Surface modification of FePt(Ag, C) granular film by ultrathin B4C capping layer

Journal Pre-proofs Full Length Article Surface modification of FePt(Ag, C) granular film by ultrathin B4C capping layer Jai-Lin Tsai, Shi-Min Weng, Cheng Dai, Jyun-You Chen, Lin-Chen Huang, Ting-Wei Hsu PII: DOI: Reference:

S0169-4332(20)30093-3 https://doi.org/10.1016/j.apsusc.2020.145337 APSUSC 145337

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

10 September 2019 6 January 2020 8 January 2020

Please cite this article as: J-L. Tsai, S-M. Weng, C. Dai, J-Y. Chen, L-C. Huang, T-W. Hsu, Surface modification of FePt(Ag, C) granular film by ultrathin B4C capping layer, Applied Surface Science (2020), doi: https://doi.org/ 10.1016/j.apsusc.2020.145337

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Surface modification of FePt(Ag, C) granular film by ultrathin B4C capping layer Jai-Lin Tsai*, Shi-Min Weng, Cheng Dai, Jyun-You Chen, Lin-Chen Huang, Ting-Wei Hsu Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan

The in-plane magnetic component in FePt-40 vol%(Ag, C) granular film was caused by lower ordering degree which lower the magnetic anisotropy field and out-of-plane coercivity (Hc=23.6 kOe). To inhibit the FePt grains with second nucleation, random texture and in plane c-axis, the ultrathin B4C layer with thickness of 1 nm was capped. After surface modification, the FePt-40 vol% (Ag, C) film illustrates high perpendicular magnetic anisotropy (Ku=1.75x107 erg/cm3) and out-of-plane coercivity (Hc=28.8 kOe). The in-plane remanence magnetic moment is small and the tiny magnetization kink at zero field was not observed in easy-axis loop of 6 nm thick FePt-40 vol%(Ag, C) film. The ordering degree and the average surface roughness of FePt-(Ag, C) films were improved by surface diffusion of B4C which deposited at 470oC and post-annealed for 2 minutes. Further, the perpendicular magnetic anisotropy and out-of-plane coercivity of FePt-(Ag, C) films were enhanced significantly. The TiOx are more favorable than TiN in MgTiON layer due to formation enthalpy, part of N atoms was diffused and incorporated in the FePt lattice evidenced in high angle angular dark field mapping. After post annealing, the released N atoms cause lots of vacancies and defects inside FePt film which promote the ordering degree.

I. INTRODUCTION The L10 FePt film with (001) texture is a promising heat assisted magnetic 1

recordings media (HAMR) which has high magnetocrystalline anisotropy (Ku) of 5x107 erg/cm3 and prototype FePt media are currently evaluated in industry [1-4]. The proper segregants are one of the important terms for FePt media with high aspect ratio columnar grains and in-plane uniform granular grains. The ordering degree, perpendicular magnetic anisotropy and coercivity of FePt film was affected by materials of intermediate layer [5-8] and segregants [9-15]. The carbon was proved to have strong phase separation ability to refine the FePt grains into granular structure with spherical and smaller grain size [13-14]. Due to ball like grains morphology, the second nucleated grains were usually formed on the c-axis aligned FePt templates and affected the columnar growth [13]. In our previous work, the (FePt/B4C) and FePt/(B4C, Ag) multilayers deposited on glass substrate show preferred perpendicular magnetic anisotropy after rapid thermal annealing [16] and more recently the granular FePt-B4C-C dual layers deposited on MgO seed layer were reported to have columnar grain structure [17]. To find more suitable segregants for FePt media, the capped B4C thin layer was used to modify the FePt grains morphology from the top. The media noise arises from switching field distribution (SFD) and lower anisotropy field (Hk) can be overcome by the reduction of the easy axis distribution and in plane variant [18]. In this work, the ultrathin B4C layer was capped at high deposition temperature and post-annealing for minutes. After diffusion of B, C and N atoms from respective B4C and MgTiON layers, the released B, C and N atoms cause lots of vacancies and defects inside FePt film which promote the ordering degree and increase the coercivity [19-20]. Furthermore, the surface roughness and in-plane magnetization were significantly reduced in FePt(Ag, C) films as compared to our previous work.

