Submicron particles of Cr-substituted strontium hexaferrite: anomalous X-ray diffraction studies, hard magnetic properties, millimeter-wave absorption, and research prospects

Here, we report the synthesis of single-domain particles of chromium-substituted hexaferrites SrFe_12-<i>x</i>Cr_<i>x</i>O₁₉ (<i>x</i> = 0-8) <i>via</i> the citrate-nitrate auto-combustion method. The introduction of Cr³⁺ ions into the hexaferrite structure up to <i>x</i> = 5.5 led to a significant increase in the coercive force from 4.4 to 13.9 kOe and a rise in the natural ferromagnetic resonance frequency from 51 to 129 GHz, which was also accompanied by a gradual decrease in Curie temperature. According to anomalous XRD and Mössbauer spectroscopy, chromium ions predominantly enter the octahedral sites 2a, 12k, and 4f₂. This correlates with reducing magnetization and magnetocrystalline anisotropy constant; however, the resulting anisotropy field increases and leads to the enhancement of the coercivity and FMR frequency. We also summarized the data obtained on Cr substitution and compared it with Al and Ga, which are also known to improve the hard magnetic properties of hexaferrites, and showed that the differences in their properties can be explained by the distribution features of the substituting ions in the hexaferrite lattice. Our findings demonstrate that chromium-substituted hexaferrites offer an effective route to enhance hard magnetic properties and high-frequency performance, paving the way for their integration into rare-earth-free permanent magnets, spintronic devices, and future generations of wireless technologies.