Magnetics Business & Technology - Spring 2015 - (Page 20)
RESEARCH & DEVELOPMENT
Berkeley Lab Reports on Electric Field Switching of Ferromagnetism at Room Temp
In a development that holds promise for future magnetic memory and logic devices, researchers with the US Department of Energy
(DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) and
Cornell University successfully used an electric field to reverse the
magnetization 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° magnetization 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," said Ramamoorthy Ramesh, Berkeley Lab's Associate Laboratory Director for Energy Technologies, who led this
research. "We exploited this multi-step switching process to demonstrate energy-efficient control of a spintronic device."
Ramesh, who also holds the Purnendu Chatterjee Endowed Chair
in Energy Technologies at the University of California (UC) Berkeley,
is the senior author of a paper describing this research in Nature.
The paper is titled "Deterministic switching of ferromagnetism at
room temperature using an electric field." 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
"The electrical currents 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 magnetization 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
"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 said.
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 polarization and the rotation of the oxygen octahedral.
"The two-step switching process is key as it allows the octahedral rotation to couple to the polarization," Heron said. "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
To demonstrate the potential technological applicability of their
technique, Ramesh, Heron and their co-authors used heterostruc-
Magnetics Business & Technology * Spring 2015
Conceptual illustration of how magnetism is reversed (see compass)
by the application of an electric field (blue dots) applied across gold
capacitors. Blurring of compass needles under electric field represents
two-step process. (Image courtesy of John Heron, Cornell)
tures 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 magnetization 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 spinpolarized current," Ramesh said.
In addition to Ramesh and Heron, other co-authors of the Nature
paper were James Bosse, Qing He, Ya Gao, Morgan Trassin, Linghan
Ye, James Clarkson, Chen Wang, Jian Liu, Sayeef Salahuddin, Dan
Ralph, Darrell Schlom, Jorge Iniguez and Bryan Huey.
Table of Contents for the Digital Edition of Magnetics Business & Technology - Spring 2015
First-Order-Reversal-Curve Analysis of Multi-Phase Ferrite Magnets
Magnetics Design Tool for Power Applications
Magnet Inspection Tool with High Magnetic and Mechanic Accuracy
Research & Development
Software & Design
Marketplace / Advertising Index
Spontaneous Thoughts: The Patent Challenge
Magnetics Business & Technology - Spring 2015