Reciprocity principles for magnetoresistive heads

A generalized reciprocity principle for magnetoresistive (MR) heads is derived. The results apply primarily to the linear (small signal) reproduce performance of any MR head type (shielded or unshielded, single or multiple active MR elements) of arbitrary three-dimensional geometry. Reciprocity principles provide a powerful technique for calculating the intrinsic sensitivity function that is characteristic of a given MR reproduce head, and which in principle contains all information regarding the linearized reproduce response of the MR head with respect to any record medium of arbitrary recorded magnetization. The computational work required to evaluate the sensitivity function via reciprocity is a small fraction of that which would be required by direct means     

Long wavelength response of elliptical reproduce heads

Long wavelength response characteristics for the class of elliptical reproduce heads are calculated. Simple series solutions are obtained for a flat recorded medium, and numerical methods are used for a tape wrapped over the head at arbitrary contact angles. Special eccentricities of the ellipse yield classical results. Many geometries lead to low frequency bumps on the response curve.     

Accurate approximation of fields and spectral response functionsfor perpendicular recording heads

Solutions for the magnetic potential in rectangular regions with simple boundary conditions are used to approximate the two-dimensional potential in the head face plane of a recording head. This method of approximation can be applied to any recording head with rectangular corners, in the presence of an underlayer. The slot approximation is demonstrated for ring heads and shielded (magnetoresistive) pole heads. The quarter plane approximation is given for application to finite-length heads. Estimates of the field components and the spectral response function are derived from the approximate potential. The simple slot approximation is shown to be at least as accurate as other, often more complicated, methods of approximation. Most significantly, both the slot and the quarter-plane approximations are easy to apply and need no prior knowledge of the head field of the particular geometry under consideration     

Writer pole tip remanence in perpendicular recording

We studied the remanent field from perpendicular writer pole tips by micromagnetic modeling. The pole tip remanence has two causes: 1) the residual flux from the yoke due to undesirable material properties and/or yoke design and 2) the geometry of the pole tip. An optimal yoke design for low remanence includes a wide and short yoke with relatively shallow yoke flare angle around 30/spl deg/ that favors transverse major domains. Horizontal anisotropy in the yoke and the pole tip also helps suppress the vertical magnetization, mainly in the yoke, which yields less residual flux into the pole tip, hence lower remanence. The remanent field is also a strong function of the throat height, i.e., the pole tip length, due to the shape anisotropy effect. Micromagnetic modeling shows that multilayer lamination, both in the yoke and in the pole tip, is an effective way to achieve low remanence. It also eliminates the sensitivity of remanence to the throat height. To improve the robustness of the remanence against the stray field, an antiferromagnetic coupling (AFC) between the lamination layers may also be necessary. Our results apply to both single pole heads and shielded pole heads.     

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.     

Frequency response function of a gapped infinite length pole piece recording head

The frequency (wavelength) response function of a gapped infinite length, infinite permeability magnetic reproduce head is calculated analytically.     

High bit density (100 Mb/cm2) with single-layer Co-Crmedia and ring heads in perpendicular magnetic recording

The signal and noise of single-layer Co₇₉Cr₂₁ media are measured with ring heads to estimate the area density that can be achieved. Densities as high as 100 Mb/cm² (1 μm²/bit) are expected when a signal-to-noise ratio sufficient for an error probability less than 10 ^-5 is required. As a comparison, densities estimated from data from metal-evaporated tape and CrO₂ tape are given. In the frequency response of the single-layer media, an additional minimum was observed for a wavelength slightly larger than the gap length. This is probably caused by the bipolar nature of the perpendicular recording field of a ring head     

Monte Carlo simulation of thin film head read-write performance

Read-write simulations of thin-film head longitudinal recording performance with thin-film disks are described. Acceptable electrical performance of thin-film heads depends on tight control of numerous parameters, and Monte Carlo simulation is used to quantify read-write sensitivity to head variations and to identify the critical parameters. A population of similar heads is assumed, and ten head parameters are subjected to random variations. The resultant electrical performance is dominated by head-disk spacing, with throat height, gap length, trackwidth, and pole thickness following in order of importance. Reasonable variations in coil resistance insulation thickness, yoke thickness, magnetic permeability, or inductance show relatively little influence on recording performance     

