IEEE TRANSACTIONS ON MAGNETICS, VOL. 26, NO. 6, NOVEMBER 1990 3001

DESIGN ISSUES FOR PRACTICAL RIGID DISK MAGNETORESISTIVE HEADS

P. - I. Bonyhard IBM General Products Division

5600 Cottle Road, San Jose, CA 95193, USA

Abstract-Some of the issues of MR head de- sisn are considered from the point of view of practical, high reliability, low cost, manufacturable heads for rigid disk files.

INTRODUCTION

The major advantages of magnetoresistive- read ( M R ) heads over single element inductive thin film (TF) heads are that the MR head gives an ideal, easy to equalize output waveshape, can be used on disks with high coercivities for high linear densities, and is less subject to stability problems at nar- row trackwidths. The major advantages of M R heads over metal-in-gap ferrite (MIG) heads are in narrow trackwidth and high frequency potential. Additional advantages of MR heads over any inductive heads include larger out- puts, especially at lower disk speeds, and additional potential to design for better offtrack performance by utilizing narrower read heads. Disadvantages of MR heads in the rigid disk environment include concerns about reliability common to any new technology and, more specifically, possible environmental fragility, loss of performance due to imperfect alignment between the read and write elements and/or due to signal asymmetry, and the possibility of higher cost/price.

MECHANICAL INTEGRITY

Issues of mechanical integrity are the most important foqheads with the M R sensor stripe having one edge exposed at the air bearing surface (ABS), especially in conjunction with particulate disks. Potential problem areas include partial electrical shorting of the sensor by the smearing of conductive material, and thermally induced spurious output signals [ I ] . Smearing effects may be contained by us- ing a ferrite slider as the first shield [ 2 ] , and by using less ductile materials exposed on the ABS. Spurious thermal signals are dif- ficult or impossible to contain by head design within the bounds of simpler, i.e., other than differential designs and, in practice, must therefore be generally contained by channel designs tolerant to, or at least able to re- cover from the occurrence of thermal signals [3]. Adequately low electromigration failure rates can be achieved by trading off current

density and temperature rise against output amplitude by a proper choice of stripe height and thickness. Resolution is also a factor in the electrical reliability tradeoff through both the read gap and the disk Mr.t. Stripe temperature rise is more strongly affected by read track width than by read gap, which makes achieving narrower read heads somewhat less difficult .

PERFORMANCE ASSESSMENT PARAMETERS AND THE DESIGN PROCEDURE

In assessing the performance of MR heads, the important perameters are ontrack output amplitude, resolution, asymmetry, and stabil- ity, all of which, together with disk and channel related parameters, determine the ontrack error rate. Offtrack performance, usually characterized by the distance the head can move off track before some specified un- acceptable error rate is reached, is equally important. The majority of these parameters play the same role in MR as in inductive head design. The role of resolution, however, is unlike that for TF heads, as the latter ex- hibit undershoots in the isolated output pulse waveform in positions corresponding to the outer poletip edges. As the undershoots in general make equalization more difficult and less effective, a lower unequalized resolution is acceptable in M R heads, as it is in MIG heads, than in TF heads for equivalent equal- ized performance. The role of signal asymmetry is also different, in that such asymmetry is inherent in most MR heads. For a practical, high yield, low cost MR head design it is es- sential that the effective flux input into the sensor be kept low enough to avoid saturation effects. This must be accomplished by the combination of keeping the disk Mr.t low, the read gap thickness also low, and sensor stripe thickness sufficiently high. The sensor stripe thickness affects saturation, as almost all the flux that enters the gap between the two shields is concentrated in the sensor stripe, so that the effective magnetic field inside the stripe is approximately inversely propor- tional to the thickness of the stripe. As all the measures aimed at avoiding head saturation tend to lower the output amplitude, this will be done at the expense of performance. Low read gap and/or low disk Mr.t will, however,

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improve resolution, so that the loss of per- formance is, to some extent, offset. In many practical situations the resolution is more sensitive to read gap than to disk Mr.t, and in this case it is good to design for the lowest read gap that is feasible (usually limited by the increased occurance of sensor to shield shorts). The disk Mr. t can then be ma& as high as possible without compromising resolution which, of course, implies that the disk coercivity is also made as high as pos- sible, as limited either by writeability or by disk availability. Finally, the MR film thickness is made as low as possible, limited either by process control or, more likely, when this design approach is followed, by head saturation. Avoiding the need for very thin MR films is an advantage of this procedure. Magnetic stability is, of course, a major concern for any new head technology, but it has not been found to be an overwhelming is- sue, provided some design precautions, as discussed below, are taken.

The written track width (magnetic core width) is not affected by the read width, so the latter can be chosen so as to trade off ontrack error rate against offtrack perform- ance. There is a detailed discussion of this topic by the Author [4]. Offtrack performance is greatly affected by read-to-write head alignment, a parameter with no equivalent for single element heads. It is sometimes asserted that the misalignment is similar in nature to the other components of mechanical track mis- alignment (TMR), so that its contribution to error rate may be taken ito account adequately by statistically adding read-to-write head misalignment to TMR. If this were the case, misalignment would be a relatively minor per- formance detractor in most practical cases. Unfortunately, however, in most practical systems each individual head/disk combination must pass some error rate and/or offtrack performance criteria, regardless how good the average error rate of the whole subsystem is, and this tends to result in a stringent alignment requirement for all heads. One pos- sible way to relax this requirement is by the utilization of a "microjog" that positions the head nearly optimally for reading the track written with the known offset of the imperfectly aligned write head. This, however, compromises performance, especially in the case of the count-key-data (0) architecture, to a point that may be unacceptable. Conse- quently, the imperfect alignment must be com- pensated for by the provision of additional offtrack capability, and by the detection and elimination of less well aligned wafers early in the process, both of which tend to reduce the performance advantage of MR heads.

