Gallium oxide (Ga2O3), an emerging ultra-wide bandgap semiconductor material with a bandgap ~ 4.8 eV, occurs in the trigonal , monoclinic , cubic and orthorhombic polymorphs. The thermodynamic stable phase of β-Ga2O3 has been the primary focus for future power electronic devices due to the availability of semi-insulating to n-type doped free-standing substrates. Although γ-Ga2O3 has been identified as the least stable phase, γ-Ga2O3 inclusions have been found in many β-Ga2O3 films, particularly when Al or Mg is present. 1–3 These γ-phase inclusions are recognized as a potential problem for devices based on β-Ga2O3.1 In this study, Ga2O3 films were deposited on (100) MgAl2O4 in the temperature range of 550 °C to 850 °C via metal-organic chemical vapor deposition (MOCVD). Post-deposition thermal treatments were conducted in air at temperatures between 800 °C to 1000 °C. The phase composition and nanostructure of the as-grown and annealed films were investigated using X-ray diffraction and transmission electron microscopy (TEM). Phase-pure β-Ga2O3 was observed in the films deposited at 550°C and 650 °C, whereas γ-Ga2O3 -based solid solutions (hereafter referred to as γ-phase) were formed at 850°C, and a mixture of β-Ga2O3 and γ-phase were observed in the films grown at 750°C. The transformation of β-Ga2O3 to γ-phase originated at the film/substrate interface and was accompanied by interdiffusion of Al and Mg from the substrate. The minimum atomic percent of Al + Mg associated with the phase transformation was determined to be ~ 4.6 at%. Atomic scale TEM characterization indicates that Al and Mg have a strong preference for octahedral sites whereas Ga prefers the tetrahedral sites, to form an inverse spinel structure. It is postulated that the octahedral site preference of Al and Mg drives the β-Ga2O3 to γ-phase transition and, as a result, enhances the observed thermal stability of the γ-phase at elevated temperatures. 1 C.S. Chang, N. Tanen, V. Protasenko, T.J. Asel, S. Mou, H.G. Xing, D. Jena, and D.A. Muller, APL Mater. 9, (2021). 2 A.F.M.A.U. Bhuiyan, Z. Feng, J.M. Johnson, H.L. Huang, J. Sarker, M. Zhu, M.R. Karim, B. Mazumder, J. Hwang, and H. Zhao, APL Mater. 8, (2020). 3 Q. Hou, K. Liu, X. Chen, J. Yang, Q. Ai, Z. Cheng, Y. Zhu, B. Li, L. Liu, and D. Shen, Physica Status Solidi - Rapid Research Letters 16, (2022).

Phase Transformation of β-Ga2O3 to Ga2O3 -Based γ-Phase Spinels on (100) MgAl2O4 Substrate

The key to realizing high area recording density capability (ADC) for heat assisted magnetic recording (HAMR) is to achieve small grain size (and pitch) in the granular L10 FePt media with a sufficiently high grain aspect ratio [1]. Media such as FePt-C, FePt-SiOx, and FePt-TaOx, or multilayers made of a combination of them, appear not able to reach the desired goal because of their failure to achieve either high grain aspect ratio, small grain size, or high L10 ordering, respectively [2, 3]. In this study, using the co-sputtering technique, we demonstrate the fabrication of FePt-BN granular media with much reduced FePt grain size and high grain aspect ratio. In particular, an underlayer stack of Ta(5nm)/Cr(50)/MgO(9) is first deposited on Corning LotusTM NXT glass followed by co-sputtering of FePt and BN at an elevated substrate temperature along. A RF bias is applied to the substrate during the FePt-BN deposition with a short and optimized delay. Figure 1C shows the order parameter as a function of substrate temperature for the FePt-BN granular film media, calculated from XRD measurements. The volumetric concentration of BN is at 38%. Figure 1A on the left of the figure shows a cross-section TEM micrograph of the FePt-BN layer formed at a substrate temperature of 725 oC. Figure 1B shows a cross-section TEM picture of FePt-BN/FePt-SiOx multilayer granular film with temperature and composition graded over the grain height for producing tall grains [4] for comparison. The grain size of single layer FePt-BN film (Figure1 A) is around 4 nm with an aspect ratio of 2 while the grain size of the FePt-BN/FePt-SiOx multilayer film is around 7nm and grain height 11nm. The small grain size with a relatively high grain aspect ratio in the single layer FePt-BN film is promising for enabling high area recording density capability. 
**References** 
[1]Y. Kubota et al., “Heat-Assisted Magnetic Recording’s Extensibility to High Linear and Areal Density,” IEEE Transactions on Magnetics, vol. 54, no. 11, Nov. 2018. 
[2] A. Perumal, Y.K. Takahashi, K. Hono, “L10 FePt-C Nanogranular Perpendicular Anisotropy Films with Narrow Size Distribution”, Appl. Phys. Express 1 101301 (2008). 
[3] B. S. D. C. S. Varaprasad, M Chen, YK Takahashi, K Hono “L1-Ordered FePt-Based Perpendicular Magnetic Recording Media for Heat-Assisted Magnetic Recording” IEEE
transactions on magnetics 49 (2), 718-722 (2013) 
[4] C. Xu, B. Zhou, T. Du, B.S.D.C.S. Varaprasad, D.E. Laughlin, J.-G. Zhu, Understanding the growth of high-aspect-ratio grains in granular L10-FePt thin-film magnetic media, APL Materials. 10 (2022). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b46f142fcddcff5ebe95523b1fdfc98e.png) 
Figure 1 (A) Out of plane micrographs of FePt-BN (8 nm) (B) Out of plane micrographs of FePt-BN/FePt-SiO 2 (11 nm) and (C) Order parameter Vs set temperature of FePt-BN sample

