Data density on hard disks has roughly doubled every year for the past 30 years, and to keep up, researchers have made smaller and smaller sensors to read the tiny bits stored on a disk. Today’s hard disks pack a mind-boggling amount of data–more than 200 gigabits in a square inch. But as the industry gears up for densities of up to one terabit per square inch, the sensors are reaching their physical limits.
Researchers at the National Physical Laboratory, in Teddington, UK, are now proposing a novel sensor design to read the bits on a hard disk. The design, published in the Journal of Applied Physics, is based on a different magnetic effect than current read heads. It could lead to much thinner and smaller read heads that are suitable for data densities as high as one terabit per square inch, says lead researcher Marian Vopsaroiu.
The new sensor would also use slightly less power than current read heads–an especially useful feature for laptops and MP3 players. And it could improve the speed of the reader. “You could read back data ten times faster,” Vopsaroiu says. “Instead of one GHz, you can read at five to ten GHz.”
Laptops and computers currently use the magneto-resistance effect to read hard-disk data. Hard disks store bits magnetically; depending on the direction of a bit’s magnetic field, it can represent a bit 1 or 0. As the read head flies over the disk, the magnetic fields of the bits cause a corresponding resistance change in the read head’s sensor. The resistance can’t be measured directly, so it’s first converted into a voltage using a direct current. (The voltage is equal to the current multiplied by the resistance.) In order for the whole thing to work, a current must run continuously through the sensor.
The new sensor will not need this constant current because it uses the magneto-electric effect. Materials that display this effect have coupled electric and magnetic fields: their electric field changes in response to an external magnetic field, and vice versa. So in the new sensor, a data bit’s magnetic field will directly generate a voltage instead of a resistance. “Each time you fly on top of a recorded bit, [it] would induce a pulse voltage which is positive or negative depending on the orientation of a bit,” Vopsaroiu says.
The sensor is a stack of seven layers made of materials with different magnetic and electric properties. Together, they interact and display the magneto-electric effect.
Current read-head sensors, by contrast, contain 15 layers, so they have to be thicker. “It is almost impossible to make a 15-to-20-layer stack in a 10-to-15-nanometer space,” Vopsaroiu says. His design, he calculates, could lead to sensors thinner than 10 nanometers, with a data density of one terabit per square inch.
He cautions that these numbers are theoretical at this point. Whether or not the design will actually work depends on the materials used in the sensor stack. The materials that have the right magnetic and electric properties are complex alloys, such as lead zirconium titanate, cobalt iron vanadium, and platinum manganese. So far, only micrometers-thick layers of these materials have been shown to have the necessary magnetic and electric properties.
To make a practical read head, the layers in the sensor stack will have to be two to three nanometers thick. It’s not clear if the materials will retain their properties at those dimensions. “When you go to such small thicknesses … the behavior can change tremendously,” says MIT physicist Jagadeesh Moodera, one of the discoverers of the tunnel magneto-resistance effect used in current read heads.
Moreover, putting together the complex alloys in a few-nanometers-thick sensor could be a challenge. The materials all have different properties, and they don’t necessarily agree with each other, Moodera says. For example, one material might be sensitive to oxygen, while another requires oxygen. Nevertheless, the idea is sound, he says, and “it makes sense to pursue it [experimentally].”
Vopsaroiu agrees that his design will have to meet many challenges. But the read-head sensors used today are just as complicated, and manufacturers have developed ways to produce them easily. Besides, he says, to reach the milestone of one-terabit-per-square-inch disk density, the industry will have to experiment with newer reading technologies.
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