Back in 1966, Japanese physicist Yosuke Nagaoka came up with an idea for an unusual new mechanism that could cause ferromagnetism – the phenomenon that drives magnets.
His idea made sense in theory, but it was never observed in natural materials. We now have the first signs that this is happening in the laboratory.
Once again, we owe quantum physics for the discovery. Scientists were able to create what they call the 'experimental signatures' of Nagaoka ferromagnetism (as it came to be called) in a tightly controlled, custom-made quantum electrical system.
While it is too early to use this new magnetism setup in practice, the discovery suggests that Nagaoki's 54-year prediction is correct; and this could have a big impact on how the quantum systems of the future will develop.
“The results were crystal clear: we demonstrated ferromagnetism,” says quantum physicist Lieven Wandersiepen of the Delft University of Technology in the Netherlands.
“When we started working on this project, I was not sure if the experiment would be possible, because physics is so different from anything we have ever studied in our laboratory.”
The easiest way to imagine ferromagnetism is with a children's puzzle game in which you insert sliding blocks into a drawing. In this analogy, each block is an electron with its own spin or alignment.
Nagaoke's ferromagnetism is in a puzzle shape, with all spins aligned to the right. (Scixel de Groot for QuTech)
When electrons align in one direction, a magnetic field is created. Nagaoka described a kind of ideal version of itinerant ferromagnetism, in which electrons can move freely and the material remains magnetic.
In the Nagaoki version of the puzzle, all electrons are aligned in the same direction, which means that even though the puzzle pieces are shuffled, the magnetism of the system as a whole remains constant.
Since the shuffling of electrons (or mosaics) is irrelevant to the overall configuration, the system requires less power.
To show Nagaoka's ferromagnetism in action, scientists actually built a two-by-two two-dimensional lattice of quantum dots, tiny semiconductor particles that could potentially form next-generation quantum computers.
The entire system was cooled to near absolute zero (-272.99 ° C or -459.382 ° F), then three electrons were trapped inside it (leaving one 'puzzle block' empty). The next step was to demonstrate that the grid behaves like a magnet, as suggested by Nagaoka.
“We used a very sensitive electrical sensor that could decode the spin orientation of electrons and convert it into an electrical signal that we could measure in the laboratory,” says quantum physicist Udittendu Muhopadhyay of Delft University of Technology.
The sensor showed that the system of ultra-small supersensitive quantum dots did indeed align the electron spins, as expected, naturally preferring the lowest energy state.
Previously described as one of the most difficult problems in physics, this is a significant step forward in our understanding of both magnetism and quantum mechanics, showing that the long-standing idea of how ferromagnetism works at the nanoscale is indeed true.
Going forward, the discovery should help develop our own quantum computers, devices capable of performing computations beyond our current technology.
“These systems allow you to study problems that are too complex to solve with today's most advanced supercomputer, such as complex chemical processes,” says Vanderspen.
“Experimental experiments such as the realization of Nagaoke ferromagnetism provide important guidelines for the development of quantum computers and simulators of the future.”
The study was published in the journal Nature.
Sources: Photo: Sofía Navarrete and María Mondragón De la Sierra for QuTech