FePt BN Granular HAMR Media with High Grain Aspect Ratio and High L10 Ordering on Corning Lotus™ NXT Glass

Sputtering technique has been the choice for fabricating L10 FePt granular thin-film media for heat-assisted magnetic recording (HAMR). A variety of amorphous grain boundary materials have been studied extensively [1,2]. Among them, SiOx has been shown to facilitate columnar growth of FePt grains; however, its limitation on deposition temperature prevents high ordering. Amorphous carbon can yield high-temperature-enduring microstructure, however, with a limitation on grain height leveling to an aspect ratio ~1. Amorphous BN was also reported to affect the ordering of FePt [3]. In this paper, we report a study for creating a granular FePt-L10 film with crystalline BN grain boundaries that facilitate small grains with high aspect ratios and high ordering. By applying RF substrate bias with a substrate temperature of 700 oC, a microstructure of columnar FePt grains separated and wrapped around by hexagonal boron nitride(h-BN) layers is produced, as shown in Fig. 1(a). The spacing of the lattice fringes in grain boundaries is ~0.34 nm, close to that of h-BN basal planes (0.331 nm). The electron energy loss spectrum (EELS) shows the π* peak at 191.8 eV in the B-K edge, confirming the formation of turbostratic h-BN. Fig.1(e) shows columnar grains with an aspect ratio of ~1.85. A similar nanostructure was observed in FePt-Carbon film deposited with RF substrate bias, in which the graphite phase was likely formed in grain boundary regions. The covalent binding within the honeycomb structured nanosheets imparts the grain boundary materials high stability at high temperatures. This study provides an extendibility of FePt-based granular media to utilize partial crystalline grain boundary materials to improve microstructure. It also presents more understanding of why carbon and BN can maintain good grain isolation at quite high temperatures. 
**References** 
[1] A. Perumal, Y.K. Takahashi, K. Hono, L10 FePt-C Nanogranular Perpendicular Anisotropy Films with Narrow Size Distribution, 1 (2008) 101301.
https://doi.org/10.1143/apex.1.101301. 
[2] C. Xu, B. Zhou, T. Du, B.S.D.C.S. Varaprasad, D.E. Laughlin, J.-G. Zhu, Understanding the growth of high-aspect-ratio grains in granular L10
-FePt thin-film magnetic media, APL
Materials. 10 (2022). https://doi.org/10.1063/5.0089009. 
[3] B.H. Li, C. Feng, X. Gao, J. Teng, G.H. Yu, X. Xing, Z.Y. Liu, Magnetic properties and microstructure of FePt/BN nanocomposite films with perpendicular magnetic anisotropy,
Applied Physics Letters. 91 (2007). https://doi.org/10.1063/1.2798584. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/07f5a5208e585f6f71b6c269715f7b17.png) 
Fig.1 (a) Plane-view HRTEM of FePt-hBN film, (b) FFT shows a ring pattern of h-BN (002), corresponding to a d-spacing of 0.33~ 0.35 nm; (c) and (d) grain size and center-to-center
pitch distance distributions; (e) cross-sectional TEM image of the 8-nm-thick FePt-hBN film grown on MgO seed layer. An order parameter of 0.84 was achieved with substrate
temperature around 700oC.

