Unshielded Magnetoresistive Head Research Articles

Analytical formulas for the unshielded magnetoresistive head

The magnetostatic and frequency response characteristics of the unshielded magnetoresistive (UMR) head have been previously calculated by means of a numerical simulation which utilized the principle of reciprocity to reduce the computational task of solving the relevant micromagnetic equations. The results of the simulation were found to be in excellent agreement with measurements made on actual UMR heads [N. Smith and D. Wachenschwanz, IEEE Trans. Magn., MAG-23, p. 2494 (1987)]. In the present paper, we analyze an idealized UMR head, following in spirit the procedure outlined in the previous work; however, we formulate the problem by introducing simplifying approximations which retain the essential physics, deriving simple equations describing the bias magnetization distribution, and the frequency response. The equations obtained in this way are analytic and simple, yet in agreement with the micromagnetic simulation and experimental measurements. Furthermore, the analytic formulas provide a source of insight which may be of use in the design or optimization of UMR recording heads.

Contact magnetoresistive head for perpendicular magnetic recording

Contact-type magnetoresistive (MR) inductive head designs and recording characteristics have been discussed in perpendicular magnetic recording. The shielded-type MR head offered a high D50 of 100–135 kFRPI, and the unshielded MR head had the largest output in a low density region. However when the recorded bit length became shorter than the shield-to-shield spacing, the shield had little effect and the output of the shielded head was no longer almost equal to that of the unshielded MR head. In result, the shielded-type and even unshielded-type MR heads have good high density recording characteristics in the head-to-medium contact state.

Signal processing for single and dual element unshielded magnetoresistive heads

Single and dual element unshielded magnetoresistive (UMR and DMR) heads have potential for extremely high density magnetic recording, but saturation and unusual readback waveforms must be dealt with. Two new peak detector channels take advantage of the unusual waveforms: (1) The quasi-integrating channel for the UMR, whose output is similar to that of integrating channels with inductive read heads. Performance may be enhanced by adding short pulses to the write current at distances greater than T/sub min/ from code 1 transitions. (2) Controlled polarity recording with decision feedback, which is a special case of write equalization where successive pulses may have the same polarity. The detection window is twice that of conventional recording. Simulated performance is comparable to PRML channels. >

(1+D)/sup n/ partial response channels for UMR heads

Most partial response channels described in the magnetic recording literature are characterized by the (1-D)(1+D)/sup n/ pulse response polynomial, where n=1 for PR4 and 2 for EPR4. Partial response channels with (1+D)/sup n/ pulse response polynomials, which include partial response classes 1 and 2 (PR1(1+D) and PR2(1+D)/sup 2/), have received scant attention because magnetic recording systems lack the DC component implicit in these polynomials. A new channel developed for the unshielded magnetoresistive (UMR) head that closely approximates a (1+D)/sup n/ channel at all but the very lowest frequencies and works well when used with DC-free modulation codes is discussed. This channel is an excellent match to the UMR head response, which has greater long wavelength response than the more familiar inductive and shielded MR heads. >

Magnetoresistive heads for magnetic tape and disk recording

The beginnings and important developments in magnetoresistive (MR) head technology are recalled, starting with the unshielded MR head by R.P. Hunt (1970, 1971). The advantages and shortcomings of the Hunt head for data recording applications are identified. Head structures that made possible the introduction of MR heads in tape storage in 1985 are shown. A general problem set is described for the developers of MR heads for either tape or disk storage, and a variety of solutions are identified for each problem from the patent and technical literature. The course of future progress in realizing and improving MR heads for both tape and disk storage is predicted. >

Signal processing in recording channels utilizing unshielded magnetoresistive heads

