show good promise for integration into microwave systems that require high values of saturation magnetization, squareness, and coercivity with a good degree of self-biasing. 5. Conclusion We have deposited barium hexaferrite polycrystalline films utilizing aerosol deposition at room temperature. Magnetic orientation in the films was achieved by using a 4 kOe permanent magnet mounted in close proximity to the substrate during deposition. We find that the resulting films show crystallographic texture along the {0 0 2l} diffraction planes and magnetic orientation out of plane. We performed sintering treatments up to 1000◦ C on the films and found that the magnetic properties are further improved. The best film properties with the films deposited under MB conditions and after 1000◦ C sintering treatment show promise for integration into microwave circuitry. 6. Acknowledgments This research was sponsored by the U.S. Naval Research Laboratory, Washington, DC base program, Korean Institute of Material Science, and the Office of Naval Research. 7. References [1] V. G. Harris, A. Geiler, Y. Chen, S. D. Yoon, M. Wu, A. Yang, Z. Chen, P. He, P. V. Parimi, X. Zuo, C. E. Patton, M. Abe, O. Acher, C. Vittoria, 14

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42 (3) (2009) 513–518. doi:10.1107/S0021889809013727. URL https://doi.org/10.1107/S0021889809013727

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x x x x x x

Aerosol deposition is used to grow barium hexaferrite films A 4 kOe magnetic field was placed during deposition to magnetically orient the films Films are found to be significantly oriented out-of-plane by VSM and XRD measurements Sintering the films further improved the magnetic properties March-Dollase factor is used to quantify the percentage of aligned grains to be 29% We draw a direct comparison between the March-Dollase factor and the ratio of inplane to out-of-plane remanence