Magnetic Properties and Microstructure of FePt(BN, X, C) (X = Ag, Re) Films

A sputtered FePt(BN, Re, C) film, here boron nitride (BN), was compared to a reference sample FePt(BN, Ag, C). Intrinsically, these films illustrate a high anisotropy field (Hk) and perpendicular magnetocrystalline anisotropy (Ku),although the reference sample shows a higher value (Hk = 69.5 kOe, Ku = 1.74 × 107 erg/cm3) than the FePt(BN, Re, C) film (Hk = 66.9 kOe, Ku = 1.46 × 107 erg/cm3). However, the small difference in the anisotropy constant (K2/K1) ratio presents a close tendency in the angular dependence of the switching field. Extrinsically, the out-of-plane coercivity for the reference sample is 32 kOe, which is also higher than the FePt(BN, Re, C) film (Hc = 27 kOe), and both films present lower remanence (Mr(parallel)/Mr(perpendicular) = 0.08~0.12), that is, the index for perpendicular magnetic anisotropy. The higher perpendicular magnetization for both films was due to highly (001) textured FePt films, which was also evidenced by the tight rocking width of 4.1°/3.0° for (001)/(002) X-ray diffraction peaks, respectively, and high-enough ordering degree. The reference sample was measured to have a higher ordering degree (S = 0.84) than FePt(BN, Re, C) (S = 0.63). As a result, the Ag segregant shows stronger ability to promote the ordering of the FePt film; however, the FePt(BN, Re, C) film still has comparable magnetic properties without Ag doping. From the surface and elemental composition analysis, the metallic Re atoms found in the FePt lattice result in a strong spin–orbital coupling between transition metal Fe (3d electron) and heavy metals (Re, Pt) (5d electron) and we conducted high magnetocrystalline anisotropy (Ku). Above is the explanation that the lower-ordered FePt(BN, Re, C) film still has high-enough Ku and out-of-plane Hc. Regarding the microstructure, both the reference sample and FePt(BN, Re, C) show granular structure and columnar grains, and the respective average grain size and distributions are 6.60 nm (12.5%) and 11.2 nm (15.9%). The average widths of the grain boundaries and the aspect ratio of the columnar grain height are 2.05 nm, 1.00 nm, 2.35 nm, and 1.70 nm, respectively.


Introduction
Energy-assisted magnetic recording is an alternative way to overcome the trilemma effect (superparamagnetic effect of CoCrPt oxides) in conventional perpendicular magnetic recording (PMR) and heat-assisted magnetic recording (HAMR) with L10 FePt media is the mainstream technology to target the hard disk area density beyond 4 Tb/in2 in the near future. The L10 FePt film, which has higher magnetocrystalline anisotropy (Ku), is the optimal media in the HAMR system [1][2][3][4] and the hard disk drive (HDD) based on FePt media is currently being tested on data center customers [5][6][7][8].
To apply FePt media to HAMR, many works have discussed this extensively for decades. For HAMR media, perpendicular magnetic anisotropy (PMA) of the FePt film was a key requirement to extend the area density to ultrahigh and increase the capacity in HDD [1][2][3][4]. After annealing the disordered face-centered cubic FePt alloy with nearly

Materials and Methods
The FePt(BN, Ag, C) and FePt(BN, Re, C) films were deposited by direct current magnetron sputtering on MgO (100) single-crystal substrate. The sputtering deposition equipment was designed to have main and pre-chambers and the substrate was moved via the load-lock feedthrough to the substrate holder, which has a heating system formed of a halogen lamp (OSRAM, 1000W) and cathodes (AJA, A320, North Scituate, MA, USA) set up in the main chamber. The two-inch-diameter composite targets FePt(BN, Ag, C) and FePt(BN, Re, C) were used. In order to compare the elemental Ag and Re doping effects precisely, the MgO (100) single crystal was used to provide the optimal baseline for the epitaxial growth of (001) textured FePt film. The MgO (100) substrate (HF-Kejing, 10 mm × 10 mm × 0.5 mm) was heated at 470 • C (the substrate surface temperature) for dry cleaning and then the magnetic layers with thickness of 15 nm were sputtered under an Argon working pressure of 10 mTorr at 470 oC by the composite targets. The deposition rate of FePt(BN, Ag, C) (15 nm) and FePt(BN, Re, C) (15 nm) was 0.0372 nm/s and 0.0421 nm/s, respectively. In this study, the FePt(BN, Ag, C) was named as the reference sample because of the typical segregants (BN, Ag, C) used in HAMR media.
