Spin-fluctuation transitions (SFTs) are generally understood as a change in the characteristics of spin fluctuations in a magnet under the effect of control parameters (for example, temperature or composition of the material), not directly related to the formation of phases with long-range magnetic order. Therefore, SFTs in most cases go beyond the standard theory of phase transitions, where fluctuations are typically considered a phenomenon accompanying a magnetic transition. We examine the current state of the SFT issue from the theoretical and experimental standpoints, including the example of MnSi and Mn_1-xFe_xSi helical magnets, Hg_1-xMn_xTe magnetic semiconductors, doped compensated Ge:As(Ga) semiconductors, and a strongly correlated metal with hidden order, CeB₆. The main methods for studying SFTs (neutron scattering and electron paramagnetic resonance) are discussed. We consider SFTs in the spin nematic phase, Ising disordered systems, paramagnetic and magnetically ordered phases, and quantum critical systems, as well as SFTs caused by changes in the spin dynamics. In discussing the SFT issue, we use the spin-polaron model, which unites objects of study that look disparate at first glance. We note that SFTs have not been studied in sufficient detail, and further research in this area may be one of the growth points in the modern physics of magnetic phenomena.

Keywords: magnetic phenomena, spin-fluctuation transitions, spin fluctuations, spin polarons, spin nematics, systems with hidden order, quantum critical systems, helical magnets, neutron scattering, electron paramagnetic resonance, MnSi, Mn1-xFexSi, Hg1-xMnx, Ge:As(Ga), CeB6 PACS: 05.40.−a, 05.70.Fh, 05.90.+m, 61.05.fg, 71.27.+a, 75.20.Ck, 75.20.En, 75.25.Dk, 75.30.Kz, 75.47.Np, 75.50.Pp, 76.30.−v (all) DOI: 10.3367/UFNe.2023.05.039363 URL: https://ufn.ru/en/articles/2024/1/c/ 001198734600007 2-s2.0-85183194631 2024PhyU...67...22D Citation: Demishev S V "Spin-fluctuation transitions" Phys. Usp. 67 22–43 (2024) BibTexBibNote ® (generic)BibNote ® (RIS)MedlineRefWorks Received: 19th, January 2023, revised: 15th, April 2023, accepted: 31st, May 2023 Оригинал: Демишев С В «Спин-флуктуационные переходы» УФН 194 23–47 (2024); DOI: 10.3367/UFNr.2023.05.039363