FePt–BN granular HAMR media with high grain aspect ratio and high L1 ordering on corning Lotus^TM NXT glass

Chemically-ordered FePt–X granular films are being used as the recording media for heat-assisted magnetic recording (HAMR), where X stands for amorphous grain boundary materials, such as carbon (C), metal oxides, nitrides, etc.^1–5 To achieve the desired areal recording density capability (ADC), the FePt grains need to be well-isolated ensuring lateral thermal and magnetic isolation.⁶ As ADC continues to increase, the grain size and grain-to-grain pitch distance in the media need to be scaled down while maintaining the desirable microstructure.⁷ The grain volume can be maintained by increasing the grain height while the grain diameter reduction is necessary to increase the ADC. Achieving columnar grains that form well-isolated microstructures becomes the key to the advancement of granular L1₀–FePt media.

Fabrication of granular L1₀–FePt media with high-aspect-ratio columnar grains and well-defined grain boundaries is still challenging. FePt–carbon system is well known to generate well-isolated small FePt grains with amorphous carbon as grain boundaries.² Carbon’s high mobility and high interfacial energy with FePt results in sphere-like FePt grains surrounded by amorphous grain boundaries.⁸ Metal oxides as segregants have shown columnar growth, including SiO₂, TaO_x, and TiO₂ but most display an in-plane maze-like microstructure observed by TEM analysis.⁵ One of the reasons to develop interconnected maze-like structures with metal oxides as a grain boundary material (GBM) is to have less mobility. Producing stoichiometric thin metal oxides using the sputtering technique is difficult. The melting temperature of the sputtered non-stoichiometric SiO_x used as the grain boundary material (GBM) might be substantially lower than that of the stochiometric bulk SiO₂.¹² To achieve well isolated highly ordered tall FePt grains, we need a GBM with high thermal and chemical stability. The GBM interacting with the FePt grains can result in a lowering degree of L1₀ ordering and a soft magnetic shell. To overcome these issues, researchers have tried mixing the segregants and varying compositions through their depth, by grading the grain boundary materials during deposition.^8–10 Our recent studies have shown the FePt–BN/FePt–SiO_x bilayer structure can achieve well-isolated grains with reasonable chemical ordering.^10–12 Amorphous BN_x is not chemically stable and it forms alloys with Fe at high temperatures. Whereas hexagonal boron nitride (h–BN), the most stable crystalline polymorph of BN, has a graphite-like structure made up of hexagonal 2D nano-sheets with sp₂ bonds. Its excellent chemical and thermal stabilities could be advantageous over other amorphous GBM. In this work, we explored the crystalline h-BN as a GBM for FePt granular HAMR media. h–BN planes can be produced by a sputtering machine employing proper RF bias at high temperatures.¹³ h–BN sheets two-dimensional material has many unique properties, one of them being, its anisotropic conductivity conductive in-plane and almost insulating in perpendicular direction. We want to fabricate h–BN layers around the FePt particles to isolate the grains magnetically and thermally.

A film stack of Ta (2 nm) | Cr (50 nm) | MgO (8 nm) | FePt (0.4 nm) | FePt–BN_{26 vol. %} (1.2 nm) (without RF bias) | FePt–BN_{26 vol. %} (2 nm) (with RF bias) | FePt–BN_{22 vol. %} (3.5 nm) (with RF bias) | FePt–BN_{20 vol. %} (3.7 nm) (with RF bias) (Going forward we will use FePt–BN–ML as defined above) is deposited on Corning NXT™ glass (hereafter we call NXT) using an ultra-high vacuum AJA sputtering system with 1 × 10⁻⁸ Torr base pressure. Ta, Cr, MgO, FePt alloy, and BN targets with 2-inch size and 99.9% purity are used to deposit films. Ta as deposited at room temperature as an adhesion and barrier layer for NXT glass. A 50 nm thick Cr layer was deposited at 250 °C to realize the good (200) texture on Ta. We performed post-annealing of Cr at 700 °C to attain a smooth surface with a large grain size. The MgO underlayer was deposited at room temperature with 100 W RF, 10 m Torr, and the target to substrate distance was around 80 mm. In our previous study, we used FePt (0.4 nm) | FePt–BN_{26 vol. %} (1.2 nm) (without RF bias) as a templet layer for FePt–SiO_x media. We used the same templet in this study also for consistency. The ultra-thin 0.4 nm FePt layer was used to increase the nucleation density. RF bias and high-temperature deposition both are necessary to realize h–BN. We used a low 3 W RF bias to realize h–BN as we learned from our research that, without the bias BN_x will be amorphous. We used the same sputtering conditions to deposit FePt–BN–ML granular films which were deposited at 600, 650, 700, and 750 °C to promote L1₀ ordering. Using a Copper source, the film structure was examined by standard x-ray diffraction (XRD). The microstructure of the samples was evaluated by in-plane and out-of-plane transmission electron microscopy (TEM) imaging, using bright-field TEM (BF-TEM), high-resolution TEM (HR-TEM), and scanning TEM-high angle annular dark field (STEM-HAADF) techniques using FEI Titan Themis 200. The grain size and grain center-to-center pitch distances were analyzed using the plane-view STEM-HAADF images and the image processing software (MIPAR) (not presented in this paper). The moment (M_s) vs field (H) curves of the film samples were measured with a Quantum Design Superconducting quantum interference device-vibrating sample magnetometer (SQUID-VSM).

