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Magnetic nanoparticles evenly distributed in ferrofluids show unusual behavior: small particles obey the collective behavior of large particles. This is reported in an article published in the Journal of Molecular Liquids scientific journal. The discovery plays an important role in applications of magnetic nanoparticles, such as magnetic technologies for targeted medication delivery and treatment of cancer. The work was financially supported by the Ministry of Science and Higher Education of the Russian Federation as part of the Priority 2030 program.

During the study, the co-authors of the article, Aleksey Ivanov, professor at the Ural Federal University, and Philip Camp, professor at the University of Edinburgh, first in mathematical calculations and then in computer simulations ordered magnetic nanoparticles of different sizes by the influence of an external magnetic field, and then turned it off.

“An unexpected effect was discovered. It is known that the smaller the magnetic particles, the faster their disordering (relaxation) occurs, and vice versa: the larger the particles, the longer their relaxation takes. However, our theoretical calculations and the following computer simulations show that in the presence of large particles, relaxation of small particles occurs irregularly. In the state of low residual magnetization of small particles, i.e. when 5-10% of these particles are unordered, they suddenly begin to obey the collective behavior of large particles. Relaxation time of small particles lengthens and approaches the relaxation time of large particles. Although the number of small particles in ferrofluids is always many times greater than the large ones. This is similar to the behavior of naughty children. The smaller the child, the faster he goes wild in the absence of adults. However, under the influence of adults, looking back at them, fidgety one behaves much more modestly,” explains Aleksey Ivanov.

Another effect discovered: at low temperatures, long chains of nanoparticles form ring-shaped aggregates in which the magnetic moments of the particles, closing in a circle, cease to respond to a weak external magnetic field.

“This is important to consider in magnetic hyperthermia, when magnetic particles are used to provide local heating of areas of an organ affected by cancerous cells. The larger the particles, the greater the heating, at the same time for a greater therapeutic effect the particles must be distributed so that they do not stick together and do not form large aggregates, lumps. In other words, it is necessary to look for the golden mean,” emphasizes Aleksey Ivanov.

Further research by the article’s co-authors will focus on identifying the causes of the detected phenomena. The discovery by Aleksey Ivanov and Philip Camp is of great practical importance for the development of so-called smart materials-capable of changing their properties in a controlled manner under the influence of a constant or alternating magnetic field. In addition to magnetic hyperthermia, such materials are used to increase the contrast of X-rays and tomographic images of internal organs, in technologies for targeted medication delivery to specific areas of the body, in the manufacture of liquid crystal screens, photodetectors, heat conductors, sealants, and hydraulic shock absorbers.

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