Researchers from ITMO University, together with Nobel Prize-winning physicist Frank Wilczek, have proposed a new way to study the ultrafast dynamics of magnetization in magnetic materials using low-frequency signals. The method will allow you to see processes that are difficult to observe in other ways. The study was supported by the Priority 2030 program and the Russian Science Foundation, and was published in the journal Physical Review Letters.
Magnetic materials are widely used in devices: they can be found in transformers, electric motors, sensors, household and medical equipment, and memory devices. The key difference between such materials and others is the non—zero magnetization inside the structure. Understanding how ferromagnets and antiferromagnets work has become an important achievement of twentieth-century quantum physics, but new research and discoveries are still being made.
For high-performance devices based on magnetic materials, it is important that the magnetization of the structure can switch quickly. This property is studied by a special branch of physics — ultrafast magnetism. Traditionally, to "switch" the magnetization, the material is irradiated with light at a frequency higher than the frequency of magnetization fluctuations. But if the magnetization changes so quickly — for some materials it lies in the terahertz region (1012 Hz) — this approach almost stops working. ITMO researchers, in collaboration with Nobel Prize-winning physicist Frank Wilczek, have proposed a new approach to solving the problem.
"Imagine that you are looking at a rotating disk with white and black fields. If the disc is spinning slowly, you will have time to distinguish between contrasting areas. If it rotates quickly, everything merges into a solid gray field, and it is impossible to distinguish between areas of different colors. It doesn't matter if you're looking at the disc with your eyes or, say, with a camera. The only important thing is the ratio of the rotation frequency and the characteristic frame change frequencies that the eye or the camera matrix can perceive," explains Maxim Gorlach, head of the scientific team, a leading researcher at ITMO. "Figuratively speaking, the idea of our work is that it is possible to see something besides the gray field, even if the disk is rotating very fast."
Instead of a rotating disk, the researchers examined a magnetic material in which ultrafast magnetization fluctuations occur under the influence of a pulse. Unlike the classical method, physicists used a small—amplitude electromagnetic wave to observe these vibrations. Passing through a magnetic material, it reflects, refracts, and changes the state of polarization (that is, the direction of the electric field fluctuations). By analyzing the transmitted and reflected light, information about the magnetization fluctuations can be obtained.
"At first glance, it seems that if the magnetization fluctuations occur very quickly compared to the frequency of the incident wave, they will not affect its propagation in any way. But this is not the case: calculations have shown that rapid fluctuations in magnetization can be approximately replaced by an effective axion field. Its magnitude depends not only on the amplitude of the magnetization, but also on the initial phase of the oscillations. This axion field, in turn, affects the propagation of an electromagnetic wave and changes its polarization. As a result, it becomes possible to find out the initial phase of the magnetization fluctuations, even if the signal used is low—frequency," says Leon Shaposhnikov, ITMO graduate student and the first author of the article.
At the same time, the electrodynamics of some magnetic materials is described by the same equations as for the hypothetical particles first proposed by Prof. Wilczek is an axion. In this case, they speak of an effective axion response.
In addition to contributing to ultrafast magnetism, the ideas of scientists from ITMO complement another field of science that has emerged recently — time metamaterials:
"In recent years, experimenters have learned how to quickly change the parameters of artificial media — metamaterials — over time. And the physics of such environments turned out to be very rich. For example, light amplification may occur in them. The mechanism is similar to swinging a swing with the help of inclines in swinging legs. In both cases, parametric resonance occurs. However, the intersection of the domains of temporal metamaterials and ultrafast magnetism remained completely unexplored. As our work shows, there are great prospects in this direction," explains Eduardo Barredo, ITMO graduate student and the second author of the article.
By the way, earlier the ITMO team was able to understand for the first time how an effective axion field arises in metamaterials. The researchers described this result in a previous article published in Physical Review B.
The research was supported by the Priority 2030 program and the Russian Science Foundation.
Please note that this press release is based on materials provided by the company. AK&M Information Agency shall not be held liable for its contents, nor for the legal and other consequences of its publication.