II. EXPERIMENTAL The B4C/FePt(Ag, C)/MgTiON/CrRu films were co-sputtered by direct-current (dc) 2

and radio-frequency (RF) magnetron sputtering onto Corning (Eagle 2000) glass substrate with base pressure around 1x10-7 Torr and under a argon pressure of 10-2-10-3 Torr. The Fe52Pt48, Ag, C, (Mg0.5Ti0.5)(O0.9N0.1), Cr83Ru17, B4C alloy and composite targets with 2 inches diameter were used. The 130 nm thick CrRu seed layer was sputtered at 290oC on glass substrate and the MgTiON intermediate layer with thickness of 30 nm was deposited at 435oC. The FePt magnetic layer with thicknesses of 6 and 8 nm were fixed and co-sputtered with 40 vol% (Ag, C) at 470oC. [The volume concentration of (Ag, C) was changed from 10 to 50 vol% and optimal condition for FePt (001) texturing was 40 vol%.] The capping B4C layer with thickness of 0.5 and 1 nm were sputtered at 470oC and keep post annealing for 2 minutes. After high temperature deposition, the FePt(B4C, Ag, C) film was in the granular structure. The reference FePt(Ag, C) samples were deposited under the same parameters and without B4C capping layers. The crystallographic structure, texture and ordering parameters of FePt were examined and calculated by X-ray diffraction (XRD), and the θ/2θ diffraction patterns were collected using a standard X-ray diffractometer (BRUKER, D8 Discover). Magnetic hysteresis loops with in-plane and out-of-plane directions were measured at room temperature by using a superconducting quantum interference device (SQUID) magnetometer (MPMS-XL). The film microstructure and the surface roughness were quantified by using transmission electron microscopy (TEM, JEOL JEM-2010) and atomic force microscopy (AFM, Veeco DI-3100), respectively. The compositions and chemical state of B4C/FePt-(Ag, C) film was determined by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 5000).

III. RESULTS AND DISCUSSION Figure. 1 illustrates the XRD patterns of B4C (x nm)/FePt-(Ag, C) (y nm)/MgTiON 3

/CrRu films, (x, y) = (a) (0, 8), (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6). The CrRu seed layer and MgTiON intermediate layer illustrate (002) reflection and the lattice misfit between MgTiON/CrRu and FePt/MgTiON layer is around 3% and 9%, respectively. From the XRD pattern, the FePt(Ag, C) layer exhibited the (001)/(003) superlatticeand (002) fundamental-reflection peaks which indicating that the (002) textured MgTiON/CrRu underlayers promote the formation of ordered FePt with (001) prefer orientation. However, the diffraction peak around 49.06° in Fig. 1(a) (reference sample) is the convolution of the Bragg peaks of the (200) fcc (at 47.75°) and (002) L10 (at 49.27°) phases. The in-plane anisotropy is due to the contribution of a disordered soft magnetic [17] or plane variant structure. For example, the volume ratio of L10(100) to L10(001) variant which usually appeared for 10 nm thick FePt film and included in the out-of-plane XRD [18]. The chemical ordering degree of FePt is proportional to the ratio of [I(001)/ I(002)]1/2, here I(001) and I(002) are the integrated intensities of the FePt (001) and (002) diffraction peaks and the ratio was increased respectively from 2.76 (reference sample) to 2.97, 2.99 and 2.87 (samples with B4C capping layers) shown in Fig. 1. The ordering parameter for (001) textured FePt thin film, the Lorentz and absorption factors were further corrected and the theoretical ratio of FePt (001)/(002) was estimated as functions of film thickness and FWHM measured by rocking width [21-22]. The ordering value estimated from [(I(001)/I(002)/(I*(001)/I*(002))1/2 [21-22] is 0.89 in Fig 1(c). The increased ordering degree can be explained by two parts. First, the L10 FePt film with B4C capping layer shows smaller lattice constant c which means more compressive stress along the c-axis during sputtering. Second, the B, C and N atoms were diffused out from respective B4C and MgTiON layers during post annealing process and the released B, C and N atoms cause lots of vacancies and defects inside FePt film which promote the ordering degree [19-20]. 4

