Whenever you use electricity, you lose some power as heat. This may be obvious when you’re standing under a heat lamp, but it holds for very efficient electronics, such as LEDs or computer chips. This heat generation has been a major obstacle in improving computer chips, which generate more heat as they work faster. Researchers at Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center, have now demonstrated a way to use this waste heat to transmit information and power using a device made of a non-magnetic conductor and a magnetic insulator. Interactions at the interface between these two dissimilar materials act to selectively allow certain information to be passed from one material to the next.
Before diving into the gritty details, it is important to address what exactly is carrying information in the middle of all this heat flow. The answer is spin, a form of angular momentum possessed by electrons. The amount of spin is the same for each electron, but the spin can point in any direction. Spins interact with magnetic fields. In magnets, all of the electrons have aligned spins (that is, all electron spins are pointing in the same direction). You can read more on the EFRC research being done on systems where spins are ordered but not all pointing in the same direction in “Harnessing Quantum Vortices.” By applying an external magnetic field, we can also control spin direction. If we say spin up is “1” and spin down is “0,” we can use controllable magnetic fields to create the binary code that computers use to transmit information. This type of device is called a “spintronic” device. The difficulty so far in realizing such a device has been generating and controlling the flow of the spins.
There are a couple of exciting prospects that would use electronic waste heat to control spins. One is the spin Seebeck effect, in which heat flow is accompanied by a flow of aligned spins. The heat flow can be independent of electronic flow, which means that these spin flows can occur even in electrically insulating materials. Another is the spin Hall effect, in which a flowing current will cause electrons to deflect depending on the orientation of their spin. For example, if there was a current flowing from left to right across this page, electrons with their spin pointed away from your face would be deflected towards the bottom of the page by the spin Hall effect.
Electrons striking the interface between a metal and a magnetic insulator can undergo a “spin-flip” process, transferring angular momentum into the insulator. This angular momentum transfer causes electron spins to precess, with subsequent spin-flips increasing the amplitude of precession.
Video courtesy of Max Grossnickle, SHINES
To take advantage of these effects, the SHINES team designed a device consisting of a strip of platinum on top of a thin piece of yttrium iron garnet (YIG). Platinum is an electrical conductor with strong spin properties, while YIG is a magnetic insulator. In their experiment, the researchers passed an electric current through the platinum, causing it to heat up. The YIG layer below remained cooler, creating a temperature gradient across the two layers. At this point, the spin Seebeck effect kicked in, driving a spin current from the platinum into the YIG. Simultaneously, spin Hall effect-deflected electrons struck the platinum-YIG interface. Because YIG is an insulator, the electrons were reflected at the interface, but because YIG is also magnetic, the electrons’ angular momentum was transferred into the bottom layer (see figure).
Angular momentum transfer into YIG is not a simple process. It turns out that the standard picture of a magnet (where every spin is pointing in the same direction) is not completely accurate. In a normal magnet, there is one force driving the spins to precess around the common magnetization direction, and another force acting to damp this precession. If a spin flow is present, there is a “spin transfer torque” acting to drive the precession even harder. Running electricity through the platinum ensures that there is always a spin flow in the YIG, so the magnetic spin precession is driven faster until the spins are completing 3.2 billion precessions every second. This frequency, 3.2 GHz, is more commonly known as the microwave frequency range.
The observation of narrow-bandwidth microwave generation from this system is a striking one. Heat is an inherently random process, but the interactions at the interface of the platinum-YIG system served to deliver very precise outputs that could be used for efficient microwave signaling or spintronic applications. The ubiquity of heat generation in electronics means that these devices could be easily integrated with existing technologies. Extracting order from noise also extends to an even more basic level. There are some tantalizing clues that the transition from disordered to ordered spin states may indicate bosonic condensation, a phenomenon that has been observed (in other systems) to have incredible properties such as superfluidity (the flow of liquid without friction) or superconductivity. Future studies on the precise interactions among electrons, spin current oscillations, and the heat flow will shed light on phenomena with far-reaching impacts on our understanding of fundamental physical processes.
Video produced by Max Grossnickle, SHINES EFRC
Safranski C, I Barsukov, HK Lee, T Schneider, AA Jara, A Smith, H Chang, K Lenz, J Lindner, Y Tserkovnyak, M Wu, and IN Krivorotov. 2017. “Spin Caloritrionic Nano-Oscillator.” Nature Communications 8:117. DOI: 10.1038/s41467-017-00184-5
This work was primarily supported as part of Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the Department of Energy (DOE), Office of Science, Basic Energy Sciences. Y.T. acknowledges support by DOE, Office of Science, Basic Energy Sciences (theory) and the Army Research Office (modeling). A.A.J. (sample nanofabrication) was supported by DOE, Office of Science, Basic Energy Sciences.