Novel ferrite materials possessing record magnetic hardness

Abstract

Hard magnetic ferrites have broad applications ranging from permanent magnets and data storage media to high-frequency devices. The only known ferrite material exhibiting giant coercivity over 20 kOe at room temperatures is epsilon-iron oxide (ε-Fe2O3). In addition, it displays sub-THz natural ferromagnetic resonance (NFMR) frequency of 182 GHz. The Rh-doping of the material leads to an increase in both the NFMR up to 209 GHz and the coercivity up to 27 kOe [1]. However, since ε-Fe2O3 is a metastable polymorph and its production as a pure phase is quite complex, it still has not been applied in industry. Recently we have reported [2] an effective method for production of single-domain M-type hexaferrite particles with composition of Sr0.67Ca0.33Fe8Al4O19, which demonstrated coercivity of 21.3 kOe and saturation magnetization of 15 emu/g, which is close to ε-Fe2O3. 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. Herein, we show 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 ferrites to date [3]. Furthermore, 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 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 NFMR. 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 grant No. 20-02-00887.