Bismuth-ferrite could make spin-valves that use 1/10th the power of STT
A research team led by folks at Cornel University (along with University of California, Berkeley; Tsinghua University; and Swiss Federal Institute of Technology in Zurich) have discovered how to make a single-phase multiferroic switch out of bismuth ferrite (BiFeO3) as shown in an online letter toNature. Multiferroics, allowing for the control of magnetism with an electric field, have been investigated as a potential solid-state memory cell for many years but this is the first time that reversible room-temperature switching has been reportedly achieved at room temperature. Most importantly, the energy per unit area required to switch these new cells is approximately an order of magnitude less than that needed for spin-transfer torque (STT) switching.
“The advantage here is low energy consumption,” said Cornell postdoctoral associate John Heron, in a press release. “It requires a low voltage, without current, to switch it. Devices that use currents consume more energy and dissipate a significant amount of that energy in the form of heat.”
The trick that Heron and others discovered involves a two-step sequence of partial switching events—using only applied voltages—that add up to full magnetic reversal. Previous theory had shown that single-step switching was thermodynamically impossible, and no other groups had reported work on similar two-step switching. Also published in the News & Views section of Nature is “Materials science: Two steps for a magnetoelectric switch” written by other researchers, which explores the possibilities of using this phenomenon in nanoscale memory chips.
While the thermodynamics of all of this seem incredibly positive, the kinetics of this two-step process have yet to be reported. Also, the effect seems to require specific crystal stuctures such as that of SrRuO3 in a particular orientation as electrical contacts, instead of the inherently less-expensive randomly oriented metal contacts to STT cells. Consequently, this could be inherently slow and expensive technology, and thus limited to niche applications.
Sharon C. Glotzer and Nicholas A. Kotov are both researchers at the University of Michigan who were just awarded a MRS Medal at the Materials Research Society (MRS) Fall Meeting in San Francisco for their work on “Integration of Computation and Experiment for Discovery and Design of Nanoparticle Self-Assembly.” Due to the fact that surface atoms compose a large percent of the mass of nanoparticles, the functional properties of quasi-1D nanoparticles differ significantly from 2D thin-films and from 3D bulk materials. An example of such a unique functional property is seen in self-assembly of nanoparticles to form complex structures, which could find applications in renewable energy production, optoelectronics, and medical electronics.
While self-assembly has been understood as an emergent property of nanoparticles, research and development (R&D) has been somewhat limited to experimental trial-and-error due to a lack of theory. Glotzer and Kotov along with their colleagues have moved past this limit using a tight collaboration between computational prediction and experimental observation. The computational theorist Glotzer provides modeling on shapes and symmetric structures, while the experimentalist Kotov’s explores areas involving atomic composition and finite interactions. Kotov and his students create a nanoparticle and look for Glotzer and her group to explan the structure. Conversely, Glotzer predicts the formation of certain structures and has those predictions confirmed experimentally by Kotov.
One specific area the two scientists have explored is the formation of supraparticles—agglomerations of tightly packed nanoparticles that are self-limiting in size. The supraparticles are so regular in size and sphericality that they would actually pack to form face-centered-cubic (fcc) lattice-like structures. The theoretical and computational work, followed by experimental verification, further proved that these supraparticles could be formed from a vast variety of nanoparticles and even proteins, provided they were small enough and had significant van der Waals and electronic repulsion forces. This exciting development creates a whole new class of “bionic” materials that may combine biomaterials and inorganics. —E.K.