Influence of Tm–Tb substitution on magnetic and optical properties of Ba–Sr hexaferrites prepared by ultrasonic assisted citrate sol-gel approach

Tm–Tb co-substituted Sr–Ba hexaferrite (HFs), Sr0.5Ba0.5TmxTbxFe12-2xO19 (x = 0.00–0.05), were produced by ultrasonic assisted citrate sol-gel approach. The compositions were characterized using diverse characterization techniques including X-rays diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning and transmission electron microscopes (SEM and TEM), diffuse reflectance spectroscopy (DRS), and physical property measurement system with vibrating sample magnetometer head (PPMS-VSM). All products are pure and consist of only hexaferrite phase except x = 0.05 composition, which contains minor impurity Fe2O3 phase. Co-doping of Tm3+ and Tb3+ ions increases the 1.73 eV band gap of Sr0.5Ba0.5Fe12O19 to maximum value of 2.65 eV belonging Sr0.5Ba0.5Tm0.03Tb0.03Fe11.94O19 HF. The magnetic properties were investigated at 300 K (RT) and 10 K. Magnetization measurements indicated ferrimagnetic behaviors for different prepared hexaferrites. It was noticed that the Ms (saturation magnetization) and Mr (remanence) grew with rising Tb–Tm co-substitution proportion up to x = 0.03 and then started decreasing. The intensification in the Ms and Mr values up to x = 0.03 was mainly ascribed to the magnetic moments of Tb3+ ions (9.7 μB) and Tm3+ ions (7.5 μB) that are superior than that of Fe3+ ions (5 μB) and to strengthening of exchange coupling among Fe sites. At higher co-substitution contents, the formation of impurity phase resulted in reducing the magnetization. The coercivity is larger for lower Tb–Tm co-substitution level compared to non-substituted one and reduced for highest level. It was revealed that the coercivity is mainly dominated by the magneto-crystalline anisotropy in different prepared hexaferrites. The examination of M-T curves showed ferrimagnetic behavior in the whole temperature range of 325 down to 10K and the existence of super-spin glass-like behavior at very low temperatures. The structure, morphology, optical and magnetic properties of Sr0.5Ba0.5TmxTbxFe12-2xO19 (x = 0.00–0.05) hexaferrites produced via ultrasonic assisted citrate sol-gel approach were investigated. Tb–Tm substitution effect could enhance the magnetic properties of SrBa hexaferrites. [Display omitted] •Sr0.5Ba0.5TmxTbxFe12-2xO19 (x = 0.00–0.05) hexaferrites were produced by ultrasonic assisted citrate sol-gel approach.•All products are pure except x = 0.05 composition that contains minor Fe2O3 phase.•Optical band gap energy increased with increasing x up to 0