Jiang Wanjun's research team from the Department of Physics of Tsinghua University has made research progress in thermoelectric manipulation and detection of topological magnetic structures. The researchers found that the use of chip in-situ heating technology can produce nano-scale skyrmions in magnetic devices; and skyrmions will diffuse unidirectionally from the high-temperature region to the low-temperature region in the direction of heat flow; at the same time, they can pass through The pyroelectric detection method detects a single skyrmion in situ.

Magnetic skyrmions are a kind of chiral spin structure with quasi-particle characteristics. Figure 1 (A) is a schematic diagram of the spin structure of Neel configuration skyrmions. In the magnetic metal film, the current or the spin-orbit torque generated by the current can be used to generate and drive skyrmions. However, the research work on using thermal effects to generate and drive skyrmions has not been reported yet. Jiang Wanjun’s team found in early research that skyrmions will undergo spinal Brownian motion related to their topological number under random thermal fluctuations, revealing the non-equilibrium thermodynamics related to skyrmions’ topology, for the purpose of studying heat generation and heat manipulation. Akiko laid the foundation.

Figure 1 (A) Schematic diagram of the spin structure of a magnetic skyrmion in the Neel configuration; (B) a scanning electron microscope picture of a device composed of a magnetic nanostructure and an in-situ heating resistor wire based on the second gate of Tsinghua University; (C) ) Using a soft X-ray full-field transmission microscope at room temperature and an out-of-plane magnetic field of -20mT, the striped magnetic domain morphology was observed in the magnetic nanostructure of the two-column gate configuration; (D) Through the resistive filament on the device After the bit is heated, skyrmions appear in close-packed magnetic nanostructures.

Recently, a soft X-ray full-field transmission microscope using the synchrotron radiation source of the Lorenz-Berkeley National Laboratory in the US In [Pt/Co/Ta]15, Jiang Wanjun’s team successfully observed the generation and movement of nano-scale skyrmions through chip in-situ heating technology. Figure 1 (B) shows a scanning electron microscope photo of this type of device, which is composed of a magnetic multilayer nanostructure and an in-situ heating resistance wire in the second gate of Tsinghua University. Under a 20mT magnetic field perpendicular to the device plane, the magnetic domain shape of the multilayer magnetic nanostructure is striped (Figure 1C). When a pulse current is applied to the resistance wire, that is, a pulse heat flow is applied to the multilayer film nanostructure, the fringe domains suddenly become densely packed skyrmions (Figure 1D).

Figure 2 (Upper left) Photo of the experimental device and schematic diagram of the pulsed heat flow application method; (Lower left) The imaging result of a rectangular magnetic nanostructure using a soft X-ray full-field transmission microscope. As the pulse voltage in the resistance wire increases, the heating temperature rises, and more skyrmions can be generated from the topological deformation of the hot end defects and fringe domains of the sample, and are accompanied by unidirectional diffusion to the cold end. (Right) is the anomalous Nernst measurement result that evolves over time. It can be seen that the annihilation of a single skyrmion produces an anomalous Nernst voltage signal of about 90nV.

In order to more accurately study the generation and movement process of skyrmions, the team increased the heating temperature of the resistance wire in the rectangular magnetic nanostructure, and clearly observed that skyrmions were generated from the hot end boundary of the nanostructure, and the striped magnetic After the domain is heated, it becomes a skyrmion, and the skyrmion diffuses in a single direction from the hot end to the cold end (Figure 2 left). Theoretical analysis and numerical simulation show that in-situ heating can produce skyrmions at locations with low energy barriers, such as multilayer film boundaries, defects, or through the deformation of fringe domains. The unidirectional diffusion of skyrmions from the high temperature region to the low temperature region originates from the repulsive force between skyrmions, the thermal spin orbit torque, the spin torque of the magnon and the equivalent force of the entropy gradient. In this integrated device, the team further used the abnormal Nernst pyroelectric detection technology to detect a pyroelectric signal of a single skyrmion of approximately 90 nV in situ (Figure 2 right). This method of heat generation, manipulation and pyroelectric detection of skyrmions can not only be integrated with existing electrical control schemes, but also can be used in insulating skyrmions that cannot be applied with current. Therefore, this research not only contributes to the study of skyrmion dynamics, but also provides new ideas for the design of new topological spintronic devices.

The above-mentioned related research results were published in "Nature Electronics" on October 26 under the title of "Thermal generation, manipulation and thermoelectric detection of skyrmions". superior.

Wang Zidong, postdoctoral fellow in the Department of Physics, Tsinghua University, Guo Minghua, postdoctoral fellow in the Institute of Microelectronics, Zhou Hengan, a postdoctoral fellow in the Department of Physics, and Zhao Le, a postdoctoral fellow in the Department of Physics of 2018, are the co-first authors of the article. Associate Professor Jiang Wanjun from the Department of Physics of Tsinghua University and Los Alamos National Laboratory in the United States Researcher Lin Shizeng is the co-corresponding author. The co-authors of the thesis include Professor Wu Huaqiang from the Institute of Microelectronics of Tsinghua University, Associate Professor Song Cheng from the School of Materials, Researcher Han Wei from the Center for Quantum Materials of Peking University, Professor Ki-Suk Lee from the Ulsan Institute of Science and Technology in South Korea, Professor Mario Carpentieri from the University of Technology in Bari, Italy, Messi, Italy Take the University Professor Giovanni Finocchio and the United States Lawrence Berkeley National Laboratory Mi-Young Im researcher. This work was supported by the National Natural Science Foundation of China, the Key R&D Program of the Ministry of Science and Technology, the Beijing Natural Science Foundation, the Science Special Project of Tsinghua University's Independent Scientific Research Program, and the Beijing High-precision Chip Center (ICFC).