MgO-C interlayer for grain size control in FePt-C media for heat assisted magnetic recording

Currently, the most common recording media used for heat assisted magnetic recording is granular L10 based FePt-C directly deposited on a highly textured polycrystalline MgO underlayer. In this study, a thin granular MgO-C interlayer is inserted between the MgO underlayer and the granular L10-FePt-C magnetic layer to control the grain size of the magnetic media. By varying the deposition conditions of the MgO-C interlayer, we can vary the size of the L10-ordered FePt grains from about 10.5 nm to 7.6nm, while keeping the carbon composition in the magnetic layer unchanged. With the optimized interlayer, the L10-FePt-C media of grain size to 7.6 nm shows good squareness and perpendicular coercivity of 47 kOe as well as an order parameter of S = 0.85.Currently, the most common recording media used for heat assisted magnetic recording is granular L10 based FePt-C directly deposited on a highly textured polycrystalline MgO underlayer. In this study, a thin granular MgO-C interlayer is inserted between the MgO underlayer and the granular L10-FePt-C magnetic layer to control the grain size of the magnetic media. By varying the deposition conditions of the MgO-C interlayer, we can vary the size of the L10-ordered FePt grains from about 10.5 nm to 7.6nm, while keeping the carbon composition in the magnetic layer unchanged. With the optimized interlayer, the L10-FePt-C media of grain size to 7.6 nm shows good squareness and perpendicular coercivity of 47 kOe as well as an order parameter of S = 0.85.


INTRODUCTION
The L1 0 -ordered FePt magnetic material possesses many important properties suited for heat assisted magnetic recording. 1,2Carbon and various oxides have been used as segregants to create a granular microstructure in the magnetic layer and to form both magnetic and thermal isolations between adjacent FePt grains.4][5] One important reason is the lattice epitaxy relationship between the MgO and FePt.The slightly larger MgO lattice constant compared with that of the FePt yields a stretching tensile strain to the FePt grain, 6 promoting ordering in the perpendicular direction in particular when FePt is deposited at sufficiently high temperature, ∼ 600 • C. 7 The FePt grains usually do not have one-to-one grain matching with that of the MgO layer.The segregants which formed at the grain boundaries in the granular FePt layer do not match that of MgO grain boundaries in the underlayer and thus the FePt grains can easily grow over the MgO grain boundaries, creating a significant probability for either multi-variant or poorly ordered FePt-L1 0 grains. 8,9Hence, the grain size of the FePt-X layer is usually controlled by the volume percentage of the segregant(s) X: a higher percentage of segregants(s) usually lead to smaller FePt grain sizes, or the deposition condition of the FePt-X layer itself.Changing the MgO grain size of the underlayer usually has little effect on the grain size of the FePt-X layer.
If we want to create one-to-one grain matching between MgO underlayer grain and the FePt grain directly above, one strategy would be to have a matching of their grain boundaries.By inserting a thin MgO-C granular interlayer layer between the MgO underlayer and the FePt-X granular layer, we have succeeded in controlling the FePt grain size through this thin interlayer while keeping the FePt-X composition unchanged.

EXPERIMENTAL
A film stack of Si/SiO 2 |MgO (7nm) |MgO-C Xvol.% (2nm) | FePt (0.2nm) | FePt-C 30vol.%(8nm) (X= 0, 20, 30, 40) is deposited using an ultra-high vacuum AJA sputtering system with 2×10 -9 Torr base pressure.Si with 100 nm thick thermally grown SiO 2 substrates are used to prepare these film stacks.MgO, FePt alloy and carbon targets with 2 inch size and 99.9% purity are used deposit films.All substrates were cleaned with acetone, IPA, DI water and finally dry etching with oxygen plasma.The MgO underlayer is deposited at room temperature with 50W RF, 10 mTorr and target to substrate distance is around 70mm.A 2 nm of MgO-C interlayer was deposited at room temperature using co-sputtering technique at 20, 30 mTorr with 120W RF for MgO whereas for carbon we varied the DC power to attain various volume percentages.We used an ultra-thin 0.2 nm FePt layer to prevent carbon coating between MgO-C and FePt-C granular layers.We used exactly the same sputtering conditions to deposit FePt-C 30vol.%granular films.All FePt-C 30vol.%films were deposited at 600 • C to promote L1 0 ordering.The film structure and microstructure were examined by the standard xray diffraction (XRD) and transmission electron microscopy (TEM) techniques.The moment (M s ) vs field (H) curves of the film samples were measured with a Quantum Design physical property measurement system-vibrating sample magnetometer (PPMS-VSM).

