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IBM's New Magnetic Hard-Disk-Drive Media Delays Superparamagnetic Effects
Superparamagnetic Effect
The superparamagnetic effect originates from the shrinking volume of magnetic grains that compose the hard-disk media, in which data bits are stored as alternating magnetic orientations. To increase data-storage densities while maintaining acceptable performance, designers have shrunk the media's grain diameters and decreased the thickness of the media. The resulting smaller grain volume makes them increasingly susceptible to thermal fluctuations, which decreases the signal sensed by the drive's read/write head. If the signal reduction is great enough, data could be lost in time to this superparamagnetic effect.
Fig. 1. TEM of the grain structures in magnetic media. (magnification = 1 million)
In Figure 1 are transmission electron micrographs (TEM) for two different disk media illustrating how the grain structure has changed over time. The TEM on the left is a magnetic media that supports a data density of about 10 gigabits/inch2 with an average grain diameter of about 13 nanometers. The magnetic media on the right supports a data density of 25 gigabit/inch2 with an average grain diameter of about 8.5 nanometers.
Historically, disk drive designers have had only two ways to maintain thermal stability as the media's grain volume decreases with increasing areal density: 1) Improve the signal processing and error-correction codes (ECC) so fewer grains are needed per data bit, and 2) develop new magnetic materials that resist more strongly any change to their magnetization, known technically as higher coercivity. But higher coercivity alloys also are more difficult to write on. While improvements in coding and ECC are ongoing, IBM's new AFC media is a major advancement because it allows disk-drive designers to have their cake and eat it too: It is easy to write at very high areal densities but is much more stable than conventional media.
How Does AFC Media Work?
Conventional disk media stores data in only one magnetic layer, typically of a complex magnetic alloy (such as coblat-platinum-chromium-boron, CoPtCrB). AFC media is a multi-layer structure in which two magnetic layers are separated by an extraordinarily thin -- just three atoms thick -- layer of the nonmagnetic metal, ruthenium. This precise thickness of the ruthenium causes the magnetization in each of the magnetic layers to be coupled in opposite directions -- anti-parallel -- which constitutes antiferromagnetic coupling. A schematic representation of this structure is shown in Figure2.
Fig. 2. Schematic representation of AFC media with single magnetic transition
When reading data as it flies over the rapidly rotating disk, a disk drive's recording head senses the magnetic transitions in the magnetic media that coats the disk. The amplitude of this signal is proportional to the media's "magnetic thickness" -- product of the media's remanent magnetic moment density ("Mr") and its physical thickness ("t"). As data densities increase, the media's magnetic thickness (known technically as Mrt) must be decreased proportionately so the closely packed transitions will be sharp enough to be read clearly. For conventional media, this means a decrease in the physical thickness of the media.
The key to AFC media is the anti-parallel alignment of the two magnetic layers across each magnetic transition between two bits. As it flies over a transition, the recording head senses an effective Mrt of the composite structure (Mrteff) that is the difference in Mrt values for each of the two magnetic layers:
Mrteff = Mrttop - Mrtbottom
This property of the AFC media permits its overall Mrt to be reduced -- and its data density increased -- independently of its overall physical thickness. Thus for a given areal density, the Mrt of the top magnetic layer of AFC media can be relatively large compared with single-layer media, permitting inherently more thermally stable larger grain volumes.
Figure 3 compares projections made based on measurements of the expected signal amplitude loss after 10 years in conventional single-layer media with that in AFC media. As the Mrt of the conventional media decreases with reduced film thickness and grain diameter, thermal effects rapidly shrink its magnetic amplitude. This dramatic signal loss is at the heart of the superparamagnetic effect. Acceptable levels of signal decay vary depending on system design but typically range between 10-20%. In comparison, AFC media has the thermal stability of conventional media having about twice its magnetic thickness. In the future, AFC media structures are expected to enable thermally stable data storage at densities of 100 gigabits per square inch and possibly beyond.