The crystal structure was measured using standard X-ray diffraction (XRD) (BRUKER, D8 Discover). The magnetization curves with in-plane and out-of-plane measured hysteresis loops were performed with a superconducting quantum interference device (SQUID) magnetometer (MPMS-XL). The sample microstructure was observed using transmission electron microscopy (TEM, JEOL JEM-2010). The surface analysis was measured using X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 5000). Figure 1 shows XRD patterns of (a) FePt(BN, Ag, C) (reference sample) and (b) FePt(BN, Re, C) films. Based on the crystal structure, the strong (001) and weak (003) superlattice peaks were present because of the ordered L10 FePt phase (JCPDS 43-1359) and the (002) fundamental reflection peak was also indexed (fcc FePt, JCPDS card no. 00-029-0718). The lattice constant "c" was calculated using the (001) diffraction peaks and the values are 0.3728 nm and 0.3743 nm, respectively. In addition, the (110) diffraction peak was measured using grazing incident diffraction (GID) that is similar to the in-plane XRD and the lattice constant "a" that was estimated with values of 0.3889 nm and 0.3857. The reference sample presents a smaller c/a ratio (0.959), which means higher ordering (more tensile stress on axis "a" and compressive force on axis "c") as compared to the FePt(BN, Re, C) film with c/a= 0.970. Another way to estimate the FePt ordering degree is by using B. E. Warren's equation [29,30]; the ordering degree is (I (001) /I( 002) ) 1/2 /(I* (002) /I* (001) ) 1/2 and the values are 0.84 and 0.63, respectively. The (I* (002) /I* (001) ) 1/2 value was calculated using film thickness and the width of half intensity in rocking curve, in theory [29,30], and I (001) I (002) is the peak's integrated intensity ratio for textured (001) FePt films and the respective values are 2.04 and 1.15 in (a) and (b). In Figure 1b, there is the fct/fcc structured FePt (200) tiny peak, indexed at 47.0 • , which contributes to the c-axis misalignment and lowers the ordering degree of the FePt(BN, Re, C) film. The c-axis misalignment mentioned above can be measured using the rocking curves and width at a half intensity of FePt (001)/(002); the peaks are 4.09 • /3.03 • and 4.09 • /3.07 • in Figure 1c,d. The (002) rocking curve was asymmetric due to the overlap of (200) and (002) peaks and a little wider inthe FePt(BN, Re, C) sample. In summary, the reference sample and FePt(BN, Re, C) film were hetero-epitaxially grown on MgO(100) and have a strong (001) texture. The crystal structure was measured using standard X-ray diffraction (XRD) (BRUKER, D8 Discover). The magnetization curves with in-plane and out-of-plane measured hysteresis loops were performed with a superconducting quantum interference device (SQUID) magnetometer (MPMS-XL). The sample microstructure was observed using transmission electron microscopy (TEM, JEOL JEM-2010). The surface analysis was measured using X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 5000). Figure 1 shows XRD patterns of (a) FePt(BN, Ag, C) (reference sample) and (b) FePt(BN, Re, C) films. Based on the crystal structure, the strong (001) and weak (003) superlattice peaks were present because of the ordered L10 FePt phase (JCPDS 43-1359) and the (002) fundamental reflection peak was also indexed (fcc FePt, JCPDS card no. 00-029-0718). The lattice constant "c" was calculated using the (001) diffraction peaks and the values are 0.3728nm and 0.3743nm, respectively. In addition, the (110) diffraction peak was measured using grazing incident diffraction (GID) that is similar to the in-plane XRD and the lattice constant "a" that was estimated with values of 0.3889 nm and 0.3857. The reference sample presents a smaller c/a ratio (0.959), which means higher ordering (more tensile stress on axis "a" and compressive force on axis "c") as compared to the FePt(BN, Re, C) film with c/a= 0.970. Another way to estimate the FePt ordering degree is by using B. E. Warren's equation [29,30]; the ordering degree is (I(001)/I(002)) 1/2 /(I*(002)/I*(001)) 1/2 and the values are 0.84 and 0.63, respectively. The (I*(002)/I*(001)) 1/2 value was calculated using film thickness and the width of half intensity in rocking curve, in theory [29,30], and I(001)I(002) is the peak's integrated intensity ratio for textured (001) FePt films and the respective values are 2.04 and 1.15 in (a) and (b). In Figure 1b, there is the fct/fcc structured FePt (200) tiny peak, indexed at 47.0°, which contributes to the c-axis misalignment and lowers the ordering degree of the FePt(BN, Re, C) film. The c-axis misalignment mentioned above can be measured using the rocking curves and width at a half intensity of FePt (001)/(002); the peaks are 4.09°/3.03° and 4.09°/3.07° in Figure 1c,d. The (002) rocking curve was asymmetric due to the overlap of (200) and (002) peaks and a little wider inthe FePt(BN, Re, C) sample. In