Investigation of Magnetic Properties of Sr-Doped Ba3–xSrxCo2Fe24O41 Z-type Hexaferrite by Mössbauer Spectroscopy

The polycrystalline Ba_3-xSr_xCo₂Fe₂₄O₄₁ (x = 0.0, 0.5, 1.0, and 1.5) samples were synthesized by the solid-state reaction method. The crystalline structure of the samples was found to be a hexagonal structure with a space group of P63/mmc. The lattice constants a₀, c₀, and Vu of the samples decrease with increasing Sr concentration. From the field-dependent hysteresis curves under 10 kOe at 295 K, the values of Ms increase while the values of H_c decrease. The zero-field-cooled magnetization curves under 100 Oe between 4.2 and 300 K show the spin transition, and the value of T_s decreases from 230 K for x = 0.0 to 135 K for x = 1.5 with increasing Sr concentration due to the reduction of planar anisotropy with the difference in ionic radius between Ba²⁺ and Sr²⁺ ions. From the isomer shift value (δ) obtained from Mössbauer spectra, the charge states of all samples are determined to be Fe³⁺ high spin state. With increasing Sr concentration, the reduction of hyperfine field (H_hf) suggests the change in hyperfine interaction between Fe ions because of the larger ionic radius of Ba²⁺ ion than that of Sr²⁺ ion, leading to decreasing M_s.