# Fileset [FePt_HAMR_Media_AuthorCopy.pdf](https://mdr.nims.go.jp/filesets/a78cda54-f66c-4452-82ea-d574e7f1242a/download) ## Creator [HONO, Kazuhiro](https://orcid.org/0000-0001-7367-0193), [TAKAHASHI, K. Yukiko](https://orcid.org/0000-0001-9197-7236) ## Rights ## Other metadata [L10-FePt granular films for heat assisted magnetic recording media](https://mdr.nims.go.jp/datasets/d14ae10f-cb89-4ba8-bf32-bbcaa66c415c) ## Fulltext Microsoft Word - FePt_HAMR_Media_AuthorCopy.docxAuthors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 1 Chapter 5 L10-FePt granular films for heat assisted magnetic recording media Kazuhiro Hono and Yukiko K. Takahashi Magnetic Materials Unit, National Institute for Materials Science 1-2-1 Tsukuba, 305-0047, Japan kazuhiro.hono@nims.go.jp FePt-C nanogranular films are coming close to the actual application as recording media for the next generation high density recording system, heat assisted magnetic recording (HAMR). To optimize the L10-FePt nanogranular structure suitable for high areal density recording, various combination of segregants and seed layer materials have been explored. In this chapter, we review recent investigations aiming at achieving the ideal nanogranular structure for HAMR media. 1.1 INTRODUCTION Hard disk drives (HDD) have been a major storage device in computers for the last 50 years and more recently in home video recording systems and cloud computing due to their large storage capacity, low bit cost and non-volatility. Although solid state devices (SSD) are now replacing the storage devices for notebook computers, HDDs will continue to be a major player in the data storage industry. The number of HDDs sold in 2012 was approximately 800 million, nearly one tenth of the world population! To meet the demand for rapidly increasing digital data and to keep the competitiveness of HDD with other emerging storage devices, the areal density of the HDDs must keep increasing. The areal density of the current HDD is approximately 800 Gbit/in2. In the past, it had shown an increase of 100% every year, but the recent growth rate has slowed down to 30%, reaching saturation [1]. Although various technological improvements have been made in both heads and media in the current perpendicular recording (PMR) system, ~1 Tbit/in2 is considered to be the limit for the PMR method. Due to the recent improvements in signal processing using shingled magnetic recording, the areal density of PMR is expected to increase marginally above 1 Tbit/in2. However, a transition to a new magnetic recording method must be realized in the near future to sustain the growth of the recording industry. Heat assisted magnetic recording (HAMR) is a technology close to commercialization. However, a new medium that is totally different from the current one must be developed using a high magnetocrystalline anisotropy ferromagnetic material. The current PMR recording media are CoCrPt-SiO2 nanogranular films, in which nanosized CoCrPt columnar grains with the hcp structure are dispersed uniformly with a strong [0001] fiber texture as shown in Fig. 1. Such films are deposited on glass substrates with an amorphous CoTaZr soft magnetic underlayer and the Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 2 Ru seed layer that optimizes the grain size and the crystallographic orientation of the CoCrPt grains. Since the easy axis of magnetization of the CoCrPt alloy is [0001], this crystallographic texture gives rise to a strong perpendicular magnetic anisotropy (PMA). Each grain of approximately 8 nm in diameter is magnetically isolated and one bit contains multiple CoCrPt grains that are magnetized in the same direction as shown in Fig. 2. The bit size for the areal density of 1 Tbit/in2 will be ~25 × 25 nm2, in which more than 10 ferromagnetic grains must be contained to attain a sufficiently high signal to noise ratio. This suggests that the ferromagnetic grains must be refined further to ~4 nm for an areal density above 1 Tbit/in2. This requirement makes the ferromagnetic grains thermally unstable as the magnetocrystalline energy KuV becomes comparable to the thermal energy kBT, where Ku is the uniaxial magnetocrystalline constant, V is the volume of the grain, kB is the Boltzmann constant. In order to keep the recorded information for longer than 10 years, KuV/kBT must be larger than 60. This means that the ferromagnetic material with high magnetocrystalline anisotropy such as L10-FePt, L10-CoPt, Nd2Fe14B, and SmCo5 must be used for high density recording, where V becomes as small as ~4 × 10-26 m3 [2, 3]. As the media require long-term endurance, the rare earth compounds are unlikely to be selected as they are very susceptible to oxidation and corrosion. As a result, L10-FePt phase is currently considered to be the most promising material for HAMR media. While Ku of the CoCrPt alloy is ~0.3 MJ/m3, the L10-ordered FePt has an order of magnitude larger magnetocrystalline anisotropy of 6.6 MJ/m3, which makes the minimum size of stable ferromagnetic