summary, the reference sample and FePt(BN, Re, C) film were hetero-epitaxially grown on MgO(100) and have a strong (001) texture. The perpendicular Hk magnetic anisotropy of the reference sample and FePt(BN, Re, C) film was proved by the in-plane and out-of-plane magnetic hysteresis loop measurements. The easy magnetization curve (out-of-plane) shows full and huge hysteresis and the hard direction is a linear-like curve with small loop areas in Figure 2. The out-of-plane coercivity (Hc) is 32 kOe, 27 kOe, and the (Mr(in-plane))/Mr(out-of-plane) ratio, which qualifies the perpendicular anisotropy as 0.12, 0.08, and the negative nucleation field, which is defined at 0.95 Ms, is −2.5 kOe and −1.8 kOe, respectively. In Table 1, the reference sample illustrates a higher magnetic anisotropic field (Hk), which equals to 2Ku/Ms (Hk = 69.5 kOe), magneto-crystalline anisotropy (Ku= 1.74 × 10 7 erg/cm 3 ) and saturation of magnetization (Ms = 502 kG), than the FePt(BN, Re, C) film intrinsically. The magnetic anisotropy field Hk was defined as the intersection point of easy-and hard-axes magnetic hysteresis loops. The reference sample FePt(BN, Ag, C) film shows a higher ordering degree (S) and also has higher magnetic anisotropy (Ku than the FePt(BN, Re, C) film. Only 63% of the FePt(BN, Re, C) grains are ordered but this film still has high-enough Ku and Hc. For reference, the 5% concentration Co-Re shows higher Ku than Co-Pt [26]. It is suggested that Fe-Re also contributes to extra Ku magnetic anisotropy due to the higher spin-orbital coupling of heavy metal Re and hybridization with 3d Fe electrons. As a result, the high magnetic anisotropy (Ku and out-of-plane Hc were proved in the relative lower ordered (S = 0.63) FePt(BN, Re, C) film without Ag segregant.  The perpendicular Hk magnetic anisotropy of the reference sample and FePt(BN, Re, C) film was proved by the in-plane and out-of-plane magnetic hysteresis loop measurements. The easy magnetization curve (out-of-plane) shows full and huge hysteresis and the hard direction is a linear-like curve with small loop areas in Figure 2. The out-of-plane coercivity (Hc) is 32 kOe, 27 kOe, and the (Mr (in-plane) )/Mr (out-of-plane) ratio, which qualifies the perpendicular anisotropy as 0.12, 0.08, and the negative nucleation field, which is defined at 0.95 Ms, is −2.5 kOe and −1.8 kOe, respectively. In Table 1, the reference sample illustrates a higher magnetic anisotropic field (Hk), which equals to 2Ku/Ms (Hk = 69.5 kOe), magneto-crystalline anisotropy (Ku= 1.74 × 10 7 erg/cm 3 ) and saturation of magnetization (Ms = 502 kG), than the FePt(BN, Re, C) film intrinsically. The magnetic anisotropy field Hk was defined as the intersection point of easy-and hard-axes magnetic hysteresis loops. The reference sample FePt(BN, Ag, C) film shows a higher ordering degree (S) and also has higher magnetic anisotropy (Ku than the FePt(BN, Re, C) film. Only 63% of the FePt(BN, Re, C) grains are ordered but this film still has high-enough Ku and Hc. For reference, the 5% concentration Co-Re shows higher Ku than Co-Pt [26]. It is suggested that Fe-Re also contributes to extra Ku magnetic anisotropy due to the higher spin-orbital coupling of heavy metal Re and hybridization with 3d Fe electrons. As a result, the high magnetic anisotropy (Ku and out-of-plane Hc were proved in the relative lower ordered (S = 0.63) FePt(BN, Re, C) film without Ag segregant. The perpendicular Hk magnetic anisotropy of the reference sample and FePt(BN, Re, C) film was proved by the in-plane and out-of-plane magnetic hysteresis loop measurements. The easy magnetization curve (out-of-plane) shows full and huge hysteresis and the hard direction is a linear-like curve with small loop areas in Figure 2. The out-of-plane coercivity (Hc) is 32 kOe, 27 kOe, and the (Mr(in-plane))/Mr(out-of-plane) ratio, which qualifies the perpendicular anisotropy as 0.12, 0.08, and the negative nucleation field, which is defined at 0.95 Ms, is −2.5 kOe and −1.8 kOe, respectively. In Table 1, the reference sample illustrates a higher magnetic anisotropic field (Hk), which equals to 2Ku/Ms (Hk = 69.5 kOe), magneto-crystalline anisotropy (Ku= 1.74 × 10 7 erg/cm 3 ) and saturation of magnetization (Ms = 502 kG), than the FePt(BN, Re, C) film intrinsically. The magnetic anisotropy field Hk was defined as the intersection point of easy-and hard-axes magnetic hysteresis loops. The reference sample FePt(BN, Ag, C) film shows a higher ordering degree (S) and also has higher magnetic anisotropy (Ku than the FePt(BN, Re, C) film. Only 63% of the FePt(BN, Re, C) grains are ordered but this film still has high-enough Ku and Hc. For reference, the 5% concentration Co-Re shows higher Ku than Co-Pt [26]. It is suggested that Fe-Re also contributes to extra Ku magnetic anisotropy due to the higher spin-orbital coupling of heavy metal Re and hybridization with 3d Fe electrons. As a result, the high magnetic anisotropy (Ku and out-of-plane Hc were proved in the relative lower ordered (S = 0.63) FePt(BN, Re, C) film without Ag segregant.   