Sharat Batra

Heat-Assisted Magnetic Recording

Due to the limits of conventional perpendicular magnetic recording, it appears that alternative technologies are needed at areal densities >500 Gb/in2. Heat-assisted magnetic recording (HAMR) is a promising approach to extend areal densities to 1 Tb/in2 and beyond. All of the unique components necessary for a working HAMR system have been demonstrated. Although HAMR permits writing on high Hc media with lower magnetic fields and can produce higher write gradients than conventional magnetic recording, head/media spacing and the development of high Hc media with small grains remains challenging

Micromagnetic modeling of head field rise time for high data-rate recording

We have developed a finite-element micromagnetics model to investigate the dynamics of write heads for perpendicular recording at high density and high data-rates. The model includes the entire head geometry, with the large return pole and the soft underlayer. The response of the head to the coil current is determined by the current waveform shape and duration, Gilbert damping constant, and presence of soft underlayer and shields. Large damping leads to a large phase shift between the coil current and the head field while small damping causes strong gyromagnetic precession. We find that an intermediate value of the damping constant gives the fastest head field rise time. The intrinsic reversal time decreases from 540 to 250 ps by reducing coil-turns from two to one and shortening the yoke length. Thus, an intermediate value of the damping constant, short yoke length, and fast current rise time are needed for maximum data rate.

Investigations of perpendicular write head design for 1 Tb/in/sup 2/

We have revisited the considerations of perpendicular recording at 1 Tb/in/sup 2/. We have extensively modeled head designs with the down-track shield (DS) and head designs with the down-track and side shields (DS-SS). The side shields are needed to reduce the thermal erase width, however, there is a substantial reduction in effective write field for the DS-SS heads compared to the DS heads. This reduction in effective write field defines an upper limit for the anisotropy of the medium that can be written. As a consequence, it is hard to simultaneously achieve writability and the thermal stability requirement for 1 Tb/in/sup 2/.

Micromagnetic modeling of ferromagnetic resonance assisted switching

We studied the steady state behavior and magnetization switching process of single domain particles subject to ac and dc magnetic fields using analytical and numerical models based on the Landau–Lifshitz–Gilbert equation. We compared the analytical solutions for circularly polarized fields with a numerical single spin model and circularly and linearly polarized ac magnetic fields. It has been found, that the initial conditions and the dynamics of the external fields (field ramps and amplitude changes) strongly determine which precession orbit the magnetization converges to, if the magnetization precession is stable, and if the magnetization switches. We also studied the effects of field amplitudes, field angles, and damping on the switching behavior. The presented results can be applied to high power ferromagnetic resonance experiments and ferromagnetic resonance assisted magnetic recording schemes.We studied the steady state behavior and magnetization switching process of single domain particles subject to ac and dc magnetic fields using analytical and numerical models based on the Landau–Lifshitz–Gilbert equation. We compared the analytical solutions for circularly polarized fields with a numerical single spin model and circularly and linearly polarized ac magnetic fields. It has been found, that the initial conditions and the dynamics of the external fields (field ramps and amplitude changes) strongly determine which precession orbit the magnetization converges to, if the magnetization precession is stable, and if the magnetization switches. We also studied the effects of field amplitudes, field angles, and damping on the switching behavior. The presented results can be applied to high power ferromagnetic resonance experiments and ferromagnetic resonance assisted magnetic recording schemes.

Three‐dimensional transmission line model for flux conduction in thin‐film recording heads

Flux spreading in thin‐film recording heads has been studied using a three‐dimensional transmission line model. This model uses the observed domain wall structure in a thin‐film head as the map on to which a set of electrical circuit nodes in the model are placed. These nodes are connected in the transverse direction by inductive circuit elements which simulate the domain wall mobility for transverse flux conduction by wall motion. In the longitudinal direction skin‐effect‐limited conduction by magnetization rotation is simulated by a R/L series element. At frequencies above 8 MHz this model begins to exhibits a concentration of flux on the axis of the head.

The effect of media background on reading and writing in perpendicular recording

The effect of stray fields from the recording layer on the reading and writing process has been studied for a perpendicular recording system. The system consists of a single pole writer with a wide return pole, a conventional spin-valve reader and a double layer recording medium. A writer- and reader-induced asymmetry is observed in the recording process for a dc erased background. The asymmetry is interpreted as stray fields of the dc background giving rise to reader saturation and an offset in the current driven inductive write field.

A perpendicular write head design for high-density recording

In this paper, we discuss a single-pole perpendicular head design and process that is suitable for densities of the order of 100 Gb/in/sup 2/. The single-pole write head was integrated with a narrow-track bottom spin valve reader. The design uses a single-turn coil to generate magnetomotive force in the head. Because of the very short yoke length that is achieved by using a single-coil turn, this writer design has a very low head inductance. Low magnetic impedance of the head makes it suitable for high data rate writing. Using the finite element model (FEM), the head geometry was optimized to write on media with coercivity (H/sub c/) of 5000 Oe. Because of the very efficient head structure, a write current below 100 mA was sufficient. As trackwidths are reduced, the field contours at the media show significant curvature, resulting in written-in transition curvature. Because of the very small yoke structure, no degradation of low-frequency amplitude up to /spl plusmn/90 Oe of external field is observed.

Effect of pole tip anisotropy on the recording performance of a high density perpendicular head

A perpendicular recording head for an areal density of 1 Tbit per square inches has been developed using a self-consistent three-dimensional (3D) micromagnetic model. The soft underlayer and the recording layer have been included in the model. The head consists of a probe type tip protruding from a collar. The tip has saturation magnetization (Ms) of 24 kG while the collar has lower Ms of 10 kG. The magnitude and orientation of anisotropy field (Hk) in the tip is varied to obtain the best recording performance. A perpendicularly oriented Hk in the tip reduces flux spreading, thereby enhancing the recording field in the writing track while reducing the offtrack field. However, simulations show that the head’s performance suffers in terms of high remanent field and slower frequency response. Simulations show that a lower remanent field can be achieved by applying ac demag pulse to the tip. A detailed comparison has been made between two cases of perpendicular Hk of 1 kOe and longitudinal Hk of 10 Oe in the ...

Effect of Write Current Waveform on Magnetization and Head-Field Dynamics of Perpendicular Recording Heads

The response of perpendicular magnetic recording heads with a single turn and two turn coils is calculated using a full micromagnetic model, including return pole and soft underlayer. We study the effect of coil current waveforms with different rise times and overshoots. For fast coil current rise times, the head field shows very little response, until the coil current changes its polarity and it is limited by the intrinsic magnetization dynamics. Even the effect of overshoot is limited by the same mechanism. Shorter yoke length and coil turns close to the air-bearing surface improve the head-field dynamics

Micromagnetic Simulation of head-field and write bubble dynamics in perpendicular recording

We have developed a finite-element micromagnetics model to investigate the magnetization and field dynamics of perpendicular recording heads. We have used a finite-difference micromagnetic recording model to study the influence of the head-field rise time and media velocity on the recording process. An analytical expression for the write bubble expansion velocity has been derived based on a linearized model, and we have compared it with dynamic micromagnetic simulations. It has been found to be directly proportional to the rate of change of field with time and inversely proportional to the head-field gradient. The write bubble expansion velocity needs to match or exceed the media velocity to minimize transition curvature.