Effects of critical fluctuations on Gilbert damping in iron near T C

Low field ferromagnetic resonance (FMR) is measured near TC. The whisker is magnetized along its long axis (001), but not uniformly so because of demagnetizing effects and the nonlinear response of the magnetization to the internal field. A current at fm frequencies is passed along the axis creating a fm magnetic field which is transverse to and circulating about the axis in a skin depth of 0.01 mm. The condition of FMR is reflected in the measured impedance. The frequency and temperature dependence of the Gilbert damping is extracted using a five parameter parametric equation of state taking into account the FMR equation of motion and the magnetostatics. The Gilbert damping is greater than that measured in high field microwave FMR and it peaks near TC. The results are consistent with extrapolation of neutron measurements to extract the wave vector zero loss.