From Table 1, the respective anisotropy fields (HK) of the reference FePt(BN, Ag, C) and FePt(BN, Re, C) films are 69.5 and 66.9 kOe, and it is noteworthy that the values of Ku were often estimated from the slopes on the magnetization curves measured using a maximum applied field lower than the magnetic anisotropy Hk of L10-FePt, which was likely to result in certain errors in the estimation of Ku and which might confuse the discussion. Figure 3 illustrates the individual linear part in hard-axis curves for the reference sample, and the FePt(BN, Re, C) film and the K1 and a high-order anisotropy constant K2 were estimated using the following equation, respectively, [31,32]

Results and Discussion
where m = M/M s is the normalized magnetization and K 1 is the second-order anisotropy constant (Sucksmith-Thompson method). From the H/m versus m 2 plot, the K 1 and K 2 constants were conducted by the interception and the slope of the fitting curve [31,32]. The  From Table 1, the respective anisotropy fields (HK) of the reference FePt(BN, Ag, C) and FePt(BN, Re, C) films are 69.5 and 66.9 kOe, and it is noteworthy that the values of Ku were often estimated from the slopes on the magnetization curves measured using a maximum applied field lower than the magnetic anisotropy Hk of L10-FePt, which was likely to result in certain errors in the estimation of Ku and which might confuse the discussion. Figure 3 illustrates the individual linear part in hard-axis curves for the reference sample, and the FePt(BN, Re, C) film and the K1 and a high-order anisotropy constant K2 were estimated using the following equation, respectively, [31,32]   The (dynamic) ordering temperature, degree, and magnetic anisotropic Ku constant were influenced by the FePt chemical composition, which was fixed to avoid any misleading. The MgO (100) single-crystal substrate was used and it is suggested that the structure variant of the FePt film was due to the slight dissolution of segregants at a high deposition temperature. The quite different ordering degree in the reference sample (S = 0.84) and FePt (Re, C, BN) film (S = 0.63) was because of the structure variant that also influenced the magnetic anisotropy Ku. Based on the reference reports [31,32], the Ku1 and Ku2 are proportional and independent to the ordering degree (S), respectively. In this study, the values of the ordering degree and magnetic anisotropic constant were a direct response to the Ag and Re segregants because the concentration of C and BN doping was the same in both samples. Adding the Ag element truly promotes the ordering degree and has The (dynamic) ordering temperature, degree, and magnetic anisotropic Ku constant were influenced by the FePt chemical composition, which was fixed to avoid any misleading. The MgO (100) single-crystal substrate was used and it is suggested that the structure variant of the FePt film was due to the slight dissolution of segregants at a high deposition temperature. The quite different ordering degree in the reference sample (S = 0.84) and FePt (Re, C, BN) film (S = 0.63) was because of the structure variant that also influenced the magnetic anisotropy Ku. Based on the reference reports [31,32], the K u1 and K u2 are proportional and independent to the ordering degree (S), respectively. In this study, the values of the ordering degree and magnetic anisotropic constant were a direct response to the Ag and Re segregants because the concentration of C and BN doping was the same in both samples. Adding the Ag element truly promotes the ordering degree and has higher magnetic anisotropic constant (K u ). In the Sucksmith-Thompson method, the K 1 , which determines the switching behavior, has close values for two samples (dK 1~0 .02) and the K 2 , which dominates the thermal stability, has large differences (dK 2~0 .47). The K 1 is proportional to the ordering degree but the FePt(Re, C, BN) film presents extra spin-orbital coupling between Fe and Pt(Re), which enhances the K 1 value. As a result, the FePt(BN, Re, C) film shows a lower K 2 /K 1 value (0.22 < 0.26). Figure 4 presents the major and minor (recoil in variant field) magnetization loops to study the relationships between the magnetic switching behavior and grains' morphology, and the intrinsic and extrinsic switching field distribution was analyzed. When the loops recoiled in the coercivity field, half of the grains (50%) were switched in the easy axial direction and the parameters ∆H int and ∆H ext were collected after comparison with the major magnetic hysteresis curves. For reference FePt(BN, Ag, C) and FePt(BN, Re, C) films, the intrinsic (∆H int = 15.5 kOe, 9.30 kOe) and extrinsic (∆H ext = 3.54 kOe, 3.06 kOe) switching field distributions were determined in Figure 4a,b, respectively.
