Dielectric and temperature dependent magnetic studies of Al3+ substituted Ba0.4La0.1Sr0.5AlxFe12-xO19 hexaferrite for microwave application

Abstract

The Al3+ substituted polycrystalline Ba0.4La0.1Sr0.5AlxFe12-xO19 (0 ≤ x ≤ 1) hexaferrite sample were prepared by the solid-state method. The structural, morphology, dielectric, and temperature dependent magnetic properties are studied systematically. The XRD and Rietveld refinement analysis revealed that all the samples have magneto plumbite hexaferrite structure with space group P6/mmc. The lattice parameters a and c are decreased from 5.888 Å to 5.874 Å and 23.109 Å to 23.085 Å, respectively. The c/a ratio is found to be between 3.92 and 3.93, which is ideal for M−type hexaferrite. It is observed that all the Raman peaks are shifted along the higher wavenumber with increasing Al3+ substitution. The surface morphology of these samples has uniform grain sizes and is closely packed. Also, all the constituent elements are present in proper composition except barium, which has a little higher atomic percentage than the calculated value. The complex dielectric constant (ε', ε“) decreased with Al3+ insertion (for x = 0: ε' = 3.44 k, ε” = 0.993 k; x = 1: ε' = 0.744 k, ε“ = 0.211 k @ 1 MHz). The hysteresis isotherm curves were analyzed by the law of approach to saturation magnetization in the temperature range of 10 K − 300 K. Further, the saturation magnetization and magnetic anisotropy constant K1(T) are found to be decreased, whereas anisotropy field Ha(T) and coercivity HC(T) are rises with an increase in temperature as well as Al3+ concentration. The Bloch’s spin wave theory is used to analyze temperature dependent saturation magnetization, and all the samples get deviated from Bloch’s T3/2 law as well as modified Kobler’s Bloch law. It might be due to finite size effects, long wavelength excitation, and the introduction of constant energy gap into spin wave spectrum, geometrical asymmetries, and other atomic disorders. Finally, Dyson’s approximation which includes T5/2 terms is perfectly fitted with the experimental data. The analysis makes it possible to predict the curie temperature and visualize the nature of the ferromagnetic coupling with the increasing Al3+ concentrations. Above that, the temperature dependent magnetic anisotropy constant K1(T) is linearly fitted with cubic of temperature dependent saturation magnetization MS(T), which confirmed that all the samples have uniaxial hexagonal crystal symmetry. The combination of magnetic and dielectric responses made the material suitable for permanent magnets and microwave applications.