Effect of Tb3+ substitution on the structural and magnetic properties of M-type hexaferrites synthesized by sol–gel auto-combustion technique

M-type hexaferrites with new substitution of rare-earth element Tb3+ having nominal composition Ba0.5Sr0.5−xTbxAlFe11O19 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) were synthesized by the sol–gel auto-combustion technique. The main aim of study is to investigate the effect of terbium (Tb3+) ions substitution at Sr2+ site on the structural and magnetic properties. The materials were characterized by Differential Scanning Calorimetry, Thermogravimetry, Fourier Transform Infra-Red spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and vibrating sample magnetometery analyses. The XRD patterns confirm the formation of M-type hexaferrite phase. The crystallite size was found to be in the range of 17–45 nm which is good enough to obtain the suitable signal-to-noise ratio in the high density recording media. The magnetic properties such as saturation magnetization (Ms), retentivity (Mr), squareness ratio (Mr/Ms) and coercivity (Hc) were calculated from the MH-loops. The saturation magnetization (Ms) and retentivity (Mr) decreased from 48.9–26.9 and 36.8–18.1 emu/g, respectively which may be due to spin canting and hence reduction in the super-exchange interactions. The enhancement of coercivity (Hc) from 1825 to 4440 G may be due to higher magnetocrystalline anisotropy, which is due to the Fe2+ ions located on a 2a site. The magnetic properties such as retentivity (Mr) and coercivity (Hc) make the synthesized materials useful for high density recording media and permanent magnets.