RF Biased sputtering induced formation of hexagonal boron nitride (h

In granular FePt-L10 HAMR media. sufficient grain volume is critical for achieving expected areal density capability (ADC) [1][2]. Grain height needs to be increased as the grain size decreases. We present an experimental study on the fabrication of HAMR media with tall/high aspect ratio FePt-L10 grains. The underlayer stack consists of Ta(5nm)/Cr(40nm)/MgO(8nm) prior to the magnetic layer deposition. A 0.5 nm FePt layer is deposited on the MgO underlayer for initial grain nucleation, followed by the deposition of 2 nm FePt-BN (35% boron nitride) both at 700 oC. Over the FePt/FePt-BN stack, FePt-SiOx of 10nm thickness is deposited in the following manner: The first 3.5nm are deposited at 600 oC followed by immediately turning-off of the substrate heating and the rest of the layer is then deposited while the substrate undergoes naturally cooling. The ending temperature is 430 oC . The volumetric SiOx concentration of the FePt-SiOx layer is also graded from 40% at the bottom to 18% at the top. The L10 order parameter is S=0.72, calculated based the XRD measurement. Figure 1 shows the plan view HAADF TEM image (left) and cross-section view TEM image (right). The plan view image clearly shows well separated grains with average grain size around 6nm. The cross-section image shows columnar grains with well-defined grain boundaries and a grain aspect ratio ranging from 1.7 to 2. In this film stack, the SiOx grain boundaries are vertically matched with boron-nitride grain boundaries at the interface of the two layers. If the deposition temperature exceeds 650 oC, the majority of the adjacent FePt grains could become laterally connected over the entire thickness of the FePt-SiOx layer whereas the BN grain boundaries remains intact. While we vary the deposition conditions of the FePt-SiOx layer, the FePt/FePt-BN stack acts as a fixed template layer for the grain growth on top. For the FePt-SiOx layer, the graded SiOx composition combined with the graded deposition temperature serves the key to (1) ensure columnar grains that are well separated by SiOx grain boundaries and (2) avoid the formation of 2nd layer FePt grains.

![](https://assets.underline.io/uploads/markdown_image/1/image/89ab165f3824e2641ad4a23daad4429d.jpg)

Fig. 1. Planview HAADF (Left) and Cross-section (Right) TEM image of the entire medium stack.