grains ~ 4 nm for spheres and 2.4 nm for cylinders [4]. In addition, the magnetic polarization of the L10-FePt phase is µ0Ms~1.4 T, which is large enough to gain sufficient signal to noise ratio with the film thickness of less than 8 nm. However, if hard magnetic grains become so small, the magnetic field required to switch the magnetization of the grain (HN) or coercivity (Hc) will increase enormously. The typical switching field or µ0Hc for the current recording media is about 0.8 T, while that of the nanosized L10-ordered FePt will be higher than 3 T. The highest magnetic field that can be generated using a write head is limited to about 1.5 T using the soft ferromagnet with the highest saturation magnetic flux density of 2.35 T for Fe65Co35 [5]. Since the reduction of grains of hard magnetic materials to a nanosized dimension leads to substantial increase of coercivity larger than 1.5 T, these grains will not be writable with the head field. This is known as "trilemma" of magnetic recording. Ultrahigh density magnetic recording requires nanosized ferromagnetic grains, which become thermally unstable below certain sizes. To overcome this problem, a high Ku material must be used; then the switching field of the grains becomes too large to write. To overcome the trilemma problem, the magnetization switching must be assisted by some external energy such as heat or microwave. HAMR technology uses heat to assist the magnetization switching of high Ku grains using a well-focused laser beam [6, 7]. As temperature increases, the coercivity of the grains decreases because of reduced anisotropy field. Writing is done at the temperature slightly lower than Curie temperature, where the coercive field becomes smaller than the head field of about 1 T. When temperature decreases rapidly after writing, the high Ku nanosized grain has sufficient thermal stability for permanent recording. To realize HAMR, new technologies are required in both the write head and the medium. The HAMR head has been demonstrated recently using a plasmonic antenna [6]. The tolerance of near field optical transducers is the most critical issue for improving the reliability of HAMR HDD to a practical level. As to the media, the FePt-C system is the only viable system for HAMR media at this moment. However, the grain size and in-plane magnetic component are not reduced to the desired level yet. Although the first HAMR HDD is anticipated to use the FePt-C based media with a recording density slightly higher than the upper limit of PMR, further increase in the areal density towards 4 Tbit/in2 depends on how we can optimize the nanogranular structure of the Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 3 FePt-C based media on glass disks through various buffer layers for heat sinking and crystal orientation alignments. In this paper, we review the history of the development and optimization of the L10-FePt based nanogranular films based on published literature.* 1.2 IDEAL STRUCTURE FOR HAMR MEDIA The desirable HAMR media should be comprised of densely dispersed ferromagnetic grains exhibiting high magnetocrystalline anisotropy of larger than 5 MJ/m3 with a uniform grain size of ~4 nm with a center to center distance of 5 nm. The size distribution of the grains must be lower than 10% for attaining sufficiently low switching field distribution (SFD). For sufficient signal to noise ratio, the grain shape should be columnar with the grain aspect ratio h/D>1.5, where h is the height and D is the diameter of FePt grains. Such an ideal film structure is schematically shown in Fig. 2 [8] and the proposed HAMR media requirements for 2 and 4 Tbit/in2 released from the Advanced Storage Technology Consortium (ASTC) is shown in Table 1 [9]. The film must exhibit high coercivity above 3 T, more than 4 times larger than that of the current recording media. In addition, the microstructure should be realized on commercially viable substrates such as glass. The key factors to control the microstructure of HAMR media are (i) ordering the A1-FePt phase to the L10 structure by annealing without grain coarsening, (ii) obtaining strong perpendicular magnetic anisotropy (PMA) with low in-plane component, (iii) attaining nanometer diameter (ideally 4 nm) with a narrow size distribution of less than 10%, (iv) low surface roughness (Ra<0.4 nm) because of narrow head-media distance for high density recording. Fulfilling all of the above requirements has been challenging, and many investigations on the optimization of FePt-based nanogranular structure have been made since 2000. Seagate has recently demonstrated HAMR over the areal density of 1 Tbit/in2 using supposedly FePt based granular media [10]. However, further microstructure optimization is essential to extend the areal density to 4 Tbit/in2. 