Needs of the data storage industry

Transcription

1 Needs of the data storage industry Focus on magnetic media first, and, as in the lecture on the electronics industry, will try to keep an eye on areas where nano will be relevant. Historic trends State-of-the-art Media Read and write mechanisms Scaling concerns Nonvolatile approaches Where it all began 1878 Oberlin Smith invents magnetic recording - patterns of domains in steel wire Valdemar Poulsen invents reel-to-reel metal tape recording, and the telephone answering machine Sony introduces reel-to-reel recorder using coated tape RCA introduces stereo tape - cartridge needs special player Phillips introduces cassettes Motorola, RCA introduce 8-tracks Cassette outsell LP records. 1

2 Where it all began Core memory - use magnetization of ferrite cores as computer memory. $6000 per 1kb bit line 6.6 kb in 8 x8 x8 word line Image from Columbia Univ. ACIS Image from pcbiography.net Where it all began Image from pcbiography.net RAMAC (1956) First hard disk drive disks Stored a total of 5 MB of information. Areal density = 2kb/in 2 Data rate = 70 kb/sec. 2

3 Historical trends Image from IBM presentation Historical trends Image from IBM presentation 3

4 Historical trends Image from IBM presentation Magnetic storage: state-of-the art Image from IBM website Disk medium: 2.5 diameter, 34 Gb/in 2 (typical size ~ 140 nm) CoPtCr alloy (M sat = 4 x 10 5 A/m, H c = 2.7 x 10 5 A/m, K = 1.5 x 10 5 J/m 3 ) Layered structure, including special AFM layer: 4

5 Magnetic media Why the AFM layer? Consider superparamagnetism for this disk material. Individual grain in a bit ~ 10 nm on a side. Energy barrier = KV ~ 1.5 x J. Attempt frequency ~ 10 9 leads to effective rate of τ 1 ~ Ω exp( KV / k T ) ~ Means typical timescale for a particular grain to thermally reverse itself would be 2 months. B 7 s 1 Need some way of reducing superparamagnetic effects! Magnetic media Image from IBM website Antiferromagnetic pixie dust layer stiffens disk medium without strongly altering its coercivity (write-ability). 5

6 Hard drives - read heads Permalloy yoke with integrated, microfabricated Cu coils. Shield of high permeability material between yoke and GMR sensor - guides B to prevent record head from affecting GMR FM layers. Minimum size of transverse bit set by write-gap width. Head attached to end of piezoactuated arm. Aerodynamics keeps head suspended above surface at flight height. Images from IBM website Hard drives - read heads Image from IBM website Basic spin valve design. Exchange layer pins one of the GMR FM layers. Note that this is a standard current-in-plane geometry. 6

7 Where are we headed? Image from IBM presentation Advances coming: Different media Thermally assisted recording Vertical recording Patterned media Different read heads CPP GMR heads CPP magnetic tunnel junction heads CMR? EMR? Different technology altogether MRAM Millipede Holographic storage 7

8 Thermally assisted recording Image from IBM website Simple idea: use media with significantly higher anisotropies. Benefit: added thermal stability against superparamagnetism. Downside: harder to write bits. Solution: locally heat medium (optically) to drop coercive field. Tricky - need short thermal relaxation times. Vertical media 8

9 Vertical media Potential advantages: Can get better thermal stability by larger bit volumes without sacrificing bit area density. Better signal to noise under some circumstances because fringing fields over larger area tend to influence read head. Disadvantages: Textured growth of medium on substrate can be quite tricky. Patterned media image from Nat. Univ. Singapore Don t rely just on film growth: make nanostructured media with prearranged bit locations - single domain particles. Several different approaches: E-beam lithography (far too slow) Ion beam patterning through mask Nanoimprint lithography Electrochemistry through porous mask Self-organization / self-assembly Real possibility of > 1TB/in 2! 9

10 CPP read heads Competing storage technologies Other approaches (nonmagnetic): AFM-based storage ( Millipede - IBM) Electron-based storage (HP) STM-based storage (IBM) Holographic storage 10

11 Millipede AFM storage Image from IBM website Millipede AFM storage Array of 1024 piezoresistively sensed AFM cantilevers. Physical deformation of polymer medium to write, using integrated resistor to heat individual tip by ~ 100 degrees. Healed by local heating to erase. Demonstrated density of 200 Gb/in 2, with an eye toward 1Tb by summer, Can this ever be cheap and reliable? IBM really seems to think so. Think 300 DVDs in a space the size of a credit card. 11

12 Electron-based storage image from Scientific American, May 2000 HP plan: nanofabricated field emission tips. In principle, can get very high resolution, approaching atomic scale. Holographic storage image from Bell Labs Use phase-sensitive interference patterns from lasers to write data pages into an optically changeable medium. Can be read pages at a time. Different pages can be stored at different depths in medium. Potential storage densities and speeds are enormous! Problems: Needs serious lasers and optics. Materials problems with media. 12

13 Conclusions Magnetic data storage still has a lot of life left to it, but progress rates are so fast they make Moore s law look relaxed. Physics (superparamagnetism) demands changes in media soon. New read head technologies also likely to be relevant. Competing technologies have incredible potential, and will eventually supplant magnetic storage for certain applications. 13