Micromagnetic predictions for signal and noise in barium ferriterecording media

A dynamic, micromagnetic recording process simulation is used to predict signal and noise for barium ferrite media. The model has a high spatial (particulate level) and temporal (≈10^-9 s) resolution. The theory includes head saturation, image charge, particle magnetostatic interactions, and particle anisotropy. Most input media and head characteristics are taken from direct measurements, such as electron microscopy or magnetometry, or from the literature. However, several variables, such as head-medium spacing, can only be estimated from the available data; errors in these estimates could affect the predicted numbers by several dBV. Overall, the calculations yield very high accuracy; both signal and bulk erase noise match experiment to within ≈1 dBV. This unusually close agreement for the bulk erase noise is obtained without the benefit of particle-particle interactions. This latter point places barium ferrite in interesting contrast to acicular particle media where interactions are believed to substantially reduce short-wavelength bulk erase noise     

A new theory of recording media noise

Previous investigations of the noise in particulate or grainy recording media have considered statistical variations in the processes by which the particles become magnetized. The theory of noise presented includes also statistical variations in the packing density of the particles. An extremely simple analysis shows that, when both of these phenomena are included properly, the noise power of recording media may always be expected to depend upon the magnetization, or signal level, and the particle packing factor. It is found that the recording media should always provide higher signal-to-noise ratios (SNRs) than was previously supposed. It is pointed out that the signal recovery or detection techniques employed today in magnetic storage devices cannot yield optimum SNRs or bit error rates. Some algebraic and/or conceptual errors in the published literature on noise are discussed.<>     

Media limitations on achievement of maximum detection window

The recording performance of a digital recording system is primarily dependent upon the isolated half pulse width (T50), signal amplitude, intersymbol interference (ISI), signal to noise ratio (SNR) and the horizontal detection window margin (Tm). These characteristics are related to the thickness and magnetic properties of the media, as well as the head/media interface. This paper addresses the relationship of the media characteristics in longitudinal digital magnetic recording for both particulate and thin film media to the overall recording system performance. The results are shown to proceed from further development of the quasi-optimum time containment filter analysis for media noise dominant and electronic (white) noise dominant recording channels [1], [2].     

Basic properties of AC noise

The noise characteristics of ac demagnetized particulate media with special emphasis on correlation function analysis are discussed. The recording samples were specially prepared iron-oxide media coated on glass substrates by means of a spin coating technique. The media were nonoriented, which meant that the particles in the dried coating constituted a virgin state which is different from the ac erased state. The noise spectra for the virgin state and the ac demagnetized state appear to be equal for wavelengths longer than about 20 μm; for shorter wavelengths, the difference is about 2 dB. A correlation function measurement was made for the long wavelength region. The noise voltage was digitized and stored in a computer. After the measurements, the correlation between different noise voltages was computed. As a result, no correlation was found at all (ρ = 0) between virgin noise and ac noise. When the sample was ac demagnetized again, there was a well-defined correlation (ρ = 0.32) between successive ac noise levels. This can be explained on the basis of the ac demagnetization process.     

Media requirements and recording physics for high density magneticrecording

Relevant aspects concerning the ultimate achievable recording densities for particulate as well as for thin-film media are discussed. This review covers the entire range starting from micromagnetics of individual single domain particles, moving on to their magnetic behavior in a particle assembly under particular consideration of the structure being actually obtained in the process of manufacturing recording media, and finally embarking on an outline of recording physics. These considerations are not only carried out for longitudinally and perpendicularly oriented recording media but also for media having an arbitrary orientation of the easy axis of magnetization. All aspects are discussed and illustrated for the first commercially available thin-film medium on a flexible substrate, which is the metal evaporated tape, i.e., the obliquely deposited Co-Ni-O layer for the Hi8 video system     

Statistical analysis of signal and noise in magnetic recording

A general theory is developed for the power spectrum of the induced voltage at the read head in magnetic recording, originating from the particulate nature of the tape. The theory is general in that it yields the signal power as well as the noise spectrum, including the effects of clustered particulate media. The statistics of all relevant parameters such as particle length, particle magnetic moment, and partial penetration of the signal into the layer have been taken into account. Experiments on nonoriented Fe2O3showed a discrepancy at short wavelengths of the order of 10 dB. This is qualitatively explained by particle interaction mechanisms.     