RESULTS AND DISCUSSION
From the literature, it is well-known that carbon and FePt will form a well isolated granular microstructure due to strong phase separation tendency. 3We use the same idea to prepare a phase separated interlayer on which we can grow the FePt-C media.We choose the MgO and carbon system.We varied the carbon volume % and Ar pressure for MgO-C interlayer to refine the MgO grain size.Figure 1(a) and (b) show in-plane TEM micrographs of MgO-C interlayer grown at 30 mTorr with 20, 30 volume% of carbon respectively.These samples showed a well separated granular microstructure.The MgO grain size estimated from the micrographs were 12, and 7 nm for 20, and 30 volume% carbon in MgO-C interlayer respectively.From this study we believe that we can vary MgO grain size in the interlayer by changing deposition conditions.We have changed both the pressure and carbon volume % to find the best MgO-C interlayer for FePt-C 30vol% .From our experiments MgO-C 40vol.%deposited 20 mTorr gave optimized microstructure and good perpendicular coercivity.We used an ultrathin FePt layer used to prevent carbon coating and improve texture.Without the ultrathin FePt layer, the FePt-C layer is more interconnected and less ordered (results were not included).The order parameter is calculated using the method of En Yang, etc., 10 by considering the geometric features of the X-ray diffractometer, the crystallographic texture of FePt films, and film thickness.In the case of FePt-C 30vol.%grown on polycrystalline MgO and MgO-C 20vol% , there are small "bumps" at (111) FePt and (200) FePt peak due to the fact that some of FePt grains grown on polycrystalline MgO grain boundaries are less ordered and misaligned.However, (111) FePt disappears when the MgO-C 40vol% interlayer is used and the intensity of (200) FePt reduces when the carbon content in From the in-plane measurement we can say that this sample have strong perpendicular anisotropy with H k of 85 kOe and ⊥ H c of 38 kOe.This supports the XRD analysis these films have a high L1 0 ordering of (S=0.84).The soft kink observed at zero field can be accounted for the small grains below 3 nm size present in the film.The open loop in the in-plane measurements suggest that these samples have misaligned grains, and agree with our XRD measurements.We see "bumps" at 41 • (111) and 47.7 • (200) for this sample.Fig. 4(b) shows M-H loops Si/SiO 2 |MgO (7nm) |MgO-C 40vol.%(2nm) | FePt (0.2nm) | FePt-C 30vol.%(8nm).Out-of-plane M-H loops of these samples have good squareness and ⊥ H c of 47 kOe, which suggest that these films have narrow size distribution as well as good L1 0 ordering (S=0.85).The in-plane loop of this sample has H c of 9 kOe, which suggests that the texture was improved for the M-C 40vol% templated layer.Fig. 5 summarizes the average FePt grain size in the FePt-C layer for different carbon concentration of the MgO-C interlayer deposited at different Ar pressures while the deposition condition and the composition of the FePt-X layer remain unchanged.The significant range of the FePt grain size change by just varying the MgO-C interlayer condition along with the obtained excellent L1 0 ordering and good magnetic properties strongly indicates the one-on-one grain growth between the MgO-C interlayer and the FePt grain in the FePt-C layer.Further investigation is needed to confirm this hypothesis.

SUMMARY
We have developed a granular MgO-C interlayer inserted in between the (001) textured MgO underlayer and the granular FePt-C recording layer.The effect of a granular MgO-C interlayer on the microstructure, especially grain size and its distribution, of the granular recording layer has been investigated.It is found that the FePt grain size can be controlled by the deposition conditions and the carbon concentration of the MgO-C interlayer alone.The FePt-C layer shows good magnetic properties with L1 0 order parameter as high as S=0.85.It is reasonable to suggest that the granular MgO-C interlayer enables a one-to-one grain growth match between the MgO grains within and the FePt grains in the granular FePt-C layer directly above.

Fig. 2
Fig. 2 shows the XRD patterns of Si/SiO 2 |MgO (7nm) |MgO-C Xvol.% (2nm) | FePt (0.2nm) | FePt-C 30vol.%(8nm) with X vol.%C x=0, 20, 40.Strong (001) FePt and (002) FePt peaks indicate all thin films are well textured.The large integrated intensity ratio of (001) FePt /(002) FePt indicates high L1 0 ordering in all thin films.The order parameter is S = 0.90 for the one with only polycrystalline MgO.The MgO-C interlayer with 20, 40 volume% carbon containing films have S = 0.84, 85 respectively.The order parameter is calculated using the method of En Yang, etc.,10 by considering the geometric features of the X-ray diffractometer, the crystallographic texture of FePt films, and film thickness.In the case of FePt-C 30vol.%grown on polycrystalline MgO and MgO-C 20vol% , there are small "bumps" at (111) FePt and (200) FePt peak due to the fact that some of FePt grains grown on polycrystalline MgO grain boundaries are less ordered and misaligned.However, (111) FePt disappears when the MgO-C 40vol% interlayer is used and the intensity of (200) FePt reduces when the carbon content in