Anisotropy-Graded L10 FePt(001) Magnetic Films ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015

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Anisotropy-Graded L10 FePt(001) Magnetic Films Obtained by Graded Working Pressures Yi-Hung Lin1,2 , Jen-Hwa Hsu1 , P. Saravanan1,3, An-Cheng Sun4 , and Po-Cheng Kuo2 1 Department

of Physics, National Taiwan University, Taipei 106, Taiwan of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan 3 Defence Metallurgical Research Laboratory, Hyderabad 500058, India 4 Department of Chemical Engineering and Materials Science, Yuan Ze University, Zhongli 32003, Taiwan 2 Department

We herein investigate the performance of L10 -FePt(001) films consisting of magnetically anisotropic-graded FePt layers grown by gradient working pressure ( Pw ). The graded structures typically consist of 5 nm thick FePt hard layer followed by five sequential layers of 1 nm thick graded-FePt layers deposited with Pw of 30, 20, 10, 7, and 3 mtorr, respectively. The structural and magnetic properties of Pw -graded-FePt layers thus fabricated on glass substrates are compared with that of those grown at different deposition temperatures, Td of 300 °C, 350 °C, 400 °C, and 450 °C. The FePt( Pw )-graded films exhibited a very high (001)-texture, island-like morphology, and strong perpendicular magnetic anisotropy and these performances are found to be consistent with that of the L10 -FePt(001) hard layer. For Td < 400 °C, the FePt( Pw )-graded films not only showed remarkable declining trend for out-of-plane coercivity (Hc⊥ ) but also demonstrated single-phase magnetization reversal—suggesting the existence of strong exchange coupling between the hard and the graded layers. In contrast, at higher Td (≥400 °C) ledge- or maze-type morphology with evidence of in-plane magnetic component is observed. Further, higher Td leads to the occurrence of intensive interlayer diffusion across the graded layers, which resulted in declining exchange-coupling behavior. The cross-sectional transmission electron microscopy images revealed epitaxial growth for the Pw -graded and the L10 -FePt layers grown using MgO under layer. The results of this paper demonstrate the feasibility of obtaining magnetic gradation in the FePt( Pw )-graded films, satisfying the requirements of future magnetic recording with ultrahigh density. Index Terms— Anisotropy, magnetic films and multilayers, magnetic measurements, metals and alloys, X-ray diffraction.

I. I NTRODUCTION

M

AGNETIC recording technology is currently facing the new challenge of pushing the storage density higher. Films of L10 -FePt have been identified as one of the promising candidates for next-generation media with recording density exceeding 1 Tbit/in2 , owing to their large magnetocrystalline anisotropy constant, K u (7 × 106 J/m3 ). However, the high K u value of this material poses the difficulty of writing for recording media applications, as these films demand sufficiently larger switching fields. A significant reduction in the switching field without compromising the K u values and thermal stability for the FePt films has therefore been considered as one of the major challenges [1]. Several film methods have been proposed to overcome this issue, viz., tilted recording head [2] or media [3], heat-assisted magnetic recording (HAMR) [4], [5], and exchange-coupled composite (ECC) media [1], [6]. Among the above, the ECC media-wherein the soft layer reduces the switching field of high-anisotropic hard layer, has been continuously investigated for several years, owing to their ability to impart a gain factor (ξ ) of about 2 [7]. Nevertheless, a large difference in K u between the hard and soft layers results in step-wise magnetization reversal. To overcome this issue, a graded variation in K u for the soft layer has been proposed, which effectively reduces the coercivities without compromising the thermal stability [8]. In this context, Manuscript received March 18, 2015; revised April 28, 2015; accepted May 13, 2015. Date of publication May 19, 2015; date of current version October 22, 2015. Corresponding author: J.-H. Hsu (e-mail: jhhsu@phys. ntu.edu.tw). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2015.2435054

