September 1, 1999

Berkeley Lab Science Beat

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The recent announcement of molecular-sized computer switches has renewed predictions of hand-held supercomputers in the near future. Practical benefits from dramatically reducing the size of data processors, however, will only be realized if there is an equally dramatic reduction in the size of data storage devices.

A significant step towards downsizing magnetic storage disks has been achieved by researchers working at Berkeley Lab's Advanced Light Source (ALS). They are producing the first high quality images of "quantum wells" – an energy state in which an electron is sandwiched between two layers of atoms so that its motion is confined to a single dimension.

"As magnetic storage devices become more densely packed with information, reading the data becomes more difficult," says Zi Qiang Qiu, a solid state physicist who holds a joint appointment with the Lab's Materials Sciences Division (MSD) and the Physics Department of the University of California at Berkeley. "A better understanding of the physics behind quantum well states should enable us to make much more sensitive reading heads for magnetic disks, which in turn, should make it possible to increase disk storage density."

Today's typical magnetic storage device is a plastic disk coated with multiple layers of metallic thin films. Data is stored by converting electrical signals representing the 0s and 1s of the binary code into magnetized areas, called "bits," on the disk's metallic film. Orienting a magnetic bit in one direction represents a 0 and in the opposite direction a 1. The smaller the bits that can be written and read, the more densely a disk can be packed with data.

If all magnetic devices could be pushed into the wavelength limit of an electron moving through a solid -- the nanometer scale -- storage capacity would be significantly raised above that of today's record best. The key is to confine the movement of the electron.

"In our experiments, the movement of the electron is restricted so that it bounces back and forth at an interface, perpendicular to the surfaces, to form a standing wave," says Qiu.

Images obtained at the ALS by Qiu and his colleagues revealed that the amplitude of a confined electron wave is modulated by a mathematical function which surrounds the electron wave like a longer wavelength "envelope." This envelope surrounding and modulating the electron wave determines the energy states of the quantum well. As the electron's wavelength gets shorter, quantum well energies increase and vice-versa.

Quantum well energy states are believed to be the basis for "oscillatory coupling" and giant magnetic resistance (GMR). Oscillatory coupling relates magnetic orientations in multiple layers of metallic films to film thickness. GMR is the phenomenon whereby exposure to a small magnetic field induces a large change in electrical resistance. Both effects could be crucial to boosting disk storage density.

Manufacturers of magnetic storage devices are already beginning to exploit these effects through structures in which two magnetic layers are separated by a metallic but nonmagnetic spacer layer. The technology, however, is in its infancy and there is much room for developmental improvement. The wave function data being gathered at Berkeley Lab is considered vital to successful development.

For their studies, the Berkeley researchers varied the thickness of their samples by making them wedge-shaped. This technique enabled the scientists to simultaneously vary the thickness of their metallic layers, creating nano-sized electron interferometers for observing quantum well states in each layer.

In one study, they deposited a wedge-shaped layer of magnetized cobalt over a nickel substrate then covered it at right angles with a spacer wedge made of copper. The variation in photemission intensity from a single scan across the sample with ALS light was the result of wave interferences from quantum well states in the two layers.

In another study, a layer of nickel only one atom thick was embedded between two wedges of copper. This enabled the Berkeley scientists to map the amplitude of their wave functions in much the same way that vibrations of a string can be analyzed by lightly touching it at different spots.

"Touching the string at an antinode or at a node causes considerably different effects in the vibration," says Qiu. "By systemically changing the contact position along the string, it is possible to map out the spatial variation of the vibration amplitude. The nickel layer is analogous to the touch of a finger, causing the vibrational modes of our wave functions to respond accordingly."


Critical to the success of Qiu and his colleagues in this work has been the quality of the x-ray beams generated at the ALS. One of the first of the "third-generation" synchrotron light sources, the ALS produces the world's brightest beams of ultraviolet light and low-energy x-rays. These beams are ideal for the study of surfaces and interfaces on a nanometer scale.

Qiu and his colleagues worked at an undulator beamline called "The Spectromicroscopy Facility." The beam at this facility delivers a trillion photons per second, focused down to a spot size of 50 to 100 microns (about the diameter of a human hair). This tiny spot size makes the detection of quantum interference effects possible.

"We now have sufficient understanding of quantum well states to begin learning more about how these states are coupled with magnetic properties," Qiu says. "Our aim is to reach a level of theoretical understanding and experimental control that will make the wave function engineering of magnetic materials comparable to the bandgap engineering of semiconductors."

Collaborating with Qiu on this project are Roland Kawakami, Hyuk Choi, Ernesto Escorcia-Aparicio, Maria Bowen, Jason Wolfe, Elke Arenholz, and Z.D. Zhang from the UC Berkeley Physics Department, plus Eli Rotenberg, who oversees use of The Spectromicroscopy Facility for the ALS, and Neville Smith, the deputy for the ALS scientific programs. Discussions of their work have already appeared in Science, Nature, and other scientific journals.