The preferential alignment degree of FePt c-axis perpendicular to the film surface was evaluated by the rocking curve. The rocking width (50) was determined by measuring the full width at half maximum (FWHM) of (001)/(002) reflection peaks and the measured values are 5.4o/4.9o, 5.5o/5.2o, 5.9o/5.7o and 5.7o/5.4o in Fig. 2. The rocking width (50) of samples capped with thinner B4C layer were slightly increased within ± 0.5o which indicates the c-axis misalignment of FePt grains were tiny deteriorated after post annealing and the B4C capping layer just has limited impact on the c-axis distribution. In Fig. 2(a) and (d), the FePt (001)/(002) reflection peaks shift to higher angle and the lattice constant of c-axis decreased from 0.376 nm to 0.356 nm. With B4C capping layer, more compressive stress along the c-axis during sputtering could promote the ordering process of L10 FePt film. The room temperature out-of-plane and in-plane magnetic hysteresis loops are shown in Fig. 3. All the samples exhibit a perpendicular magnetic anisotropy, although a clear hysteretic contribution is also present in the film plane (hard-axis) for reference sample (Fig. 3(a)), the origin of which is due to lower ordering degree. The values of the out-of-plane coercivity (Hc) and the magnetocrystalline anisotropy constant (Ku) were extracted from the magnetic hysteresis loops and summarized in Table 1. The present of B4C capping layer leads to a totally improvement of the magnetic properties (Hc ⊥ , Ku) with respect to the reference sample, together with a significant reduction of the in-plane magnetic moments at zero field which is due to the comparable easy-axis dispersion and higher ordering degree of FePt grains. The in-plane component was estimated by the ratio of in-plane to out-of-plane remanence (Mr(in-plane)/Mr(out-of-plane)) [6] and the values are 0.27, 0.11, 0.06 and 0.02, respectively in Fig. 3. In Fig. 3(c), the tiny kink at zero field caused by the c-axis misalignment is evidenced in the wider rocking width (50= 5.9o/5.7 o) for the B4C(1 nm)/FePt-(Ag, C) (8 nm)/MgTiON/CrRu sample. The increase of perpendicular magnetic anisotropy 5

was clearly observed from the reduction of in-plane hysteretic contribution as compared to reference sample. The above magnetic characteristics of FePt film were mainly influenced by the increased ordering degree and slightly affected by the c-axis misalignment. Figures 4 & 5 show the plane view and cross sectional TEM images of reference sample, (a) FePt-(Ag, C) (8 nm)/MgTiON/CrRu films, and B4C(x nm)/FePt-(Ag, C) (y nm)/MgTiON/CrRu films, (x, y)= (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6). From plane view TEM images of reference sample in Fig. 4(a), part of the FePt grains are inter- connected in maze-like structure thus indicating that the grains separation by (Ag, C) segregants are not effective in two-dimensional planes. The FePt grains are in the ball-like from cross-sectional view image illustrated in Fig. 5(a) and the grains width are around 14.7 nm. The larger FePt grains are observed in samples with B4C capping layer illustrated in Figs. 4(b-d) and the average grains size estimated in Fig. 6 was changed from maze-like structure to 30.6 nm, 28.2 nm and 23.3 nm, respectively. To investigate the effect of the capping layer on the grains’ separation, the average grain boundaries width was estimated by measuring the grains’ pitch  (i.e. the centerto-center distance). Using a capping layer leads to a larger grain separation ( = 3.44, 3.97, 4.21 nm in Figs. 4(b-d)) with respect to the reference sample ( = 1.82 nm in Fig. 4(a)) and the increased out-of-plane Hc can be explained by the higher grains’ pitch which has lower intergranular interaction and higher ordering degree. After diffusion of capped B4C layer, the FePt grains were interconnected in irregular shape and present larger and broaden grain size distribution in TEM in-plane images. However, the FePt grains are cross-sectional denser with higher contact angle and flat surface for (1.0, 8) and (1.0, 6) samples presented in Fig. 5. Based on the XRD result, the crystallographic structure was not influenced by the rough MgTiON/CrRu interface shown in Fig. 5(a). The reference sample shows smaller rocking widths (5.4o/4.9o) of 6

