Power/Performance Bits: Aug. 11

Tilting magnets for memory

UC Berkeley researchers discovered a new way to switch the polarization of nanomagnets, which may offer a way for high-density storage to move from hard disks onto integrated circuits and potentially open the door to a memory system that can be packed onto a microprocessor.

Creating and switching polarity in magnets without an external magnetic field has been a key focus in the field of spintronics. Generating a magnetic field takes power and space, the problem preventing the integration of magnets onto chips.

This image taken from a computer simulation shows nanomagnets tilted at various angles, with the white regions indicating greater angles of tilt. Researchers have found that even a small tilt of 2 degrees will facilitate magnetic switching. (Source: Samuel Smith/UC Berkeley)

In past research, the team found that directing electrical current through the rare metal tantalum creates polarity in magnets without an external magnetic field. But the battle wasn’t over: packing a sufficient number of nanomagnets onto a chip meant aligning them perpendicularly, but that vertical orientation negated the switching effects of tantalum.

So the team looked at the problem from a different angle. According to Sayeef Salahuddin, an associate professor of electrical engineering and computer sciences at UC Berkley, “We found that by tilting the magnet – just 2 degrees was enough – you get all the benefits of a high-density magnetic switch without the need for an external magnetic field.”

Graphene nanotube hybrid switches

A research team from Michigan Technological University created digital switches constructed of graphene and boron nitride nanotubes.

Graphene is a molecule-thick sheet of carbon atoms; the nanotubes are like straws made of boron and nitrogen. As a conductor, graphene lets electrons zip too fast—there’s no controlling or stopping them—while boron nitride nanotubes are so insulating that electrons are rebuffed like an overeager dog hitting the patio door.

“The question is: How do you fuse these two materials together?” asked Yoke Khin Yap, a professor of physics at MTU. The key was found when the team maximized their existing chemical structures and exploited their mismatched features. The researchers exfoliated graphene and modified the material’s surface with tiny pinholes, then grew the nanotubes up and through the pinholes.

The chemical structures of graphene (gray) and boron nitride nanotubes (pink and purple) were key to creating the digital switch. (Source: Michigan Tech)

“When we put these two aliens together, we create something better,” said Yap. “When we put them together, you form a band gap mismatch—that creates a so-called ‘potential barrier’ that stops electrons.”

The band gap mismatch results from the materials’ structure: graphene’s flat sheet conducts electricity quickly, and the atomic structure in the nanotubes halts electric currents. This disparity creates a barrier, caused by the difference in electron movement as currents move next to and past the hair-like boron nitride nanotubes. These points of contact between the materials—called heterojunctions—are what make the switch possible.

The research team also showed that because the materials are respectively so effective at conducting or stopping electricity, the resulting switching ratio is high: several orders of magnitude greater than current graphene switches. Ultimately, the team hopes to further research into making transistors without semiconductors.

Stretchy conductivity

An international research team based at the University of Texas at Dallas constructed electrically conducting fibers that can be reversibly stretched to over 14 times their initial length and whose electrical conductivity increases 200-fold when stretched.

The research team is using the new fibers to make artificial muscles, as well as capacitors whose energy storage capacity increases about tenfold when the fibers are stretched.

The new fibers differ from conventional materials in several ways. When conventional fibers are stretched, the resulting increase in length and decrease in cross-sectional area restricts the flow of electrons through the material. But even a “giant” stretch of the new conducting sheath-core fibers causes little change in their electrical resistance.

This illustration shows complex two-dimensional buckling, shown in yellow, of the carbon nanotube sheath/rubber-core fiber. The buckling results in a conductive fiber with super elasticity and novel electronic properties. (Source: Alan G. MacDiarmid/NanoTech Institute UT Dallas)

One key to the performance of the new conducting elastic fibers is the introduction of buckling into the carbon nanotube sheets. Because the rubber core is stretched along its length as the sheets are being wrapped around it, when the wrapped rubber relaxes, the carbon nanofibers form a complex buckled structure, which allows for repeated stretching of the fiber.

By adding a thin overcoat of rubber to the sheath-core fibers and then another carbon nanotube sheath, the researchers made strain sensors and artificial muscles in which the buckled nanotube sheaths serve as electrodes and the thin rubber layer is a dielectric, resulting in a fiber capacitor. These fiber capacitors exhibited a capacitance change of 860 percent when the fiber was stretched 950 percent.