the ECC media with graded PMA or K u , referred as graded media, have gained considerable importance—as these structures can have the gain factor up to 4, associated with high thermal stability and smooth magnetization reversal [7], [9]. Although there exist several approaches to realize a gradation in the K u values of L10 -FePt films with smooth magnetization reversal behavior, they still lack in producing island-like structures. The significance of graded-FePt structures thus lies in producing films of island-like morphology, comprising only single-domain grains. Along these lines, we have recently grown L10 -FePt graded films by means of gradient temperature (Tg ) and composition (C g ) [10], [11] and demonstrated that the possibility of achieving island-like morphology with strong (001)-texture associated with a significant reduction in Hc⊥ values with respect to L10 -FePt hard layer. As continuation of above efforts, herein we exploit the advantages of gradient working pressure (Pw ) to produce the FePt-graded layers with K u gradation along the film plane normal direction. The FePt(Pw )-graded films thus produced seems to be promising in reducing the Hc⊥ of hard layer from 1.59 to 0.41 MA/m without sacrificing high Hk (∼3.98 MA/m), while still retaining the large thermal stability factor (K u V ∗ /k B T ) of above 100. II. E XPERIMENTS The graded L10 -FePt(001) magnetic films were prepared by ultrahigh vacuum magnetron sputtering system. Prior to the deposition of FePt layer, an underlayer of 10 nm thick MgO was initially grown on glass substrate at room temperature (RT) and postannealed at 550 °C to induce (001)-texture in the FePt hard layer. Then a 5 nm thick FePt hard layer

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Fig. 2. XRD patterns of (a) 5 nm thick L10 -FePt hard layer and the FePt(Pw )-graded layers grown at different Td 300 °C, 350 °C, and 400 °C. (b) Their corresponding (001)-rocking curve. (c) FWHM values as a function of Td .

Fig. 1. SEM images of L10 -FePt hard layer with two different thicknesses (a) 5 nm (b) 10 nm and the FePt(Pw )-graded structures grown at different Td (c) 300 °C, (d) 350 °C, (e) 400 °C, and (f) 450 °C. The corresponding histograms are included in the inset of each figure.

was deposited at 550 °C and subsequently, five 1 nm thick FePt layers were sequentially grown at lower temperatures using Pw of 30, 20, 10, 7, and 3 mtorr. The FePt(Pw )-graded films were fabricated by varying the deposition temperature, Td = 300 °C, 350 °C, 400 °C, and 450 °C. The crystal structure of the FePt(Pw )-graded films was examined by X-ray diffractometer (XRD) using CuK α radiation; while the surface and cross section morphologies were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The magnetic properties of 5 nm thick L10 -FePt hard layer and FePt(Pw )-graded structures were analyzed by superconducting quantum interference device vibrating sample magnetometer at RT with a maximum applied field of 5.57 MA/m. III. R ESULTS AND D ISCUSSION Prior to evaluating the performance of Pw -graded layers, it is important to understand the effect of Pw -sequence on the K u variation. For this purpose, a series of FePt (5 nm)/MgO (10 nm)/glass samples were deposited at 550 °C with Pw = 3, 10, 20, and 30 mtorr (not shown). In our studied range of Pw , higher Pw results in larger K u —indicating that K u can be gradually controlled by varying Pw from 3 to 30 mtorr. Magnetic gradient has also been achieved in Co/Pd pressuregraded system [12]. The observed graded magnetic structure is believed due to a decrease in saturation magnetization for regions deposited at progressive higher working pressure. However, a different origin causes the dependence of K u on the working pressure in our studied system. The increase of K u is attributed to the improved chemical ordering Sord with higher working pressure Pw [13], [14]. Fig. 1(a)–(f) depicts the SEM morphologies along with histograms showing grain-size distribution for the 5 and 10 nm

