Chris Rea

Exchange tab stabilized readback transducers for areal densities exceeding 20 Gb/in/sup 2/

We present results from a high-density giant magnetoresistive magnetic recording reader using exchange bias stabilization. This novel reader design approach reduces the amount of parasitic resistance, as the sense current is not delivered through high resistivity permanent magnets. Heads were demonstrated to deliver areal densities in excess of 24 Gb/inch/sup 2/. The electrical performance of these heads, in particular, amplitude sensitivity, microtrack profiles and areal density capability are presented. Reader film properties and manufacturability of this approach are discussed in detail.

HAMR Performance and Integration Challenges

Heat-assisted magnetic recording (HAMR) is a fast evolving technology, and has been established as the next enabler of higher areal density in magnetic recording. After achieving high areal density capability, HAMR drive integration was recently demonstrated. In this paper, we discuss some of the recent learning from component performance and drive integration. We identify a key challenge in HAMR integration: erasure due to thermal background heating. The background heating was introduced to improve near-field transducer reliability and reduce laser power requirement. We present experimental and modeling data on the impact of thermal background, both in the cross-track (adjacent-track erasure) and down-track (self-erasure) dimensions.

High Track Pitch Capability for HAMR Recording

Differences in the areal density capability limits for heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording (PMR) are explored using spinstand measurements and micromagnetic modeling. The written track curvature and the transition width are measured with a special technique that mitigates cross-track averaging effects due to finite read sensor width. Tracks written with HAMR heads are shown to have more curvature compared with those written with modern PMR writers. Both broadening and transition curvatures are present cross track, that can be described reasonably well using Landau–Lifshitz–Gilbert modeling and indicates the effective gradient profile dominates. However, as we get to the track edge, we see disagreement between the model and the experiment, which is not currently reconciled. The curvature and broadening effects appear to challenge not only the downtrack bit resolution during readback, but also the cross-track written width with increased linear density. We discuss different near field transducer (NFT) designs, and comparing narrow versus wide thermal profile NFT designs, we can see that track pitch appears to weakly optimize to a maximal cross-track gradient point. Experimental measurements of constant bit error rate for different linear and track densities indicate a significant opportunity for high track density recording using HAMR. The difference appears to be related to the ability for HAMR to address high track pitches with a minimal increase in risk of adjacent track interference compared with PMR. We revise NFT head-to-media spacing requirements when we attempt to account for interference effects.

Areal-Density Limits for Heat-Assisted Magnetic Recording and Perpendicular Magnetic Recording

Differences in the areal-density capability limits for heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording (PMR) are explored using spinstand measurements, drive footprinting, and micromagnetic modeling. The written track curvature is measured with a special technique that mitigates the cross-track averaging effects due to a finite read sensor width. Tracks written with HAMR heads are shown to have more curvatures compared with those written with modern PMR writers. Mitigation of written track curvature is demonstrated with two different HAMR writer designs. The curvature effect appears to challenge not only the downtrack bit resolution during readback, but also the cross-track written width with increased linear density (LD). Experimental measurements of a constant bit error rate for different LDs and track densities (TDs) indicate a significant opportunity for high TD recording using HAMR. The difference appears to be related to the ability for HAMR to address high track pitches with a minimal increase in risk of adjacent track interference compared with PMR.

Side-Track Erasure Processes in Perpendicular Recording

In perpendicular recording, substantial erasure of the stored data patterns could occur during writing process. Among all those erasure processes, the sidetrack erasure (STE) is one of the critical erasure issues. To sort out the intrinsic characteristics of the STE process and deepen our understanding of the underlying physics in the erasure processes, in this work, we have experimentally investigated the general attributes of STE process in various situations and quantified some of its distinctive signatures as well as some of its origins. Particularly, some STE behaviors have been characterized thoroughly by employing both the amplitude and BER based STE measurement methods in combination with other unique characterization techniques, in order to unambiguously reveal some of inherent features of the STE processes.

High frequency magnetization reversal in rotating fields

Micromagnetic simulations of polycrystalline thin films predict that rotational magnetization processes are much more strongly affected by interactions and much less strongly affected by damping than 1-D switching processes. Interactions significantly affect the rotational hysteresis loss and reduce the magnitude of the field required for reversal. Reversal in rotating fields is predicted to occur at frequencies up to an order of magnitude higher than in 1-D switching fields. Track edge erasure in longitudinal recording is therefore predicted to persist to higher frequencies than the recorded signal, and we suggest that perpendicular recording may be possible at higher frequencies and with higher anisotropy materials than longitudinal recording.

Writer and Reader Head-to-Media Spacing Sensitivity Assessment in HAMR

We collate multiple experimental measurements of heat-assisted magnetic recording (HAMR) near-field transducer (NFT) and reader sensitivity measurements on spinstand to compare and contrast with the conventional perpendicular magnetic recording (PMR). The readback process shares many similarities, but differences appear due to the increased measured curvature of the prewritten track, which increases the observed pulsewidth (Pw50), lower readback amplitude due to reduced flux from the media transition (MrT), and increased coating thicknesses. We find that the reader head-to-media spacing (HMS) sensitivities and requirements converge toward the conventional scaling requirements. The HAMR write process is more complex due to the uncertainties associated with the optical properties and protrusion position of the NFT. However, accumulating multiple studies varying write HMS with coatings, NFT changes, and media changes, we consistently observe lower sensitivities compared with the conventional writer HMS, in line with modeled comparisons, leading to more relaxed requirements on the NFT clearance than the PMR write HMS. A 1.5 Tbpsi basic technology demonstration demo is shared using the HMS numbers in the bounds of the claims.

Effect of seedlayer and junction geometry on permanent magnets stabilisation of magnetoresistive heads

The dependence of CoPt stabilization properties on surface topography and seedlayer thickness has been investigated. The coercivity degrades drastically without a Cr seed, showing a dependence upon surface angle. Using the resulting data wafer level sensor structures are micromagnetic simulated. Poor seedlayer coverage on the junction edge yields open transfer curve loops and degraded sensor response.

HAMR Thermal Gradient Measurements and Analysis

The edge gradient of the thermal spot used to define bits in heat-assisted magnetic recording is a critical parameter in determining the quality of the magnetic transitions. We review the laser current modulation method used to measure this parameter and introduce the sideband ratio (SBR) approach as a fast, inexpensive technique for getting data on large volumes of heads or media. Mathematical derivation of the SBR method is given along with the details of implementation. Production level data on thousands of recording heads are used to illustrate its usefulness.

Write position shifts in heat-assisted magnetic recording

Micromagnetic simulations have been performed to investigate the write position shifts in heat-assisted magnetic recording (HAMR) under variation of variables including the thermal profile, write field magnitude, head velocity, media damping, and write field rise time. Simulations reveal that a larger head velocity or a larger media damping tends to shift the write positions towards the heat center. Simulation results are also compared to the predictions of an analytical model based on classical mean-field theory. It is found that, with smaller thermal gradients the analytical model tends to give write positions closer to the heat center; whereas when thermal gradients are larger it tends to shift the write positions away from the heat center. All these effects appear to be dynamic, and can be understood by recognizing the lag between spin temperature and lattice temperature that cannot be captured by the analytical model that assumes equilibrium at all times.