Fig. 3. Thermal stability of AFC and conventional media
Two additional advantages of AFC media are that it can be made using existing production equipment at little or no additional cost, and that its writing and readback characteristics are similar to conventional longitudinal media. The output pulse sensed by the recording head is a superposition of the fields from transitions in both the top and bottom magnetic layers. As with conventional media, this output is detected as a single pulse, so no changes to the disk drive's recording head or electronic data channel components are required.
The invention of AFC media was just the starting point in its development for use in IBM products. New physical insights had to be developed to understand how the various properties of the two magnetic layers and the ruthenium layer should be optimized. Secondly, it was necessary to find a way to fabricate the multi-layer structures that maintained the proper microcrystalline growth characteristics in each layer. Lastly, it was a significant challenge to modify existing, conventional, high-volume manufacturing tools to deposit the 6-Å ruthenium layer with suitable uniformity over the entire disk surface.
IBM is a pioneer in the research, development and manufacture of antiferromagnetically coupled structures, which have remarkable properties due to the "spintronic" interactions between the materials' electrons and magnetic fields. In 1990, IBM researchers discovered that a thin layer of ruthenium atoms created the strongest anti-parallel coupling between adjacent ferromagnetic layers of any nonmagnetic spacer-layer element. The structure was used in the first giant magnetoresistive read element for disk drives,which was introduced by IBM in 1997. GMR heads are now used in virtually all disk drives. IBM is considering antiferromagnetically coupled structures for use in other applications, such as magnetic random access memory.
Summary
In summary, IBM has developed and is now mass-producing a promising new disk-drive media technology based on antiferromagnetically coupled multilayers that can enable significant areal density increases while maintaining the thermal stability of recorded data. This advancement will permit magnetic hard-disk drive technology to extend far beyond the previously predicted "limits" imposed by the superparamagnetic effect.
IBM's new AFC media is featured in the following products designed for the 2.5-inch-form-factor portable hard-disk-drive market segment: the 15-gigabyte-per-platter (25.7 gigabits/inch2 data density) Travelstar 15GN and 30GN and the 12-gigabyte-per-platter Travelstar 48GH (21.7 gigabits/inch2 ) disk drive products. As disk-drive areal densities continue to increase, we expect AFC media will be used in future products within all HDD segments.
References:
(1) Fullerton, E.E., Margulies, D.T., Schabes, M.E., Carey, M., Gurney, B., Moser, A., Best, M., Zeltzer, G., Rubin, K., Rosen, H., Doerner, M., Antiferromagnetically Coupled Magnetic Media Layers For Thermally Stable High Density Recording, Appl. Phys. Lett., 77, 3806 (2000).
(2) M. F. Doerner, X. Bian, K. Tang, M. F. Toney, K. Rubin, D. Weller, A. Moser, M. Mirzamaani, A. Polcyn, T. Minvielle, K. Takano, R. White; Advanced media on glass substrates for 30 Gbits/in2 and beyond; The 2000 IEEE International Magnetics Conference, Invited Paper GA-01, Toronto, Canada, 13 April 2000.
(3) Madison, M., Doerner, M., Tang, K., Peng, Q., Polcyn, A., Arnoldussen, T., Toney, M.F., Bian, X., Takano, K., Fullerton, E.E., Margulies, D.T., Rubin, K., Weller, D., Demonstration of 35 Gbits/in2 Using Media On Glass Substrates, published in IEEE Trans. Mag., 37, 1052 (2001).
(4) Fullerton, E.E., Margulies, D.T., Schabes, M.E., Doerner, M., Antiferromagnetically Coupled Recording Media, MMM/Intermag Conference, Invited Paper BA-01, San Antonio, Texas, 8 January 2001.
(5) Schabes, M.E., Fullerton, E.E., Margulies, D.T., Theory of Antiferromagnetically Coupled Magnetic Recording Media, J. Appl. Phys., in press (2001).
(6) Lohau, J., Moser, A., Margulies, D.T., Fullerton, E.E., Schabes, M.E., Dynamic Coercivity Measurements of Antiferromagnetically Coupled Magnetic Media Layers, Appl. Phys. Lett., 78, 2748 (2001).
(7) S. S. P. Parkin, N. More, K. P. Roche; Oscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/Ru, Co/Cr and Fe/Cr; Phys. Rev. Lett., 64, 2304 (1990).