is not unfamiliar with pomegranate Chinese. According to legend, pomegranate cultivation was introduced into China by Zhang Qian , who was sent to Western Regions . Their tightly arranged grains have also become the inspiration for scientists when naming chemicals.
The earth we live in is mainly composed of circles such as the crust, mantle, and core. The mantle is divided into the upper mantle and the lower mantle. In the upper mantle, rock-making minerals - are the general term for the minerals that make up rocks.
garnet is one of the minerals in the upper mantle. The crystal composition of this mineral is very similar to the shape and color of pomegranate seeds, so it is called garnet.
yttrium iron garnet (yttrium iron garnet, YIG), is a type of garnet. Its chemical formula is Y3Fe2 (FeO4)3. With its magnetic and magneto-optical properties, it has been used in microwave instruments and optical communication instruments. Although yttrium iron garnet is an insulator, we can still use quasi-particle , which is called spin wave or magneton (magnon) (magnetic oscillator is a quantized form of spin wave, which will not be distinguished below.) to transmit signals.
Recently, Wei Xiangyang, alumnus of the University of Science and Technology of China and a doctoral student in the Nano Device Physics Research Group of the University of Groningen, Netherlands, and his team reported the following findings: at room temperature, the magnetic spin conductivity of ultra-thin yttrium iron garnet film with a thickness of 3.7nm has greatly improved compared with ordinary yttrium iron garnet materials. At the same time, in the process of thinning the thickness, the number of two-dimensional subbands occupied by magnetic oscillators has dropped from a very small amount, which may be accompanied by the transformation from three-dimensional to two-dimensional spin wave transport.
At the same time, magnetic oscillators may still have high spin conductivity in a two-dimensional system at room temperature, which can open up new ideas for the development of low-dissipation, magnetic oscillator-based spin electronic devices.
(Source: Nature Materials)
In the process of submission of related papers, the review experts believed that: "This topic is very valuable for contemporary spin electronics. Long-distance spin transport makes new spin-based device design possible and opens up new opportunities for spin wave dynamics. The interesting physical phenomena introduced by the author in the paper and the very high spin conductivity found, which makes this job not only arouse the interest of experts in related fields, but also attracts everyone's interest in a wider range.
's application prospects mainly revolve around the unique quasi-particle of magnetic oscillators. In contrast, when electrons are used as carriers, Joule heat will be generated in information transmission. 4; Using magnetic oscillators as information carriers, very little energy is dissipated during the transmission process.
On the other hand, magnetic oscillators can exhibit the characteristics of the quantum world, spin wave color Einstein condensation, and spin wave superfluid state under room temperature.
Therefore, Wei Xiangyang and others found that the ultra-thin yttrium iron garnet system has better performance, and In the ultra-thin yttrium iron garnet system, the spin wave transport capacity is stronger, and it is easier to control the characteristics of quantum .
" The conclusion of this work is counterintuitive. My colleagues and I found that the thinner the thickness of the yttrium iron garnet film, the more the spin conductance and spin conductivity increase. "It means.
Figure | Wei Xiangyang (Source: Wei Xiangyang)
Take the familiar conductivity and conductivity as an example. Ohm's law describes the relationship between the conductivity of an electric conductivity and the size of the conductor. When a conductor is thinner, its conductivity will become smaller. The conductivity, as the property of the material itself, will not change with the size. (The macroscopic law described by Ohm's law is not suitable for materials with quantum characteristics such as two-dimensional electron gas.)
For ferromagnetic insulators such as yttrium iron garnet, when magnetic oscillators are used as carrier , signal transmission can be performed in the insulator. This itself does not rely on the movement of free electrons , and the directional movement of free electrons in the conductor will generate Joule heat, which in turn will lead to energy loss.
Wei Xiangyang pointed out that the energy dissipation of magnetic vibrators in yttrium iron garnet will be much lower. This study found that as the thickness of yttrium iron garnet continues to thin, the subbands involved in magnetic oscillator transportation have decreased from tens of thousands to only a few, while the spin conductivity has increased by nearly three orders of magnitude.
This shows that during the thinning of the thickness of yttrium iron garnet, the transportation of magnetic oscillators may undergo a transition from three-dimensional to two-dimensional. This may be because magnetic oscillators have better properties in two-dimensional systems, such as longer scattering time.
This study currently only reports such a phenomenon, and there may still be very complex physical rules behind it. The electrical testing methods used in the experiment have certain limitations, such as the inability to specifically analyze spin waves with different momentums and their contribution to spin conductivity. Therefore, in order to fully and accurately explain the above phenomenon, further research must be done from both experimental and theoretical aspects.
