EFFECT OF ULTRAFINE PARTICLE-SIZE AND CRYSTALLINITY BREAKDOWN ON THE ORDERED MAGNETIC STATE IN BARIUM FERRITE

The relationship between crystallinity and magnetocrystalline anisotro py has been studied in a series of ultrafine barium ferrite samples pr epared using the citrate precursor method. Mossbauer data have been co llected at 4.2 K in an applied field of 9 T, and effective anisotropie s B-A have been estimated using a simple two sublattice model. For rel atively well-crystallised particles (as evidenced by sharp XRD and Mos sbauer lines) a correlation is observed between particle size and anis otropy, with B-A approximate to 2.5 T in 50 nm particles, and B-A appr oximate to 1.3 T in 100 m, particles. A non-crystalline phase (evidenc ed by strongly broadened Mossbauer lines) seen in some of the smallest samples had a larger anisotropy, B-A approximate to 7.3 T.