Because unshielded magnetoresistive (UMR) heads offer excellent short-wavelength sensitivity, immunity from Barkhausen noise, a velocity-independent readback signal, and simple construction, they were chosen for use in a new high-performance, multichannel tape system that operates at 80 kbit/in with a 1.5-mil track width. UMR heads have two properties that present a challenge to system designers: (1) a tendency to saturate with large applied magnetic fields, and (2) a high sensitivity to long-wavelength signals and noise which corresponds to an isolated pulse that extends far beyond the system's minimum transition time. These problems were solved through a combination of write and read equalization. System performance, measured by amplitude and timing margins, is shown to be excellent. In selecting write equalization parameters for amplitude-qualified peak-detecting systems, both amplitude and timing margins are significant. Write-equalized pulses that superficially appear to be highly irregular can frequently be effectively read equalized. The only caution in using write equalization is that write current risetime must be short enough to fully record the added pulses. This should not be a problem in systems with separately optimized write and read heads. >

Improved bias and stability in dual-exchange-biased unshielded magnetoresistive heads

Data are presented on dual-exchange-biased NiFe-TbCo UMR (unshielded magnetoresistive) tape heads with improved signal response and process stability. It was found that a Si/sub 3/N/sub 4/ passivation layer offered better stability of the exchange bias through head processing than SiO/sub 2/. In order to improve sensor sensitivity, the longitudinal component of the exchange bias was reduced by increasing the exchange angle to 75 degrees from the sense current direction. This resulted in a +6-dB improvement in signal over previous heads without sacrificing Barkhausen-noise-free operation in narrow trackwidths. Heads with trackwidths as narrow as 8 mu m were shown to operate at linear densities of up to 50 kFCI with excellent signal-to-noise ratios (>45 dB). Investigations of different passivation materials showed Si/sub 3/N/sub 4/ to provide the best protection against environmental corrosion and thermally induced oxidation of the TbCo layer. >

Dual exchange biased NiFe-TbCo unshielded MR heads for high density recording

Narrow-trackwidth unshielded MR (magnetoresistive) heads with both longitudinal and transverse bias supplied by a single TbCo exchange-bias layer were constructed. These preliminary dual-biased NiFe-TbCo MR heads exhibit Barkhausen-noise-free, low-distortion response characteristics over a wide range of recording densities (up to 80 kFCI). A 64- mu m track width, 7- mu m height dual-exchange-biased element exhibited a phase margin of 50% at a raw bit error rate of 10/sup -5/ in a recording channel reproducing

The influence of substrate edge stress on magnetoresistive head anisotropy

It is found that the film anisotropy H/sub k/, in an unshielded magnetoresistive (UMR) sensor is changed after the establishment of a tape-bearing surface. It is demonstrated experimentally and by a theoretical 2-D stress/micromagnetic treatment of the UMR head configuration that the undesirable change in H/sub k/ is caused by the interaction of an anisotropic stress localized in the vicinity of the lapped tape-bearing surface with the UMR element, which inevitably possesses nonzero magnetostriction. The theory predicts that the change in H/sub k/ can be minimized by minimizing the thickness of the SiO/sub 2/ layers used above and beneath the UMR element. by this means and by minimization of MR element magnetostriction, the intrinsic high sensitivity of the UMR sensor can be preserved in a lapped head. >

Unshielded magnetoresistive heads in very high-density recording

Unshielded magnetoresistive (UMR) heads provide very high signal levels and low noise but, because of their relatively large element height and an insensitive "dead zone" at the sensor edges, they have poor resolution. As a consequence, the signal diminishes dramatically as the recorded density increases and may be as much as 30 dB or a factor of 30 down at very high density. Various techniques have been used to increase the resolution and reduce the "peak-to-bandedge" ratio but they all reduce the bandedge signal as well and hence tend to lower the signal-to-noise ratio. We have found that a peak-to-bandedge ratio of more than 30 dB can be equalized and hence the standard UMR described by Hunt can be used to advantage in very high-density recording. This report describes results obtained with a UMR head reproducing 80 kFCI (3150 FC/mm) signals recorded on Kodak Isomax tape. Bandedge signal and low-density distortion were plotted versus bias field. Surprisingly, maximum high-density signal and minimum distortion occur at about the same bias field. Electronic, thermal, and magnetic noise were measured and tape-noise-limited performance was obtained. Equalized signals from a pseudo-random data sequence were examined with a transition interval analyzer as well as by eye pattern photograph. The transitions were well separated, and the eye pattern was well defined in both phase and amplitude.