. In response to the above-mentioned research, Wei Xiangyang and others have sorted it into papers. Recently, the relevant paper was published on Nature Materials with the title "Giant magnet spin conductivity in ultrathin yttrium iron garnet films", with Wei Xiangyang as the member of the communication.
Figure | Related papers (Source: Nature Materials)
The transportation mechanism changes from three-dimensional to two-dimensional, resulting in an increase in spin conductivity
In the winter of 2019, Wei Xiangyang and others began to work on this research. At that time, he and his colleagues did not know very well about the properties of the ultra-thin yttrium iron garnet system. However, in another work on ultrathin yttrium iron garnet spin-wave transistors, the team found that ultrathin yttrium iron garnet systems may possess some interesting properties.
Therefore, the research team decided to conduct a systematic study on the relationship between the magnetic oscillator transport properties and thickness of yttrium iron garnet.
Wei Xiangyang said: "French collaborator Dr. Jamal Ben Youssef is responsible for the growth of the ultra-thin film of yttrium iron garnet, and we conducted device preparation and testing in Groningen, Netherlands."
By measuring the spin wave transport signals of yttrium iron garnet of different thicknesses, the team initially found that in thinner films, larger spin wave transport signals can be observed.
(Source: Nature Materials)
After observing this phenomenon, the researchers thought about the reasons and established a model to extract the spin conductivity from the experimental data.
At the beginning, Wei Xiangyang and others thought that the reason might be: the surface and body phase of the film have different contributions to spin wave transport. Compared with the bulk phase, the surface has a higher possibility of spin conductivity. In the process of thinning of the film, the contribution proportion from the surface becomes larger and dominates.
"Low we found that this assumption is wrong, because from thick film to ultra-thin film, the signal increases many times. If there is such a dominant layer, it should always dominate and the signal will not change with the thickness, just like two resistors with many times different phases are connected in parallel, and the overall resistance is determined by the part with smaller resistance," it said.
Later, the research team found through calculations that the number of spin wave subbands decreased sharply during the reduction of the thickness of yttrium iron garnet. Test data for strong magnetic fields and low temperatures also show that in ultra-thin yttrium iron garnet, transportation is dominated by spin waves with low energy.
Therefore, they believe that in the process of thinning film thickness, the increase in spin conductivity may be related to the changes in the transport mechanism of spin waves from three-dimensional to two-dimensional.
(Source: Nature Materials)
is using the high spin conductivity of ultra-thin yttrium iron garnet for further research
Wei Xiangyang added that this work is an extension and in-depth basis for everyone's previous work. The research team used many existing methods to explore the spin conductivity of yttrium iron garnet films.
Many of the concepts and models have been introduced in the previous work of yttrium iron garnet films in the research group. Therefore, this study on the ultra-thin yttrium iron garnet system can explore the spin conductivity of yttrium iron garnet more deeply and systematically. The transportation of
spin waves in ferromagnetic insulators is a field of vigorous development in recent years. "My former colleagues, Dr. Ludo Cornelissen, Dr. Shan Juan, and Dr. Liu Jing, have done a lot of foundational work. For example, electrical excitation and detection of spin waves in ferromagnetic insulators [1, 2], proposed methods to study spin wave transport driven by chemical potential [3], current-controlled spin wave transistors [4], etc.," said Wei Xiangyang.
Currently, he is using the excellent spin conductivity of ultra-thin yttrium iron garnet to conduct research on electrical excitation and detection of spin waves Einstein condensation.
Although he served as the corresponding author of this time, Wei Xiangyang is also a doctoral student, studying under Professor Bart van Wees from the Nanodepartment Physics Research Group of the University of Groningen, Netherlands. Wei Xiangyang graduated from the University of Science and Technology of China with a bachelor's degree in materials chemistry. After graduation, he went to the University of Groningen to study for a master's degree and doctoral degree. After graduating from his PhD, he will go to Singapore to do a postdoctoral research. After the end, he hopes to have the opportunity to return to China to continue doing scientific research.
1.Cornelissen, L. J., et al. "Long-distance transport of magneton spin information in a magnetic insulator at room temperature." Nature Physics 11.12 (2015): 1022-1026.
2.Shan, J, et al. "Criteria for accurate determination of the magnon relaxation length from the nonlocal spin Seebeck effect." Physical Review B 96.18 (2017): 184427.
3.Cornelissen, L. J., et al. "Magnon spin transport driven by the magnon chemical potential in a magnetic insulator." Physical Review B 94.1 (2016): 014412.
4.Cornelissen, L. J., et al. "Spin-current-controlled module of the magnon spin conductance in a three-terminal magnon transistor." Physical review letters 120.9 (2018): 097702.