thick L10 -FePt hard layers and the FePt(Pw )-graded structures processed at Td = 300 °C–450 °C, respectively. The 5 nm thick FePt hard layer shows an island-like morphology with grain size of 16 ± 7 nm and average intergranular distance of 10 nm, respectively [Fig. 1(a)]; while in the case of a 10 nm thick FePt hard layer, both grain size (38 ± 16 nm) and intergranular distance (10–15 nm) were considerably increased [Fig. 1(b)]. In contrast, the FePt(Pw )-graded films follow the morphology of 5 nm thick hard layer until Td ≥ 400 °C [Fig. 1(c) and (d)]. It is worth mentioning that though the total thickness of the FePt(Pw )-graded/FePt(5 nm) is 10 nm; their grain sizes remain almost identical with that of the 5 nm thick FePt hard layer at Td 400 °C. Furthermore, the magnetic domains of island-like graded layer follow their underneath L10 -FePt hard layer [10], [11] (figures not shown). When Td ≥ 400 °C, the grains tend to connect each other and they become ledge- or even maze-like morphology [1] [Fig. 1(e) and (f)], wherein the graded films have a larger horizontal length than the underneath hard magnetic island structure. In addition, a different size distribution is observed. Fig. 2(a) shows the XRD patterns of 5 nm thick L10 -FePt and the FePt(Pw )-graded structures. It can be seen that the L10 (001) texture can be enhanced by MgO(200). The graded films obtained at Td < 400 °C have strong (001) and (002) peaks—indicating the presence of highly ordered L10 phase. However, the width of L10 (002) peak increased along with increasing (200)-peak intensity at Td ≥ 400 °C. The increase in (200)-texture is accompanied by a lateral growth of FePt grains. The alignment of crystal axes was studied using the rocking curves of L10 (001) peak [Fig. 2(b)]. The full-width at half-maximum (FWHM) values were calculated by fitting Gaussian function and the results are plotted as a function of Td in Fig. 2(c). For comparison, the FWHM of single 5 nm thick L10 FePt is included in Fig. 2(c). It can be noticed that the FWHM increases substantially from 3.1° to 3.9° as Td increases from 300 °C to 400 °C and then level off with further increase in Td . Only a small difference is found between those of the graded layers deposited at 300 °C and the bare 5 nm thick L10 FePt hard layer. The increasing values of FWHM reflect a degraded (001)-texture with the deposition of graded layers at elevated Td . Both in-plane and out-of-plane hysteresis loops for the 5 nm thick FePt hard layer and FePt(Pw )-graded films are

LIN et al.: ANISOTROPY-GRADED L10 FePt(001) MAGNETIC FILMS

Fig. 3. (a) In-plane and out-of-plane hysteresis loops for the samples of 5 nm thick FePt hard layer. FePt(Pw )-graded films grown at different Td (b) 300 °C, (c) 350 °C, and (d) 400 °C.

shown in Fig. 3(a)–(d), respectively. To demonstrate the detail of coercivity reduction in the graded films, the scale of the applied field (x-axis) in Fig. 3(a) is different from those in Fig. 3(b)–(d). The FePt hard layer exhibits a large Hc⊥ of 1.59 MA/m and high perpendicular anisotropy field of >5.57 MA/m with a small in-plane magnetic component— which is a typical feature of L1-FePt with strong (001) texture. The graded samples deposited at Td = 300 °C and 350 °C show a significant decrease in Hc⊥ from 1.59 to 0.41 MA/m and both films exhibit a similar magnetization reversal behavior [Fig. 3(b) and (c)], but quite different from that of the bare FePt hard layer. At large fields, all the magnetic moments are directed toward the saturation magnetization. When the field is decreased to the nucleation field [denoted by Hn in Fig. 3(b)] of the top soft layer, small reverse domains are being nucleated. Subsequently, the magnetization reversal in the graded part is induced by the propagation of antiparallel domains. Accordingly, the magnetic moments of softest layer tend to deviate from the easy axis with reduced applied field [8]. The switching of the upper part of the soft layer occurs at the applied field close to the soft-layer anisotropy field [15]. The domain wall is then pinned near the hard/soft interface with a pinned field (H p ) [15]. As the field is further increased the domain wall in the soft layer is compressed and penetrates more and more into the hard layer [15]. Once the applied field reaches 1.07 MA/m the domain wall depins from the hard/soft interface and propagates immediately through the entire hard layer [15]. Upon further increasing Td (400 °C), a large in-plane component appears accompanied by an enhancement in the Hc value, as shown in Fig. 3(d). It can be noticed that the Hc⊥ drastically dropped by a factor of 3.9 for the FePt(Pw )-graded layer deposited at Td 350 °C and these values tend to increase gradually with further increase in Td (not shown). Both decreasing and increasing trends in Hc⊥ values can be explained as follows. When Td 350 °C, formation of perpendicular graded structure leads to significant reduction in Hc⊥ [Fig. 3(b) and (c)]. Besides, this film exhibits minimal in-plane component. The diffusion process starts to occur when Td ≥ 400 °C. As discussed previously, the fct FePt(001)-texture is destroyed

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Fig. 4. Cross-sectional TEM images of (a) 5 nm L10 -FePt hard layer. (b) Magnified image from FePt/MgO interface in Fig. 4(a). The FePt(Pw )-graded films grown at different Td (c) 350 °C and (d) 450 °C.

