Hard disks available in the current market ar makde of ferromagnetic materials like cobalt alloys. Data bits are stored as magnetized regions of the material, alignment of magnetic filed defining 0's and 1's. As data densities (number of bits per square inch) goes up, bit sizes goes down. For 1Tb per square inch, bit size reaches few tens of nanometers across. In this situation, the grains become unstable, a small amount of heat can flip magnetization directions.
Today's hard disks mostly have 500 gigabits per square inch and the above hints that 1Tb is the upper limit here. This is called the data density barrier.
But wait, there may be a way to break the barrier. A new scheme of data-recording is being studies (mainly in Segate). The new technology is HAMR, Heat Assisted Magnetic Recording.
The main problem with traditional ferromagnetic materials are their unstability in high data density. But, there exists some other material which can hold stability of the data bits. They can hold magnetization in spots just a few nanometers across.
Then why are we using cobalt alloy instead of those materials? Because, recording on such material requires a very large magnetic field. This is a problem, such large magnetic field cannot be used inside a hard disk. If we could change magnetic field directions of the single data bit areas on our will, the problem would have been solved.
This is where HAMR comes to rescue. Study showed that if the area is heated up first, data can be recorded with small magnetic field! In HAMR, the bit area is heated up with tightly focused light sopt. Sounds like a problem solved? Not fully really. Another challenge comes in when we try to heat up a single bit area with light. Light's diffraction limit prevents lenses from focusing it down to less than half of its wavelength. So, the lower limit here is around 200nm with the best lenses around.
“One of the main issues is to be able to deliver an adequate amount of light energy into a small spot size,” says Sakhrat Khizroev, an electrical engineering professor at the University of California, Riverside
Segate designed a recording-head which is reported to be able to concentrate light onto spots just 70nm wide.
Other technologies are also competing for a role in future hard-disk drive recording. Khizroev, for example, is making lasers for a HAMR disk that focus power on 30-nm spots. He deposits aluminum films on semiconducting diode lasers and then etches tiny apertures in the film to focus the emitted light. The technique, which he says has generated commercial interest, can be pushed to 10 nm or even less.
Meanwhile, hard-disk maker Hitachi Global Storage Technologies, in San Jose, Calif., is working on bit-patterned media. The idea is to create isolated islands of magnetic material on a nonmagnetic disk to serve as bits. Instead of hundreds of grains, each bit could then contain just a few magnetic grains, which stay strongly coupled, or even one grain, so they don’t flip their magnetization due to heat. Bit-patterned media could even be combined with HAMR.
Which of these technologies will eventually replace present-day magnetic recording is anyone’s guess.
IEEE Spectrum March 2009
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