Hard Disk Reading Process Hard Disk Reading Process Becky Jaiven - - PowerPoint PPT Presentation

Hard Disk Reading Process Hard Disk Reading Process

Becky Jaiven Becky Jaiven Dini Dini Vemuri Vemuri Jason Jason Naso Naso Gabi Gabi Nunes Nunes

Reading Components Reading Components

Modern disk drives typically consist of Modern disk drives typically consist of Magnetoresistive Magnetoresistive (MR) or Giant (MR) or Giant Magnetoresistive Magnetoresistive (GMR) heads. (GMR) heads. The read heads are the interface between the magnetic The read heads are the interface between the magnetic physical media on which the data is stored and the physical media on which the data is stored and the electronic components that make up the rest of the hard electronic components that make up the rest of the hard disk. disk.

– – Heads are energy converters: they transform electrical signals t Heads are energy converters: they transform electrical signals to

• magnetic signals, and magnetic signals back to electrical ones

magnetic signals, and magnetic signals back to electrical ones again again – – The heads do the work of converting bits to magnetic pulses and The heads do the work of converting bits to magnetic pulses and storing them on the platters, and then reversing the process storing them on the platters, and then reversing the process when the data needs to be read back. when the data needs to be read back. – – The read/write heads are essentially tiny electromagnet The read/write heads are essentially tiny electromagnet

How It Works How It Works

Data is written to a disk Data is written to a disk The Actuator looks through a table of The Actuator looks through a table of stored data locations to find the where the stored data locations to find the where the chosen data is located chosen data is located When the correct sectors pass beneath When the correct sectors pass beneath the head, the magnetic fields from the bits the head, the magnetic fields from the bits induce resistively changes in the sensitive induce resistively changes in the sensitive MR or GMR materials located in the MR or GMR materials located in the reading elements within the head. reading elements within the head.

Ferrite Heads: How they work Ferrite Heads: How they work

Ferrite heads are Ferrite heads are classic classic electromagnets electromagnets

– – Applying a magnetic Applying a magnetic field to a coil will cause field to a coil will cause an electrical current to an electrical current to flow; this is used when flow; this is used when reading back the reading back the previously written previously written information. information.

MR & GMR Heads: How they work MR & GMR Heads: How they work

Newer MR and GMR Newer MR and GMR heads don heads don’ ’t induce t induce current on the coil to read current on the coil to read back the information, they back the information, they

• perate by
• perate by

magnetoresistance magnetoresistance. . Magnetoresistance Magnetoresistance is is happens when materials happens when materials change their resistance change their resistance when subjected to when subjected to different magnetic fields. different magnetic fields.

Detailed structure diagram of a GMR head assembly

MR & GMR Heads: How they work MR & GMR Heads: How they work Cont. Cont.

As the head passes over the surface As the head passes over the surface fo fo the disk, the disk, the material changes resistance as the magnetic the material changes resistance as the magnetic fields change corresponding to the stored fields change corresponding to the stored patters on the disk. patters on the disk. A sensor is used to detect these changes in A sensor is used to detect these changes in resistance, allowing the bits to be read. resistance, allowing the bits to be read. Be cause the MR head isn Be cause the MR head isn’ ’t generating a current t generating a current directly the way Ferrite head do, it is more directly the way Ferrite head do, it is more sensitive to magnetic flux changes. sensitive to magnetic flux changes.

Inside a GMR Head Inside a GMR Head

• 1. Free Layer: the sensing layer of nickel-

iron allow, and is passes over the surface of the data bits to be read. It is free to rotate in response to the magnetic patterns on the disk.

• 2. Spacer: nonmagnetic layer
• 3. Pinned Layer: held in a fixed magnetic
• rientation by cobalt. This is possible

because its attached to the exchange layer.

• 4. Exchange layer: Made from

antiferromagnetic material and fixes the pinned layer’s magnetic orientation.

Reading Reading

When the head passes over a magnetic fields of When the head passes over a magnetic fields of

• ne polarity (i.e.
• ne polarity (i.e. “

“0 0” ”), the free layer has its ), the free layer has its electrons turned to be aligned with those of the electrons turned to be aligned with those of the pinned layer, creating a lower resistance. pinned layer, creating a lower resistance. When the head passes over a magnetic field of When the head passes over a magnetic field of the opposite polarity (i.e. the opposite polarity (i.e. “ “1 1” ”), the electrons in ), the electrons in the free layer rotate so they are not aligned with the free layer rotate so they are not aligned with the pinned layer. This causes an increase in the the pinned layer. This causes an increase in the resistance of the overall structure. resistance of the overall structure.

Demonstration Demonstration

http://www.research.ibm.com/research/de http://www.research.ibm.com/research/de mos/gmr/index.html mos/gmr/index.html

Areas for Improvement Areas for Improvement

Magnetic Materials Magnetic Materials

• n the disk must be
• n the disk must be

made finer so made finer so smaller bits can be smaller bits can be written and still read written and still read Reading elements Reading elements must be made more must be made more sensitive so they can sensitive so they can read smaller bits read smaller bits

References References

http://www.pcguide.com/ref/hdd/op/heads/ http://www.pcguide.com/ref/hdd/op/heads/

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www.wikipedia www.wikipedia. com . com