ИСТИНА

M-type hexaferrites are solid oxides with the general formula MFe12O19 (M = Ba, Sr, Pb). They exhibit hard magnetic properties at room temperature and contain available and low-cost elements. They are already used as materials for magnetic recording and considered to be an alternative for expensive rare-earth alloys (Nd-Fe-B, SmCo) due to a number of important advantages. First, they are insulators, therefore can be applied as hardmagnetic probes for magnetic force microscopy. Second, they are chemically and thermally more stable than metal alloys. Third, they absorb the electromagnetic radiation at frequencies up to 100 GHz, which is not inherent in metal alloys [1]. The magnetic properties of hexaferrites can be tuned via partial substitution of Fe3+ ions. Recently, it was shown that single-domain high-Al-substituted hexaferrites can demonstrate giant coercivities (20 — 36 kOe) and sub-terahertz natural ferromagnetic resonance frequencies (160 — 250 GHz) at room temperature [2]. Natural ferromagnetic resonance frequencies, huge coercive forces, and the possibility of large-scale fabrication of these materials opens opportunities for them to be used in industry. For example, sub-THz detectors and other spintronic devices operating via spin pumping mechanism even in the absence of an applied magnetic fi eld can be created on the basis of the hexaferrites, which is extremely in demand today [3]. However, to understand the potential applications of a material, it is important to know the dynamics of its properties with temperature. Herein, we demonstrate a study of the magnetostatic properties and natural ferromagnetic resonance of single-domain hexaferrites with the following compounds Sr1-x/12Cax/12Fe12-xAlxO19 (x = 1.5 — 5.5) in temperature range of 5 — 300 K. The hexaferrite samples were synthesized via a citrate-nitrate auto-combustion method. According to X-ray powder diff raction (XRD) all samples are singe-phase and consist of M-type hexaferrite. We have investigated the dependence of natural ferromagnetic resonance frequencies and magnetostatic properties of Al-substituted single-domain hexaferrites on the temperature. Throughout the range of the measured temperatures 5 — 300 K all samples keep their hard magnetic properties and possess hysteresis loops typical of an ensemble of randomly oriented Stoner-Wohlfarth particles. The saturation magnetization (MS) monotonously increases with a decrease in a temperature. The maximum of HC(T) shifts to the low temperature region. The NFMR frequency raises with an aluminum concentration and the maximum of fr(T) also shifts to low temperatures. The highest both coercivity of 42 kOe and natural ferromagnetic resonance frequency of 297 GHz were observed for Sr0.54Ca0.46Fe6.5Fe5.5O19 sample at 180 K. The sample possess the highest NFMR frequency among all materials so far and the highest coercivity value among non-textured ferrites. The work was supported by the Russian Science Foundation (RSF) grant № 20-73-10129. [1] Gorbachev E.A. et al. Design of modern magnetic materials with giant coercivity // Russ. Chem. Rev. 2021. Vol. 90, № 10. P. 1287 ± 1329. [2] Gorbachev E.A. et al. Hexaferrite materials displaying ultra-high coercivity and sub-terahertz ferromagnetic resonance frequencies // Mater. Today. Elsevier Ltd, 2019. Vol. 32, № 20. P. 13–18. [3] Lev A. Trusov, Evgeny A. Gorbachev, Vasily A. Lebedev, Anastasia E. Sleptsova b, Ilya V. Roslyakov a, Ekaterina S. Kozlyakova, Alexander V. Vasiliev a, Robert E. Dinnebier M.J. and P.E.K. Ca-Al double substituted strontium hexaferrites with giant coercivity // ChemComm. 2018. Vol. 54, № 5. P. 479–482.