1.2.1 A1→→L10 ordering Although the L10-FePt phase is thermally stable below 1300°C, the FePt films sputter-deposited below 400 °C have the disordered A1 structure because of a kinetic constraint. Since the melting temperature of equiatomic FePt alloy is about 1830 K, the volume diffusion is very sluggish below 917 K (~640 °C). This means that the sputtered A1 film has to be annealed above 640 °C to make them transform to the L10 structure by volume diffusion [11] or must be deposited on heated substrates [12]. Hence, lowering the kinetic ordering temperature from the A1 to L10 structure was considered to be essential, and the effect of various alloying elements to enhance the kinetics of L10 ordering was investigated. Among various elements, Cu and Ag were reported to be effective to reduce the kinetic ordering temperatures in continuous polycrystalline films [12-14]. By alloying FePt with Cu, the ordering temperature decreases to 400 °C while binary A1 FePt does not order below 600 °C [12]. The Cu alloying, however, leads to the reduction of the magnetocrystalline anisotropy energy [15]. Note that most of these investigations on the effect of additives on ordering were carried out on continuous polycrystalline films, in which ordering was accompanied by recrystallization of nanocrystalline films [16]. So the elements that are effective in reducing the ordering temperature of continuous films do not necessarily work on * Many investigations carried out in industry are kept confidential. It is beyond the authors' knowledge to make comprehensive review including the efforts carried out in industrial laboratories. Hence, the review is based on published literatures only. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 4 the FePt-X granular films, where FePt grains are isolated by segregants [17]. It should be also noted that there is a size dependence of L10 ordering when the grain size becomes smaller than 4 nm [18-20]. This is because the surface energy become dominant compared to the volume free energy, where A1 phase become more stable than the L10 phase. This size dependence of order may become significant when the grain size is successfully reduced to less than 4 nm for 4 Tbit/in2 areal density in the future. 1.2.2 [001] texture To apply the L10-ordered FePt thin film to HAMR media, the magnetic easy axis must be aligned normal to the film plane. However, the FePt thin films deposited onto amorphous substrates tend to develop a [111] texture because the (111) plane of A1 phase has the lowest surface energy. In early studies of FePt thin films, (001) oriented MgO single crystalline substrates were used to grow PMA films [21-27]. MgO has the NaCl structure with the lattice parameter of 0.421 nm, while the lattice parameters of L10-FePt are a=0.386 nm and c=0.373 nm in the fct lattice or a=0.273 nm and c=0.373 nm in the bct lattice. The (001) FePt grows on (001) MgO with the cube-to-cube orientation relationship with the lattice mismatch of about 8.3%. Because of this large misfit, the FePt is subject to receive expansion stress to the a- and b-axis directions, thereby growing only one variant for A1→L10 ordering on heated MgO substrate. This gives rise to the strong PMA to the L10-ordered FePt thin films. Based on detailed TEM observations, Shima et al. showed the L10-ordered FePt film grows with the Volmer-Weber (VW) mode on the (001) single crystalline MgO at 700 °C [26]. The coercivity of these films was as high as 4 T, which decreases abruptly when the isolated grains are percolated for nominal film thickness of larger than 40 nm. The growth mode changes from the VW mode to the Frank-Van der Merwe (FV) mode when the substrate temperature is decreased below 350 °C [26] as shown in Fig. 3. This nature of the Volmer-Weber growth on the MgO substrate is essential for the formation of well-isolated FePt grains in recording media. The degree of order increases above 450 °C, about 200 °C lower than Tm/2, where Tm is the melting temperature of FePt. This is because the L10 ordering progresses in-situ on heated substrate by surface diffusion. The coercivity of the films depends on both the morphology of the FePt grains and the degree of L10 order. Even if the degree of L10 order is high, Hc can be low if the FePt becomes continuous, as the magnetization switching can occur by the domain wall motion without any pinning after a nucleation. Single domain grains of about 50 nm show very large coercivity of 7 T at room temperature, which is nearly 50% of the anisotropy field HA [28]. This level of coercivity with respect to the anisotropy field has never been observed in other hard magnetic systems. One may wonder if the large mismatch between MgO and FePt may influences the intrinsic magnetic properties of the L10-oredered FePt films such as magnetization, Ms and Ku. Hotta et al. investigated the thickness dependence of the magnetic properties of FePt films deposited on heated MgO, MgAl2O4, and SrTiO3 (001) single crystalline substrates that induce the lattice mismatches of ~9, 5, and 1 %, respectively [29]. Based on the results, they concluded that the magnitude of the lattice mismatch does not give any influence on Ms, Ku and the long range order parameter, S. On the other hand, Ding et al. investigated the Ku of the FePt film sputter-deposited on MgO(001) substrates through various buffer layers, such as Pt, Cr, CrMo, and concluded that the critical lattice mismatch around 6% is the most suitable for improving the L10 ordering of FePt [30]. However, the results need to be carefully interpreted because of the possibility of interdiffusion of Cr into the FePt layer. