Alex Roesler

Experimental observations of thermally excited ferromagnetic resonance and mag-noise spectra in spin valve heads

Thermally excited ferromagnetic resonance (FMR) gives rise to so-called mag-noise in the spin valve read heads used in disk drives. In this article, an experimental method of measuring the mag-noise power spectrum density (PSD) in a frequency range of 2.5–6 GHz is presented. The thermally excited FMR modes in the spin valve and the corresponding resonant frequencies have been observed. It is found that the resonance frequencies occur in the range from 3 to 5 GHz in today’s spin valve heads at the quiescent state. Multiple resonance peaks were observed for some spin valve read heads while others only show a single resonance peak. An external field was applied to the spin valve heads to study the field dependence of the FMR spectrum. The mag-noise level at low frequencies is at the similar level of the head’s Johnson noise for heads of track width W=0.3 μm and 50 Ω resistance. It is suggested that the thermally excited FMR PSD could be used as a rapid diagnostic tool for spin valve heads.

Quantifying advanced tape medium noise

An investigation of the tape medium noise mechanism analyzed the magnetization fluctuation produced by an incremental dc erase process in a variety of modern metal-particle tape media. It examined the wavelength characteristics of the noise and compared the integrated noise during the reverse dc erase process with in situ remanent hysteresis curves. The integrated noise power followed the square of the first field derivative of the remanent hysteresis curve, (dM/dH)/sup 2/, showing that spatial fluctuation of the recording field inside the medium is a major source of the tape medium noise. It excluded spatial fluctuation of the medium coercivity as a possible noise source by comparing results from recording heads with two very different gap lengths. A quantitative analysis of the medium noise spectra at different erase currents indicated that interfacial roughness at the magnetic coating backside in some dual-layer media is a major source of medium noise. The conclusion: The main mechanism of the medium noise in advanced tape media is the head medium spacing fluctuation due to surface roughness.

Experimental analysis of tape noise

The mechanism of the tape medium noise is investigated by comparing the magnetization fluctuation resulting from an incremental DC erase process. It is found that the produced magnetization fluctuation displays contrasting behavior for recording heads with two very different gap lengths. A quantitative analysis of the medium noise mechanisms indicates that the sensitivity of the record head to spacing fluctuations explains these differences, and it is concluded that the main mechanism of the medium noise in advanced tape media is the head medium spacing fluctuation due to surface roughness.

Study of narrow tracks written with FeAlN write heads

Thin film inductive write heads with track-widths down to 0.7 /spl mu/m were fabricated by trimming wide track-width heads using focused ion beam etching from the slider air bearing surface. These heads incorporated FeAlN high moment pole material. They were observed to saturate media with coercivity of 2950 Oe. In this work, magnetic force microscopy (MFM) is used to evaluate tracks written by these heads. Sharply defined track edges were observed even for the narrow track-width heads up to high linear density. Simulations using accepted models for wide track writing agree with some of the experimentally observed trends for the submicron tracks.

Correlation of surface roughness with the recording characteristics of thin metal-particle tape

The presence of surface roughness in tape media results in a head-to-medium spacing that fluctuates throughout the recording process, which can have a large impact on the recording performance of the tape. To provide an understanding of the fluctuations introduced by the roughness, a self-consistent recording model combined with a micromagnetic MR readback module was used to study the recording (write) process on thin-layer metal-particle media with spatially correlated surface roughness. Correlation was made between the surface roughness and the recording characteristics through a spatial analysis of the recordings. Media with both long and short roughness correlation were investigated. Discussions on recording schemes that offer better immunity to the fluctuations introduced by the surface roughness are provided, and in particular the impact on write equalization is evaluated.

Understanding the effect of the tape surface on the metal-particle tape medium noise

Spacing fluctuations resulting from the head–tape interface dominate the metal-particle tape medium noise. Utilizing narrow-gap record heads, the spatial correlation of the head-medium interface has been completely and accurately characterized using a magnetic recording measurement. The measurements show that the spacing fluctuations damp with shorter wavelength, or in other words, the tape head-medium noise possesses a long spatial correlation length. In addition, the results demonstrate that the roughness-induced spacing fluctuations originate from portions of the tape surface that contact the tape, i.e., the load-bearing surface. This understanding of the head-medium noise reveals that the majority of the tape medium noise occurs during the recording process, and not during signal playback. Spatial correlation measurements on low-density (5 kfci) recordings provide verification of these conclusions. The study offers a thorough understanding of the surface roughness role on producing the head-medium noise.

Track-width dependence of medium noise in metal-particle tape systems

Using a high-resolution positioning stage to produce narrow-track recordings, we investigated the track-width dependence of metal-particle tape medium noise. Measurements showed that the medium noise power varies with the square of the track width, creating a linear relationship between the signal power and noise power. The observed track-width dependence results from a long spatial crosstrack correlation length, leading to the conclusion that the head-medium noise continues to dominate the noise in the transition, even at high-track density. Binary noise, which produces a noise power directly proportional to the track width, does not become significant even for track widths below 2 /spl mu/m.

Impact of write equalization on high-density particulate recording systems

Write equalization (WE), used extensively in tape recording systems, has offered many improvements to the recording channel. These include improved resolution, better overwrite, and reduced distortion. As linear density in these systems has increased with the use of thinner metal-particle (MP) media, which significantly improves read nonlinearity in magnetoresistive (MR) heads, it is not clear that the effects of the WE scheme are still advantageous. In this paper, the impact that WE has on high-density advanced particulate recording systems is investigated. It is found that surface roughness is much more detrimental to recordings using WE because of the higher frequency behavior and reduced channel output of the recording scheme.

Noise due to particle distributions in metal-particle media

Noise is a concern when it comes to storing data because noise can cause a corrupted signal and, therefore, lost data. Micromagnetic simulation was used to gain insight and understanding of particulate media noise due to nonuniform particle distribution. These distributions result in a spatial fluctuation of the anisotropy field and orientation of the particles, which introduces randomness into the recording process. The effect of the randomness (i.e., noise) on the recording process was studied, as were effects of reduced media thickness and improved particle alignment on the noise level and record nonlinearity. Effect of the noise on write-equalized recordings was also investigated. The focus of the paper is on thin metal-particle media.

Evaluation of MP tape packing homogeneity through depth-probing

The particle packing of MP tape media is explored by means of a new measurement technique. Utilizing a narrow gap length record head, the tape is evaluated throughout its thickness with the use of an incremental DC erase measurement. A quantitative analysis of the medium noise spectra at different erase currents allows for the packing fluctuations at different depths into the tape to be proportioned. The technique provides a method for evaluating the interface between the magnetic recording layer and non-magnetic underlayer in dual-layer tape, and measurements indicate that interfacial roughness at the magnetic coating backside in some dual-layer media is a major source of medium noise.