An enhancement behavior of coercivity near T _C in ferromagnetic films with uniaxial magnetic anisotropy

The temperature dependence of coercivity was investigated in CrO ₂ and La _0.7Sr _0.3MnO ₃ films with and without uniaxial magnetic anisotropy. When the magnetic field was applied along the uniaxial magnetic anisotropy axis, the coercivities of the CrO ₂ and La _0.7Sr _0.3MnO ₃ films showed remarkable enhancement behaviors near ferromagnetic transition temperature (T _C). In contrast, CrO ₂ and La _0.7Sr _0.3MnO ₃ films without uniaxial magnetic anisotropy did not exhibit such an enhancement behavior. The temperature dependence of coercivity H _C(T) for an aligned Stoner-Wohlfarth ferromagnet was theoretically studied based on a modified model of previously reported ones by He [Phys. Rev. B 75, 184424 (2007)] and Grimsditch [Phys. Rev. Lett. 90, 257201 (2003)]. Experimentally observed enhancement of coercivity near T _C could be explained by an effect of a distribution of the local ordering temperatures ρ(T _B) for magnetic grains. The qualitative behavior of H _C(T) was investigated for three different types of uniaxial anisotropies: (i) magnetocrystalline anisotropy, (ii) strain-induced anisotropy, and (iii) shape anisotropy.