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  1. 1. Introduction to Optoelectronics Optical storage (1) Prof. Katsuaki Sato
  2. 2. Let’s talk on optical storages! <ul><li>Can you tell difference between storages and memories? </li></ul><ul><li>There are a lot of different information storage techniques. What sort of storage devices do you know? </li></ul><ul><li>Can you tell the peculiarity of optical storages in these storages? </li></ul>Point of discussion Density, capacity, transfer rate, size, removability
  3. 3. Storages <ul><li>Old storage: stones, paper, films, photographs, record </li></ul><ul><li>Advanced storage </li></ul><ul><li>Audio/Video use </li></ul><ul><ul><li>Analog: audio cassette, video tape </li></ul></ul><ul><ul><li>Digital: CD, MD, Digital video tape, DVD, HD </li></ul></ul><ul><li>Computer use </li></ul><ul><ul><li>Magnetic: MT, FD, HD </li></ul></ul><ul><ul><li>Optical: CD-ROM, CD-R, CD-RW, MO, DVD-ROM, DVD-R, DVD-RW </li></ul></ul><ul><ul><li>Semiconductor: Flash memory (USB memory) </li></ul></ul>
  4. 4. Old storages <ul><li>Woods, Bamboo </li></ul><ul><li>Stone: example Rosetta Stone </li></ul><ul><li>Paper: books, notebooks, etc. </li></ul><ul><li>Films: movies, photographs </li></ul>
  5. 5. Magnetic Tape (MT) <ul><li>Tape recorder </li></ul>
  6. 6. Magnetic recording <ul><li>History </li></ul><ul><li>Magnetic tape and magnetic disk </li></ul><ul><li>Recording media and recording head </li></ul><ul><li>GMR head for high density </li></ul><ul><li>Magneto-optical recording </li></ul><ul><li>Hybrid magnetic recording </li></ul><ul><li>Solid state nonvolatile magnetic memory (MRAM) </li></ul>
  7. 7. History of magnetic recording <ul><li>1898 V. Poulsen (Denmark) invented wire recorder; Information storage technology by control of magnetic state. </li></ul><ul><li>1900 The magnetic recorder was exhibited at the Paris EXPO and was praised as “the most interesting invention of recent years”. </li></ul><ul><li>Invention of vacuum tube amplifier by L. De Forest (USA) in 1921, together with development of the ring-type magnetic head and the fine magnetic powder applied tape bring about practical magnetic recorder. </li></ul>
  8. 8. Recording process K. Sato ed., Applied Materials Science (Ohm publishing) Fig. 5.18 Recording current time moving direction of recording media Recorded wavelength
  9. 9. Recording process <ul><li>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. </li></ul><ul><li>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. </li></ul><ul><li>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) </li></ul>
  10. 10. Read out of recorded signal ( 1 ) Inductive head <ul><li>Electromagnetic induction Electric voltage proportional to the derivative of the magnetic flux is generated </li></ul><ul><li>Output has the differential form of the recorded signal </li></ul><ul><li>The readout voltage is proportional to the product of the recorded wavelength and relative velocity between the head and the medium. </li></ul>K. Sato ed., Applied Materials Science (Ohm publishing) Fig. 5.19, 5.20 Running direction Spacing loss Principle of read-out induction
  11. 11. Read out of recorded signal ( 2 ) MR (magneto-resistance) head <ul><li>Change of the electric resistance of the head by the magnetic flux from the medium is utilized. </li></ul><ul><li>AMR (anisotropic magneto-resistance) was utilized in the early stage and was replaced to GMR (giant magneto-resistance). </li></ul>N S N S N S leakage flux MR head N S
  12. 12. Magnetization curve and GMR <ul><li>If F1 and F2 have different Hc then high resistivity state is realized for H between Hc1 and Hc2 </li></ul>H C2 H C1 Resistance is high for anti-parllel configuration H M R H F1 F2 F1 F2 F1 F2 F1 F2 F1 F2
  13. 13. What is GMR? <ul><li>Ferromag(F 1 )/Nonmag(N)/Ferromag(F2) multilayer </li></ul><ul><li>Small resistance for parallel spin direction of F1 and F2, while high resistance for antiparallel direction. </li></ul>Pinned layer Free layer
  14. 14. Spin valve <ul><li>NiFe(free)/Cu/NiFe(pinned)/AF(FeMn) uncoupled sandwich structure </li></ul>Exchange bias Free layer Nonmagnetoc layer Pinned layer Antiferromagnetic ( 例 FeMn) Synthetic antiferro
  15. 15. Head clearance
  16. 16. Increase of areal recorded density Superparamagnetic limit MR head GMR head
  17. 17. Limit of increase in density is coming <ul><li>Until 2000 the increase rate was 100 times per 10 years but it becomes slower. </li></ul><ul><li>The reason of slowing is due to superparamagnetism due to smallness of the recorded region for one bit. </li></ul><ul><li>By the use of perpendicular recording the drawback will be overcome. </li></ul>

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