BALTIMORE The refinement of techniques for "locking in" memory values with magnetic spin could help magnetic random-access memories mount a challenge to volatile DRAMs.
"DRAMs use the charge on tiny capacitors to store current memory values, but they leak and must be electronically refreshed periodically to prevent data loss," said Chia-Ling Chien, professor of physics at the Krieger School of Arts and Sciences at Johns Hopkins University. "MRAM chips will rely on magnetic spin orientations to lock in stored values, so even a loss of power will not cause the MRAM to lose the data it stores."
Chien collaborated with Gang Xiao, professor of physics at Brown University, to create a new material that makes the magnetic-spin memory-locking technique possible. The research was funded by the National Science Foundation through the Materials Research Science and Engineering Center at Johns Hopkins.
The researchers call the new material, derived from chromium dioxide (CrO2), a half-metallic ferromagnet. Half-metallic materials use the spin orientation of their magnetic domains, a characteristic that previously has been leveraged to manipulate the sensitivity of magnetic sensors. For instance, magnetic heads for hard disks today comprise materials that allow the spin orientation to be precisely controlled.
"Others have already created half-metallic materials that allow scientists and engineers to make superior magnetic sensors, so that, for instance, more information can be stored in hard drives," said Xiao, whose lab, with help from IBM Corp., developed the single-crystal films of the CrO2 material.
"The simple spintronic devices invented for hard drives were so advantageous that they're now found in practically all hard drives' heads," added Chien.
Few conventional electronic circuits, other than new disk drive heads, use spin. But in fact all electrons have both a charge and a spin direction that can be harnessed to enable a new generation of magneto-electronic spintronic devices a specialty of the Johns Hopkins Materials Research Science and Engineering Center, where Chien is a director.
"Because electron spin and a material's magnetic properties are linked, devices based on this technology can manipulate both the charge of electrons and their spin," said Chien.
He noted that the spin of each electron behaves like a tiny magnet, with north and south poles, described by the orientation of the magnetic pole as either "up" or "down." To determine the "spin polarization" of a material, scientists measure the percentage of electrons in a metal with spin "up" as opposed to the percentage with spin "down." For instance, measurements to determine the spin polarization of copper reveal that it has zero spin polarization, whereas common magnets have about a 40 percent spin polarization.
More than a decade ago, scientists predicted that materials could theoretically be produced that have 100 percent spin polarization, but it was not until the recent successes of magneto-electronic disk drive heads that scientists made a concerted effort to produce them, according to Chien. At 100 percent spin polarization, a material has the same spin orientation either all up or all down thereby eliminating one of the two possibilities for electron spin. Scientists call the condition "leaving only one spin band."
In a normal metal, by contrast, both spin bands are present, which is why scientists call a material that is 100 percent spin-polarized "half-metallic."
Xiao and colleagues at Brown and IBM realized their half-metallic material by growing a single-crystal film of chromium dioxide on a silicon substrate using chemical-vapor deposition. A superconducting sensor was then used to measure the spin polarization of the chromium dioxide film. The film proved to be 96 percent spin-polarized.
"Chromium dioxide is a gift from nature with its simple yet exciting properties, but even so it took more than two years to perfect our method of making this material with high yields," Xiao said.
Researchers at IBM are now working to incorporate the chromium dioxide films into a memory structure that can be used as the heart of a magnetic RAM. The result is a technology known as a magnetic tunnel junction, consisting of an insulator sandwiched between spintronic electrodes, or "plates." By controlling the polarization of the magnetization on each plate, the scientists hope to make the junction switch between high resistance to electricity and low resistance, thereby "locking" in the stored charge.
By using the new half-metallic ferromagnet for the plates, it should be possible to lock in the charge so that it remains even when the power is turned off.