Michael Alex

Characteristics of thermally assisted magnetic recording

Extensive thermally assisted recording measurements were performed on isotropic longitudinal media. The dynamic coercivity decrease provided by thermal assistance was quantified by spin stand measurements. Thermally assisted recording performance was found to equal the performance of conventional (nonthermally assisted) recording at significantly reduced values of write current. This reduction in write current is consistent with the coercivity decrease that occurs during the thermally assisted writing process.

Thermal effects and recording performance at high recording densities

Thermal effects in thin film media have interesting. if not potentially serious implications for both short~tenn writing and long-term storage of recorded data. Remanent-moment thickness product (M,T) and grain size reduction for enhanced media performance, in conjunction with increasing areal densities, make thermal effects important considerations in the design and characterization of advanced recording systems. This talk will discuss both the short- and longterm manifestations of thermal effects in thin-film media, dynamic coercivity and thermal decay, as well as the measurement of, and correlation between these two phenomena.

Relationship Between Equalized SNR and Jitter—Theory and Application to PMR and HAMR

A simple analytical relationship between equalized signal-to-noise ratio (SNR) and jitter is derived theoretically. Equalization is applied to the spectra of pseudorandom sequence signals, by whitening the frequency content of the noise. Jitter is evaluated in the frequency domain with square waveform signals. We verify experimentally that, for a wide range of linear densities, we can predict well-equalized SNR (and thus overall system performance) by using the measured jitter value.

HAMR recording challenges at high linear densities

HAMR technology has experienced big advances in just this last year but hurdles still abound and need to be resolved before HAMR can fulfill its full potential for both lifetime and areal density capability. A few of these challenges are directly tied to component optical, thermal and magnetic designs. Specifically, from a media perspective, in-plane grains and grain size distribution are a major concern limiting the linear density capability of the HAMR system. Assuming that high enough thermal gradients are achieved, bit curvature also limits the resolution of the system and needs to be better characterized and addressed. Many of these issues are exclusive of HAMR recording. Thus, they present new challenges but also new opportunities for workarounds and innovation in the magnetic recording industry; something our industry is famous for overcoming.

HAMR Noise Mechanism Study With Spin-Stand Testing

In this paper, we present a spin-stand testing study of heat-assisted magnetic recording (HAMR). Motivated by the understanding of the major recording noise mechanisms predicted by previous modeling, experiments were made to investigate the recording field/write current dependence of signal-to-noise ratio. The two significant noise mechanisms in HAMR, incomplete switching and erase-after-write, have been observed in the experiments and shown to be similar to the previous modeling results. Previous simulations indicate that system performance strongly correlates with recording time window (RTW). Therefore, head/media relative velocity was varied to change the RTW, and the effects of linear velocity are also compared. Further analysis has been applied to cases with different writer and media components. These results help understand the field/media requirements to optimize the HAMR recording system performance.

2-D Visualization in Magnetic Recording

The quality of magnetic recording can be assessed from 2-D distributions of the recorded readback signal, media, and read sensor noise. The procedure for generating spatial signal and noise distributions is based on cross-track data alignment using media noise as a reference. In order to analyze signal and noise statistics, multiple data periods are processed using synchronous averaging, resulting in average readback signal and rms noise voltage maps. Using multiple signal acquisitions from the same media location allows separate measurements of media and read sensor noise distributions. Fast 2-D mapping algorithms allow high-resolution visualization of arbitrary data patterns, pole footprint imaging, and encroachment of the writer field onto adjacent tracks, providing detailed measurements of recording parameters (media jitter, dc noise saturation, signal-dependent read sensor noise, transition curvature, edge noise, and others). Recently, we have introduced a highly sensitive switching probability averaging measurement (SPAM), which allows the detection of media grain switching with unprecedented sensitivity (better than one part in 1000). This 2-D technique provides quantitative assessment of the magnetic field coming out of the write head and domain wall activity, which contributes to undesirable adjacent track erasure.

Correlative Magnetic Imaging of Heat-Assisted Magnetic Recording Media in Cross Section Using Lorentz TEM and MFM

In order to increase the storage density of hard disk drives, a detailed understanding of the magnetic structure of the granular magnetic layer is essential. Here, we demonstrate an experimental procedure of imaging recorded bits on heat-assisted magnetic recording (HAMR) media in cross section using Lorentz transmission electron microscopy (TEM). With magnetic force microscopy and focused ion beam (FIB), we successfully targeted a single track to prepare cross-sectional TEM specimens. Then, we characterized the magnetic structure of bits with their precise location and orientation using Fresnel mode of Lorentz TEM. This method can promote understanding of the correlation between bits and their material structure in HAMR media to design better the magnetic layer.

Distinguishing Random and Spatially Deterministic Noise Components in Heat-Assisted Magnetic Recording

By averaging multiple writes of opposite polarity waveforms captured synchronously from heat-assisted magnetic recording (HAMR) media, we distinguish random noise of the HAMR writing process from the spatially anti-correlated repeatable noise due to media microstructure variations. We show both analytically and experimentally how the normalized cross correlation coefficient, R, of two opposite polarity waveforms quantifies the random and repeatable dc-noise fractions, showing where dc-noise reduction efforts should be concentrated. The value of R also reflects the degree of media saturation. At optimal recording conditions for the head and media studied here, the total dc-noise comprises appreciable fractions of both random and repeatable noise. The experimental results are supplemented with micromagnetic modeling and analytic calculations.

Comparative study of micromagnetic modeling and experiment in heat-assisted magnetic recording

Comparative observations between heat-assisted magnetic recording (HAMR) experiments and micromagnetic modelling are presented and discussed in this work. Modeling results and experimental measurements show that the optimum HAMR recording is a tradeoff between two dominant noise mechanisms: erase-after-write and incomplete media saturation that are manifested as transition jitter and DC-media noise, respectively.

XXV Magnetic Recording Conference 2014 Foreword

On behalf of the organizing committee of the XXV Magnetic Recording Conference, TMRC 2014, we are pleased to present selected papers from the conference in this issue of the IEEE TRANSACTIONS ON MAGNETICS. The conference convened at the University of California, Berkeley, CA, USA, on August 11–13, 2014. The hard work and hospitality of the hosting institution in making this event so successful is greatly appreciated.