All About Hard Drives

1
All About Hard Drives

• Lili Ji
• 2005. 4

2
Outlines

• Hard Drive History
• Hard Drive Structures
• Hard Drive Disk Media
• Hard Drive Writing Heads
• Hard Drive Reading Heads
• Hard Drive Limits
• Whats next?
• Summary

3
Hard Drive History

• 1956, IBM, RAMAC
• 5Mb Storage
• 50 disks, 24 inch in diameter

4
Hard Drive History

• First Modern Hard Disk Design (1973)
• IBM's model 3340, nicknamed the "Winchester",
  is introduced. With a capacity of 60 MB it
  introduces several key technologies that lead to
  it being considered by many the ancestor
• First 3.5" Form Factor Disk Drive (1983)
• Rodime introduces the RO352, the first disk
  drive to use the 3.5" form factor, which became
  one of the most important industry standards
• First Drive to use Magnetoresistive Heads (1990)
  
• IBM's model 681 (Redwing), an 857 MB drive.

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Hard Drive History
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Hard Drive Structures
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Hard Drive Disk Media

• The gap between the head and the disk surface is
  about 15 nm.
• Surface roughness should be a few nanometers.
• Traditionally it is Al-Mg substrate with Ni-P on
  it. Now glass substrate is increasingly used.
• Cr or Cr-V alloy are used as under layers to
  control the crystallographic orientation of the
  magnetic layer.
• Co based alloy is used as the top magnetic
  layer,1030nm in thickness.
• Limited by grain size, areal density lt35
  Gb/sq.inch

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Hard Drive Disk Media
Topography AFM picture RMS is about 8-12Å
MFM image, dark and white Represents the bit
information
Sadamichi, Spin Dependent Transport in Magnetic
Nanostructure
9
Hard Drive Writing Head---Longitudinal Writing
Head
S. Khizroev and D. Litvinov, J.A.P Vol 95,Num 9,
May 2004
10
Hard Disk Writing Head Perpendicular Writing
The first one use perpendicular is Toshibas mini
hard drive MK8007GAH , which will be used in
IPod, 80GB 1.8in
S. Khizroev and D. Litvinov, J.A.P Vol 95,Num 9,
May 2004
11
Hard Drive Reading Heads---AMR Reading Heads

• Introduced by IBM in 1991.
• ?R/R25,providing areal density 15Gb/sq.inch
• RR0 ?Rcos2?

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AMR Origin

• Spin-Orbit coupling leads to spin dependent
  scattering of conduction electrons.(3d and 4s
  electrons)
• 3d orbitals will be affected by magnetization.
  They will mix and reorient, and show a larger
  scattering cross sections when electrons are
  moving parallel to M. And more scattering, or
  resistance!

http//www.owlnet.rice.edu/phys533/notes/week14_l
ectures.pdf
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Hard Drive Reading Heads---GMR Reading Heads

• ?R/R1050, providing areal density larger than
  10Gb/sq.inch

http//www.owlnet.rice.edu/phys533/notes/week14_l
ectures.pdf
14
GMR Origin

• Spin-dependent transmission of carriers at
  interface between non-magnetic layer and magnetic
  layer.

http//www.owlnet.rice.edu/phys533/notes/week14_l
ectures.pdf
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Hard Drive Bit Size
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Is there a limit?----Yes. Super Paramagnetic

• Transmission Electron Micrograph of a Co-Cr-Pr-B
  magnetic media.
• Fine grain size is around 85Å
• Capable of supporting areal density 35Gb/sq.inch

Sadamichi, Spin Dependent Transport in Magnetic
Nanostructure
17
Whats the problem?

• Each bit usually contains hundreds of grains.
  Magnetic recording relies on the statistically
  averaging over those grains to get a satisfactory
  signal to noise ratio .
• As bits size continue decrease, grain size need
  to be reduced, too. This can be achieved by under
  layer control.
• However, eventually, the grains will become super
  paramagnetic.

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Super Paramagnetic

• Definition
• Magnetic information of the grain undergoes
  spontaneous switching by assistance of thermal
  energies.
• Ms ----Saturation magnetization
• Ku ---- Uniaxial anisotropy
• V --- Volume of the grain
• KuV---Magnetic anisotropy energy of the grain
• To save information more than 10 years,
  KuVgt4050kT
• As V decreasing, Ku need to be increased to avoid
  super paramagnetic!
• Hc ----Switching field, Hc2Ku/?0Ms, so a larger
  field is needed to write information.

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Possible Solutions

• Engineering media with narrower grain size
  distribution, so magnetic anisotropy Ku can be
  increased.
• Perpendicular writing will have larger writing
  field, and supporting smaller bit size while at
  the same time allows more amount of grains in
  each bit.
• Thermally assisted writing is to use laser to
  locally heat the media, to lower the coercivity
  Hc in that spot.

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Whats next?---Patterned Magnetic Media

• Bit size is decided by lithography.
• Information is stored in a single domain magnet
  particle.
• A 50 nm-period square dot array gives 250 Gb
  /sq.inch

http//eltweb.mit.edu/3.063/lecturenotes/Lec.16.4.
5.05.pdf
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Whats Next?--BMR Reading Heads

• Ballistic Magnetoresistive can provide a ?R/R of
  more than 300.

Edward Price, CMRR UCSD Physics.
22
Summary

• Smaller Bit Size More sensitive reading heads
• ?Larger Hard Drive!
• Now let's have some fun!