Spintronics to control the Bose-Einstein Condensation of Magnons

06.12.2021

An international team of scientists have found an easy way to control an unusual state of matter called a Bose-Einstein condensate by spin-orbit torque. | New paper in "Nature Nanotechnology".

The team of physicists from the Technische Universität Kaiserslautern (TUK) and the University of Vienna were able to create the Bose-Einstein condensate (BEC) using a rapid temperature shift: quasi-particl The team of physicists from the Technische Universität Kaiserslautern (TUK) and the University of Vienna were able to create the Bose-Einstein condensate (BEC) using a rapid temperature shift: quasi-particles were slowly heated up and then swiftly cooled down to room temperature. The demonstration of the method used magnons, which are quasi-particles representing the quanta of magnetic excitations of a solid body.

"Spintronics is one of the largest and the most successful fields in solid-state physics," said Professor Andrii Chumak from the University of Vienna. "Our next task was to check if we can use spintronics phenomena to control this fascinating state of matter. Fortunately, our structures were already miniaturized to 100 nm scale, and the door was open. We combined our expertise and know-how in nanomagnetism and spintronics with the expertise of Prof. Burkard Hillebrands, one of the leading experts in magnon BEC."

The existence of Bose-Einstein condensates was proposed by Albert Einstein and Satyendra Nath Bose. They are a perplexing type of matter: they consist of particles that spontaneously and in unison act the same way on the quantum level, effectively becoming one entity. Formerly they were used to describe ideal gas particles, however now they have been established with atoms and quasi particles such as magnons, bosons and phonons.

Michael Schneider, the PhD researcher at the TUK who executed the experiments, pointed out to the difficulty of creating BECs due to the fact that they have to happen spontaneously. He explained that when setting up the right conditions to generate these condensates, one has to avoid introducing order or coherence: the particles have to attain this state on their own. The method he and his team discovered to create BECs was easier than others reported before: they only had to cool the nano-structure quickly enough.

Continuing this research, the same team has found a way to control the formation of a magnon BEC using the spin-orbit torque (SOT). Electrons, in addition to their charge, also have a fundamental property named spin. This is somehow similar to the rotating of the Earth around itself, which creates the field we can detect with a compass. At the same time, electrons rotate around atomic cores as the Earth rotates around the Sun. In materials such as platinum, the electron's orbital motion is strongly coupled to the electron spin via the spin-orbit interaction. As a result, sending the usual electric current through platinum film accumulates electrons with spin-up at the one side of the film and with spin-down at the opposite side. This spin accumulation, consequently, acts as a source of magnons in addition to the magnons redistributed by the rapid cooling mechanism. The critical experimental finding is that the SOT-driven change in the magnon population during the pulse action modifies the threshold of the BEC formation process developing after the pulse is turned off. "We have found a threshold voltage shift of around 8% only," Chumak says, "but when the applied voltage is close to the threshold, SOT allows us to switch the formation of BEC on or off completely. The nature of the BEC of magnons is still under intensive investigation, and we still have many questions. The possibility to control it by new means will help to answer some of them".

Publication in Nature Nanotechnology:
M. Schneider, et al., Control of the Bose-Einstein Condensation of Magnons by the Spin Hall Effect, Physical Review Letters 127, 237203 (2021). DOI: https://doi.org/10.1103/PhysRevLett.127.237203

Funding information: The research was performed in the frames of ERC Starting Grant MagnonCircuits (A. Chumak), ERC Advanced Grant Super-Magnonics (B. Hillebrands) and Collaborative Research Center SFB 173 Spin+X.

Caption. YIG/Pt structure for the investigation of SOT-based control of magnon BEC. The colour map shows the measured magnons density as a function of time and frequency. The injection of magnons is clearly seen during the applied pulse - see the right grey shaded region. BEC is formed just after the end of the current pulse if SOT injects magnons, and there is no BEC if SOT annihilates the magnons, like it is shown in the left panel. © Michael Schneider (TU Kaiserslautern)