Taking a low energy spin
We add gadget to gadget in our carpetbag of electronic goodies well aware that the devices can only be themselves when there is enough power at hand. Read and write operations, which are computing tasks that include getting information from a computer’s hard drive and putting it there, all take energy to run. Researchers are exploring lower power ways to do the job.
The idea is to one day lose conventional switching methods, which require a magnetic field, so that an electric current is enough to handle the switching. Solutions that have been tried up to now have unfortunately puffed out too much energy as heat loss, the authors of a new study write.
As scientists hunt for the right materials a number of groups focus on multiferroics, which have garnered much interest among physicists. These materials are multitaskers: they have both electric and magnetic properties. Applying an electric field to these multiferroics can reverse their magnetization. But getting them to do so in an orderly fashion is hard.
Materials scientists at the University of California Berkeley and the National Chiao Tung University in Taiwan along with Intel Corporation have now managed the feat and published their work in the journal Physical Review Letters. With their, in their words, “heretofore unreported” method of magnetization reversal with an electric field, they may well have changed the outlook in this field.
Their method could “pave the way to a new generation of compact magnetic devices with unmatched energy efficiency,” writes materials scientist Manfred Fiebig of the ETH Zurich in an accompanying editorial to their study.
It has long been known that an electric field can be used to change a material’s magnetic properties. This quality is one the data storage industry likes, which is why multiferroic compounds hold promise. Yet the marriage between magnetic and electric order in these materials is not “a happy one,” writes Fiebig.
Multiferroic compounds generally require low temperatures, which is one challenge to working with them and creating applications. And even under those conditions, they do not deliver much in terms of energy-saving effects. Some scientists have been focusing on a multiferroic called BiFeO3, Fiebig explains, which shows “robust” multiferroicity at room temperature.
In their new paper, the Berkeley researchers explain how they lowered the energy needs of writing a magnetic state. They leveraged the goings-on at the interface between a ferromagnetic layer and the mutiferroic BiFeO3.
Since nature does not deliver a wealth of materials with desired multiple electric and magnetic traits, researchers like the Berkeley group have been fabricating heterostructures, thin-film composite materials. They hook up ferromagnets and ferroelectrics into interacting heterostructures. Apply a magnetic field to one partner in such a structure creates strain, which becomes electric current to the other partner in the structure.
In this new work, the scientists created a composite by depositing a ferromagnetic alloy, CoFe, onto films of the multiferroic BiFeO3. When they applied an electric field to this heterostructure, they were able to reversibly switch the ferromagnet’s magnetization at room temperature and in the absence of a magnetic field.
Fiebig says that the Berkeley group has now achieved “one of the longstanding goals” in the field of multiferroics research. The heterostructure creates electric and magnetic behavior that is “mediated by an interfacial magnetic coupling dictated by the mutiferroic,” state the scientists in their paper. The composite can be directed to show its effects with precision.
The Berkeley researchers call their work “a critical advancement” offering a “unique” pathway in writing a magnetic state. They suggest that a next step would be to connect a spin valve device to the BiFeO3 layer. “Such a design would allow for the read and write operations to be performed on the same device using only small currents for reading and an electric field for writing,” they note.
Fiebig points out that it will be interesting to see how fast the group can get the magnetoswitching to run since fast switching is “essential” for memory applications.
Petrus Peregrinus, a 13th century engineer, experimentally showed the magnetic properties of ore and that “from the poles of the world” do the poles of the lodestone “receive their virtue.”
In his letter explaining his work and the subject, he wrote about the “hidden virtue of the lodestone,” which was shrouded in darkness until brought to light “by application to public utility.” His treatise dates back to 1269. It appears materials still have many virtues for scientists to bring to light.
References and links:
Electric-Field-Induced Magnetization Reversal in a Ferromagnet-Multiferroic Heterostructure
J. T. Heron, M. Trassin, K. Ashraf, M. Gajek,2 Q. He, S.Y. Yang, D. E. Nikonov, Y-H. Chu, S. Salahuddin, and R. Ramesh Physical Review Letters, 107, 217202 (2011) .