Magnetic relaxation in Zn-Sn-doped barium ferrite nanoparticles for recording

Magnetic relaxation analysis has been carried out on Zn-Sn-doped barium ferrite BaFe_12-2xZn_xSn_xO₁₉ nanoparticles with 0 ≤ x ≤ 0.5 at room temperature. The choice of these two ions combination was based on the consideration that Zn²⁺ substitution could help to increase saturation magnetization M_s and Sn⁴⁺ substitution could decrease the temperature dependence of coercivity dH_c/dT. Logarithmic behavior on the time dependence of the magnetization for these particles has been observed. The measured viscosity coefficient S was found to vary with the applied field and peaked around the coercive field H_c. With increased doping concentration x, thermal relaxation of magnetization increased, mainly due to the decrease of particle size and anisotropy field, as well as their wider distributions. The activation volume V_ac and fluctuation field H_f were determined from the magnetic viscosity coefficient S together with the irreversible magnetic susceptibility χ_irr. V_ac was found to be smaller than the particle's physical volume, indicating incoherent magnetic reversals in these particles.