A New Approach to Superconducting Memory

Despite the potential of superconductor-based electronics to significantly impact the electronics industry – for example, a superconducting computer chip is a thousand times faster than the one within the laptop I'm using right now – they have not taken off the way some researchers anticipated. One reason may be that superconducting memory chips are just not dense enough; they can only hold up to 16 kilobits of information, compared to up to several gigabits delivered by other chips.

But now, a team of scientists from the University of Augsburg in Germany and Stanford University has taken steps to help fix this problem. They have proposed a superconducting memory concept that uses a ferromagnetic material to store data.

“We have made significant improvement to superconducting memory, and we hope that our design can be integrated into current devices on a large scale and operated at high speeds,” said the study's corresponding scientist, University of Augsburg physicist Jochen Mannhart, to PhysOrg.com.

Current superconducting memory devices have several disadvantages, but perhaps the most prevalent is their large size. This is due to the way these devices store data: Bits of information take the form of basic units, or “quanta,” of magnetic flux (flux being a measure of the strength and extent of a magnetic field). These flux quanta occupy a relatively large amount of space, however, and are stored in circuit components called superconducting quantum interference devices (SQUIDs) that must be large to accommodate them.

Mannhart and his team overcame this size limitation by rethinking memory cell design. Their concept stores data by coupling tiny ferromagnetic “dots” to electronic components called Josephson junctions – current-conducting “sandwiches” formed by a thin layer of an insulating material between two superconducting layers.

To write bits of data to the cell, a current flowing down a “write line” creates a magnetic field that magnetizes the dots. The direction of each dot's magnetic field – pointing toward the Josephson junction or away from it – determines whether the dot will correspond to a “0” or a “1.” It is the job of the Josephson junction to make that determination. It does this by adding its own magnetic field to a background magnetic field, which is generated by a direct current running under it. Because the strength of the resulting field depends on the direction of the dot's field, knowing the former allows one to know the latter.

This memory concept gets around the size issue because the dimensions of the ferromagnetic dots and their distance from the Josephson junctions do not compromise the operation of the cell. The dots can be very small and very close to the junctions, and the cell will still function properly.

The group tested their concept by fabricating a memory chip out of the metal niobium (Nb). The chip consists of a pattern of several Nb Josephson junction arrays, with eight junctions per array. Each junction is coupled to a ferromagnetic dot that is 6 micrometers wide, 9 micrometers long, and 600 nanometers thick. There are many ferromagnetic materials that would work, but for this first test the group chose a nickel-iron compound commonly known as Permalloy.

“Our analysis of the test cell showed that it did achieve memory function – it can store data,” said Mannhart. “These first cells showed some problems, such as the large write current needed to magnetize the Permalloy, but by optimizing the cell design and the materials, these problems can hopefully be resolved.”

This work is reported in the October 19, 2006, online edition of Applied Physics Letters.

Citation: R. Held, J. Xu, A. Schmehl, C.W. Schneider, J. Mannhart, and M.R. Beasley, “Superconducting memory based on ferromagnetism.” Appl. Phys. Lett. 89, 163509

By Laura Mgrdichian, Copyright 2006 PhysOrg.com

Related stories:

A fresh spin in quantum physics: The 'spin triplet' supercurrent
For the first time, scientists have created a “spin triplet” supercurrent through a ferromagnet over a long distance. Achieved with a magnet developed at Brown University and the University of Alabama, the feat upends long-standing theories of quantum physics – and may be a boon to the budding field of “spintronics,” where the spin of electrons, along with their charge, is harnessed to power computer chips and circuits. Results are published in Nature.
Breakthrough in high-temperature superconductivity
Scientists at the University of Aberdeen have made a major breakthrough towards the mechanism of high-temperature superconductivity. Results from studies of a crystal structure of a new chemical compound containing copper and ruthenium have provided valuable insight into the mechanism of high-temperature superconductivity. The new results have shown for the first time that the mechanism of high-temperature superconductivity (when there is no resistance to the flow of electrical current) is actually coupled to the crystal lattice.



National Academy honors 17 for major contributions to science
The National Academy of Sciences (NAS) has selected 17 individuals to receive awards honoring their outstanding scientific achievements.
The awards will be presented on May 2 at a ceremony in Washington, D.C., during the Academy's 142nd annual meeting.
IBM, Stanford Collaborate on World-Class Spintronics Research
SAN JOSE, Calif. -- April 26, 2004 -- IBM and Stanford University are joining forces on the advanced research and creation of new high-performance, low-power electronics in the emerging field of nanotechnology called "spintronics." To formalize the effort, scientists at IBM's Almaden Research Center and Stanford University today announced the formation of the IBM-Stanford Spintronic Science and Applications Center (SpinAps, for short).
Long-Lasting Quantum Memory Leads to Long-Distance Quantum Communication
(PhysOrg.com) -- Physicists have taken a step closer to realizing long-distance quantum communication, in which a quantum state is transferred from one location to another by becoming entangled with a traveling photon.
Researchers propose minocycline as a promising drug for patients with Fragile X syndrome
A UC Riverside-led team of biomedical scientists has found that a readily available drug called minocycline, used widely to treat acne and skin infections, can be used to treat Fragile X syndrome, the most common inherited cause of mental impairment and the most common cause of autism.
Zooming way in, technique offers close-ups of electrons, nuclei
Providing a glimpse into the infinitesimal, physicists have found a novel way of spying on some of the universe's tiniest building blocks.Their "camera," described this week in the journal Nature, consists of a special "flaw" in diamonds that can be manipulated into sensitively monitoring magnetic signals from individual electrons and atomic nuclei placed nearby.
Lava flows reveal clues to magnetic field reversals
(PhysOrg.com) -- Ancient lava flows are guiding a better understanding of what generates and controls the Earth's magnetic field — and what may drive it to occasionally reverse direction.

News discussion:

Physics news