Recently, scientists at the University of Illinois at Urbana-Champaign revealed a new physical mechanism through which scientists can use heat to manipulate the formation of magnetism. Unlike traditional magnetic fields, the new mechanism relies on thermal energy transmission to provide people with a new way of manipulating magnetization at the nanometer scale. Related papers were published in the recently published "Nature Physics".
According to the report of the physicists' organization network on the 8th, the researchers produced a multi-layered metal spin valve structure consisting of two magnetic layers and one heat transfer layer. “When the heat flow passes through the first layer of magnetic material, it will produce electron spin separation. Our research is to use this point. With this process of forming a magnetic bipolar flow, we can manipulate the direction of the second magnetic layer.†Champagne Professor David Kasir, director of the Division of Materials Science and Engineering at the branch said.
“We use the spin flow induced by the ultra-fast thermal conduction to generate the spin transfer torque (STT). The spin transfer torque is a transfer of the spin angular momentum of a ferromagnetic body from conduction to magnetization, allowing people to use spin flow. Instead of a magnetic field, the nano-magnets are manipulated," said Zhou Jiangmin, Ph.D., the first author of the paper and a Ph.D. candidate in the Department of Materials Science and Engineering. Normally, a spin transfer torque can be generated by passing current through the magnetic layer. They now demonstrate the existence of a mechanism for generating a spin transfer torque using a strong heat flow. This mechanism is mainly driven by the spin-dependent Seebeck effect. The Seebeck effect is a thermoelectric phenomenon in which the temperature difference between two different materials in the same circuit generates a voltage. The spin-dependent Seebeck effect refers to the similar phenomenon produced by spintronics in a ferromagnetic body.
In the metal spin valve structure, the researchers used a picosecond (one trillionth of a second) laser pulse to create a strong ultrafast heat flow that quantifies the thermal spin transfer. This heat flow reaches 100 GW per square meter (Gigawatts, meaning 1 billion watts) and lasts about 50 picoseconds. "The sign and value of the thermally driven spin current can be controlled by the composition of the ferromagnetic layer and the thickness of the heat transfer layer," said Cassirer.
In nanospin devices, the coupling of spin and heat creates a new physical phenomenon. The spin transfer torque driven by heat transfer can provide a new way to manipulate local magnetization. Cassirer said: “The physical mechanism for separating electron spins by heat flow is related to thermocouples and thermal generators. Thermal generators can power deep-space detectors. In thermoelectric devices, heat flow causes charge separation. Separation can be used to detect temperature and can also be used to supply power.†(Reporter Chang Lijun)
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