Switching to spintronics

Researchers at the US DOE’s Berkeley Lab and Cornell University have successfully used an electric field to reverse the magnetisation direction in a multiferroic spintronic device at room temperature.

This demonstration, which runs counter to conventional scientific wisdom, points a new way towards spintronics and smaller, faster and cheaper ways of storing and processing data. “Our work shows that 180° magnetisation switching in the multiferroic bismuth ferrite can be achieved at room temperature with an external electric field when the kinetics of the switching involves a two-step process,” says Ramamoorthy Ramesh, Berkeley Lab’s associate laboratory director for energy technologies, who led this research. “We exploited this multistep switching process to demonstrate energy-efficient control of a spintronic device.” Ramesh is the senior author of a paper describing this research in Nature. John Heron, now with Cornell University, is the lead and corresponding author.

Multiferroics are materials in which unique combinations of electric and magnetic properties can simultaneously coexist. They are viewed as potential cornerstones in future data storage and processing devices because their magnetism can be controlled by an electric field rather than an electric current, a distinct advantage as Heron explains. “The electrical current that today’s memory and logic devices rely on to generate a magnetic field are the primary source of power consumption and heating in these devices,” he said. 

“This has triggered significant interest in multiferroics for their potential to reduce energy consumption while also adding functionality to devices.” Nature, however, has imposed thermodynamic barriers and material symmetry constrains that theorists believed would prevent the reversal of magnetisation in a multiferroic by an applied electric field. Earlier work by Ramesh and his group with bismuth ferrite, the only known thermodynamically stable room-temperature multiferroic, in which an electric field was used as on/off switch for magnetism, suggested that the kinetics of the switching process might be a way to overcome these barriers, something not considered in prior theoretical work.

“Having made devices and done on/off switching with in-plane electric fields in the past, it was a natural extension to study what happens when an out-of-plane electric field is applied,” Ramesh says.

Ramesh, Heron and their co-authors set up a theoretical study in which an out-of-plane electric field - meaning it ran perpendicular to the orientation of the sample - was applied to bismuth ferrite films. They discovered a two-step switching process that relies on ferroelectric polarisation and the rotation of the oxygen octahedral. “The oxygen octahedral rotation is also critical because it is the mechanism responsible for the ferromagnetism in bismuth ferrite. Rotation of the oxygen octahedral also allows us to couple bismuth ferrite to a good ferromagnet such as cobalt-iron for use in a spintronic device.”

To demonstrate the potential technological applicability of their technique, Ramesh, Heron and their co-authors used heterostructures of bismuth ferrite and cobalt iron to fabricate a spin-valve, a spintronic device consisting of a non-magnetic material sandwiched between two ferromagnets whose electrical resistance can be readily changed.

X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) images showed a clear correlation between magnetisation switching and the switching from high-to-low electrical resistance in the spin-valve. The XMCD-PEEM measurements were completed at PEEM-3, an aberration corrected photoemission electron microscope at beamline 11.0.1 of Berkeley Lab’s Advanced Light Source.

“We also demonstrated that using an out-of-plane electric field to control the spin-valve consumed energy at a rate of about one order of magnitude lower than switching the device using a spin-polarised current,” Ramesh says.