Introduction to OptoelectronicsOptical 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 discussionDensity, 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

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

Magnetic Tape (MT)

• Tape recorder

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)

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.

Recording process

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

Recording currenttime

moving directionof recording media

Recorded wavelength

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 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.

tE

Spacing loss

Principle of read-out

induction

K. Sato ed., Applied Materials Science(Ohm publishing) Fig. 5.19, 5.20

Running direction

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).

N S N SN S

leakage flux

MR head

N S

Magnetization curve and GMR

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

H

M

R

H

F1

F2

F1

F2

HC2HC1

F1

F2

F1

F2

F1

F2

Resistance is high for anti-parllel configuration

What is GMR?

• Ferromag(F １ )/Nonmag(N)/Ferromag(F2) multilayer• Small resistance for parallel spin direction of F1 and

F2, while high resistance for antiparallel direction.

Pinned layer

Free layer

Spin valve

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

Exchange bias

Free layer

NonmagnetoclayerPinned layer

Antiferromagnetic ( 例 FeMn)

Synthetic antiferro

Head clearance

Increase of areal recorded density

Superparamagnetic limit

MR head

GMR 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.