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 5 In recording applications, single crystalline substrates cannot be used because of their high cost. While FePt thin films were prepared on heated single crystalline (001) MgO substrate in early fundamental studies, practical seed layer must be deposited on either glass or amorphous substrates. Jeon et al. reported the growth of PMA FePt polycrystalline films on the (001) textured MgO seed layer that was deposited on a thermally oxidized Si substrate [31]. Thereafter, Xu et al. reported the successful growth of (001) textured FePt polycrystalline films (t=50 nm) on Cr91Ru9 substrate with PMA [32]. However, since the Cr diffuse into FePt at elevated substrate temperature, the growth temperature was limited to 400 °C. Hence, only a low perpendicular coercivity of ~0.4 T was reported. Although the (110) Cr plane is the most close-packed plane, a [001] texture develops in a Cr buffer layer at an optimized condition, which led to the [001] texture of FePt on CrRu. The lattice mismatch of 5.8 % with the FePt (001) causes the expansion of the in-plane FePt (100) lattice. Since the c/a ratio of L10 FePt phase is 0.964, the slightly higher lattice mismatch between Cr (110) and FePt (100) assist the transformation from the A1 phase to the L10 phase. By adding Ru into Cr thin films, the lattice mismatch can be tuned as the atomic radius of Ru is larger than that of Cr. Xu et al. showed that 10 at.% Ru optimized the lattice mismatch for L10 ordering [32]. With in-situ annealing, perpendicular FePt [001] texture with good L10 order was obtained at a relatively lower temperature of 350°C, whereas a temperature over 600 °C was required to achieve high L10 order by post-deposition annealing. This means that a reduction of the ordering temperature by 200 °C was achieved by the in-situ annealing using the CrRu underlayer. Although Cr based underlayer reduces the ordering temperature, Cr diffusion into FePt deteriorates the magnetocrystalline anisotropy. Chen et al. inserted a thin MgO layer as a diffusion barrier of Cr and reported the growth of (001) textured L10-FePt polycrystalline films with strong PMA on glass substrates [33]. In their film stack, the orientation relationships of FePt(001)[100]// MgO(001)[100] // CrRu(200)[110] were confirmed. The coercivity was enhanced to 1.2 T in this film stack. Later, Perumal et al. reported [001]-textured FePt particulate films grown on [001] textured MgO seed layer that was pre-deposited on thermally oxidized Si [34]. This work has shown that the intermediate layer such as CrRu is not essential to control the texture and the microstructure of FePt films. However, there is one drawback in the MgO seed layer. Since it is insulator, the MgO seed layer must be grown using RF sputtering, which causes slow sputtering rates and some undesired grains. Ideally, the seed layer could be sputtered with a DC power, so electrically conductive materials are more desirable as a seed layer. Varaprasad et al. reported [001] textured FePt film grows on a (Mg0.2Ti0.8)O seed layer. (Mg1-xTix)O is electrical conductive having the NaCl structure for a wide range of x [35]. This seed layer shows strong [001] texture on amorphous substrates and can induce the growth of [001] FePt ferromagnetic layer. However, the surface energy appears to be lower than that of MgO(001). Hence, the wettability of FePt on MTO is good, hindering the particulate growth of FePt layers using the FePt-C system. (Mg1-xTix)O (MTO) has the identical rock salt structure for a wide range of x, i.e., 01.5 as shown in Fig. 2. These two problems stem from the strong driving force for the phase separation between FePt and C. The reason why the FePt-C system shows well-isolated planar structure is that the FePt and C are entirely immiscible with strong repulsive nature. Hence, while the plan-view show nice grain separation, the cross sectional image show the formation of second layer on the top of the FePt grains that were grown on MgO seed layer epitaxially as shown in Fig. 4. Since the second layer has no orientation relationship with the underlayer being separated by amorphous carbon, the perpendicular anisotropy is substantially degraded. This is because the C has too strong driving force for phase separation. Although strong phase separation makes the grain isolation better in the lateral direction, this in turn interrupts the columnar growth in the growth direction and causes another grain to nucleate Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 8 on an underlying grain. In order to overcome this issue, various segregants other than C have been explored. 