Effect of temperature-dependent shape anisotropy on coercivity for aligned Stoner-Wohlfarth soft ferromagnets

The temperature variation effect of shape anisotropy on the coercivity, H-C(T), for the aligned Stoner-Wohlfarth (SW) soft ferromagnets, such as fcc Ni, fcc Co, and bcc Fe, are investigated within the framework of Neel-Brown (N-B) analysis. An extended N-B equation is thus proposed, H-C(T)=H(0)m(tau) x {1-[k(B)T ln (t/t(0))/E(0)m(2)(tau)](1/alpha)},by introducing a single dimensionless correction function, the reduced magnetization, m(tau)=M-S(T)/M-S(0), in which tau=T/T-C is the reduced temperature, M-S(T) is the saturation magnetization, and T-C is the Curie temperature. The factor, m(tau), accounts for the temperature-dependent effect of the shape anisotropy. The constants, H-0 and E-0, are for the switching field at zero temperature and the potential barrier at zero field, respectively. According to this newly derived equation, the blocking temperature above which the properties of superparamagnetism show up is described by the expression, T-B=E(0)m(2)(tau(B))/[k(B) ln(t/t(0))], with the extra correction factor m(2)(tau(B)). The possible effect on H-C(T) and the blocking temperature, T-B, attributed to the downshift of T-C resulting from the finite size effect has been discussed also.