Double magnet master media for magnetic printing onto energy-assisted magnetic recording media

Abstract

Recently, the problem of information storage becomes more serious due to the information explosion. It is widely known that future advances in the areal density of hard disk drives (HDDs) are essentially limited by the magnetic recording trilemma arising from the conflicting requirements between signal-to-noise ratio, writeability, and stability[1]. Energy-assisted magnetic recording (EAMR) such as heat-assisted magnetic recording[2] and microwave-assisted magnetic recording[3] is thought to solve the trilemma by improving writability and enhancing areal recording density. Two-dimensional magnetic recording with shingled writing[4] has also been proposed for increasing track density, which is enabled by partial overlapping of recorded tracks. EAMR combined with SMR is expected to be achieved for the next generation recording schemes of HDDs.On the other hand, servo signal writing is one of serious problems for high areal recording density in hard disks. The self-servo track writing technology, which regenerated from previous written servo patterns, has been adopted during head-disk assembly. However, since the servo signal writing in the present hard disks takes more than a few days per one drives on manufacturing process, and the supply of HDDs can not keep up with the social demand. Magnetic printing with perpendicularly magnetized patterns is a strong candidate for servo track writing with extremely high speed and low cost[5]. However, the recording field strength by utilizing the conventional magnetic printing master medium is not enough to write servo-signals on HDDs with large coercivity for EAMR, and it becomes much weaker as the pattern size or the track pitch decreases. The decrease in the recording field or the field gradient deteriorates the signal-to-noise ratio of servo-signals, and this problem becomes more serious in HDDs which has narrower track width. In order to accurately write servo-signals on HDDs for EAMR combined with SMR, it is essential to enhance the recording field. I propose a new master structure, herein called double magnet master (DMM) medium[6], for magnetic printing technique to improve recording characteristics.Figure 1 shows the schematic illustration of double magnet master medium and recording layer. The DMM medium consists of two magnetic materials with different coercivity. Both materials have perpendicular magnetic anisotropy but exchange interaction between both materials does not act. One material has a coercivity higher than the coercivity of the recording layer, while the other has a lower coercivity. During application of printing field Hp, the magnetization of low coercivity parts (magnet 1) orients to the direction of printing field while high coercivity parts (magnet 2) is not subject to magnetization reversal. As a result, the recording field can be enhanced.The recording field distributions of L/S patterns with pattern length of 10 nm, which corresponds to bit length, were calculated by micromagnetic simulation for conventional perpendicular master (CPM)[5] and DMM media. Tha magnetic spacing is 2 nm in this case. The printing field Hp was varied from 5 kOe to 50 kOe. In CPM, the recording field Hr consists of the magnetostatic field generated from only the magnet 1, while in DMM, the recording field Hr consists of the magnetostatic fields generated from both magnets 1 and 2. The recording field difference ΔHr of about 11 kOe for DMM is twice as that for CPM in various printing field. It was clearly found that the recording field can be enhanced by DMM. Since the magnetostatic field generated from double magnets is effective when the magnetization in the harder magnet does not reverse, the printing field is less than the coercivity of the harder magnet, which is about 73kOe by using L10-FePt patterms as magnet 2.In order to evaluate printing characteristics of CPM and DMM, the printed magnetization distributions of ECC-type EAMR media [7] with the coercivities of 15kOe and 22kOe were calculated. Figure 2 shows printing performance [8] as a function of printing field Hp. The printing performance of 1 means the perfect printing. The optimum printing fields are slightly less than those of the coercivity of recording layer. The printing performances of DMM were dramatically improved comparing with CPM printing characteristics. It was clearly confirmed that the printing performances of DMM are superior to that of CPM. Therefore, the magnetic printing by utilizing DMM is a promising way to realized high-speed servo-track writing for EAMR combined with SMR. **