The effect of the angular dependence of the particle nucleation field on the magnetic recording processes

The magnetic recording process is examined by using a model which easily clarifies the role of the particle nucleation field. An inverse cosine and constant angular dependence of the nucleation field are utilized to encompass measured single particle switching behavior. The model neglects demagnetization fields and is applied to contact recording on thick media. The primary conclusion is that the angular dependence affects the output of longitudinally well-oriented particles by raising the voltage maximum approximately 5 dB above that for a constant nucleation field. Vertically oriented particles exhibit large maximum voltages independent of the nucleation field angular dependence.     

Model for demagnetization-induced noise in thin-film magneticrecording media

A linear, statistical model is described which predicts the power spectrum of measured noise in bulk-demagnetized (i.e. AC-erased) thin-film magnetic recording media. It is shown that the noise is the result of magnetic flux which is ascribed to erasure-induced transitions along the track length in the medium. The noise power spectrum for a rigid disk medium is shown to correspond to the power spectrum of Poisson-distributed induced transitions along the track length, while noise along the track width is sufficiently described in terms of a uniform, average magnetization with small variance. Experimental data from two thin-film disks are used with the model to estimate the Poisson parameter for each disk. It is demonstrated that AC-erased noise from particulate media can be considered as a limiting case of the Poisson model     

Bounds on the capacity of a peak power constrained Gaussian channel

The authors present an upper bound and conjecture a lower bound on the information theoretic capacity of a digital magnetic recording channel. The channel is modeled as a linear time-invariant filter with a peak constrained input, and with additive non-white Gaussian noise at the output. The bounds are evaluated numerically using typical head and media parameters. The effects of write current, track width, media noise, and electronics noise on the channel capacity are examined     

An investigation of high-frequency bias-induced tape noise

Bias-induced tape noise remains a major limitation of the SNR in audio magnetic tape recording systems. Defined as the increment in system noise incurred when the bias oscillator is turned on, the noise can originate from a number of different causes; namely, bias oscillator harmonic distortion, magnetized heads, the earth's magnetic field, and an intrinsic noise source. The latter noise source is our primary concern here. Such record system parameters as head-to-tape spacing, gap length, bias current, and bias frequency were investigated with regard to their influence on this intrinsic bias noise source. Two models of the mechanism of intrinsic bias noise are examined. The first, the "amplitude modulation model." proposes that bias noise is generated by amplitude modulation of the recorded bias signal by the physical and magnetic variations of the head-tape system. In this model, bias noise is merely the lower AM sidebands of the recorded bias signal. The second model relates bias noise to the interaction fields in erased tape. This model proposes that these fields behave similarly to normal recording fields and can be "re-recorded" on the tape at an enhanced level. The two proposed mechanisms are examined in the light of the experimental data. The amplitude modulation model is shown to agree with all the observed data with the exception of the existence of bias noise at bias wavelengths smaller than the particle size. The second model, which does not incorporate a wavelength dependency of bias noise, is in qualitative agreement with the observed data. Methods are discussed for reducing the bias noise without materially affecting the system performance.     

Recording performance characteristics of granular perpendicular media

The recording performance of CoCrPtO granular-type perpendicular media was examined with two types of perpendicular heads to demonstrate the importance of matching head and media designs in perpendicular recording. Shielded-pole heads with high write field gradients, field angles, and sufficient write field magnitude yielded superior writability and signal-to-noise ratio as compared to mono-pole heads. The recording performance dependences on head-to-medium spacing, interlayer thickness, and soft-underlayer (SUL) thickness were also weaker with the shielded-pole heads. In addition, the effect of stray fields on the SUL domain noise was investigated for a synthetic antiferromagnetically-coupled (SAF) SUL. A radial field close to the exchange field of the SAF SUL was found to induce domain noises that could potentially cause errors in recording systems.     

L10 ordered FePt based double-layered perpendicular recording media with (002) oriented FeCo films as a soft magnetic underlayer

The introduction of the soft magnetic underlayer (SUL) in perpendicular recording technology is to further increase the recording areal density. However, problems such as growth of the uncontrollable recording layer and additional media noise contributed from the SUL could be resulted. In this work, a synthetic antiferromagnetically (SAF) coupled (002) oriented Fe₆₅Co₃₅ film as an SUL was developed for L1₀ ordered FePt based double-layered recording media. The crystallography of hetero-epitaxially grown double-layered media CrRu/(Ru/FeCo)₂/Pt/FePt/Ru was demonstrated. The L1₀ ordered FePt based double-layered perpendicular recording media with SAF coupled FeCo films as the SUL were developed.     