From our previous work, we know that amorphous BN_x is structurally stable even at high temperatures and forms a stable grain boundary. High deposition temperatures lead us to have good L1₀ ordered FePt. From the literature, h–BN can be deposited in a sputter tool using proper deposition conditions. We learned from our previous studies that a higher segregant percentage in the initial layer effectively reduces the grain size, however to avoid the over-coating GBM on the FePt grains we have reduced the BN volume in the later layers. We reduced gradually the BN volume percentage during the film growth from 26 vol. % to 20 vol. %. The stack used in this work is the optimized film stack that gave a better microstructure with decent magnetic properties. In this research, our focus is h–BN as GBM, which is chemically and structurally stable even at very high temperatures. We varied the deposition temperature to study the effect of deposition temperature on the grain growth of the FePt. The same film stack was deposited at various 600, 650, 700, and 750 °C temperatures. Fig.1 (a) shows XRD micrographs of FePt–BN–ML deposited at various temperatures. An 11 nm thick FePt–BN–ML film showed a strong (001) texture, Strong (001)_FePt, and (002)_FePt peaks indicating all thin films are well textured from 600 °C. We observe a clear peak shift of 001 and 002 to the higher angle suggesting the decrease in the c-axis with the temperature. The sample deposited at 750 °C shows (002)_FePt peak is observed at 48.8 which is very close to the ideal value. Fig.1 (b) shows the order parameter Vs temperature. The order parameter (S) was calculated following Yang et al., by considering the geometric features of the X-ray diffractometer, the crystallographic texture of FePt films, and film thickness.¹⁴ It is evident that the integrated intensity ratio is high for the sample deposited at 750 °C compared with the sample deposited at 600 °C resulting in improved ordering.

Fig.2 (a) shows in-plane and out-plane room temperature moment vs the field (M-H) curves for the 750 °C FePt–BN–ML sample. From the in-plane measurement, we can say that this sample has strong perpendicular anisotropy with H_k of 70 kOe and perpendicular H_c of 35 kOe. The in-plane loop of this sample has a H_c of 4.5 kOe, which suggests that the texture was improved for the 750 °C. This supports the XRD analysis that these films have a high L1₀ ordering of (S = 0.85). The open loop in the in-plane measurements suggests that these samples have misaligned grains. Fig. 2(b) shows the coercivity Vs deposition temperature. The sample deposited at 600 temperature showed a perpendicular coercivity of 9.5 kOe and the sample deposited at 650 is 27 kOe. It is also observed in the XRD analysis (002) peak shift from 48.0° to 48.6° for 600 and 650 °C respectively.

The in-plane bright field TEM micrograph shown in Fig. 3(a) shows the optimized microstructure of the sample deposited at 750 °C with FePt–BN–ML (11 nm) thickness, which showed a well isolated FePt grain surrounded by BN. While Fig. 3(b) shows the cross-section of the complete film stack. All the films showed a single-layer structure from the out-of-plane observation (not included in this paper). The cross-section shows nice 11 nm tall columnar grains. Grain size distribution and pitch distance were estimated for the samples using STEM analysis. MIPAR image software was used to estimate the grain size from STEM images. In all our samples, we observed large interconnected scattered FePt grains (all large grains are connected in the top layers only) and we also observed randomly arrayed small FePt grains buried under h–BN GBM. The FePt grain size distribution is estimated to be 6.5 ± 1.9 nm for the sample deposited at 750 °C. This large deviation is due to the randomly present small and large interconnected FePt grains. Furthermore, optimization is required to reduce the size distribution. From our experimental results, we do not observe a large change in the average grain size of samples deposited at 600 to 750 °C.

Fig. 4 summarizes the average FePt grain size in the FePt–BN–ML layer deposited at different temperatures, while the deposition condition and the composition of the FePt–BN–ML layer remain unchanged. There is no significant changes in the FePt grain size by varying the deposition temperature, hence we can say there is no lateral growth of the grains, whereas, from our previous studies, carbon as a segregant showed strong lateral growth of FePt grains. This suggests the h–BN layer around the FePt grains is able to stop the lateral growth of FePt grains. Further investigation is needed to confirm this hypothesis.

Highly ordered tall L1₀–FePt–BN–ML with a small grain pitch distance on a commercially available glass substrate for heat-assisted magnetic recording (HAMR) media is developed. Well-isolated L1₀–FePt–BN granular media with a grain diameter of 6.5 nm and height of 11 nm is achieved. The lateral growth of FePt grains is suppressed by crystalized h–BN formed around the grains. Samples deposited at 750 °C films exhibit a high order parameter of 0.85 with good squareness in M-H loops with a perpendicular coercivity of 35 kOe.

This research was funded in part by the Data Storage Systems Center at Carnegie Mellon University and all its industrial sponsors and by the Kavcic-Moura Fund at Carnegie Mellon University. The authors acknowledge the use of the Materials Characterization Facility at Carnegie Mellon University supported by Grant No. MCF-677785.

The authors have no conflicts to disclose.

B. S. D. Ch. S. Varaprasad: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Chengchao Xu: Data curation (equal); Formal analysis (equal); Visualization (equal). Ming-Huang Huang: Funding acquisition (supporting); Investigation (equal); Resources (supporting). David E. Laughlin: Conceptualization (equal); Data curation (equal); Methodology (equal); Project administration (equal); Writing – original draft (equal); Writing – review & editing (equal). Jian-Gang Zhu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Methodology (equal); Project administration (equal); Resources (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.