Magnetic printing characteristics of burst signals by using double magnet master media

By the explosive increase in the amount of information due to the development of information and communication technology, the supply of storage devices is becoming increasingly important. Because the demand for hard disk drives (HDDs), especially in data centers, is increasing, the problem of information storage becomes more serious due to the information explosion. One solution is to increase the areal recording density of HDDs by overcoming the trilemma, which is arising from the conflicting requirements between signal-to-noise ratio, writability, and stability.¹ Energy-assisted magnetic recording (EAMR) such as heat-assisted magnetic recording (HAMR)² and microwave-assisted magnetic recording (MAMR)^3–5 is feasible to overcome the trilemma by improving writability and enhancing areal recording density. The commercialization of EAMR also begins recently.

On the other hand, servo signal writing is one of serious issues for high areal recording density HDDs in order to supply the demand for HDDs. The self-servo track writing technology, which regenerated from previous written servo patterns, has been adopted during head-disk assembly.^6,7 However, the servo signal writing in the present hard disks takes a few days per drive in the manufacturing process, and thus HDD manufacturing speed is limited.

To overcome manufacturing process speed, magnetic printing technique is a strong candidate for servo track writing with extremely high speed and low cost.⁸ Recently, the magnet printing technique with double magnet master (DMM) was proposed for EAMR media with high coercivity of recording layer.^9–11 The DMM media consists of two magnetic materials with different magnetic properties⁹ while the previously reported master media,⁸ herein called the conventional master media, has only one type of magnetic material. One magnet is L1₀–FePt which has a coercivity higher than the coercivity of recording layer, while the other magnet is FeCo alloy with a lower coercivity and a large saturation magnetization. Two magnetic materials have no exchange interaction between these materials by insertion of non-magnetic separation (NMS) layer.¹⁰ The patterned soft or hard magnet in DMM media corresponds to the servo patterns.

The magnetic printing procedure with DMM media is as follows: first, the recording layer is uniformly magnetized by applying the initial magnetic field along the perpendicular direction of the recording layer. The DMM media is in contact with the recording layer, and then the external magnetic field, herein called printing field, is applied along the opposite direction to the initial magnetic field. Because the magnetic flux is concentrated into soft magnet patterns and simultaneously avoids hard magnet patterns, the magnetization of the recording layer underneath the soft magnet pattern area is reversed while the initial magnetization at the hard magnet pattern area is preserved. In other words, the sum of the printing field and the stray fields generated from the magnetizations of soft- and hard-magnets is applied onto the recording layer as the recording field. Since the recording field can be enhanced compared with the conventional master media, the DMM media can transfer servo signals into the recording layer of EAMR. Finally, the servo patterns are printed into the recording layer. Magnetic printing is notable for its recording speed, requiring only the applications of the initial and the printing fields.

The previous works showed that magnetic printing with DMM media can achieve superior characteristics for periodical line/space, dot, and checkerboard patterns.^9–11 The practical servo-signals include non-periodic patterns such as servo-burst, automatic-gain control (AGC), and address part with Gray code. However, printing characteristics for such non-periodic patterns have not been discussed yet. In this study, as the more realistic servo signals, the printing characteristics of non-periodic patterns including AGC and burst signals have been demonstrated by utilizing micromagnetic simulation.

Figure 1 shows the calculation model for master media and recording layer. In this study, the amplitude burst of A/B burst is assumed in the simulation. The servo information typically includes track centering information and quadrature reference information providing position error signal, namely PES. The master pattern includes AGC and A/B burst patterns in the simulation. The standard bit length L and track width T_w are 20 and 50 nm, respectively. Two possible master media structures of DMM media were considered, which are positive-(p-) and negative-(n-) DMM media as shown in Figs. 1(a) and 1(b), respectively. The p-DMM divides soft magnet parts which is located directly above the reversal region of the recording layer while soft magnet parts are connected with the exchange coupling in the n-DMM. In order to calculate the printed magnetization distributions of the recording layer, the micromagnetic simulation has been carried out. For comparison, the conventional master with soft magnet of FeCo alloy was also calculated in this study. Figure 1(d) shows the schematic illustrations of DMM medium and recording layer during printing process. The magnetic pattern thickness was fixed to be 10 nm, and the magnetic spacing between the master medium and the recording layer is set constant to 2 nm in each case, which is assumed to correspond to the total thickness of the lubricant and the protection layers.

The exchange coupled composite media, which consists of hard- and soft-magnetic layers with granular structure, were utilized as the recording layer. In this study, the hexagonal column grains with the average grain diameter of 4.6 nm, the grain spacing of 0.2 nm, the height of 12 nm were assumed as the next-generation EAMR media.^11,12 The hexagonal column grains are divided into the calculation cells with a thickness of 1 nm along the out-of-plane direction. This is an ideal granular structure but the purpose of this study is to confirm magnetic printing characteristics. The exchange coupling between stacked calculation cells inside hard- or soft-magnetic layers is 1.0 × 10⁻⁶ erg/cm, the exchange coupling between hard- and soft-layers is 70 erg/cm², and the exchange coupling between hexagonal grains is 1.0 erg/cm². Material parameters of recording layer are shown in Table I. The thicknesses of soft- and hard-layers are denoted t_S and t_H, respectively. Soft layer thickness was varied from 2 to 5 nm while the overall thickness of 12 nm was fixed. The corresponding hysteresis loops are shown in Fig. 2. These recording layers showed the coercivity of the recording layer ranges from about 15 to 35 kOe, corresponding to the thickness of the soft layer.

