Kazan University physicists continue magnonics research promising to blaze new trails in microelectronics

The team continues experiments to investigate the possibility of controlling the spectrum of standing spin waves (SSW) in ferromagnetic films of Pd-Fe alloy with variable thickness composition. The new advances are presented in the American Physical Society journal Physical Review B.

The work was carried out in the Laboratory of Heterostructures for Post-Silicon Electronics of the Institute of Physics by lead researcher Roman Yusupov, senior researchers Igor Yanilkin and Amir Gumarov. Also contributing is leading researcher of the Laboratory of Nonlinear Optics of the Kazan Physical-Technical Institute, Academician of the Academy of Sciences of the Republic of Tatarstan Lenar Tagirov, and researcher of the Laboratory of Prospective Platforms for Spin Quantum Manipulation of the Institute of Physics Bulat Gabbasov.

Magnonics is a new branch of electronics, in which information is transferred not by charge carriers—electrons or "holes", but by magnons—quanta of spin waves. Available developments in this field allow us to assert that magnonics can be used to build microprocessors that are more energy-efficient than semiconductor microprocessors, Mr. Gumarov notes.

In the course of this research, scientists have achieved two fundamentally important results.

"First, it is a careful and flexible control of the composition of the 'gradient' magnetic material, which has a significant impact on the magnon spectrum. A 'gradient' refers to a change in properties along a particular direction in space. Second, modeling of magnetic properties that describes its spin-wave spectrum with high accuracy at all experimental temperatures," reports Mr. Yanilkin.

The process was described by Mr. Yusupov: high-precision molecular beam epitaxy (MBE) technology was used, which is characterized by the highest purity (i.e., the absence of extraneous impurities and oxidation), as well as the almost perfect crystalline structure of the resulting materials, the perfection of their interfaces with the substrate. Plus, says the researcher, these distinctive properties are superimposed on the possibility of programmable control of the intensity of molecular beams for the synthesis of magnetic material with a continuously variable composition profile.

The publication is devoted to a detailed study of the resonance spectrum of SSVs in inhomogeneous films at temperatures from 10 to 300 degrees Kelvin (the latter value is room temperature). The key thesis of the work is that in the thickness gradient film Pd-Fe in each local sublayer will be its own ferromagnetic transition temperature (Curie temperature), determined by the local concentration of iron in the alloy: where there is less iron, the temperature is lower. Consequently, at a certain temperature, the sublayer with lower iron concentration becomes paramagnetic. As the temperature increases further, the paramagnetic region will increase by reducing the thickness of the ferromagnetic part of the film until the whole film becomes paramagnetic. Depending on the complexity of the thickness profile of the composition, there may be several such regions, they may appear at different temperatures, merge with each other, etc. All these processes of phase composition change strongly affect the spectrum of standing spin waves, which can occur only in the ferromagnetic part of the film. Thus, the SSW spectrum can be effectively controlled by changing the sample temperature.

The phenomenon is comparable to controlling the tones of strings on a guitar using a capodastra, a special clip on the fingerboard that changes the length of a freely oscillating piece of string. By using a freely movable capo when playing Hawaiian guitar, a continuous change of tones is achieved over a wide range of frequencies.

"Before we started our research, we of course guessed that temperature should affect the SSV spectrum. However, we did not even anticipate that over the entire temperature range our measurement results would be so well described by the semi-classical approach based on the Landau-Lifshitz equation. This means that it is possible to predict in advance the spectrum of standing spin waves for any technologically specified profile of iron concentration in the ferromagnetic film of the Pd-Fe alloy at any temperature, which means that temperature becomes an additional tool for controlling the spectrum of spin waves in a wide range," emphasizes Yusupov.

The MLE method was first used by Kazan University physicists in 2022 to create 'gradient' magnetic materials. Then the study confirmed the fundamental possibility of controlling the spectrum of standing exchange spin waves.

"The obtained knowledge and technologies bring to magnonics a new material with a tunable spectrum of magnons, possessing the property of nonreciprocity (a large difference in the properties of magnons propagating in opposite directions), on which magnon diodes and logic gates are realized," Academician Tagirov explained the relevance of the research.

The next significant step for the development of this area, according to scientists, is the creation of nanostructures of 'gradient' materials in which it will be possible to induce running spin waves necessary for the transfer and processing of information. The creation of such structures requires electronic or optical lithography. The formation of nanostructures is planned to be carried out at the Center for Advanced Methods of Mesophysics and Nanotechnology of the Moscow Institute of Physics and Technology.

The research was supported by the Priority-2030 program and a megagrant from the Ministry of Science and Higher Education of the Russian Federation.

More information:
Temperature control of spin-wave spectra in continuously graded epitaxial Pd-Fe alloy films
journals.aps.org/prb/abstract/ … /PhysRevB.111.094428

Provided by Kazan Federal University