Different dynamic and static magnetic anisotropy in thin Permalloy™ films

The values of uniaxial anisotropy Hk in thin polycrystalline Permalloy™ films measured by static and dynamic methods differ by as much as a factor of 1.5. The anisotropy obtained with a pulsed inductive microwave magnetometer in 2.5 to 100 nm thick Permalloy films exhibits an additional isotropic component of 120 to 240 A/m not observed in static measurements. The static value of anisotropy was obtained with an inductive magnetic hysteresis loop tracer. The time-resolved precessional response was measured as a function of in-plane applied magnetic bias field and the angle between the easy axis and that of the applied bias field. We interpret the constant-offset field as a transient component of the magnetic susceptibility that affects only dynamical response at time scales below 10 ns.