Cutting magnets down to quantum-effect size.

The rules of quantum mechannics allow the safe passage of a truck through a mountain instead of over it. But for objects larger than an atom, such extraordinary events have an extremely low probability of happening because of the objects' size.

Nonetheless, theorists have suggested that quantum tunneling may occur not only in submicroscopic systems, but also on a macroscopic scale - in tiny magnets made up of several thousand atoms each. Using naturally produced magnets encased in protein molecules, researchers have now obtained a hint that quantum tunneling within a magnet allows a transition from one magnetic field direction to another to occur in large aggregations of atoms or ions.

"Our interest is in testing whether macroscopic quantum phenomena can be directly observed experimentally," says physicist David D. Awschalom of the University of California, Santa Barbara. The idea that quantum tunneling can occur in sufficiently small magnets also has important technological implications as a fundamental barrier to ongoing efforts to pack increasing amounts of information on magnetic tapes or disks.

To search for macroscopic quantum tunneling, Awschalom and his co-workers turned to a protein known as ferritin, which serves as a storehouse for iron in cells. Each protein molecule has a magnetic core containing about 4,500 iron ions. In this particular case, the spins of neighboring ions line up parallel to each other but in opposite directions to create what is known as an antiferromagnet.

"The whole magnet acts like one big quantum particle--one big spin--which could point up or down," Awschalom says.

To measure the exceedingly weak magnetic fields involved, the researchers used advanced superconducting sensors and performed their experiments at temperatures below 1 kelvin. They describe their technique in the Oct. 16 SCIENCE.

The measurements revealed that ferritin molecules strongly absorb electro-magnetic radiation at a frequency near 1 megahertz. Awschalom and his colleagues attribute that absorption to quantum tunneling back and forth between two particular magnetic states.

"It's qualitatively consistent with all of the quantum-mechanical predictions in terms of temperature, field, and density--every parameter that we varied," Awschalom says. "Though some of the numbers are not in exact agreement, there is no other self-consistent explanation that anyone's been able to provide."

Other researchers remain skeptical. "I think their technique is very interesting and promising," says Anupam Garg of Northwestern University in Evanston, Ill., "but I'm very doubtful that they're seeing macroscopic quantum coherence."

One problem involves uncertainties in the geometry of the magnetic protein cores. "If one were actually capable of making such small particles, one would have to be very careful in how one aligns them, and one would have to expend considerable effort characterizing them," Garg notes.

Similar concerns surround earlier work done by B. Barbara and co-workers at the Louis Neel Laboratory of Magnetism in Grenoble, France. Their findings also revealed an unusual magnetic effect that they attributed to quantum tunneling in ferromagnetic particles somewhat larger than those used by Awschalom's group.

"No one has yet done the conclusive experiment," says Philip C.E. Stamp of the University of British Columbia in Vancouver. "There are hopeful signs, but there is no proof."

Awschalom and his group are now looking for quantum tunneling in precisely manufactured magnetic particles about 100 times larger than the 7.5-nanometer, naturally occurring protein magnets they had previously used. "We've spent a year and half making these particles, which is the hard part," Awschalom says. "We're measuring [their magnetic properties] now."