Download Introduction to Optoelectronics Optical storage (1)

Introduction to Optoelectronics
Optical storage (1)
Prof. Katsuaki Sato
Let’s 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
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)
Old storages
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Woods, Bamboo
Stone: example Rosetta Stone
Paper: books, notebooks, etc.
Films: movies, photographs
Magnetic Tape (MT)
• Tape recorder
Magnetic recording
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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)
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 ringtype magnetic head and the fine magnetic powder
applied tape bring about practical magnetic recorder.
Recording process
Recording current
time
moving direction
of recording media
Recorded wavelength
K. Sato ed., Applied Materials Science
(Ohm publishing) Fig. 5.18
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)
Read out of recorded signal（1）
Inductive head
• Electromagnetic induction
Electric 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.
K. Sato ed., Applied Materials Science
(Ohm publishing) Fig. 5.19, 5.20
induction
Running direction

E
t
Principle of read-out
Spacing loss
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).
MR head
NS
NS
NS
NS
leakage flux
Magnetization curve and GMR
F1
F1
F1
F2
F2
F2
M
HC1 HC2
H
F1
F2
• If F1 and F2 have different
Hc then high resistivity
state is realized for H
between Hc1 and Hc2
F1
F2
R
H
Resistance is high
for anti-parllel
configuration
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.
Free layer
Pinned layer
Spin valve
• NiFe(free)/Cu/NiFe(
pinned)/AF(FeMn)
uncoupled
sandwich structure
Free layer
Nonmagnetoc
layer
Pinned layer
Antiferromagnetic
(例 FeMn)
Synthetic antiferro
Exchange bias
Head clearance
Increase of areal recorded density
Superparamagnetic limit
GMR head
MR head
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.