READ HEAD PERFORMANCE

In order to assure consistent good performance in the read head, the direction and amplitude of the magnetic anisotropy in the MR film must be accurately controlled [5]. This, in addi- tion to depositing the film in the presence of a highly homogenous bias field, implies stfingent control of magnetostriction [5], (61 . Magnetic stability can be achieved by the use of the well known "barberpole" structure (71, (81, or by controlling the magnetization at the boundaries of the device by shape anisotropy [9], or by the use of exchange coupling (lo], [ l l ] to inhibit multidomain states. Biasing of the sensor may be achieved also by the m barber pole"^ structure, or by a current shunt [12], or by the use of an adja- cent soft film [13]. In the latter case, the active region of the soft film is controlled by the bias current. The soft film wings may also be controlled by exchange, but this adds complexity. The soft film wings can be alter- natively controlled by the magnetostatic cou- pling between the MR film and soft film magnetization components parallel to the long direction of the sensor stripe. Coupling of adequate strength is achieved when the length of the inactive sensor wings are kept short [14], in addition to exercising the same care in anisotropy control as for the M R film. Magnetic states in the shields have been found to have little effect on performance or sta- bility, as long as the shields have adequate permeability 1151.

WRITE HEAD PERFORMANCE

In a practical MR head, a "trouble free" write head is important. The combined yield of the two elements will not be sufficiently high, unless one of the two causes essentially no yield losses. Also, for the "piggyback" write head [ 1 6 ] the control of the throat height is generally poor compared to TF heads, as the heads are lapped to a sensor stripe height target, so that the throat height tolerance range is broadened by the uncertainty of zero throat placement in addition to the uncer- tainty of lapping. For sufficient overwrite, it is desirable to design the write head with a large gap and thick (long) poletips. The gap length is limited by sidewriting effects and/or poor resolution due to a low field gradient. The poletip thickness is limited by pracessing considerations and by eddy current losses at high frequencies. The latter, how- ever, are not significant at data rates of about 4.5 MB/s for poletip thicknesses up to 5 um. For particulate media, resolution prob- lems also occur when the write field is too high, (171, and adequate control of the write

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field can be accomplished only if the head is ,operated below saturation [18]. For thin film media avoiding high drive fields is less critical. Thick poletips, no shaping layers, single layer coils, and a large excess in the first poletip width, P1W over the second poletip width, P2W in order to ease alignment requirements results in an easy to build write head with a nominal overwrite much higher than the acceptable minimum, for high yield and low cost .

CONCLUSION

The use of MR heads for rigid disk drives offers significant performance advantages. Many new challanges must be met in order to make the technology practical. For maximum benefits, novel design procedures need to be employed.

ACKNOWLEGEMENTS

Thanks are due to many of the Author's colleagues who kindly consented to some of their unpublished work being referred to in this paper, including those mentioned in Ref- erences [14] and [15]. Special thanks are due to L. L. Rosier for his valuable help and ecouragement with the writing of the paper.

REFERENCES

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[3] For example: S. A. Jove, K. B. Klaassen, "Protective Circuit for Magnetoresistive Element", U. S. Patent 84,879,610, 1989.

[4] P. I. Bo+nyhard, "Magnetoresistive Read Magnetic Recording Head Offtrack Performance Assessment", Paper FR-01, 1990 Intermag Conf. , to be published.

[5] Discussed in: R. Simmons, et al., "Anisotropy Changes in Magnetores istive Heads Due to Lapping and Annealing", IEEE Trans. Magn., vo1.25, pp.3212-3214, 19f9.

[a] D. Markham, N. Smith, The Influence of Substrate Edge Stress on Magnetoresistive Head Anisotropy", IEEE Trans. Magn., vo1.24, pp.2606-2608, 1988.

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[lo] R. D. Hempstead et al., "Unidirectional Anisotropy in Nicke 1 -Iron Films by Exchange Coupling with Antiferromagnetic Films", IEEE Trans. Magn., vol.MAG-14, pp.521-523, 1978.

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[13] T. Beaulieu, D. A. Nepala, "Induced Bias Magnetoresistive Read Transducer, U. S. Patent #3,864,751, 1975.

(141 D. E. Heim, D. M. Hannon, T. J. Gallagher, unpublished.

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[16] C. H. Bajorek et al., "An Integrated Magnetoresistive Read, Inductive Write High .Density Recording Head", 20th Ann. AIP Conf. Proc., no.24, pp.548-549, 1974.

[17] J. S.-Y. Feng, R. D. Harper, "Pulse Distortions in Transitions Recorded with Thin Film Pole Tips", IEEE Trans. Magn., vol.MAG-23, pp.2082-2084, 1987.

[18] J. K. Alstad, P. I. Bonyhard, "Effect of Permalloy Thickness on the Overwrite Per- formance of Thin-Film Heads", Paper FB-04, 1987 Intermag Conf.