Thermal effect limits in ultrahigh-density magnetic recording

In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in². This paper discusses thermal effects in the framework of basic Arrhenius-Neel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speed-switching phenomena. Thickness-, temperature- and bit-density dependent recording experiments reveal the onset of thermal decay at “stability ratios” (K_uV/K_BT)₀ ≃35 ± 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in ² conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains per bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits     

Statistical study of zig-zag transition boundaries in longitudinaldigital magnetic recording

In this paper, we study the statistics of zig-zag transition walls in digital magnetic recording and their relationship to transition noise defining quantities. We provide analytic results that link the statistics of zig-zag transitions to media/recording parameters. The basis of our study is the triangle zig-zag transition (TZ-ZT) model due to its well-defined triangle zig-zag shape and its cross-track stability. The results we derive here, however, are of a general nature, and given the right interpretation, apply to other zig-zag models as well, as we show in the paper. We also provide an interpretation of the cross-track correlation width, linking this quantity to the statistics of magnetized clusters in thin-film magnetic media. The paper concludes by showing how these results can be used in media noise modeling     

Optimizing tape‐burnishing/wiping process of magnetic recording media through Taguchi method

For the ever‐increasing recording density in hard disk drives, ultra‐low gliding height media are required. It is an immense challenge for the tape‐burnishing/wiping process of media to reduce the asperities effectively and efficiently while no scratches occur. The purpose of this paper is to characterize and then optimize the tape‐burnishing/wiping process as so to minimize the asperities without leaving any scratches on the media surface. A Taguchi experiment design method is adopted to analyze the data and acquire an optimal level combination of process parameters. The resulting optimal combination is practically implemented in tape‐burnishing/wiping the several magnetic recording media, which reveals that the average pass ratio of 5 nm glide avalanche testing increases nearly doubly to a level of 96%. Copyright © 2008 John Wiley & Sons, Ltd.     

The effect of cluster size on thin film media noise

A dynamic micromagnetic model of thin film magnetic recording media with generalised structure has been used to study the effect of cluster size on thin film media noise. Clusters formed by common crystallography and by intergranular exchange coupling have been simulated, and the effects of clustering on hysteresis and noise are presented. For small clusters of a few grains, crystallographic correlations are shown to have the same effect on noise as intergranular exchange coupling, giving an increase in magnetic feature size and noise     

Digital holographic particle validation via complex wave

The inability to distinguish between particle images and noise in holographic reconstruction of dense particle fields hampers the advancement of holographic particle diagnostic techniques including holographic particle image velocimetry. We developed a method to separate particles from the noise by unlocking a unique particle signature in the complex reconstructed field. This complex-wave signature is present in digital particle holograms recorded at any scattering angle. Simulations of single and multiple particle holograms, as well as preliminary laboratory particle-field experiments, not only demonstrated the existence of the particle signature but also evaluated its ability to remove noise. Regardless of particle seeding density, scattering angle of hologram recording and particle size range, the particle identification/validation routine consistently provides >50% removal of "bad" particles and <8% of good particles.     

Low-noise magnetic force microscopy with high resolution by tip cooling

Low-noise magnetic force microscopy (MFM) was realized by using a conventional high-vacuum MFM with homemade tip-cooling equipment. The noise level of the MFM at a tip temperature of 130 K was estimated at /spl mu/N/m order. High spatial resolution of 10 nm was obtained for observing high-density recording media with recording density of 1000 kfci. The improvement of resolution by tip cooling was a result of the reduction of thermodynamic noise of a cantilever and the effective reduction of tip-sample distance due to the magnetic hardening of a tip.     

Sources of noise in erasable optical disk data storage

Noise sources in the readback signal for phase-change and magneto-optical disks at red, green, and blue wavelengths are examined, and a simple model is presented to explain the observed noise spectra. For phase-change disks the media noise, which corresponds to ~0.4% fluctuation in the disk's amplitude reflection coefficient, is the limiting performance factor for the conventional detection scheme. In magneto-optical media the depolarization noise, whose fluctuations are ~0.05% of the disk's reflection coefficient, is the major contributor to the media noise in the differential detection scheme. In phase-change optical disks the main sources of noise are the roughness of the groove profiles and the graininess of the polycrystalline recording layer. In nongrooved regions of the disk the media noise measured with green light is found to be nearly the same as that obtained with the red light. In magneto-optical disks the scattering of light from the rough groove profiles, as well as media inhomogeneities, gives rise to depolarization. Measurements on nongrooved regions of a magneto-optical disk indicate that the media noise obtained with the green light is somewhat higher than that obtained with the red light.