Introduction to Optoelectronics Optical storage (1)

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Introduction to OptoelectronicsOptical storage
(1)

• Prof. Katsuaki Sato

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Lets talk on optical storages!

• Can you tell difference between storages and
  memories?
• There are a lot of different information storage
  techniques. What sort of storage devices do you
  know?
• Can you tell the peculiarity of optical storages
  in these storages?

Point of discussion Density, capacity, transfer
rate, size, removability
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Storages

• Old storage stones, paper, films, photographs,
  record
• Advanced storage
• Audio/Video use
• Analog audio cassette, video tape
• Digital CD, MD, Digital video tape, DVD, HD
• Computer use
• Magnetic MT, FD, HD
• Optical CD-ROM, CD-R, CD-RW, MO, DVD-ROM, DVD-R,
  DVD-RW
• Semiconductor Flash memory (USB memory)

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Old storages

• Woods, Bamboo
• Stone example Rosetta Stone
• Paper books, notebooks, etc.
• Films movies, photographs

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Magnetic Tape (MT)

• Tape recorder

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Magnetic recording

• History
• Magnetic tape and magnetic disk
• Recording media and recording head
• GMR head for high density
• Magneto-optical recording
• Hybrid magnetic recording
• Solid state nonvolatile magnetic memory (MRAM)

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History of magnetic recording

• 1898 V. Poulsen (Denmark) invented wire recorder
  Information storage technology by control of
  magnetic state.
• 1900 The magnetic recorder was exhibited at the
  Paris EXPO and was praised as the most
  interesting invention of recent years.
• Invention of vacuum tube amplifier by L. De
  Forest (USA) in1921, together with development of
  the ring-type magnetic head and the fine magnetic
  powder applied tape bring about practical
  magnetic recorder.

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Recording process
K. Sato ed., Applied Materials Science (Ohm
publishing) Fig. 5.18
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Recording process

• Signal current is applied to a coil in the
  magnetic head which is placed close to the
  recording medium to generate the magnetic flux,
  the intensity and direction of which is
  proportional to the signal.
• The medium is magnetized by the magnetic flux
  from the head, leading to formation of magnetic
  domain corresponding to the intensity and
  polarity of the signal.
• Recorded wavelength ?(the length of recorded
  domain corresponding to one period of the signal)
  is calculated by ?v/f where v is the relative
  velocity between head and medium, and f the
  signal frequency)

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Read out of recorded signal(1)Inductive head

• Electromagnetic inductionElectric voltage
  proportional to the derivative of the magnetic
  flux is generated
• Output has the differential form of the recorded
  signal
• The readout voltage is proportional to the
  product of the recorded wavelength and relative
  velocity between the head and the medium.

Running direction
K. Sato ed., Applied Materials Science (Ohm
publishing) Fig. 5.19, 5.20
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Read out of recorded signal(2) MR
(magneto-resistance) head

• Change of the electric resistance of the head by
  the magnetic flux from the medium is utilized.
• AMR (anisotropic magneto-resistance) was utilized
  in the early stage and was replaced to GMR (giant
  magneto-resistance).

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Magnetization curve and GMR
HC1
HC2

• If F1 and F2 have different Hc then high
  resistivity state is realized for H between Hc1
  and Hc2

Resistance is high for anti-parllel configuration
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What is GMR?

• Ferromag(F1)/Nonmag(N)/Ferromag(F2) multilayer
• Small resistance for parallel spin direction of
  F1 and F2, while high resistance for
  antiparallel direction.

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Spin valve

• NiFe(free)/Cu/NiFe(pinned)/AF(FeMn) uncoupled
  sandwich structure

Free layer
Nonmagnetoc layer
Exchange bias
Pinned layer
Antiferromagnetic (? FeMn)
Synthetic antiferro
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Head clearance
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Increase of areal recorded density
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Limit of increase in density is coming

• Until 2000 the increase rate was 100 times per
  10 years but it becomes slower.
• The reason of slowing is due to
  superparamagnetism due to smallness of the
  recorded region for one bit.
• By the use of perpendicular recording the
  drawback will be overcome.