Magnetic and electrical transport studies of polycrystalline Sr_1-xBi_xFe₁₂O₁₉ (x =0, 0.01, and 0.02)

Abstract Bismuth (Bi) substituted strontium hexaferrite, Sr 1-x Bi x Fe 12 O 19 ; for x = 0, 0.01 and 0.02 are studied via powder neutron diffraction (ND), magnetization (M) studies, Mössbauer spectroscopy and electrical transport. ND results show an indication of increasing Fe 2+ at 12k crystallographic sites (and is supported by Mössbauer results), with increasing Bi in the sample. It also suggests increase in strain due to Bi substitution for the polyhedral associated with 2a and 2b spin-up and 4f 1 spin-down sites. Magnetization measurements in a wide temperature range (3 – 823 K), shows irreversibility in zero field cooled (ZFC) and field cooled (FC) data right below the Curie temperature, along with the Hopkinson peak in the ZFC data. Temperature dependence of saturation magnetization follows the Bloch relation but that of coercive field shows an unconventional behavior. The coercive field data is fitted using an equation devised by taking into consideration of all the three anisotropies. The critical exponents at the ferromagnetic-paramagnetic phase transition boundary are calculated using modified Arrott plots, are slightly overvalued as per mean-field theory. Temperature dependence of resistivity displays nearest-neighbor hopping conduction in all the three samples. The conductivity increases with increasing Bi in the sample, due to the increasing Fe 2+ content, which facilitates the electron hopping between Fe-sites. The magnetoresistance measured at various sub-room temperatures for all the compounds shows the interplay of anisotropy magnetoresistance (AMR) and giant magnetoresistance (GMR). Low temperature data are dominated by GMR and gradual participation of AMR increases as room temperature is approached.