The intrinsic switching field distribution comes from grain-size distribution (σv olume ), magnetic anisotropy field dispersion (σ Hk ), and misalignment of the c-axis (σ axis ), and the extrinsic SFD comes from the grains' dipolar and exchange coupling [2,3,27,28]. The standard deviation (σ int ) of intrinsic SFD is estimated from the ∆Hint/1.35 [2,3,27,28], and the (σ int )2 can be written as the summary of (σ volume ) 2 + (σ axis ) 2 + (σ Hk ) 2 , because of the close values in rocking width (∆θ 50 ) and average grain size in both samples; the grain-size distribution (σ volume ) and misalignment of the c-axis (σ axis ) terms were dropped. As a result, the SFD can be evaluated by the magnetic anisotropy field dispersion (σ Hk ) and the FePt (BN, Re, C) film illustrates smaller values. The small difference in extrinsic SFD (∆H ext ) means that the equal grain decoupling in granular structure for two samples and the long-range magnetostatic coupling and inter-granular coupling were not considered in this study.
which determines the switching behavior, has close values for two sam and the K2, which dominates the thermal stability, has large differences K1 is proportional to the ordering degree but the FePt(Re, C, BN) film pre orbital coupling between Fe and Pt(Re), which enhances the K1 value. FePt(BN, Re, C) film shows a lower K2/K1 value (0.22 < 0.26). Figure 4 presents the major and minor (recoil in variant field) magn study the relationships between the magnetic switching behavior and ogy, and the intrinsic and extrinsic switching field distribution was ana loops recoiled in the coercivity field, half of the grains (50%) were swit axial direction and the parameters ΔHint and ΔHext were collected after the major magnetic hysteresis curves. For reference FePt(BN, Ag, C) and films, the intrinsic (ΔHint = 15.5 kOe, 9.30 kOe) and extrinsic (ΔHext = 3.5 switching field distributions were determined in Figure 4a,b, respectivel The intrinsic switching field distribution comes from grain-size dist magnetic anisotropy field dispersion (σHk), and misalignment of the c-ax extrinsic SFD comes from the grains' dipolar and exchange coupling standard deviation (σint) of intrinsic SFD is estimated from the ΔHint/1.3 the (σint)2 can be written as the summary of (σvolume) 2 + (σaxis) 2 + (σHk) 2 , bec values in rocking width (Δθ50) and average grain size in both samples; th tribution (σvolume) and misalignment of the c-axis (σaxis) terms were drop the SFD can be evaluated by the magnetic anisotropy field dispersion (σ (BN, Re, C) film illustrates smaller values. The small difference in extr means that the equal grain decoupling in granular structure for two samp range magnetostatic coupling and inter-granular coupling were not co study.  Figure 5, in spite of metallic Fe, Pt, C, and Ag peaks, ferrite Fe3O4 and (Sat.) were also detected from the film surface after peak resolution. From shape shown in Figures 5 and 6a, there are distinctive shoulders at the around 710 eV and 720 eV, and the Fe 2p3/2 and 2p1/2 core-level bin considered and the Fe3O4 phase was observed both in FePt(BN, Ag, C) a C) films. The magnetite Fe3O4 (ferrimagnet) is a soft magnetic material the slight kink illustrated in the out-of-plane magnetization curves at ze 2a [33,34].   Figure 5, in spite of metallic Fe, Pt, C, and Ag peaks, ferrite Fe 3 O 4 and Fe satellite peak (Sat.) were also detected from the film surface after peak resolution. From the spectra line shape shown in Figures 5 and 6a, there are distinctive shoulders at the binding energy around 710 eV and 720 eV, and the Fe 2p3/2 and 2p1/2 core-level binding energy was considered and the Fe 3 O 4 phase was observed both in FePt(BN, Ag, C) and FePt(BN, Re, C) films. The magnetite Fe 3 O 4 (ferrimagnet) is a soft magnetic material and responds to the slight kink illustrated in the out-of-plane magnetization curves at zero field in Figure 2a [33,34].      