Record characteristics of tape heads with CoZr and NiFe poles

Overwrite and recorded signal are compared for tape heads with Co ₉₁Zr₉ and Ni₈₁Fe₁₉ poles as a function of throat height. The effects of throat height variation on performance are discussed for constant current recording with and without pole saturation. The impact of pole saturation on throat height tolerance is also discussed. Co₉₁Zr₉ and Ni₈₁ Fe₁₉ tape heads have equivalent record performance as long as their poles are not saturated. When pole saturation does affect head performance, it has less impact on CoZr heads, and they record and overwrite better than NiFe heads. In a system that requires at least 25-dB 2f/1f overwrite at 2362 fc/mm (60 kFCI), for example, both CoZr and NiFe heads would perform well on oxide tape over a wide range of throat heights. On metal particle tape the overwrite falls below 25 dB at a throat height of 4.4 μm for the CoZr heads and 2.3 μm for the NiFe heads. These throat height limits could be extended with thicker poles or, in the case of NiFe, plated poles that maintain a uniform thickness on sloped areas of the underlying films. Nonetheless, for the conditions described here CoZr heads had nearly twice the throat height tolerance of otherwise identical NiFe heads     

Alternate pole materials for MR tape heads

We have investigated the suitability of a number of magnetic films for use as a pole material in MR tape heads. These include nickel iron, cobalt tantalum zirconium, and two cobalt iron alloys. Heads were fabricated using these alloys as pole materials. The heads were tested both for their electrical performance and for their wear characteristics     

Three dimensional simulation of side-writing and crosstalk for thinfilm heads with various pole shapes

Side-fringing fields of thin-film heads are studied with the three-dimensional simulation code TOSCA. Readback signals are calculated using the principle of reciprocity for various pole shapes and off-tracks. Read/write characteristics are evaluated in the presence of side-writing and crosstalk. They depend on the shape of the interface between the pole and the air-bearing surface of the sliders. The side-writing is large when the side angle is small. Crosstalk is large when the side angle is small and the projection of the pole from the track edge is large. A pole edge shape on the air bearing surface of thin-film heads that produces a narrow head field outside of the track and a sharp head field gradient is concluded to be essential to achieve large side-writing and small crosstalk     

Maximum signal-to-noise ratio of a tape recorder

Using the Wiener autocorrelation theorem, the noise power spectrum of the pole strength in a thin lamina of an erased tape is shown to be approximately white. The noise power spectrum of the reproduce head voltage is calculated for a thick tape and compared with the signal power. The wide-band signal-to-noise ratio of a tape recorder equalized flat is deduced and expressed in very simple forms, which are inversely dependent upon the square of a bandwidth. Notably, in this special case the wide-band result is independent of reproduce head-to-tape spacing. Numerical examples demonstrate that this simple theory yields results in excellent agreement with practice.     

Film-head pulse distortion due to microvariation of domain wallenergy

Experiments show that the location of spurious peaks on the trailing edges of data pulses is strongly influenced by the application of small DC bias currents to the head coil during readback. A simple model of the head response which includes both magnetization rotation and domain wall motion for the case where domain wall coercivity is comparable to signal fields in the head reproduces many of the features of the observations. Calculations show that fields in pole tips during reading can exceed the wall-motion coercive force of permalloy. It is demonstrated that the resulting domain wall motion could give rise to the spurious signals sometimes seen in thin-film heads. It is proposed that such heads contain domain walls parallel to the signal flux path. The observed oscillations in the spurious response are primarily caused by filter dynamics, not domain wall dynamics     

Diffraction theory of optimized low-resolution Fresnel encoded lenses

A mathematical model describing the behavior of low-resolution Fresnel encoded lenses (LRFEL's) encoded in any low-resolution device (e.g., a spatial light modulator) has recently been developed. From this model, an LRFEL with a short focal length was optimized by our imposing the maximum intensity of light onto the optical axis. With this model, analytical expressions for the light-amplitude distribution, the diffraction efficiency, and the frequency response of the optimized LRFEL's are derived.     

