When you make conducting wires thinner, their electrical resistance increases. This is Ohm’s law, and it is generally correct. An important exception is at very low temperatures, where electron mobility increases when the wires become so thin that they are effectively two-dimensional. Now, physicists from the University of Groningen, together with colleagues from the University of Brest, have noticed that something similar happens with the conductivity of magnons, the spin waves that travel through magnetic insulators, just like a wave through a stadium. The increase in conductivity was spectacular and occurred at ambient room temperature. This observation was published in Natural materials on September 22.
Electrons have a magnetic moment, called spin, which has an “up” or “down” value. It is possible to build up one type of spin by sending a current through a heavy metal such as platinum. When those electron-carrying spines meet the YIG (iron yttrium garnet) magnetic insulator, the electrons cannot pass through them. However, at the interface with YIG, the spin excitation is passed on: magnons (which can also carry spin) are excited. These spin waves pass through the magnetic insulator like a wave in a stadium: none of the electrons (the “spectators”) move from their place, but they still pass on the spin excitation. At the detector electrode, the reverse process takes place: the magnons make electronic spins, which then produce an electrical voltage that can be measured, explains Bart van Wees, professor of applied physics at the University of Groningen and specialist in fields such as spintronics.
Motivated by the increase in electron mobility in 2D materials, his group decided to test magnon transport in ultrathin (nanometer) YIG films. “These films are not strictly 2D materials, but when they are thin enough, the magnons can only move in two dimensions,” explains Van Wees. The measurements, carried out by PhD student Xiangyang Wei, produced a surprising result: the spin conductivity increased by three orders of magnitude compared to the bulk YIG material.
Scientists don’t use terms like “giant” lightly, but in this case, it was fully justified, says Van Wees. “We made the material 100 times thinner, and the magnon conductivity worked 1,000 times. And this did not happen at low temperatures, as is necessary for high electron mobility in 2D conductors, but at room temperature”. This result was unexpected and, until now, inexplicable. Van Wees: “In our paper we give a tentative theoretical explanation that is based on the transition from 3D to 2D magnon transport. But that cannot fully explain the dramatic effects we observe.”
So what could be done with this giant magnon lead? “We don’t understand,” says Van Wees. “Therefore, our current claims are limited. This enables research that could point the way to new but undiscovered physics. In the long term, this could also produce new devices.” First author Xiangyang Wei adds, “Because no electron transport is involved, magnon waves do not produce conventional heat dissipation. And heat production is a big problem in ever smaller electronic devices.”
And since magnons are bosons (ie have integer spin quantum values), it might be possible to create a coherent state comparable to a Bose-Einstein condensate. Van Wees: “This could even produce spin superconductivity.” All this is for the future. For now, the giant magnon conductance in YIG is well documented. “The measurements are clear. We look forward to a good collaboration of theoretical physicists and experimenters”.
Practical spin-wave transistor one step closer
X Y. Wei et al, Giant Magnon spin conductivity in ultrathin yttrium iron garnet films, Natural materials (2022). DOI: 10.1038/s41563-022-01369-0
Provided by the University of Groningen
Citation: Giant Magnon Spin Wave Conductance in Ultrathin Insulators Surprises Researchers (2022, September 23) Retrieved September 23, 2022 from https://phys.org/news/2022-09-giant-magnon-ultrathin-insulators.html
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