Time domain dynamics of the asymmetric magnetization reversal in exchange biased bilayers

We have explored the dynamics of magnetization reversal asymmetry in exchange biased Fe F2 Fe bilayers using subnanosecond time-resolved Kerr magnetometry. The data reveal an increase in the characteristic precession frequency with decreasing temperature, even above the Néel temperature of the antiferromagnet, which we interpret in terms of the previously observed anisotropy enhancement due to antiferromagnetic spin fluctuations. Below the Néel point the magnetization precession is strongly suppressed due to the damping provided by exchange coupling to the antiferromagnetic layer. Dynamic hysteresis loops measured at a fixed delay between the magnetic field pulse and the optical probe pulse reveal distinct reversal asymmetry that is not observed in the corresponding static loops. The asymmetry takes the form of a suppression of the Kerr rotation signal in the part of the hysteresis loop where nucleation of reverse domains is energetically favorable. The formation of reverse domains prevents the magnetization from rotating coherently on nanosecond time scales. The temperature dependence of this dynamic asymmetry is found to be nonmonotonic and appears to be correlated with the coercivity.