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 to Nature. 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.
—E.K.