Magnetic and electrical transport studies of polycrystalline Sr1− x Bi x Fe12O19 (x = 0, 0.01, and 0.02)

Abstract

Bismuth-substituted strontium hexaferrites, Sr1−x Bi x Fe12O19 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 Fe2+ at 12k crystallographic sites (which is supported by Mössbauer results), with increasing Bi in the sample. They also suggest an increase in strain due to Bi substitution for the polyhedral associated with 2a and 2b spin-up and 4f1 spin-down sites. The M measurements over a wide temperature range (3–823 K), shows irreversibility in zero field cooled (ZFC) and field cooled data right below the Curie temperature, along with the Hopkinson peak in the ZFC data. The temperature dependence of saturated magnetization follows the Bloch relation but that of the coercive field shows 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, calculated using modified Arrott plots, are slightly overvalued as per mean-field theory. The 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 Fe2+ 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.