and the graded behavior is degraded. When Td is further raised to 450 °C, extensive diffusion causes the grain growth in the lateral direction with network-type morphology, which results in a large amount of misaligned grains. Accordingly, poor performance is observed for the graded films at higher Td . In addition, a substantial reduction in the Hc⊥ value does not lead to the degradation of thermal stability in our FePt(Pw )-graded films. According to [16], the dynamic coercivity Hc (t) as a function of measuring time can be expressed by the following equation: kB T f o t 1/n (1) Hc (t) = H0 1 − ln K u V ∗ ln 2 where k B , T , K u , and V ∗ are the Boltzmann constant, measuring temperature, anisotropy energy constant, and switching volume, respectively. The parameters f 0 and n are accounting to attempt frequency for barrier crossing and magnetization orientation with respect to the applied field. Constant values of 109 s−1 and 1.5 were considered for the parameters f 0 and n, respectively [16]. Performing the moment-decay measurements [17], the thermal stability factor (K u V ∗ /k B T ) can be determined by fitting (1). A K u V ∗ /k B T value of 120 was estimated for the Pg -FePt films with deposition temperatures of 300 °C and 350 °C which satisfies the requirement that the thermal stability of the recording medium should be in the range of 50–70 to maintain the media noise within the acceptable limits [18]. Furthermore, even when Td is increased up to 450 °C, the thermal stability factor only drops to 96. Fig. 4(a)–(d) represents a typical cross-sectional TEM images of 5 nm thick FePt hard layer and the FePt(Pw )-graded films grown at Td = 350 °C and 450 °C, respectively. As shown in Fig. 4(a), the interface of FePt/MgO is very smooth and the FePt-layer has a distinct island-like structure with the lateral size of 15 nm, which is consistent with the SEM observations [Fig. 1(a)]. The magnified image of the dash-square region in Fig. 4(a) is displayed in Fig. 4(b). A good epitaxial growth of FePt grains, initiating from

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the MgO layer can be observed and this has promoted a well-developed (001)-texture. The planar spacing of FePt is found to be 0.37 nm, which is in agreement with the XRD results (0.37 nm), as well as with the Joint Committee on Powder Diffraction Standards data #43–1359 (0.371 nm). This further confirms the formation of L10 phase in the FePt hard layer. In Fig. 4(c), it can be noticed that the graded layers grown at Td = 350 °C also exhibits island-like morphology with (001)-texture and 0.37 nm thick planar spacing, suggesting that the gradedFePt layers are epitaxially grown along the (001) orientation of 5 nm thick L10 -FePt hard layer. However, the interfaces between each graded layers in Pg -FePt film are difficult to identify, due to the compositional similarity in the Pg -FePt-graded layers. Nevertheless, the top-graded layers, 2 nm away from the film surface, which is deposited at lower working pressures (3 and 7 mtorr), do not show any apparent layered structure. This implies that the reduction of ordering parameter occurs at the expense of K u values upon decreasing the Ar-pressure during film deposition. Further increasing the Td to 450 °C [Fig. 4(d)], the grains tend to connect with each other, leading to ledge-type morphology having larger horizontal length than the underneath hard magnetic island structure. The top 2 nm region is no longer separated from the lower 3 nm region. But, the whole graded layers are distributed in random orientations with different planar spacings (ranging between 0.33 and 0.37 nm). Intensive and extensive interdiffusion along the film plane normal direction produces large amount of defects and lattice strain, resulted in worse performance for the graded layers grown at higher Td . IV. C ONCLUSION In summary, we have investigated the structural and magnetic properties of FePt(Pw )-graded films—wherein K u -gradation was introduced by gradually decreasing the working pressure during the deposition process. The decrease in working pressure not only resulted in significantly lower Hc⊥ values but also demonstrated good PMA due to the strong exchange-coupling between the graded layers. Hc⊥ declines from 1.59 MA/m (5 nm thick L10 -FePt) to 0.41 MA/m for the graded films grown at Td = 300 °C and 350 °C. The FePt(Pw )-graded structures obtained at 300 °C and 350 °C exhibited high ordering parameters, as well as high anisotropy fields, as similar to the 5 nm thick FePt hard layer and also demonstrated a single-phase magnetization reversal with minimal in-plane magnetic component. In addition, the thermal stability factor of the recording layer is not significantly reduced with the inclusion of graded layers. The results of FePt(Pw )-graded layers obtained in this paper are quite encouraging in fulfilling the requirements of future ultrahigh recording media applications.