1.2.5 Exploration for a new segregant for FePt-X media Before the FePt-C system was found to be suitable for achieving the granular structure for a HAMR medium, various FePt-X systems including the FePt-SiOx system were thoroughly investigated. Seki et al. reported the formation of fine FePt-SiO2 granular structure [57]. However, the coercivity becomes very low when the substrate temperature was decreased to achieve the grain size of less than 6 nm. Yang et al. also reported columnar grain growth in the cross sectional TEM image of FePt-SiO2 films, but the plan-view TEM image showed the grains were interconnected in the planar direction [58]. Various other systems were investigated, but when columnar structure is observed in cross sectional TEM image, in most of the cases the grains are interconnected in the planar direction [59,60]. Fig. 7 shows typical granular microstructure observed in the FePt-TiOx system [60]. The FePt grains are interconnected in the planar direction while uniform columnar structure with a smooth surface is obtained in the cross sectional view. However, the film showed soft magnetic properties because of the suppression of the L10 ordering due to the dissolution of Ti into FePt grains. This means that the driving force for the phase separation between FePt and TiOx as well as SiO2 is too small to obtain well-isolated grains. From this stand point, Varaprasad et al. have grown FePt-(C,SiO2) and FePt-(C,TiOx) granular films and confirmed certain effect of the usage of mixtures of C and metal oxides for the columnar growth of FePt grains, although the microstructure optimization has not been fully achieved [60]. They also proposed double layer glass/a-NiTa/MgO/FePt-C/FePt-X media, where thin FePt-C (t<6 nm) was used as a template for the second FePt-X (X=SiO2 and TiOx). The initial FePt-C film form well separated granular structure. However, the second untextured layer is formed for t>6 nm. So the system with a lower driving force of phase separation, like FePt-SiO2 and FePt-TiO2, was used as the second layer to suppress the phase separation during the growth in thickness. This approach also showed some promising result. Shiroyama et al. carried out more comprehensive work on search of metal oxide segregant, i.e., FePt-MxOy granular films, where M=Cr, Ti, Zr, Nb, W, Al, and Cr [61,62]. Fig. 8 shows the change of perpendicular Hc as functions of the volume fraction of MxOy. Except for C, all oxides segregant causes the reduction of Hc with the volume fraction of MxOy. This is because Zr, Nb, W, Al, and Ti are all dissolved in FePt, which reduces the driving force for the L10 ordering. Compared to these, the degree of the degradation of Hc as function of the volume fraction is low for Cr2O3. This is because Cr has large positive enthalpy of mixing with Fe and is not dissolved in FePt. From this figure, one can understand C is a really unique segregant, i.e., it does not lead to the degradation of Hc up to the volume fraction of 40%. Fig. 9 shows the in-plane and cross-sectional TEM bright field image of FePt–40vol.%Cr2O3 film deposited on a glass substrate with a MgO seed layer [62]. The nominal thickness is 10 nm and all grains are columnarly grown with excellent planar separation. Unfortunately, PMA is not as good as those for FePt-C on MgO seed layer and the coercivity is only 10 kOe with a large in-plane component. The average grain size is 4.8 nm, very fine as compared to FePt-C. However, the size distribution is about 33%, which is too large for a recording medium. Recent investigations on FePt-ZnO2 [63], FePt/AlN [64], FePt-B-Ag [65] all suggest that no systems can match the FePt-C system in view of the planar isolation, narrow size distribution, and high coercivity. Thereafter, research has been focused on how to optimize the granular microstructure in the FePt-C system. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 9 1.3 Optimization of nanostructure of the FePt-C system The remaining issues for the FePt-C system are summarized as follows; 1) how to grow L10-FePt grains columnarly in the perpendicular direction to the film without the formation of the second ferromagnetic layer, 2) how to reduce the average grain size to achieve the pitch distance of 7 nm or less while keeping the grain size distribution of less than 15%, 3) how to remove the in-plane component of magnetization. 