Magnetic Study of M-type Co-Ti Doped Strontium Hexaferrite Nanocrystalline Particles

The effects of Co-Ti ions substituting Fe3+ ions in strontium hexaferrite nanoparticles synthesized using ball-milling technique were investigated. X-ray diffraction patterns of SrFe12-2xCoxTixO19 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) demonstrated standard SrFe12O19 patterns with space group P6(3)/mmc. The phase purity and the valence state of each ion were examined using X-ray photoelectron spectroscopy. The results indicated a Ti4+ valence state and a coexistence of 2 + and 3 + valence states for Fe and Co. An obvious irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves was observed below 300 K. The broad peaks in the ZFC curves were attributed to the movement of magnetic domains at temperature (T-M). Magnetization hysteresis loops at discrete temperatures between 4 and 300 K showed a ferromagnetic behavior, with high coercive fields and large saturation magnetizations. The change of the saturation magnetization with increasing x revealed spin reorientation behavior at x >= 0.4 and remained relatively high for practical applications. The first anisotropy constant decreased with the increase of temperature. With Co-Ti substitution, it remained almost constant up to x = 0.4, and then decreased almost linearly at higher Co-Ti concentrations. Furthermore, the coercive field decreased with increasing x, reaching values suitable for high-density magnetic recording at 300 K in the substitution range 0.4 <= x <= 0.8.