(001)/(002) reflection peaks which means the FePt (001) orientation was promoted by the (002) textured MgTiON/CrRu films and the intensities of CrRu layer were almost the same in different samples evidenced in Fig. 1. The in-plane magnetization in reference sample was due to disordered soft magnetic FePt grains or plane structure variant (L10 (100)) and the lower out-of-plane Hc was caused by lower ordering degree and smaller grains pitch. The energy dispersive X-ray (EDX) spectroscopy was used for analyzing element composition and the elemental mapping was conducted by high-angle angular dark field (HAADF) analysis, which provided atomic number (Z) dependent contrast. Figure 7 presents the plane view elements distribution of B4C(1.0 nm)/FePt-(Ag, C)(8 nm)/MgTiON/CrRu films. The Fe and Pt elements are observed in the same grains areas and the Ag element is present in the FePt grains areas. The Mg, Ti, O and N elements are appeared in MgTiON layer but part of N element is observed in FePt grains. The signal of B and C elements are unclear because this technique is much more effective to detect heavier atoms. In Fig. 8, the electron energy loss spectroscopy (EELS) with beam size smaller than 5 nm was used to detect lighter atom and the B and C elements are observed in the FePt grains and boundaries, respectively. Figure 9 shows the XPS spectra of B4C(1.0 nm)/FePt-(Ag, C) (8 nm) film deposited on MgTiON/CrRu/glass. After fitting, the Fe element is in the FeO oxide and Fe-B forms and the Pt and C elements are present in the metallic and carbon forms. The tiny kink of magnetization curve at zero field in Fig. 3(c) can be explained by the soft magnetic Fe-B and FeO shown in Fig. 9(a). Figure 10 maps the distribution of the different elements in cross sectional view within the stack of B4C(1.0 nm)/FePt-(Ag, C)(8 nm)/MgTiON/CrRu films and almost the elements are visible. The overcoat C was deposited during sample preparation by FIB and the atomic C was observed all over the FePt grains. The intermixing is not visible 7

in-between (Mg, Ti) and (Fe, Pt) layers and the atomic O, N in MgTiON layer were diffused up to the FePt layer during high temperature deposition and post-annealing. The O atoms were in the boundaries and surround the FePt grains, however, the atomic N was diffused into the FePt grains. MgTiON is the rocksalt structure and large vacancy concentration may exist on both sublattice even for stoichiometric. From XPS spectrum [5], the Ti element is present in the TiO2, TiO, TiN and TiON forms because the titanium oxides [TiO (-542.7 kJ/mol), TiO2 (-849.1 kJ/mol)] are much more favorable than TiN (-337.7 kJ/mole) based on the formation enthalpy at 298K [23]. During high deposition temperature, part of N atoms was diffused out from MgTiON layer and it’s suggested that the N atoms was incorporated in the FePt lattice and released out of the ordering phase during capping and post annealing [19-20]. The released N atoms cause lots of vacancies and defects inside film which promote the ordering degree and increase the coercivity [19-20]. Figure 11 shows the surface roughness of the reference sample, (a) FePt-(Ag, C) (8 nm)/MgTiON/CrRu films, and B4C(x nm)/FePt-(Ag, C) (y nm)/MgTiON/CrRu films, (x, y)= (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6), measured by tapping mode and the average surface roughness was 3.43 nm, 2.00 nm 2.14 nm, and 1.56 nm, respectively. After capping B4C ultrathin layer at high temperature, the FePt films show lower surface roughness because the FePt grains are denser with higher contact angle and flat surface for (1.0, 8) and (1.0, 6) samples. IV. CONCLUSIONS The ordering degree of FePt (Ag, C) film was increased by capping ultrathin B4C layer which provide more compressive stress during sputtering and create vacancies after post annealing. The enhanced perpendicular magnetic anisotropy was clearly evidenced in the hard axis hysteresis loop which has lower magnetization at zero

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field. The increased out-of-plane coercivity was due to more ordered and separated FePt grains. The advantages of doping B4C segregant by thinner capping layer has been proofed in this study.

