ИСТИНА

Here we report a simple synthesis of Ca-Al double substituted strontium hexaferrite Sr1-x/12Cax/12Fe12-xAlxO19. The rise of the substitution ratio x leads to decrease of magnetization, but significant increase of coercivity. At x = 4 the sample coercivity is 21.3 kOe, which is higher than for ε-Fe2O3 with the same magnetization of 15 emu/g. At x = 5.5 the coercivity reaches 36 kOe, which is the highest value known for ferrite materials. However, further increasing the substitution degree doesn‘t lead to improve coercivity (HC of x = 6 is 21 kOe). Nevertheless, the coercivity could be further enlarged by alignment of the hexaferrite particles dispersed in a polymer by the external magnetic field. Such oriented composites possess nearly square hysteresis loops with coercivity up to 40 kOe for x = 5.5 compound while magnetized in alignment direction. Materials with gigantic coercivity values have potential in future high-density magnetic recording media as their particle size can be considerable reduced while maintaining hard magnetic properties. Also we have studied the millimeter wave absorption properties of the samples. The high anisotropy fields result in very high natural ferromagnetic resonance frequencies. The FMR frequency increases with substitution ratio and reaches the record-high values of 180 – 240 GHz for x = 4 – 5.5. The present materials should be suitable for high frequency millimeter wave devices (absorbers and rotators) because it can curb electromagnetic interference problems as the FMR frequency corresponds to the highest window of air, which is the anticipated carrier frequency for next-generation millimeter wave wireless communications. Also we have studied the features of the crystalline structure of the samples by high precision synchrotron radiation diffraction and revealed that the presence of calcium results in shrinking of oxygen surroundings in bipyramidal iron position, which could be a reason of the increase of magnetocrystalline anisotropy compared to simple aluminum doping of strontium hexaferrite.