Adaptive asynchronous algorithm for fringe pattern demodulation

We present a spatial adaptive asynchronous algorithm for fringe pattern demodulation. The proposed algorithm is based on the standard five-step asynchronous method with the one modification that we select the best sample spacing for each point of the fringe pattern. As we show, the frequency response of any asynchronous method depends on the sample spacing. This interesting behavior is used to select the best sample spacing as the one that gives the biggest response for each location. The overall result is a spatial demodulation algorithm with an improved frequency response compared to the existing ones. We show the feasibility of the proposed method with theoretical analysis as well as experimental results.     

Irreversible wall motion in inductive recording heads

The effect of signal-induced irreversible domain wall motion on the isolated transition response of recording heads is investigated. An experimental method is presented allowing differentiation between the effects caused by irreversible wall motion and reversible wall motion. The method, utilizing the injection of a small DC current into the head coil, can also be used to determine whether the irreversible motion in a thin-film head occurs in the first pole tip, the second pole tip, or the backgap. The transients observed result from signal-induced irreversible wall motion in the second pole tip. The largest isolated impulse variations occur after saturation of the yoke. In addition, it is found that Barkhausen transitions can occur at or near the top of the isolated impulse (even on its leading edge) as well as after the impulse has decayed, and not only on the trailing edge of the impulse as reported previously. Finally, the authors observe two Barkhausen-noise-free zones, the one at the leading edge being the largest     

Thin-film inductive heads for perpendicular recording

A modified thin-film magnetic head for perpendicular recording in rigid disk drives with improved read/write characteristics, especially at high areal bit densities, is presented. The head on which the modified design is based is described. It combines the advantages of single-pole heads and thin-film heads, writing with the sharp field edge of the leading pole and reading like a thin-film head. To increase the writing efficiency and improve the yield, the sequence of magnetic layers in the head is changed; the second layer of the four-layer head is embedded in the substrate, where it can be placed much closer to the pole tip of the first layer. The improved write capability depends mainly on the position of the embedded layer. In addition, there results an improved magnetic flux guidance from the embedded layer into the pole tip layer, providing the potential for a significant improvement in fabrication yield. The embedded-layer approach also allows a further increase in areal density. The results of read/write tests and the write-wide and read-narrow characteristics are presented     

Effect of pole-tip size on perpendicular recording head remanence

To aid understanding the physical mechanisms responsible for the perpendicular write head remanent field and thereby improve the write head design, we studied the remanence of single-pole perpendicular recording heads with trailing shield by using a contact scanning recording tester. We used two different pole-tip size heads in the study. We found that the remanence areas of the heads tested usually locate at the trailing edge corners of the pole tip. We also found that the remanence areas of both types of heads are also the head field easy-rising areas when the heads are driven by low write currents. Additionally, the current level that leads to remanence is the same for both types of heads. However, the remanence of the small pole-tip heads can be demagnetized by a much lower amplitude reverse current than that of the large pole-tip heads.     

Total magnetic fields and read characteristics of infinite length pole piece magnetic recording heads

Magnetic recording head fields generated by current distributions of zero total current flux are calculated considering the two topological possibilities for the current elements being either external to infinite permeability pole pieces or contained entirely within the pole pieces. The frequency response for each of these field configurations is determined by application of the fast Fourier transform.     

Comparison of the magnetic recording performance of medium-scale ring, double-sided pole, metal-in-gap heads on high-coercivity longitudinal media

The magnetic recording performance of medium-scale ring, double-sided pole, and metal-in-gap (MIG) heads were investigated on standard longitudinal high-coercivity floppy, video tape, and isotropic floppy media using a stretched-surface multi-head magnetic recording tester. The isolated pulses for the ring and MIG heads exhibited sharp peaks near tbe edges of the gap for a head-medium separation of less than 0.1 um. Low density maximum output voltage was independent of the type of the write head. The largest output voltage was achieved from the MIG head. The isolated outputs on video media agreed with calculated values for the ring head efficiency set to 52% and the pole head efficiency set to 33% using the Karlqvist head field approximation.