Yuhui Tang

Microwave Assisted Magnetic Recording

In this paper, we present a novel mechanism for recording at a head held significantly below the medium coercivity in a perpendicular recording geometry. By applying a localized ac field at adequate frequency to the perpendicular recording medium, saturation recording can be achieved with recording field amplitudes significantly below the medium coercivity, or the medium perpendicular anisotropy field. A scheme utilizing spin torque to generate a localized ac field at high frequency (tens of gigahertz) with kilo-oersted field amplitude in the medium is proposed along with a systematic modeling analysis. Recording simulations at high linear densities are presented.

A medium microstructure for high area density perpendicular recording

A perpendicular medium microstructure consisting of fully exchange coupled grains with densely distributed nonmagnetic pinning sites, referred to as the percolated perpendicular medium, is proposed. In this medium, the ferromagnetic exchange coupling between the grains ensures sufficient thermal stability, while the densely distributed domain wall pinning sites ensure relatively smooth transition boundaries. The detailed magnetic hysteresis properties and recording characteristics of the percolated perpendicular media are investigated via micromagnetic modeling. The study found that the percolated medium could offer much improved recording properties in terms of enhanced transition sharpness and reduced transition noise while maintaining thermal stability in comparison to present granular perpendicular medium.

Media damping constant and performance characteristics in microwave assisted magnetic recording with circular ac field

Medium damping constant remains as one of the key concerns for microwave assisted magnetic recording (MAMR) since switching field reduction seems to be less evident for media with possible high damping constant. In this paper, the impact of medium damping constant on the recording performance is analyzed via dynamic micromagnetic modeling for MAMR with a polarized ac field. The analysis on isolated grains shows that when matching the precession chirality, switching field reduction is significantly greater with circular ac field than that with linear ac field, especially in high damping regime. Recording simulations show that an area density of 1.88 Tb/in.2 with a medium signal-to-noise ratio of 18.2 dB can be achieved on media with strong anisotropy of 31 kOe and a relatively high damping constant of 0.1.

Narrow Track Confinement by AC Field Generation Layer in Microwave Assisted Magnetic Recording

Microwave assisted magnetic recording (MAMR) has been proposed to provide sufficient writability in magnetic recording when the magnetic grain has a large crystalline anisotropy field in high recording density. In this paper, the relationship between the track width and recording head designs in this novel recording scheme is studied using micromagnetic modeling. By controlling the AC field generation layer width, high track density can be obtained without decreasing the main pole width in the writer. A quantitive result shows 770 KTPI can be realized in MAMR with a 120 nm wide main pole and 25 nm wide AC field generation layer.

Return Field-Induced Partial Erasure in Perpendicular Recording Using Trailing-Edge Shielded Writers

Return field-induced partial erasure (RFPE) in trailing-edge shielded perpendicular writers has been studied, both by modeling and by experiments. For a given head-media combination, the return field underneath the trailing shield increases with increasing write current. Once exceeding a certain threshold, it will cause partial erasure of the bits that have just been written by the main pole. Recording performance, such as reverse overwrite, spectral signal-to-noise ratio, and bit-error rate are all found degraded at high write currents, due to RFPE. Design optimization of both head and media together is needed, in order to maximize the advantage of a trailing-edge shielded pole head and minimize the impact of RFPE

Magnetic recording performance of keepered media

Using low flying and proximity inductive heads, keepered media show improved on- and off-track performance leading us to conclude that a greater than 20% areal density improvement is possible with a keeper layer over the magnetic storage layer. For Sendust keeper layers there is an optimal range of thickness and an optimal bias point for best performance. There are both amplitude and timing asymmetries that are functions of the read-back bias. For a peak detect channel the best performance corresponds to the minimum timing asymmetry although this is not the point where the pulses are narrowest. Keepered media may have an advantage in total jitter and partial erasure. NLTS is almost identical for keepered versus unkeepered media.

Micromagnetics of Percolated Perpendicular Media

In this paper, we present a systematic micromagnetic analysis on the magnetic properties, thermal magnetic stability, and recording performance of percolated perpendicular media (PPM), a novel film microstructure of exchange coupled small-size magnetic grains with densely distributed nonmagnetic pinning sites. The analysis shows that the PPM microstructure can substantially reduce medium noise over the conventional granular perpendicular media while maintaining sufficient magnetic stability

Integration of Servo and High Bit Aspect Ratio Data Patterns on Nanoimprint Templates for Patterned Media

Magnetic recording bit patterned media (BPM) stands as a promising technology to deliver thermally stable magnetic storage at densities beyond 1 Tb/in2. High throughput BPM fabrication will be enabled by nanoimprint lithography which relies on a high-quality master template to be able to meet pattern fidelity and low defectivity specifications. Master template fabrication for BPM can be done by e-beam lithography alone or by e-beam directed self-assembly of block copolymers. Incorporating servo features in the fabrication of master templates brings numerous nanofabrication challenges that vary depending on which method is used. Here we demonstrate the fabrication of conventional servo features at 200 Gd/in2 using e-beam lithography and we explore some nonconventional servo geometries that are compatible with self-assembly for BPM beyond 1 Td/in2.

Understanding Adjacent Track Erasure in Discrete Track Media

We present a micromagnetic modeling analysis on recording performance, in particular adjacent track erasure (ATE), in discrete track media. We have compared the ATE performance between the continuous and discrete track media, and we have modeled two types of perpendicular media: one with zero intergranular exchange coupling and the other with moderate intergranular exchange coupling. We found that in the medium with moderate intergranular exchange coupling, the ATE has significantly been suppressed; that is, the track pitch can be significantly smaller with the same recording head and recording condition for the same ATE level. This gain becomes less significant if the medium has zero intergranular exchange coupling.

Domain Wall Pinning and Corresponding Energy Barrier in Percolated Perpendicular Medium

The domain wall pinning processes in a percolated perpendicular medium are studied using micromagnetic modeling. With a given nonmagnetic pinning site, the energy barrier of the pinned domain wall reaches a maximum when the domain wall width is about twice the size of the pinning site. For transitions in the percolated perpendicular medium, the energy barrier for the transition shift is sensitive to the pinning site spacing. Considering regularly distributed pinning sites with 4.7-nm center-to-center spacing, the energy barrier for the transition shift is about twice the energy barrier of the domain wall pinned by one pinning site. The domain stability in percolated perpendicular medium is also discussed with an analytic model