ИСТИНА

Magnetic materials with large coercivity have broad applications ranging from permanent magnets and data storage media to high-frequency electromagnetic wave absorbers. The only known ferrite material exhibiting coercivity over 20 kOe (also considered as “giant coercive force”) at room temperatures is epsilon-iron oxide (ε-Fe2O3). Furthermore, it displays ultra-high natural ferromagnetic resonance (NFMR) frequency of 182 GHz. The Rh-doping of the material (ε-Fe2-xRhxO3) leads to an increase in both the NFMR up to 209 GHz and the coercivity up to 27 kOe [1]. However, since epsilon-iron oxide is a metastable polymorph and its production as a pure phase is quite complex, it still has not been applied in industrial applications. Recently we have reported [2] an effective method for production of single-domain M-type hexaferrite particles with composition of Sr1−x/12Cax/12Fe12−xAlxO19 (x = 4), which demonstrated coercivity of 21.3 kOe and saturation magnetization 15 emu/g, which is close to pure ε-Fe2O3 phase. The proposed preparation technique is based on citrate-nitrate auto-combustion method; therefore, it is simple, economical and readily scalable, and it can be efficiently integrated into modern ferrite technology. In the following work [3] it was shown that further increase of Al substitution degree leads to significant improvement of coercivity (up to 36 kOe) and NFMR frequency (up to 250 GHz), which are record-high values for ferrite powders to date. Herein, we discuss temperature dependence of the magnetic and microwave properties, crystal and magnetic structures of the ultra-hard magnetic hexaferrites Sr1−x/12Cax/12Fe12−xAlxO19 (x = 3 – 5.5) in a range of 4.2 – 700 K. Temperature dependencies of both NFMR and coercivity, which are proportional to anisotropy field, have a maximum shifted to low temperatures with increase of aluminium content. For x = 5.5 the compound coercivity is raised from 36 kOe at 300 K up to 41 kOe at 180 K. Furthermore, we developed a technique for preparation of dense ceramics with single domain grains and studied their room temperature magnetic properties and millimetre wave absorption. The obtained ceramic materials possess coercivity higher than 18 kOe at specific density of 67 - 95%. NFMR frequency of the ceramics is 200 GHz, which is 20% blue-shifted compared to that of a powder sample. The ferrite ceramics with a coercivity of more than 10 kOe and sub-terahertz absorption were obtained for the first time. The work was supported by RFBR project № 20-02-00887. [1] A. Namai, et al, Nat. Commun, 3 (2012), 1035. [2] L.A. Trusov, E.A. Gorbachev, et al, Chem. Commun, 54 (2018), 479-482. [3] E.A. Gorbachev, L.A. Trusov, et al, Mater. Today, 32 (2020), 13-18.