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The laws of electromagnetism at the scale of a few atoms

Read time: 4 mins
The laws of electromagnetism at the scale of a few atoms

Photo by Mika Baumeister via Unsplash

Physicists discovered the relationship between electricity and magnetism more than a century ago. The intricate relationship between these entities, previously thought to be separate, currently goes by the name ‘electromagnetism’. It largely governs the world in which we live. From large power grids to fans in our rooms to electronic gadgets, everything runs on these well-understood principles.

Scientists often use electrical energy to control magnetism, which enables them to increase the speed of computers. They have also achieved the reverse by generating electricity from magnetism that varies over time, first observed in the nineteenth century. In a new study, a group of researchers from the Indian Institute of Technology Bombay (IIT Bombay) have demonstrated the inverse effect for the first time in a ferromagnet, a type of naturally-occurring magnetic material, measuring only a few atomic layers thick. Thus, they have shown how the two effects are interlinked at the atomic scale. The study was supported by the Department of Science and Technology (DST), Government of India, and was published in the journal Science Advances.

To study the effect, the researchers passed an electrical current through a sophisticated electrical circuit, generating a magnetic field. According to the laws of electromagnetism, this magnetic field should affect any magnet in its vicinity. Then, they placed a device consisting of a thin, atomic layer of interface between an oxide and a ferromagnet very close to the circuit. By measuring the electric field produced by the magnetisation of the film, they showed that it varied with the magnetisation direction.

The atomic-scale interface between the oxide and the magnet is special. By working out the physical laws that govern that scale, the researchers also knew that any external magnetic field could only penetrate this interface and not beyond it. The magnetic field generated by the external circuit plays the role of the external magnetic field, which may be more than a thousand times stronger than the magnetic field of the Earth! However, making a sample of the oxide and magnetic interface in the laboratory is not easy, as it needs to be devoid of impurities.

“We had to ensure that the sample is very clean,” says Dr Ambika Shanker Shukla of IIT Bombay, the lead author of the study. In fact, they flew in the sample from their colleagues in Japan.

The inverse effect — generating current by changing the magnetisation of the film — has never been seen before at such a scale, and that is what the team was after. To observe this, they varied the current in the circuit and placed another circuit close to the thin film. “To verify the effect, however, these two circuits, and the thin film, had to be completely isolated,” shares Prof Ashwin Tulapurkar, who supervised this project. This ensures that the cause and the effect are distinguishable, conclusively demonstrating that the inverse effect is indeed taking place.

When the researchers passed a varying electrical current through the first circuit, they found that the magnetisation of the thin film changed with time. This, in turn, gave rise to an electric field strong enough to cause the flow of electric current in the other, independent, electric circuit. “This is the first time ever that the inverse effect has been observed at that scale,” asserts Dr Shukla.

The researchers have demonstrated that, with the help of this inverse effect, they could control the magnetisation as well as its direction without having to rotate it mechanically. This control, the researchers think, can help them in creating memory (or storage) devices. By controlling the external circuit, they can generate the magnetisation in two distinct states, depending on the direction of the magnetisation. This would enable them to make binary states, the building blocks of modern electronics.

The inverse effect also demonstrates how information about an external field propagates through a magnetic material using a fundamental property of its constituents, the electrons.

“Compared to traditional memory devices, this property enables us to propagate information with about a thousand times lower loss of power,” explains Dr Shukla. “We are now trying to create a memory device that uses very low power,” he signs off.

This article has been run past the researchers, whose work is covered, to ensure accuracy.