Figures 5 and 6, the Fe 2+ /Fe 3+ ratio is higher in the FePt(BN, Re, C) film, which means that larger soft-magnetic moments contributed to the surface of the hard magnetic layer and a clear kink was observed in the hysteresis loops, as evidenced in Figure 2b [35][36][37]. In addition, the metallic Re, and ReO 2 appeared after considering the Re 4f 5/2 and 4f 7/2 core-level binding energy in Figure 6c. According to reference [38], the ReO2 is dark blue or black in color and the crystal structure is in the monoclinic form below, 300 • C, and orthorhombic between 300 and 1500 • C. The FePt(BN, Ag, C) and FePt(BN, Re, C) thin-film surface oxidations were both indexed in XPS and the oxides were soft-magnetic Fe 3 O 4 and ReO 2 , which may cause the lower ordering degree in the FePt(BN, Re, C) film due to the deviation in near-equal atomic FePt. Actually, the soft ferrite Fe 3 O 4 causes the kink in magnetic hysteresis loops shown in Figure 2a,b and needs to be avoided in practical applications.
From the plane-view TEM images, the FePt(BN, Ag, C) and FePt(BN, Re, C) films show a typical uniform granular structure in which the FePt grains were separated by the segregants (Figure 7). In Figure 8a,b, the FePt grains are mono-distributed with average grain sizes <D> of 6.60 nm and 11.2 nm, and the grain-size distributions, defined by (σ: standard deviation)/<D>, are 12.6% and 16.0% in the reference sample and FePt(BN, Re, C) film, respectively. The reference sample shows smaller and more uniform grains than the FePt(BN, Re, C) film and reflects the higher nucleation field (HN) that was defined at the field of 0.95 Ms and out-of-plane coercivity (H c ), as presented in Table 1. However, the grains' density in the FePt(BN, Re, C) film was higher than in the reference sample because of the smaller grain boundaries' width (1.00 nm < 2.05 nm), which was estimated by the subtraction of <D c >(centre-to-centre average grain size) to <D> (average grain size).
The coherence between FePt and MgO(100) was confirmed by the orientation and periodicity of the bright-field Moire fringes caused by the interference of diffraction spot patterns between FePt and MgO and the periodicity (λ) is given by Equation (2) [39] where α is the relative angle between FePt(001) and MgO(002), and d FePt and d MgO are spacing between planes, ∆g vector is reciprocal vectors between g FePt and g MgO [39]. The ferrite Fe3O4 was also observed in the XPS of the FePt(BN, Re, C) film in Figure  6(a) and the kink in the magnetization curve was more clear in Figure 2b. For inverse spinel ferrite Fe3O4, the Fe 3+ ions are divided into tetrahedral (A) and octahedral (B) sites equally and the Fe 2+ ions are in B sites. In Figures 5 and 6, the Fe 2+ /Fe 3+ ratio is higher in the FePt(BN, Re, C) film, which means that larger soft-magnetic moments contributed to the surface of the hard magnetic layer and a clear kink was observed in the hysteresis loops, as evidenced in Figure 2b [35][36][37]. In addition, the metallic Re, and ReO2 appeared after considering the Re 4f 5/2 and 4f 7/2 core-level binding energy in Figure 6c. According to reference [38], the ReO2 is dark blue or black in color and the crystal structure is in the monoclinic form below, 300 °C, and orthorhombic between 300 and 1500 °C. The FePt(BN, Ag, C) and FePt(BN, Re, C) thin-film surface oxidations were both indexed in XPS and the oxides were soft-magnetic Fe3O4 and ReO2, which may cause the lower ordering degree in the FePt(BN, Re, C) film due to the deviation in near-equal atomic FePt. Actually, the soft ferrite Fe3O4 causes the kink in magnetic hysteresis loops shown in Figure 2a,b and needs to be avoided in practical applications.