1.3.1 Columnar Grain Growth To grow the FePt grains columnarly for the thickness larger than 6 nm, the suppression of the phase separation between FePt and C in the perpendicular direction to the film plane is necessary. When the thickness of FePt ferromagnetic layer becomes thicker than 6 nm, carbon appears to segregate on the surface of FePt grains by the phase separation to the growth direction, which triggers the formation of the second FePt layer. This problem appears to be overcome by modifying the composition of segregant and the sputtering conditions from the fact that the aspect ratios exceeding 1.5 were reported in recent unpublished reports from industrial laboratories [66,67]. However, no details on the materials and processing conditions for achieving the columnar grain structure have been revealed. Therefore, there is a need of getting better understandings on the condition of the columnar growth of the FePt grains. Although the process is totally different from the industrial one, Varaprasad et al. reported a way to achieve the columnar growth of FePt grains by using a concentration graded co-sputtering method [68]. Fig. 10 shows the plan-view and the cross-sectional TEM images of the FePt-30vol.%C films deposited on MgO (001) single crystals by the standard co-sputtering method (a) (b) and the compositionally graded process (c) (d) [68]. The compositionally graded process means that the films were grown with a sequence of FePt-25vol%C(2)/FePt-30vol%C(2)/FePt-35vol.%C(2)/FePt-25vol%C(2)/FePt-30vol%C(2), where the numbers in the parenthesis is the thickness of each layer in nm. The total thickness of these films is 10 nm, 1.7 times thicker than the critical thickness of the single layer grown by co-sputtering (Fig. 3). In the cross sectional TEM image of the co-sputtered sample, Fig. 10 (b), the film shows the second layer with randomly oriented FePt grains. The plan-view TEM image shows FePt grains are overlapped with other grains and are interconnected, resulting in the large average grain size of 12 nm. Because of this second layer formation, PMA of the film is very poor as shown in Fig. 10 (a). On the other hand, the film fabricated by the compositionally graded process shows a uniform microstructure with the average grain size of 7.8 nm and the aspect ratio of 1.5. The co-sputtered film has small perpendicular Hc and large in-plane hysteresis due to the relatively large grain size and second layer grains. On the other hand, the film grown with the compositionally graded process shows large perpendicular Hc of 3.9 T without in-plane hysteresis. This suggests that controlling the supply of excess amount of carbon after the nucleation of a high number density of FePt grains is effective to suppress the formation of the second ferromagnetic layer and to grow the grains columnarly. 1.3.2 Reduction of in-plane magnetic component SFD has a close correlation with the signal to noise ratio (SNR) of magnetic recording, and the SFD is influenced by the distributions of size, sD, distribution of the anisotropy field of individual grains because of some heterogeneities in chemical composition and the degree of order, and the easy axis distribution. Pisana et al. concluded that the Hk distribution is the major Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 10 reason for the SFD and its grain size dependence, and the angular distribution of the c-axis of FePt grains is the secondary reason [69]. They also point out that the in-plane and randomly oriented grains and the grain size distribution do not make much contribution to the SFD. On the other hand, Acharya reported that SNR and jitter noise receive significant influence from the in-plane component of magnetization when the grain size of FePt becomes smaller than 7 nm [66]. In the prototype HAMR media, MgO seed layer is used to induce strong [001] texture in the FePt-C magnetic layer [69,70]. However, all FePt-C granular media grown on MgO seed layers contain substantial in-plane components and the origin of the misaligned grains have been a subject of recent TEM studies [71,72,73]. In order to clarify the influence of grain boundary and textures of MgO seed layer, Wang et al. investigated the easy axis distribution in FePt-C media deposited on two kinds of MgO [73], i.e., one is single crystalline substrate and the other is a polycrystalline MgO (poly-MgO) underlayer deposited on glass substrates through an a-NiTa buffer layer. Figure 11 shows in-plane and cross-sectional TEM images and their magnetization curves of the FePt-C film deposited on (a) MgO single crystal substrate and (b) polycrystalline MgO seed layer [73]. Both of the films show similar microstructure with the average grain size of about 10 nm and the grain distribution of 1.4 nm. The nominal thickness of the FePt-C layer is 10 nm. Although the films grown on polycrystalline MgO show strong PMA and Hc as seen from the magnetization curves in Fig. 11, the squareness and the gradient of the demagnetization curve of the film deposited on the MgO single crystal substrate is far better than that on the poly-MgO seed layer. This indicates that either the angular distribution of the MgO grains or the presence of grain boundaries in the polycrystalline seed layer causes in-plane component of the magnetization in the FePt-C layer. Fig. 12 shows the orientation maps of the FePt-C granular films grown on (a) MgO(001) single-crystal and (b) poly-MgO underlayer obtained from the plane-view specimens together with the easy axis distribution. The out-of-plane orientation map for the film grown on the single