Fabrication of Granular FePt-L10 HAMR Media with High Grain Aspect Ratio

As the current recording media for the heat assisted magnetic recording (HAMR), the granular L10-FePt thin film needs to have uniform grain size with small pitch distance and well-defined grain boundaries. To achieve the targeted microstructure with proper L10 ordering using sputtering techniques, FePt along with grain boundary materials (segregants) are co-deposited on MgO underlayers at elevated temperature. The columnar growth of FePt is desired to have sufficient grain volume without grain size expansion. The fabrication of grains at small grain pitch with well-defined grain boundaries has been proven to be challenging.[1] Over the past decade, segregant materials, such as carbon, have attracted extensive attention for producing isolated FePt grains with well-defined grain boundaries but has experienced difficulty at reducing grain pitch distance and making grains with large aspect ratio. Though the columnar growth of FePt can be achieved by using some oxide segregant materials, such as TaOx[2] and TiOx[3], the resulting films always show poor L10 ordering and poor magnetic properties. Proper ordering of FePt can be achieved with SiOx[4], but the resulting microstructure tends to be maze-like. Therefore, exploiting multilayer deposition is a promising strategy to achieve the desired microstructure with proper ordering and magnetic properties. In this study, we utilize a dual-layer strategy of FePt-BN/FePt-SiOx on polycrystalline MgO underlayer in order to obtain the desired media microstructure with good L10 ordering and magnetic properties. The film stack of Ta(5)|Cr(40)|MgO(6)|Media(6.5) was deposited on Lotus NXT glass substrate with the numbers in parentheses in nm. Lotus NXT glass is one of the promising glass substrates for high-temperature deposition of HAMR media. The Ta and Cr layers were DC sputtered at room temperature and 280°C, respectively. The samples were subsequently annealed at 650°C for 1 hour in order to promote the texture and grain size of Cr buffer layer. The MgO underlayer was RF deposited on Cr buffer layer after the sample cooled down to room temperature. There are two magnetic layers in the media layer, where FePt was co-sputtered with BN and SiOx, respectively, for the first (M1) and the second (M2) magnetic layer. The deposition temperatures of the M1 layer and the M2 layer were fixed at 700°C and 550°C, respectively. From the microstructural point of view, the main objective of the M1 layer is to obtain well-isolated FePt grains to set the grain center-to-center pitch distance, whereas the M2 layer aims to suppress the lateral growth and to promote the columnar growth of the FePt grains. Fig. 1 (a) shows the in-plane HAADF image of the film stack Lotus NXT glass |Ta(5)|Cr(40)|MgO(6)|Media(6.5) with its (b) grain size distribution profile and (c) grain center-to-center pitch distance distribution profile. The figure demonstrates well-isolated FePt grains with mean grain size of 6.3 nm and mean pitch distance of 8.3 nm. As shown in the XRD pattern of this media in Fig. 1 (d), there are only (001) and (002) peaks from FePt, indicating the c-axis of FePt grains are predominantly perpendicular to the film plane. The broad peak at around 23.5° corresponds to the amorphous peak from Lotus NXT glass substrate. The order parameter (S) considering the relative integrated intensity ratio between (001) and (002), I001/I002, with the absorption factor and Lorentz factor is 0.7, suggesting good chemical ordering in FePt grains. Fig. 1 (e) shows the perpendicular and in-plane hysteresis loops of this media. The perpendicular coercivity (Hc⊥) is 26 kOe. The in-plane loop shows a small opening with the coercivity (Hc//) of 2.9 kOe and the normalized remanence magnetization (Mr//) of 0.08. The remanent ratio, which is Mr// /Mr⊥, is 0.09, suggesting a minor in-plane component of FePt grains in the sample. By exploiting the dual-layer structure of FePt-BN/FePt-SiOx, we have demonstrated well-isolated columnar FePt grains with small mean pitch distance of 8.3 nm. The XRD pattern shows that the grains are well-textured and highly ordered. As a result, the perpendicular coercivity reaches 26 kOe. The utilization of multilayer deposition strategy with segregants BN and SiOx provides a promising technique to achieve desired HAMR media microstructure without sacrificing FePt chemical ordering and magnetic properties.

**References:**

[1] Y. Kubota et al., IEEE Trans. Magn., vol. 54, no. 11, p. 3201206, 2018 [2] B. Lim et al., J. Appl. Phys., vol. 105, no. 7, p. 07A730, 2009 [3] B. S. D. C. S. Varaprasad et al., IEEE Trans. Magn., vol. 49, no. 2, p. 718–722, 2013 [4] H. Ho et al., Appl. Phys. Lett., vol. 102, no. 11, p. 112411, 2013

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/44d762af9dd6d7af7c27e30c471fccc1.jpg)
Fig. 1 (a) In-plane HAADF image of the film stack Lotus NXT glass |Ta(5)|Cr(40)|MgO(6)|FePt-38vol.% BN(2)|FePt-35vol.%SiOx with its corresponding (b) grain size distribution and (c) grain center-to-center pitch distance. Its XRD pattern and magnetic properties are in (d) and (e) respectively.

 Fabrication of FePt-BN/FePt-SiOx dual-layer structure for HAMR media on Corning Lotus™ NXT glass substrate

CC: Characterization of Gallium Oxide-Based Materials and Devices II

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IOB: Magnetic Recording II

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IA: Magnetic Recording: Energy-assisted, Media, Head, Models

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Presentations

Phase Transformation of β-Ga2O3 to Ga2O3 -Based γ-Phase Spinels on (100) MgAl2O4 Substrate

FePt BN Granular HAMR Media with High Grain Aspect Ratio and High L10 Ordering on Corning Lotus™ NXT Glass

RF Biased sputtering induced formation of hexagonal boron nitride (h

Fabrication of Granular FePt-L10 HAMR Media with High Grain Aspect Ratio

Fabrication of FePt-BN/FePt-SiOx dual-layer structure for HAMR media on Corning Lotus™ NXT glass substrate

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