AFM image of a mixed-phase bismuth ferrite sample. The red and green areas indicate phase regions oriented at 90° to each other. Credit: Ramesh Group; Berkeley Lab.
Following on the heels of our earlier story about multiferroic superconductor work at Berkeley Lab, we also received word about work there regarding spontaneous magnetization in bismuth ferrite (BFO), another multiferroic material.
What seems to have researchers really excited is that they can turn this magnetization on and off via an external electric field at room temperature, making the BFO a possible material for spintronic applications.
In a news release from the lab, Ramamoorthy Ramesh, the materials scientist with lab’s Materials Sciences Division who led this research, explains the novel approach taken:
“[W]e’ve created a new magnetic state in bismuth ferrite along with the ability to electrically control this magnetism at room temperature. An enhanced magnetization arises in the rhombohedral phases of our bismuth ferrite self-assembled nanostructures. This magnetization is strain-confined between the tetragonal phases of the material and can be erased by the application of an electric field. The magnetization is restored when the polarity of the electric field is reversed.”
In 2009, Ramesh and his research group looked at thin films of the BFO and found that although bismuth ferrite is an insulator, two of three domain wall orientations in the material conduct electricity.
Ramesh and his group subsequently found that application of a large epitaxial strain (compression in the direction of a material’s crystal planes) changes the BFO crystal structure from a predominant rhombohedral phase into a tetragonal phase.
Conversely, a partial strain reduction produces a nanoscale mixed phase composed of the rhombohedral and tetragonal phases. Ramesh says the mixture can be stable, with the rhombohedral phases mechanically confined by regions of the tetragonal phases.
Schematic of rhombohedral and tetragonal phases in BFO film. Magnetization is confined to the rhombohedral phase. Credit: Ramesh Group; Berkeley Lab.
As a result of the interaction, a different magnetic moment (30 to 40 electromagnetic units per cubic centimeter) spontaneously arises within the distorted rhombohedral phase — one that is qualitatively stronger than the magnetic moment of fully rhombohedral BFO (6 to 8 electromagnetic units/cubic centimeter).
Ramesh says these differences are large enough to be put to use, and can be manipulated using an external electrical field rather than using a magnetic field used in conventional memory devices.
Results of the group’s work is published Nature Communications (doi:10.1038/ncomms1221).