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

Abstract

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. **