crystalline MgO(001) substrate (Fig.12a) demonstrate almost homogeneous red index color, indicating a prefect [001] texture of FePt grains along the film normal direction. It confirms the good epitaxial growth of FePt nanograins on the MgO(001) single crystal substrate without any misoriented grains. On the other hand, the FePt-C film grown on the poly- MgO underlayer (Fig.12b) shows a distinctly different orientation map. While the majority of the grains are oriented to the [001] direction, variants such as [110] and even [100] (90° misorientation) poles were also detected. The easy axis distribution histograms also show the majority of the misoriented grains have either 48-52° or 90° misorientation. It is known that several FePt grains are nucleated on a MgO grain in the seed layer [73], so the easy axis distribution within the seed layer grain should be very low. This suggests that if the misalignment of [001] texture in the MgO seed layer is improved, the easy axis distribution will be substantially improved. However, even if MgO(001) single crystalline substrate is used, the strong texture does not always develop, suggesting misaligned particles may be nucleating from atomic steps on the MgO (001) surface [74]. Based on high resolution TEM observation, Wicht et al. [75] reported that the misalignment of the MgO crystal is smaller than that of the FePt grains. Their work suggests highly textured FePt grains do not develop even if the angular distribution of MgO is small. They conclude that the FePt and MgO are slightly misaligned from the perfect cube-to-cube orientation relationship because of the nucleation of FePt from step edges of MgO seed layer. As the misfit between MgO and FePt on the (001) plane is large, some of faceted planes in FePt show epitaxy with the step edge of MgO, which causes misalignment of FePt on the [001] textured MgO seed layer. On the other hand, Ho et al. [72] reported the FePt grains crossing the grain boundaries of MgO underlayer cause the formation of the L10 lattice with 90° misalignment. These recent investigations suggest the controlled surface roughness, grain size, and texture of the MgO seed Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 11 layer are essential to reduce the easy axis distribution of FePt-C media. In addition, search of an alternative seed layer is also important. Some of the promising seed layers are (Mg1-xTix)O [35] and TiN, both of which can be grown with DC sputtering. 1.4 Concluding Remarks Recent progresses made on the development of FePt based granular films for HAMR media have been reviewed. After many investigations of various FePt-X systems, only the FePt-C and (FePt)Ag-C system remain as promising candidates that can fulfill the requirements for 2 Tbit/in2 HAMR media. However, there are still several issues to be overcome: reduction of grain size, columnar growth of grains, and reduction of easy axis distribution. To resolve these issues, fundamental studies on the growth mechanisms of the FePt-C ferromagnetic layers on various seed layers is necessary. Some approaches to these issues are addressed in this review. Recent industrial demonstrations of HAMR recording over 1 Tbit/in2 and its improving endurance test result are encouraging, and the commercial implementation of HAMR appears to be right around the corner. However, further improvement of the microstructure to optimize the read and write properties as well as heat control is essential for achieving higher areal density beyond 2 Tbit/in2 in the future. Acknowledgment Financial support from IDEMA-ASTC for our HAMR media work is acknowledged. We thank A. Perumal for valuable comments. 1. Shiroishi Y., Fukuda K., Tagawa I., Iwasaki H., Takenoiri S., Tanaka H., Mutoh H. and N. Yoshikawa N. (2009). IEEE Trans. Magn., 2009, 45, pp. 3816. 2. Klemmer K., Hoydick D., Okumura H., Zhang B. and W. A. Soffa W. A. (1995). Scripta Metal. Mate. 33, pp. 1793. 3. Weller D. and Moser A. (1999). IEEE Trans. Magn. 35, pp. 4423. 4. Rottmayer R. E., Batra S., Buechel D., Challener W. A., Hohlfeld J., Kubota Y., Lu B., Michalcea C., Mountfield K., Pelhos K., Peng C., Rausch T., Seigler M. A., Weller D. and Yang X. M. (2006). IEEE Trans. Magn. 42, pp. 2417. 5. Guarisco D., and Nguy H. (2003). J Appl Phys. 93, pp. 6745. 6. Kryder M. H., Gage E. C., McDaniel T. W., Challener W. A., Rottmayer R. E., Ju G., Hsia Y. T. and Erden M. F. (2008). Proceedings of the IEEE, 96, pp. 1810 7. Stipe B. C., Strand T. C., Poon C. C., Balamane H., Boone T. D., Katine J. 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S., Wang J., Shiroyama T., Takahashi Y. K. and Hono K. upublished. 75. Wicht S., Neu V., Schultz L., Mehta V., Jain S., Reiner J., Mosendz O., Hellwig O., Weller D. and B. Rellinghaus B. (2015), Modification of the structural and magnetic properties of granular FePt films by seed layer conditioning, J. Appl. Phys. 117, pp. 013907. