Influence of Al–Cr co-substitution on physical properties of strontium hexaferrite nanoparticles synthesized by sol–gel auto combustion method

Substituted strontium hexaferrite ceramics SrCr _x Al _x Fe_12−2x O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by sol–gel auto-combustion method. X-ray diffraction (XRD) study revealed the M-type hexagonal structure of the synthesized nanoferrites with some additional peaks of Fe₂O₃. The lattice constants (a and c), unit cell volume (V), X-ray density (ρ _x ), bulk density (ρ _m ), porosity (P) and average crystallite size (t) values changes when Al–Cr ions are co-substituted in SrFe₁₂O₁₉ lattice, resulting in the structural variation. The surface morphology of the grains was examined by scanning electron microscopy (FESEM). Fourier transform infrared spectroscopy (FTIR) confirmed the formation of hexagonal ferrite structure. Ferromagnetic nature confirmed by recording M-H curves exhibited typical hysteresis loop at room temperature using pulse field hysteresis loop tracer technique. The large coercivity (H _c ) values indicate the nanocrystalline nature of the present samples. The coercivity (H _c ), saturation magnetization (M _s ), remanence magnetization (M _r ) and magneton number (n _B ) decreases with increase in Al–Cr content x. The DC electrical resistivity studies of the prepared samples were carried out in the temperature range of 300–873 K using a standard two-probe technique. Curie temperature (Tc) i.e. ferrimagnetic to paramagnetic transition temperature for all samples was obtained from resistivity data. The Curie temperature decreases linearly as the concentration of Al–Cr content is increased. The activation energy below and above Tc was calculated. The dielectric parameters such as dielectric constant (ɛ′), dielectric loss (ɛ″) and loss tangent (tan δ) were measured in the frequency range 50 Hz–5 MHz at room temperature. All the dielectrical parameters show compositional as a function of frequency dependences. At lower frequencies, it is observed that the dielectric constant (ɛ′), dielectric loss (ɛ″) and loss tangent (tan δ) are high.