Lawrence Berkeley National Laboratory

Multiferroics - Making a switch the electric way

Multiferroics are materials in which unique combinations  of electric and magnetic properties can simultaneously coexist. They are  potential cornerstones in future magnetic data storage and spintronic  devices provided a simple and fast way can be found to turn their  electric and magnetic properties on and off. In a promising new  development, researchers with the U.S. Department of Energy’s Lawrence  Berkeley National Laboratory (Berkeley Lab) working with a prototypical  multiferroic have successfully demonstrated just such a switch --  electric fields.

“Using electric fields, we have been able to create, erase and invert  p–n junctions in a calcium-doped bismuth ferrite film,” said Ramamoorthy  Ramesh of Berkeley Lab’s Materials Sciences Division (MSD), who led this  research.
“Through the combination of electronic conduction with the electric and  magnetic properties already present in the multiferroic bismuth ferrite,  our demonstration opens the door to merging magnetoelectrics and  magnetoelectronics at room temperature.”

Ramesh, who is also a professor in the Department of Materials Science  and Engineering and the Department of Physics at UC Berkeley, has  published a paper on this research that is now available in the on-line  edition of the journal /Nature Materials/. The paper is titled:  “Electric modulation of conduction in multiferroic Ca-doped BiFeO_3 films.” Co-authoring the paper with Ramesh were Chan-Ho  Yang, Jan Seidel,Sang-Yong Kim, Pim Rossen, Pu Yu, Marcin Gajek,  Ying-Hao Chu, Lane Martin, Micky Holcomb, Qing He, Petro Maksymovych,  Nina Balke, Sergei Kalinin, Arthur Baddorf, Sourav Basu and Matthew  Scullin.

The next generation of computers promises to be smaller, faster and far  more versatile than today’s devices thanks in part to the anticipated  development of memory chips that store data through electron spin and  its associated magnetic moment rather than electron charge. Because  multiferroics simultaneously exhibit two or more ferro electric or  magnetic properties in response to changes in their environment, they’re  considered prime candidates to be the materials of choice for this  technology.

Bismuth ferrite is a multiferroic comprised of bismuth, iron and oxygen  (BiFeO_3 ). It is both ferroelectric and antiferromagnetic (“ferro”  refers to magnetism in iron but the term has grown to include materials  and properties that have nothing to do with iron), and has commanded  particular interest in the spintronics field, especially after a  surprising discovery by Ramesh and his group earlier this year. They  found that although bismuth ferrite is an insulating material, running  through its crystals are ultrathin (two-dimensional) sheets called  “domain walls” that conduct electricity at room temperature. This  discovery suggested that with the right doping, the conducting states in  bismuth ferrite could be stabilized, opening the possibility of creating  p-n junctions, a crucial key to solid state electronics.

“Insulator to conductor transitions are typically controlled through the  combination of chemical doping and magnetic fields but magnetic fields  are too expensive and energy-consuming to be practical in commercial  devices,” said Ramesh. “Electric fields are much more useful control  parameters because you can easily apply a voltage across a sample and  modulate it as needed to induce insulator-conductor transitions.”

In their new study, Ramesh and his group first doped the bismuth ferrite  with calcium acceptor ions, which are known to increase the amount of  electric current that materials like bismuth ferrite can carry. The  addition of the calcium ions created positively-charged oxygen  vacancies. When an electric field was applied to the calcium-doped  bismuth ferrite films, the oxygen vacancies became mobile. The electric  field “swept” the oxygen vacancies towards the film’s top surface,  creating an n-type semiconductor in that portion of the film, while the  immobile calcium ions  created a p-type semiconductor in the bottom  portion. Reversing the direction of the electric field inverted the  n-type and p-type semiconductor regions, and a moderate field erased them.

“It is the same principle as in a CMOS device where the application of a  voltage serves as an on/off switch that controls electron transport  properties and changes electrical resistance from high (insulator) to  low (conductor),” said Ramesh.

Whereas a typical CMOS device features an on/off switching ratio (the  difference between resistance and non-resistance to electrical current)  of about one million, Ramesh and his group achieved an on/off switching  ratio of about a thousand in their calcium-doped bismuth ferrite films.  While this ratio is sufficient for device operation and double the best  ratio achieved with magnetic fields, Chan-Ho Yang, lead author on this  /Nature Materials/ paper and a post-doc in Ramesh’s group says it can be  improved.

“To make the ON state more conductive, we have many ideas  to try such  as different calcium-doping ratios, different strain states, different  growth conditions, and eventually different compounds using the same  idea,” Yang said.

A year ago, Ramesh and his group demonstrated that an electric field  could be used to control ferromagnetism in a non-doped bismuth ferrite  film. (See /Nature Materials/, “Electric-field control of local  ferromagnetism using a magnetoelectric multiferroic” by Ramesh, et. al)

With this new demonstration that the combination of doping and an  applied electric field can change the insulating-conducting state of a  multiferroic, he and his colleagues have shown one way forward in  adapting multiferroics to such phenomena as colossal magnetoresistance,  high temperature superconductivity and SQUID-type magnetic field  detectors as well as spintronics.

Said Yang, “Oxides such as bismuth ferrite are abundant and display many  exotic properties including high-temperature superconductivity and  colossal magnetoresistance, but they have not been used much in real  applications because it has been so difficult to control defects,  especially, oxygen vacancies. Our observations suggest a general  technique to make oxygen vacancy defects controllable.”

Much of the work in this latest study by Ramesh and his group was  carried out at Berkeley Lab’s Advanced Light Source (ALS), on the PEEM2  microscope. PEEM, which stands for PhotoEmission Electron Microscopy, is  an ideal technique for studying ferro magnetic and antimagnetic domains,  and PEEM2, powered by a bend magnet at ALS  beamline, is one of  the world’s best instruments, able to resolve features only a few  nanometers thick.

“Without the capabilities of PEEM2 our experiments would have been dead  in the water,” said Ramesh. “Andreas Scholl (who manages PEEM2) and his  ALS team were an enormous help.”

This research was primarily supported by the U.S. Department of Energy’s  Office of Science through its Basic Energy Sciences program.

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