Flash of Brilliance or Flash in the Pan?

Giant planar Hall effect could be the next big thing in data storage

IMAGE: BRYAN CHRISTIE

In a highly sensitive ferromagnetic semiconductor [right], with controllable magnetic and electronic properties, a voltage develops transverse to the current when a magnetic field is applied in its plane. In the classic Hall effect [left], the transverse voltage is created when a magnetic field is applied perpendicular to the current.

The sensitivity of read-heads and the capacity of hard disks could in principle be more than 1000 times greater as a result of a new effect discovered by researchers at the California Institute of Technology (Pasadena) and the University of California, Santa Barbara. Those researchers, though, are the first to caution that much depends on whether the effect can be effectively harnessed.

They detected what is now called the giant planar Hall effect in thin films of gallium manganese arsenide—one of a new class of materials, ferromagnetic semiconductors, whose magnetic and electronic properties are susceptible to control. Hong Tang of Caltech performed many of the measurements that demonstrated the effect, obtained by careful doping of gallium arsenide with atoms of manganese.

When a magnetic field is applied in the plane of a current flowing through such a film, a voltage develops transverse to the direction of the current [see above]. This contrasts with the classic Hall effect, in which a transverse voltage is created when a magnetic field is applied perpendicular to a current flowing in a material. (The ratio of the transverse voltage to the original current is known as the Hall resistance.)

”You put just a few percent of manganese atoms” into a gallium arsenide film, ”and suddenly you see these four to five orders of magnitude change in resistance,” marvels David Awschalom, one of the effect’s discoverers at UC Santa Barbara.

While the classic Hall effect can be explained in terms of the same familiar Lorentz force that makes electric motors and generators hum along, the newly discovered effect has so far defied an easy explanation. Awschalom suspects that the intrinsic magnetic properties of the manganese atoms lie at the heart of the effect.

A promising sensor technology

Whatever the exact causes of the effect, its discoverers have been quick to grasp potential uses. ”This discovery enables a whole new class of robust sensors,” says Tang’s supervisor, Michael Roukes. He notes that sensing technology based on the giant magnetoresistive effect, or GMR, is the foundation of a ”[US] $100 billion business.” Yet the planar Hall effect ”is several orders of magnitude stronger in effect” than GMR, which explains the interest companies like IBM and Motorola have taken in the discovery.

Roukes cautions, however, that it might be quite some time before you can purchase a disk drive based on the new effect: ”The shortcoming is that we don’t yet have good semiconductor ferromagnets working at room temperature.” The gallium manganese arsenide films only manifest the effect at temperatures below 40 K—higher temperatures wipe out the ferromagnetism. But, based on theoretical predictions, Roukes said, ”We believe the effect is generic to all semiconductor ferromagnets,” some of which have already demonstrated ferromagnetism at up to 180 K.

Other scientists are even more wary. ”After GMR hit, there were some papers in the mid-1990s about a colossal magnetoresistance effect [CMR],” notes Mark Johnson of the Naval Research Laboratory, ”but the bottom line is that they never got the [operating] temperature” above 150 K. So Johnson says he and others are ”just real skeptical of these materials.”

Even if exploiting the planar effect does not revolutionize hard disk technology, its discoverers believe it will be a useful tool in the effort to develop spintronic devices, which process signals by manipulating the spins of electrons, rather than by moving them around as in traditional electronics [see ”The Quest for the Spin Transistor,” IEEE Spectrum, December 2001, pp. 30-35]. Tang says he can now ”watch a single [magnetic] domain propagating” in a semiconductor ferromagnet and ”look at the dynamic and static properties of the domain walls,” where many phenomena occur that are important to spintronics.

For Roukes, part of the excitement about the new effect is precisely the fact that its ultimate application is unclear. ”Whenever you have some sort of robust phenomenon discovered in the laboratory...you have a profound opportunity for easily making new classes of electronic devices. We feel like we’re sort of on the tip of the iceberg here.”

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