Thermal fluctuation of magnetization in nanocrystalline FePt thin films with high coercivity

The effect of thermal fluctuations of magnetization on static and dynamic properties is discussed for nanocrystalline FePt films. The large magnetocrystalline anisotropy K_u of L1₀ type FePt results in large K_uV/(kT) values of more than 70 even for very fine grain sizes of 7-8 nm, indicating the potential of this alloy film to resist thermal fluctuation of magnetization. It is successfully demonstrated that larger K_uV/(kT) values of these films lead to lower magnetic viscosity. Annealing at higher temperature results in larger K_uV/kT values and smaller V_act. The remanent coercivity measured at high sweep rate by using pulsed magnetic fields indicates the high thermal stability of these alloy films at high frequencies. However, the results indicated that care should be taken to induce an adequate magnitude of K_u for the FePt alloy to be used as ultrahigh density recording media