ACKNOWLEDGMENT This work was supported in part by the National Science Council of Taiwan under Grant NSC 102-2112-M-002-007MY2 and in part by the Ministry of Economic Affairs of Taiwan under Grant 103-EC-17-A-01-S1-219. R EFERENCES [1] F. Casoli et al., “Role of interface morphology in the exchange-spring behavior of FePt/Fe perpendicular bilayers,” Acta Mater., vol. 58, no. 10, pp. 3594–3601, Jun. 2010. [2] K.-Z. Gao and H. N. Bertram, “Magnetic recording configuration for densities beyond 1 Tb/in2 and data rates beyond 1 Gb/s,” IEEE Trans. Magn., vol. 38, no. 6, pp. 3675–3683, Nov. 2002. [3] C. H. Hee, Y. Y. Zou, and J. P. Wang, “Tilted media by micromagnetic simulation: A possibility for the extension of longitudinal magnetic recording?” J. Appl. Phys., vol. 91, pp. 8004–8006, May 2002. [4] A. Q. Wu et al., “HAMR areal density demonstration of 1+ Tbpsi on spinstand,” IEEE Trans. Magn., vol. 49, no. 2, pp. 779–782, Feb. 2013. [5] X. Wang, K. Gao, H. Zhou, A. Itagi, M. Seigler, and E. Gage, “HAMR recording limitations and extendibility,” IEEE Trans. Magn., vol. 49, no. 2, pp. 686–692, Feb. 2013. [6] S. N. Piramanayagam, “Perpendicular recording media for hard disk drives,” J. Appl. Phys., vol. 102, no. 1, pp. 011301-1–011301-22, Jul. 2007. [7] H. Wang et al., “Characterization of L10 -FePt/Fe based exchange coupled composite bit pattern media,” J. Appl. Phys., vol. 111, no. 7, pp. 07B914-1–07B914-3, Mar. 2012. [8] J. Zhang et al., “Design and micromagnetic simulation of the L10 -FePt/Fe multilayer graded film,” J. Appl. Phys., vol. 111, no. 7, pp. 073910-1–073910-4, Apr. 2012. [9] Z. Lu, P. B. Visscher, and W. H. Butler, “Domain wall switching: Optimizing the energy landscape,” IEEE Trans. Magn., vol. 43, no. 6, pp. 2941–2943, Jun. 2007. [10] F.-T. Yuan, Y.-H. Lin, J. K. Mei, J.-H. Hsu, and P. C. Kuo, “Structure and magnetic properties of FePt(001) graded films deposited on glass substrates,” J. Appl. Phys., vol. 111, pp. 07B715-1–07B715-3, Mar. 2012. [11] Y.-H. Lin, J.-H. Hsu, F.-T. Yuan, P. C. Kuo, and J. K. Mei, “Microstructure and magnetic performance of perpendicularly magnetic anisotropic Fe3 Pt/Fe2 Pt/L10 -FePt(001)/MgO(002) graded films,” IEEE Trans. Magn., vol. 49, no. 7, pp. 3679–3682, Jul. 2013. [12] B. J. Kirby et al., “Direct observation of magnetic gradient in Co/Pd pressure-graded media,” J. Appl. Phys., vol. 105, no. 7, pp. 07C929-1–07C929-3, Mar. 2009. [13] S. Okamoto, N. Kikuchi, O. Kitakami, T. Miyazaki, Y. Shimada, and K. Fukamichi, “Chemical-order-dependent magnetic anisotropy and exchange stiffness constant of FePt (001) epitaxial films,” Phys. Rev. B, vol. 66, pp. 024413-1–024413-9, Jul. 2002. [14] H. Sato et al., “Fabrication of L11 type Co-Pt ordered alloy films by sputter deposition,” J. Appl. Phys., vol. 103, p. 07E114, Jan. 2008. [15] A. Y. Dobin and H. J. Richter, “Domain wall assisted magnetic recording,” Appl. Phys. Lett., vol. 89, pp. 062512-1–062512-3, Aug. 2006. [16] M. P. Sharrock, “Time dependence of switching fields in magnetic recording media,” J. Appl. Phys., vol. 76, no. 10, pp. 6413–6418, Nov. 1994. [17] Z. S. Shan et al., “Moment reversal characterization of thin magnetic film by VSM or AGFM,” IEEE Trans. Magn., vol. 37, no. 4, pp. 1944–1946, Jul. 2001. [18] S. H. Charap, P.-L. Lu, and Y. He, “Thermal stability of recorded information at high densities,” IEEE Trans. Magn., vol. 33, no. 1, pp. 978–983, Jan. 1997.