Field dependence of the temperature at the peak of the zero-field-cooled magnetization

Abstract

The effect of an applied magnetic field on the temperature at the maximum of\nthe ZFC magnetization, $M_{ZFC}$, is studied using the recently obtained\nanalytic results of Coffey et al. (Phys. Rev. Lett. {\\bf 80}(1998) 5655) for\nthe prefactor of the N\\'{e}el relaxation time which allow one to precisely\ncalculate the prefactor in the N\\'{e}el-Brown model and thus the blocking\ntemperature as a function of the coefficients of the Taylor series expansion of\nthe magnetocrystalline anisotropy. The present calculations indicate that even\na precise determination of the prefactor in the N\\'{e}el-Brown theory, which\nalways predicts a monotonic decrease of the relaxation time with increasing\nfield, is insufficient to explain the effect of an applied magnetic field on\nthe temperature at the maximum of the ZFC magnetization. On the other hand, we\nfind that the non linear field-dependence of the magnetization along with the\nmagnetocrystalline anisotropy appears to be of crucial importance to the\nexistence of this maximum.\n