24 March 2009—The density of data on hard-disk drives has doubled every three years since they were invented in 1955. Today’s hard disks pack 500 gigabits on a square inch (6.45 square centimeters). But magnetic disk recording as it is done now will run out of steam in just one more doubling, at 1 terabit. Engineers at Seagate Technology’s research arm, in Pittsburgh, have built a prototype heat-assisted magnetic recording scheme, which has the potential to allow up to 50 Tb per square inch.
Hard disks today are made of ferromagnetic materials, typically cobalt alloys. Bits are recorded as tiny magnetized regions of the material—the magnetic fields of all the grains in the area are aligned in one of two directions. As densities go up, bit sizes go down, reaching a few tens of nanometers across at 1 Tb per square inch. At those dimensions, the grains become unstable; a small amount of heat is enough to make them flip their magnetization direction.
But in a new data-recording scheme being studied mainly at Seagate, heat can actually be a good thing. Heat-assisted magnetic recording, or HAMR, promises to allow a switch to materials that are more stable than cobalt alloys and can hold magnetization in spots just a few nanometers across.
Ordinarily, recording on such material requires a very large magnetic field. But a small field will do, if you first heat it up. To do that, HAMR heats the bit area with a tightly focused light spot. And therein lies the biggest challenge: Light’s diffraction limit generally prevents lenses from focusing it down to less than half its wavelength. That means you can go down to around 200 nanometers with the best lenses.
”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, who was not involved in the Seagate project.
Seagate’s recording-head design, which the researchers report on in a Nature Photonics paper published online this week, manages to efficiently concentrate light onto spots just 70 nm wide. At the design’s heart are two main parts: One is a device called a solid immersion mirror, a parabola-shaped waveguide that focuses light down to a quarter of its wavelength. At the focusing point of this device sits the other crucial part—a transducer. This is a 200-nm-wide gold disk with a 15-nm peg jutting out at the bottom.
When light falls on the transducer, it generates surface plasmons—energy waves that oscillate along the surface and generate an intense electric field at the tip of the peg. William Challener, one of the Seagate researchers involved in the work, describes the transducer as a ”metal antenna designed to efficiently capture light focused onto it and funnel that into the recording medium through the little peg.”