From the plane-view TEM images, the FePt(BN, Ag, C) and FePt(BN, Re, C) films show a typical uniform granular structure in which the FePt grains were separated by the segregants (Figure 7). In Figure 8a,b, the FePt grains are mono-distributed with average grain sizes <D> of 6.60 nm and 11.2 nm, and the grain-size distributions, defined by (σ: standard deviation)/<D>, are 12.6% and 16.0% in the reference sample and FePt(BN, Re, C) film, respectively. The reference sample shows smaller and more uniform grains than the FePt(BN, Re, C) film and reflects the higher nucleation field (HN) that was defined at the field of 0.95 Ms and out-of-plane coercivity (Hc), as presented in Table 1. However, the grains' density in the FePt(BN, Re, C) film was higher than in the reference sample because of the smaller grain boundaries' width (1.00 nm < 2.05nm), which was estimated by the subtraction of <Dc>(centre-to-centre average grain size) to <D> (average grain size).
The coherence between FePt and MgO(100) was confirmed by the orientation and periodicity of the bright-field Moire fringes caused by the interference of diffraction spot patterns between FePt and MgO and the periodicity (λ) is given by Equation (2) (2) where α is the relative angle between FePt(001) and MgO(002), and dFePt and dMgO are spacing between planes, ∆g vector is reciprocal vectors between gFePt and gMgO [39].  The mixed Moire fringe patterns can be observed in Figure 7c and the average periodicity is 2.33 nm (63.5°); for example, the individual period pattern is 2.15 nm, as measured in Figure 7d. When the epitaxial coherency is perfect, the alpha value is 0 but the rotational angle between FePt(001) and MgO(002) in this work is around 63.5o, calculated by Equation (1), which means non-perfect epitaxial growth in the FePt(BN, Re, C)/MgO system.
For the light atoms, for example, B, C, and N elements we chose to detect them via electron energy loss spectroscopy (EELS) with beam sizes smaller than 5 nm; the EELS spectrum of FePt(BN, Re, C) is shown in Figure 9. The B element was observed both in FePt lattice and grain boundaries and the C and N elements were segregated in the grain boundaries. After overlapping, part of the BN and C was found clearly in the grain boundaries. The unexpected O element, which may come from the natural oxidation during high-temperature deposition, was also observed at the grain's boundary and the surface oxides, for example, ReOx and Fe3O4 were indexed in the XPS.  The mixed Moire fringe patterns can be observed in Figure 7c and the average periodicity is 2.33 nm (63.5°); for example, the individual period pattern is 2.15 nm, as measured in Figure 7d. When the epitaxial coherency is perfect, the alpha value is 0 but the rotational angle between FePt(001) and MgO(002) in this work is around 63.5o, calculated by Equation (1), which means non-perfect epitaxial growth in the FePt(BN, Re, C)/MgO system.
For the light atoms, for example, B, C, and N elements we chose to detect them via electron energy loss spectroscopy (EELS) with beam sizes smaller than 5 nm; the EELS spectrum of FePt(BN, Re, C) is shown in Figure 9. The B element was observed both in FePt lattice and grain boundaries and the C and N elements were segregated in the grain boundaries. After overlapping, part of the BN and C was found clearly in the grain boundaries. The unexpected O element, which may come from the natural oxidation during high-temperature deposition, was also observed at the grain's boundary and the surface oxides, for example, ReOx and Fe3O4 were indexed in the XPS. The mixed Moire fringe patterns can be observed in Figure 7c and the average periodicity is 2.33 nm (63.5 • ); for example, the individual period pattern is 2.15 nm, as measured in Figure 7d. When the epitaxial coherency is perfect, the alpha value is 0 but the rotational angle between FePt(001) and MgO(002) in this work is around 63.5o, calculated by Equation (1), which means non-perfect epitaxial growth in the FePt(BN, Re, C)/MgO system.
For the light atoms, for example, B, C, and N elements we chose to detect them via electron energy loss spectroscopy (EELS) with beam sizes smaller than 5 nm; the EELS spectrum of FePt(BN, Re, C) is shown in Figure 9. The B element was observed both in FePt lattice and grain boundaries and the C and N elements were segregated in the grain boundaries. After overlapping, part of the BN and C was found clearly in the grain boundaries. The unexpected O element, which may come from the natural oxidation during high-temperature deposition, was also observed at the grain's boundary and the surface oxides, for example, ReOx and Fe3O4 were indexed in the XPS. The columnar grain structure was observed both in the reference sample a FePt(BN, Re, C) film in Figure 10a,b and the average aspect ratio of grains was defin from the film thickness to grain width (t/w), where the values are 2.35 and 1.70, resp tively. Some columnar grains were interrupted and non-uniform due to the thicker Fe film (15 nm), but the highest-aspect-ratio FePt grains were demonstrated in this stud The insets show that the nanobeam diffraction patterns and the {0KL} planes were dexed.