ACKNOWLEDGMENT The authors would like to thank the Ministry of Science and Technology, Taiwan, for financial support under the grant number MOST 108-2221-E-005-031.

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FIGURE CAPTIONS Figure 1 XRD patterns of the reference sample, (a) FePt-(Ag, C)(8 nm)/MgTiON/ CrRu films, and B4C(x nm)/FePt-(Ag, C)(y nm)/MgTiON/CrRu films, (x, y) = (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6). Figure 2 Rocking curves of FePt(001) (left panel) and FePt (002) (right panel), (a) reference sample and B4C(x nm)/FePt-(Ag, C)(y nm)/ MgTiON/CrRu films, (x, y) = (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6). Figure 3 Out-of-plane and in-plane magnetic hysteresis loops of (a) reference sample and B4C(x nm)/FePt-(Ag, C)(y nm)/ MgTiON/CrRu films, (x, y) = (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6) measured at room temperature. Figure 4 Plane view TEM images of reference sample, (a) FePt-(Ag, C)(8 nm)/ MgTiON/CrRu films, and B4C(x nm)/ FePt-(Ag, C)(y nm)/MgTiON/ CrRu films, (x, y)= (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6). Figure 5 Cross sectional TEM images of reference sample, (a) FePt-(Ag, C)(8 nm)/ MgTiON/CrRu films, and B4C(x nm)/FePt-(Ag, C)(y nm)/MgTiON/CrRu films, (x, y)= (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6). Figure 6 Grain size distribution of reference sample, (a) FePt-(Ag, C)(8 nm) and B4C(x nm)/FePt-(Ag, C)(y nm), (x, y)= (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6) deposited on MgTiON/ CrRu/glass. Figure 7 Element mapping of B4C(1.0 nm)/FePt-(Ag, C)(8 nm)/MgTiON/CrRu films; dark field image and plane view mapping image of Fe, Pt, C, Mg, Ti, O, and N elements. Figure 8 (a) TEM image and EELS mapping of (b) B atom and (c) C atom in selected areas. Figure 9 XPS spectra of B4C(1.0 nm)/FePt-(Ag, C)(8 nm) deposited on MgTiON/ CrRu/glass, (a) the Fe element is present in the FeO and FeB forms, (b) Pt element, and (c) C element. Figure 10 Element mapping of B4C(1.0 nm)/FePt-(Ag, C)(8 nm)/MgTiON/CrRu films; dark field image and cross sectional view mapping image of and

(b–f)

mapping image of Fe, Pt, Mg, Ti, O, N, Ag elements. Figure 11 AFM image of reference sample, (a) FePt-(Ag, C)(8 nm) and B4C(x nm)/FePt-(Ag, C)(y nm), (x, y)= (b) (0.5, 8), (c) (1.0, 8) and (d) (1.0, 6) deposited on MgTiON/CrRu/glass. 11

Declaration of interest The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Author contributions J. L. Tsai was designed the experiments, analyzed the data and paper writing; C. Dai. and S. M. Weng were performed the sample preparation experiments; L. C. Huang, J. Y. Chen and T. W. Hsu were performed the microstructure and surface investigation.

Highlights 1. 2. 3. 4.

Perpendicular magnetic anisotropy was enhanced by B4C capping layer. Out-of-plane coercivity was increased by B4C capping layer. Ordering degree and surface roughness were improved by B4C capping layer. The B, C, N elements were segregated in FePt grains and boundaries.