The conductance of the giant Magnon spin wave in ultrathin insulators surprises researchers

Current (I) through the injector electrode generates magnon in the thin YIG layer. These flow to the detector electrode, where they produce an electrical voltage (V). Credit: University of Groningen / Xiangyang Wei

As the conductor wires become 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 as the wires become so thin that they are effectively two-dimensional. Now, physicists from the University of Groningen, along with colleagues from the University of Brest, have observed that something similar happens with the conductivity of magnons, spin waves that travel through magnetic insulators, just like a wave through a stage. The increase in conductivity was spectacular and occurred at room temperature. This observation was published in Materials of nature on September 22

Electrons have a magnetic moment, called spin, which has a value of “up” or “down”. It is possible to accumulate a type of spin by sending a current through a heavy metal, such as platinum. When those electron-carried spins encounter the YIG (yttrium iron garnet) magnetic insulator, the electrons cannot pass. However, at the interface with YIG, the excitation of the spin is transmitted: the magnons (which can also carry the spin) are excited. These spin waves pass through the magnetic insulator like a wave in a stage: none of the electrons (the “spectators”) move from their place, but they still transmit the spin excitation. The reverse process occurs at the detector electrode: the magnons make electronic turns, which then produce an electrical voltage that can be measured, explains Bart van Wees, professor of applied physics at the University of Groningen and a specialist in fields such as spintronics.

Motivated by the increased electron mobility in 2D materials, his group decided to test magnon transport in ultra-thin (nanometer) YIG films. “These films are not strictly 2D materials, but when they are thin enough, magnons can only move in two dimensions,” explains Van Wees. The measurements, performed by Ph.D. student Xiangyang Wei, produced a surprising result: the conductivity of the spin increased by three orders of magnitude, compared to the bulk YIG material.

Dramatic effects

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 conductivity of Magnon went up 1,000 times. And this didn’t happen at low temperatures, as is required for the high electron mobility in 2D conductors, but at room temperature.” This result was unexpected and, so far, inexplicable. Van Wees: “In our article we give a tentative theoretical explanation which is based on the transition from 3D Magnon transport to 2D. But this cannot fully explain the dramatic effects we observe.”

So what could be done with this giant Magnon conduction? “We don’t understand that,” says Van Wees. “Therefore, our current claims are limited. This allows for research that could point the way to new but unknown physics. In the long run, this could also yield new devices.” First author Xiangyang Wei adds, “Because no electron transport is involved, Magnon waves produce no conventional heat dissipation. And heat production is a big problem in ever smaller electronic devices.”


And since magnons are bosons (i.e. they 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 conductance of Magnon 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

More information:
X.-Y. Wei et al, Magnon Giant Spin Conductivity in Yttrium Iron Garnet Ultrathin Films, Materials of nature (2022). DOI: 10.1038 / s41563-022-01369-0

Provided by the University of Groningen

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