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MAGNETORESISTIVE (MR) HEADS and the EARLIEST MR HEAD-BASED DISK DRIVES: SAWMILL and CORSAIR Christopher H

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MAGNETORESISTIVE (MR) HEADS and the EARLIEST MR HEAD-BASED DISK DRIVES: SAWMILL and CORSAIR Christopher H MAGNETORESISTIVE (MR) HEADS AND THE EARLIEST MR HEAD-BASED DISK DRIVES: SAWMILL AND CORSAIR Christopher H. Bajorek, Storage Interest Group Computer History Museum, Mountain View, CA (Updated November, 2014) Why it is Important Hard disk drives (HDDs) have survived as the information storage device of choice because of the sustained and steep advances achieved in storage density. Density advances required downward scaling of the key components. The resulting smaller head-disk spacing and reduced magnetic fields from the disk were the key technology challenges. The reduction in the size of stored bits caused signal amplitude reductions which necessitated the invention of more powerful detectors. Magnetoresistive (MR) heads were the successful answer. These new film heads, together with new film disks, provided successful components for future HDDs up to the present time. They solved the joint challenge of ultra small signals and ever-reduced spacing of the heads and disks. Since 1990, this combination improved data density, with concomitant cost per bit reductions, by approximately 20,000 times. Two companies led the industry in independently developing MR heads, IBM and HP. IBM was first to market in 1990. HP was second in 1994. Seagate also benefited from the HP heads since 1995. The rest of the industry started using MR heads in 1996. Magnetoresistive (MR) Heads Prior to 1990, all hard disk drives used the same inductive head for reading and writing. The readback signal amplitude from an inductive head is proportional to the magnetic flux captured from a stored bit, to the number of turns of wire wrapped on the head, and to the velocity at which the head flies on the disk. There are engineering limits to the number of turns and the disk velocity, so as bits on the disk became smaller, with correspondingly smaller magnetic flux to couple to the head, the readback signal from an inductive head became so small that the signal-to-noise ratio became inadequate. Magnetoresistive heads use an entirely different method of readback. A tiny electrical resistor inside the head changes resistance in the presence of changing magnetic flux from the disk. This is independent of the disk velocity, so the signal is the same at any speed. The size of the signal is proportional to the percentage change in resistance, and to the amount of energy that the magnetoresistive element can dissipate without damage. As it turns out, the resultant signal can be much larger than that from an inductive head. 1 HDDs have used three classes of MR heads. Each new class provided a larger percentage change in resistance with magnetic field. Anisotropic magnetoresistive (AMR) heads were first introduced in 19901, giant magnetoresistive (GMR) heads were introduced in 19972, and tunneling magnetoresistive (TMR) heads were introduced in 2004.3 AMR heads are based on the phenomenon of anisotropic magnetoresistance, first discovered by Lord Kelvin in 1856. The effect involves a resistance change in an electrical conductor produced by application of an external magnetic field. Lord Kelvin found in experiments with iron and nickel conductors that electrical resistance increases when the applied magnetic field is parallel to the conductor, but resistance decreases when the magnetic field is perpendicular to the conductor. The changes were about 5% in these experiments. GMR heads are based on the phenomenon of giant magnetoresistance, discovered by Albert Fert and Peter Grunberg in 1988 (recognized by the Nobel Prize in Physics in 2007). GMR relies on alternating layers of magnetic and non-magnetic conductive materials. When nearby magnetic layers are magnetized in parallel, electrical resistance is lower than when layers are magnetized in opposite directions, which increases the observed resistance change effect to about 10%. TMR heads are based on the phenomenon of tunneling magnetoresistance. John Slonczewski was first to propose in 1974 that magnetic tunnel junctions could be used as a “Magnetic Gate” to detect magnetic fields such as the fields from a magnetic bubble or magnetic recording transition.4 TMR involves electrons moving through an insulating barrier between magnetic layers (a quantum mechanical tunneling effect), resulting in an even larger resistance change effect, well in excess of 100% today, between the parallel and anti parallel orientations of the magnetizations of the films. It is this latter technology that is currently used in most disk drives. A key characteristic of MR heads is that such heads enabled dual element magnetic recording heads, with one inductive (non-MR) element optimized for writing data and a second magnetoresistive element optimized for reading data. The ability to separately optimize these two head functions allowed the attainment of much higher magnetic write fields as well as much sharper read head resolutions than would have been possible with the previously used, single element inductive heads used for both writing and reading. The sequential use of the AMR, GMR and TMR phenomena enabled a systematic increase of the sensitivity of the read element over time. The first MR heads were used in the longitudinal magnetic recording (LMR) configuration. The separate write head allowed capitalizing on the full potential of LMR 1 IBM 9345 HDD. 2 IBM DTTA-351680 (Deskstar 16GP) HDD. 3 Seagate 100 GB Backup Plus circa 3Q 2004. 