To provide more evidence of the element distribution, energy-dispersive X-ray (ED mapping was performed to analyze the composition in the reference sample and FePt(B Re, C) film. The cross-sectional high-angle angular dark-field (HAADF) images are show in Figures 11 and 12. Both Fe and Pt elements are presented almost in the same positi and grain areas and most of the Ag element were distributed in the FePt columnar grai edges, as shown in Figure 11. Because the Ag atoms were rejected or diffused out at a hi deposition temperature and occupied the Fe sites that decreased the coordination numb a Ag shell was formed [1,40]. The signal of B and C elements was not observed in ED mapping but proved in the EELS image. The N element was distributed thoroughly FePt grains and boundary areas in EDX mapping in Figures 11 and 12 but more segregat at the grain boundaries in EELS mapping presented in Figure 9. In principle, the EE mapping data are more accurate for light element N. In Figure 12, the heavy metallic appeared in the FePt grains and was accumulated at the FePt(BN, Re, C) and MgO int face, and the Re was overlapped with O and formed ReO2 on the FePt grain surface. It suggested that the metallic Re will replace part of the Fe in the FePt lattice due to t formation of iron oxide on the film surface and the composition was shifted a little to t Pt-rich FePt(Re) alloy. Furthermore, the hybridization of 3d Fe and 5d Pt(Re) electro presents high spin-orbital coupling, which reflects high magnetocrystalline anisotro (Ku). The columnar grain structure was observed both in the reference sample and FePt(BN, Re, C) film in Figure 10a,b and the average aspect ratio of grains was defined from the film thickness to grain width (t/w), where the values are 2.35 and 1.70, respectively. Some columnar grains were interrupted and non-uniform due to the thicker FePt film (15 nm), but the highest-aspect-ratio FePt grains were demonstrated in this study. The insets show that the nanobeam diffraction patterns and the {0KL} planes were indexed.
To provide more evidence of the element distribution, energy-dispersive X-ray (EDX) mapping was performed to analyze the composition in the reference sample and FePt(BN, Re, C) film. The cross-sectional high-angle angular dark-field (HAADF) images are shown in Figures 11 and 12. Both Fe and Pt elements are presented almost in the same position and grain areas and most of the Ag element were distributed in the FePt columnar grains' edges, as shown in Figure 11. Because the Ag atoms were rejected or diffused out at a high deposition temperature and occupied the Fe sites that decreased the coordination number, a Ag shell was formed [1,40]. The signal of B and C elements was not observed in EDX mapping but proved in the EELS image. The N element was distributed thoroughly in FePt grains and boundary areas in EDX mapping in Figures 11 and 12 but more segregated at the grain boundaries in EELS mapping presented in Figure 9. In principle, the EELS mapping data are more accurate for light element N. In Figure 12, the heavy metallic Re appeared in the FePt grains and was accumulated at the FePt(BN, Re, C) and MgO interface, and the Re was overlapped with O and formed ReO2 on the FePt grain surface. It is suggested that the metallic Re will replace part of the Fe in the FePt lattice due to the formation of iron oxide on the film surface and the composition was shifted a little to the Pt-rich FePt(Re) alloy. Furthermore, the hybridization of 3d Fe and 5d Pt(Re) electrons presents high spin-orbital coupling, which reflects high magnetocrystalline anisotropy (K u ).

Conclusion
In conclusion, the Re segregant in the FePt film did not improve the ordering degree like Ag but induced extra Fe-Pt(Re) spin-orbital coupling. As a result, the magnetic anisotropic field Hk and Ku can still be high as compared to the reference sample FePt(BN, Ag, C). It is suggested that 5d heavy transition metal segregants such as Re may compensate for the disadvantage of lower ordering degree in the structure, and the lower ordering of the FePt(BN, Re, C) film may be better understood from growth disorientation and defects in between interfaces. Therefore, this study provides the segregant design result for HAMR using FePt media with granular and columnar grains, showing that the 5d heavy transition metal can be used to replace the Ag segregant.

Conclusions
In conclusion, the Re segregant in the FePt film did not improve the ordering degree like Ag but induced extra Fe-Pt(Re) spin-orbital coupling. As a result, the magnetic anisotropic field Hk and Ku can still be high as compared to the reference sample FePt(BN, Ag, C). It is suggested that 5d heavy transition metal segregants such as Re may compensate for the disadvantage of lower ordering degree in the structure, and the lower ordering of the FePt(BN, Re, C) film may be better understood from growth disorientation and defects in between interfaces. Therefore, this study provides the segregant design result for HAMR using FePt media with granular and columnar grains, showing that the 5d heavy transition metal can be used to replace the Ag segregant.