Laser-Heated Hard Drives Could Break Data Density Barrier

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.”

Challener and his colleagues use an iron-platinum alloy for the disk. The recording head flies over the rotating disk, heating 70-nm spots to about 350 C in a nanosecond, then altering the spots’ magnetic fields. The resulting data density is about 250 Gb per square inch. That’s half of what is possible with today’s magnetic technology, but Challener says there are many ways to focus the light further. Shrinking the transducer is one. Another is to redesign the disk by including a heat sink and a plasmonic film underneath the iron-platinum layer.

Ed Schlesinger, head of the electrical and computer engineering department at Carnegie Mellon University, in Pittsburgh, says the new work shows for the first time that ”all the elements of HAMR technology can be done. They’ve put everything together in a system.” He notes, however, that the laser is not inside the recording head right now. Instead, an off-board laser shines light on a grating that directs light to the solid-immersion mirror.

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, Schlesinger says. ”If HAMR is going to have a chance in the marketplace, then in five years I would expect to see some commercial system out there.”