First, the recording field H_rec was obtained by calculating the magnetization distribution of magnetic patterns in DMM media during the application of printing field H_p with micromagnetic simulation.⁸ By applying the calculated recording field distribution H_rec to the recording layer, the printed magnetization in the recording layer was also calculated, and the printing characteristics in each case were discussed.

Figure 3 shows the z-component of recording field, H_rec, at the surface of recording layer, and also show recording field H_rec along the center of A/B-burst patterns. The printing field H_p is 10 kOe and the thickness of NMS is 2 nm. During the printing process, the recording field difference generated by p-DMM media is about 15 kOe for both AGC and burst pattern which are much larger than that of the conventional master. On the other hand, although the n-DMM showed the recording field difference of about 14 kOe which is slightly smaller than that of the p-DMM, the undesired recording field in the off-track position of A/B-burst appears. Since the n-DMM has an undivided soft magnetic pattern corresponding to the off-center region, domains and nonuniform magnetization regions exist in the off-center region. The magnetization of the soft magnetic pattern for the periodic line/space pattern does not saturate by applying a printing field of less than 13 kOe from the previous study due to the demagnetizing field.¹¹ Thus, the soft magnetic patterns in DMM must be divided and must avoid unsaturation of the soft magnetization for EAMR media with a coercivity of less than 13 kOe. However, the influence of domain and unsaturated magnetization in the soft magnet parts is reduced for printing onto higher coercivity media because the printing field is larger than about 13 kOe.

Figure 4 shows the printed magnetization distributions onto the recording layer with the coercivity of about 15 kOe. The printing field H_p is 10 kOe. As a result, the p-DMM can clearly print burst patterns, especially even if in the off-center region the clear magnetization distribution can be achieved. Moreover, the conventional master can print burst parts onto the recording layer with a relatively lower coercivity but the reversed region slightly penetrates into the off-center regions. On the other hand, using the n-DMM shows some non-reversal regions in the off-center region and the undesired magnetization distribution because the soft magnet parts are unsaturated and generates undesired recording field as shown in Fig. 3(b). Moreover, in the AGC parts, there are some unreversed regions for the cases of the p-DMM and the conventional master.

In order to improve the unreversed area in the AGC part, the pattern width corresponding to bit length in the AGC parts, L_AGC, is varied while the pattern width of burst parts is kept to be 20 nm. Figure 5(a) shows the enlarged view of the recording field as shown in Fig. 3, and the definition of recording field differences, ΔH_max and ΔH_min, which are the field differences between the maximum/minimum values of AGC and burst parts. These values mean the effect of suppressing variations in printing characteristics, and the area ratio of hard magnet and soft magnet is important to design the DMM pattern. Figure 5 shows these recording differences ΔH_max/min as a function of AGC pattern width L_AGC for various NMS thicknesses t_NMS. The pattern width L_AGC of 16 nm gives the minimum values of ΔH_max/min. The pattern width differences of AGC and burst parts originate from the area ratio of soft and hard magnets near the surrounding region. In this way, the design of pattern widths in each part is important to improve the printing characteristics.

Figure 6 shows the magnetization distributions printed by p-DMM onto the recording layers with various coercivities of about 15, 23, and 32 kOe. The corresponding printing fields H_p are 8, 18, and 28 kOe, respectively, which are slightly smaller than the coercivity. The NMS thickness t_NMS is 5 nm, which is the optimum value suggested in the previous work.¹⁰ As a result, the p-DMM can clearly print burst patterns in any recording media, especially, even for the EAMR media with a high coercivity of 30 kOe. Further clear printed magnetization will be obtained by optimizing the bit lengths of AGC and burst parts, and the thickness of NMS. Each pattern dimension depends on the surrounding hard magnet area of each soft magnet pattern. Thus, a clear PES is expected by utilizing the positive DMM with the optimum patterns in magnetic printing onto energy-assisted magnetic recording. This study provides the feasibility of DMM printing for a realistic servo writing technique.

In this study, the printing characteristics of burst signals with DMM media onto energy-assisted magnetic recording media was investigated by the micromagnetic simulation. The DMM media were considered to be two types of configuration of positive and negative magnetic patterns of soft magnet. By utilizing the positive-DMM, the burst signals can be clearly printed, which is obviously superior to that of the conventional master. While by utilizing the negative-DMM, the printing of burst signals is insufficient due to non-uniform magnetization in the soft magnet parts. The sufficient printed magnetization onto EAMR media with high coercivity can be obtained by the optimum non-magnetic spacer thickness and the optimum printing field. Therefore, the magnetic printing with positive-DMM master is promising onto energy-assisted magnetic recording media.

This research was supported in part by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (Grant No. 20H02196), by a Grant-in-aid from the KDDI Foundation, by a Grant-in-aid from the Fujikura Foundation, and by a Grant-in-aid from the Amano Institute of Technology. This work was partially performed under the Research Program of “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” in “Network Joint Research Center for Materials and Devices.”

The author has no conflicts to disclose.

Takashi Komine: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.