Are nanowires the future of racetrack memory?
IBM racetrack memories inspired a team of researchers at the University of California at Davis (UC Davis) to develop a material that can provide performance boosts, giving this type of memory an edge over hard drives (HDs) and solid-state flash.
The researchers are attempting to build racetrack memories from nanowires, resulting in memories that are faster, higher capacity, more reliable, and energy efficient.
"The trend today is to put solid-state flash memory into computers, but it's very expensive compared to hard drives," Bob Havemann, director of Nanomanufacturing Sciences at SRC told EE Times. "But if we can succeed in configuring these racetrack memories we'll have increased speed, lower costs, higher capacity, more reliability and improved energy efficiency."
The work was prompted by UC Davis researchers who have been studying the unusual properties of a complex oxide called LSMO (La0.67 Sr0.33 Mn O3), which exhibits novel magnetic, electrical, and optical properties.
"We have been studying complex oxide materials or multi-ferroics, because their magnetic state can be changed not only by magnetic fields, but also by electric fields and by light," Yayoi Takamura, a professor at UC-Davis, told EE Times. "And with SRC we have been patterning them into the kind of geometry you would need to build a racetrack memory—one-dimensional nanowires with notches in them to trap magnetic domain walls."
A racetrack memory works similarly to a hard disc, but is completely solid state, since instead of the disc spinning its magnetic domains under the write head, in racetrack memories the magnetic domains travel around a closed track consisting of a notched nanowire. The magnetic state of the nanowire can be changed by a write head positioned over it, which can change the magnetic orientation between each notch, then moves all the patterns encoded along the length of the wire to the next notch thus invoking the metaphor of a racetrack.
Source: UC Davis
Illustrated in the picture above, the team assigned red (magnetisation pointing right) to be "1" and blue (magnetisation pointing left) to be "0"; four notches along a one-dimensional nanowire with only one domain wall are in white. In Panel (b) (shown on left), the data before the domain wall are "1"s and the data after the domain wall are "0"s, as indicated that in the detail view of Panel (b) (right side). Panel c shows a different shaped domain wall.
"IBM's previous work was on metallic materials, but we're looking at these complex oxides and trying to understand their differences and how they may work better than metallic materials if configured as a racetrack memory," Takamura told EE Times.
It will be at least a year before Takamura's group attempts to build a complete racetrack memory, for which they will need the help of one of SRC's members to fabricate the chip. Right now UC Davis is growing its own films, but they have to send the wafers to Oak Ridge National Laboratories to pattern them into notched nanometre-sized wires. Other organisations assisting the Takamura Research Group included the Centre for Nanophase Materials Sciences and the Advanced Light Source at Lawrence Berkeley National Laboratory.
Takamura told EE Times:
We don't have a [racetrack] prototype device at this point. What we have is a demonstration in which we can see similar phenomena that must occur in the racetrack memory device in a completely different material family, in this case complex oxides, compared to the metal systems that are typically employed. Our next step will be to study the geometry of the nanowires, the optimal shape of the notches and how closely together we can pack them which defines the density of the memory.
The challenges that remain include optimising the shape of the domain walls formed, controlling their position within the nanowires and mastering their movement along around the nanowire racetrack. The parameters to be optimised include the intensity of the applied magnetic and electrical fields, light irradiation levels, pressure and temperature.
- R. Colin Johnson
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