4 John Slonczewski, Presented at IBM technical meeting on January 22, 1974, personal communication. 2 film disks by supporting about a 10-fold increase in the magnetic field used to write the disk. This high intensity write field allowed use of magnetic disks with higher resistance to demagnetization (higher coercivity), resulting in a corresponding increase in stored energy seen by the read head. This, in turn, allowed the use of smaller bits. The most recent MR heads, the TMR versions introduced in 2004, have been used in the perpendicular magnetic recording (PMR) configuration. A PMR configuration superior to LMR would not have been possible without dual element MR heads. Diagrams depicting the GMR heads used for LMR and PMR can be found in Figure 1 of a recent white paper published by scientists at Hitachi.5 This figure is reproduced below. The diagrams in this figure would also apply to depicting AMR and TMR heads by substituting these terms for the read element labeled “GMR Sensor”. 5 R. Wood, Y. Hsu and M. Schultz, “Perpendicular Magnetic Recording Technology”, white paper: http://www.hgst.com/tech/techlib.nsf/techdocs/F47BF010A4D29DFD8625716C005B7F34/$file/PMR_whi te_paper_final.pdf 3 The aforementioned progress also enabled dramatic miniaturization of HDDs, from 5 ¼ in., to 3 ½ in., to 2 ½ in., to 1.8 in. and to 1.0 in. devices, while delivering extraordinary data capacity to end users in the smaller form factors. HDDs capable of storing several terabytes of data are the norm today. R. P. Hunt of Ampex was the first to propose the use of unshielded, Permalloy-based MR read elements for sensing of magnetically recorded data in 1966.6 But, the unshielded option had linear data density limitations because the minimum bit spacing was comparable to the read element’s height.7 This limitation notwithstanding, Hunt-type MR heads, packaged in hand-held wands, were commercialized by IBM in 1975, for use as readers of magnetically pre recorded price tags in consumer transaction applications.8 Macy’s is an example of a department store that used such point of sales (POS) systems with MR device readers between 1975 and the early 1980s. The potential of MR read elements for use in higher density applications was enhanced significantly with the invention of an element placed in the gap of a magnetic head, called a “shielded MR head”.9 The use of closely spaced shields, made of a high magnetic permeability material like ferrite or Permalloy, to sandwich Hunt’s MR stripe, significantly decreased inter symbol interference, i.e. increased the linear bit density that could be resolved with MR heads. Ferrite-shielded MR heads were first commercialized by IBM in 1984, for storing and reading data in the IBM 3480 magnetic tape storage system.10 However, it took another 6 years, until 1990, to successfully commercialize shielded MR heads for use in HDDs. Challenges of MR heads for HDDs There are two key requirements for the read element of AMR, GMR and TMR heads in HDDs: 1. Biasing the device to operate in a quasi-linear response mode, in order to distinguish the polarity of the sensed fields from data, and 2. Operating the device in a single domain mode to avoid magnetic switching noise, also known as “Barkhausen” noise, in order to achieve a high signal-to-noise ratio (SNR). Achieving these characteristics turned out to be much more difficult for the very narrow read elements required by HDDs. HDDs have consistently had track widths about 100 times narrower than tape recorders. 6 R.P. Hunt, “A Magnetoresistive Readout Transducer”, IEEE Trans. Mag. 7, 150 (1971); patent 3,493,694 (1970). 7 R. L. Anderson, C. H. Bajorek and D. A. Thompson, “Numerical Analysis of a Magnetoresistive Transducer”, IEEE Trans. Mag. 10, 1445 (1973). 8 C. H. Bajorek, C, Coker, L. T. Romankiw and D. A. Thompson, “Hand-held Magnetoresistive Transducer”, IBM J. Res. Dev. 18, No. 6, 541 (1974); IBM 3660/3663 POS System. 9 D. A. Thompson, “Three-Legged Magnetic Recording Head Using a Magnetoresistive Element”, patent 3,921,217 (1975). 10 D. M. Cannon, W. R. Dempwolf, J. M. Schmalhorst, F. B. Shelledy and R. D. Silkensen, “Design and Performance of a Magnetic Head for a High Density Tape Drive”, IBM J. Res. Dev. 30, No. 3, 270 (1986). 4 The first requirement is attained via two closely coupled (closely spaced) ferromagnetic films, one of the films being free to respond to the magnetic field from data, i.e.
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