Will magnetic switching by light keep storage vendors spinning?

Circularly polarised lasers - the new magnetism


Hard drive storage is based on switching magnetic states by magnetism. Switching by laser light could be much faster and could be used on non-spinning media.

Laser light switching of a magnetic medium is described in a PhD thesis, Laser-Induced Femtosecond Magnetic Recording, written by Dr Daniel Stanciu, currently a researcher for Océ Technologies in The Netherlands. He and Dr Fredrik Hansteen discovered how to use light to reverse magnetic polarity in 2006. Stanciu developed the idea more as an intern at Seagate's Pittsburgh research centre in 2007.

The PhD thesis was published last year and can be downloaded here (pdf).

Light-induced femtosecond

Front cover of Stanciu's thesis.

The background is that magnetic-induced switching of a magnetic field becomes progressively harder as the size of the magnetic field shrinks. We want to reduce the size of the magnetic area so as to increase the storage capacity of the medium, such as a hard disk drive. Current shipping hard disk drives (HDD) have a storage areal density at or in excess of 325Gbit/sq inch.

Post-perpendicular recording technologies such as discrete track recording, bit-patterned media and HAMR (Heat-Assisted Magnetic Recording) are set to take this to and perhaps beyond 1Tbit/sq inch.

Unless the write and read time per bit increases in speed as the areal density increases then the I/O rate of the disk will appear slower and slower relative to its capacity. Stanciu reckons that, in 2007, disk drives were operating at an internal data transfer rate of approximately 200 MB/sec, corresponding to a channel data rate of about 1.6 Gbit/s. He says that the writing time for a single bit is a little less than one nanosecond (billionth of a second) at that speed.

(For reference here a picosecond is one trillionth of a second. A femtosecond is one billionth of one millionth, or one quadrillionth of a second.)

Stanciu asserts that HDDs with terabit areal densities will need sub-nanosecond switching times; their data bits will need their magnetic polarities set at such speeds. How could it be done?

One way is to increase the strength of the applied magnetic field and this, he says, can get us into speeds measured in picoseconds. But the smallness of the bits requires magnetic material with a high anisotropy (directional differences in magnetic field strength).

The strength of the writing magnetic field must be high to overcome this and it must also be very narrow in focus or else contained some how so that neighbouring bit values are not affected. He cites a potential limit on magnetic switching that indicates it can never be faster than two picoseconds.

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