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 15 Table Table 1. HAMR media requirements for 2 and 4 Tbit/in2 released from the Advanced Storage Technology Consortium (ASTC) [9]. Areal Density (Tbit/in2) 2 4 Grain size, D (nm) 6 4.3 Center to center distance, Dp (nm) 7 5.1 Size distribution, s (%) 10 – 15 10 – 15 µ0Ms of FM layer (T) 0.88 1 µ0Ms of FM grain (T) 1.1 1.26 Ku (MJ/m3) 3.5 5 µ0Hk (T) 8 10 sHk/Hk 5 – 10 5 -10 Tc (°C) 480 430 sTc/Tc (%) 2 2 Angular distribution, sq (°) 2 0.8 Media thickness, t (nm) 9 8.2 Grain aspect ratio (h/D) 1.29 1.6 Grains/bit 6.7 6.2 Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 16 Fig. 1. (a) Plane-view TEM bright field images and (b) the cross sectional TEM image of the currently used perpendicular recording media. CoCrPt ferromagnetic grains of ~8 nm are dispersed in SiO2 matrix. On the glass substrate, amorphous-CoTaZr soft magnetic underlayer, Ru interlayer that align the [0001] axis of CoCrPt alloy in the perpendicular direction to the film plane are grown. Fig. 2. The ideal structure of a HAMR medium for 4 Tbit/in2 arel density. Red and blue colors show the direction of magnetic poles. The L10-FePt crystal (the upper right figure) must grow columnarly separated by the non-ferromagentic "segregant". The column height to grain diameter ratio h/D must be higher than 1.5 and the diameter of the columnar grains are desired to be less than 4 nm [8]. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 17 Fig. 3. (a) Plane-view TEM bright field images of FePt films deposited on MgO (001) single crystalline substrates at different temperatures and (b) the degree of L10 order and coercivity of the films as functions of substrate temperature (Tsub) [26]. Fig. 4. Plane-view and cross-sectional TEM bright field images and their magnetization curves of FePt-C granular films with nominal thickness, t, of 6 nm and 10 nm. Up to t=6 nm, the film grows with a single layer granular structure, but a second granular layer appears for t>6 nm [48]. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 18 Fig. 5. (a) and (b) Plane-view and cross-sectional TEM bright field images of the (FePt)0.9Ag0.1-40vol.%C granular film and (c) magnetization curves, and (d) the patterns of recording bits by the static HAMR head. The mean grain diameter of the FePt grains is 6.1 nm, and their size dispersion is 1.8 nm. Coercivity is 3.7 T, which is five times larger than that of the conventional magnetic recording media. In the recording pattern with the static HAMR head, 15 nm bits were observed with a bit width of 92 nm. Converting to a recording density, this is equivalent to 550 Gbpsi [53]. Fig. 6. Scanning transmission electron microscope energy dispersive X-ray spectroscopy (STEM-EDS) elemental maps of Fe and Ag in the (FePt)0.9Ag0.1-28vol%C granular film [54]. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 19 Fig. 7. Plane-view and cross-sectional TEM bright field images of a 8- nm-thick FePt-40vol%TiO2 thin film deposited on a glass substrate through a-NiTa (60 nm)/MgO (10nm) buffer layers. The cross-sectional microstructure shows good columnar structure with aspect ratio of ~2 with less surface roughness whereas plane-view microstructure shows interconnected granular structure [60]. Fig. 8. Change in Hc as a function of the volume fraction of ZrO, NbO, WO, AlO, TiO, Cr2O3 and C [61]. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 20 Fig. 9. In-plane and cross-sectional TEM bright field images of the FePt–40vol.%Cr2O3 film deposited on a glass substrate with a MgO seed layer [62]. Fig. 10. Plane-view and cross-sectional TEM bright field images of FePt-30vol.%C films deposited by the standard co-sputtering method, (a) and (b), and the compositionally graded process, (c) and (d). The inset figures show their magnetization curves and size distributions of the grains. The compositionally graded process means that the films were grown with a sequence of FePt-25vol%C(2)/FePt-30vol%C(2)/FePt-35vol.%C(2)/FePt-25vol%C(2)/FePt-30vol%C(2), where the numbers in the parenthesis is the thickness of each layer in nm [68]. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 21 Fig. 11. Plane-view and cross-sectional TEM bright field images of FePt–C granular film on (a) MgO single crystal and (b) polycrystalline MgO underlayer. (Inset: FePt grain size distribution, unit in nm.) The FePt-C magnetic layer was deposited with the concentration graded process of [FePt-48vol.%C (0.25nm)/FePt (0.15nm)]25 to obtain the nominal thickness of the FePt-C layer as 10 nm. In-plane and out-of-plane magnetization curves of FePt–C granular film on (a) MgO single crystal and (b) polycrystalline MgO underlayer [73]. Authors' Copy from Ultrahigh-Density Magnetic Recording: Storage Materials and Media Designs, Edited by Gaspare Varvaro and Francesca Casoli Pan Stanford Publishing Pte. Ltd., 2016, ISBN 978-981-4669-58-0 (Hardcover), 978-981-4669-59-7 (eBook) 22 Fig. 12. Orientation maps of FePt–C granular films deposited on (a) a MgO single crystal substrate and (b) a polycrystalline MgO underlayer. The orientation of each grain is given by the reference triangle [73].