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

Abstract

We have explored the dynamics of magnetization reversal asymmetry in exchange biased FeF 2 / 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.

Effect of Interface Roughness on Exchange Bias of an Uncompensated Interface: Monte Carlo Simulation

“…This asymmetry results in a reduction of the exchange bias. Our results are in good qualitative agreement with experimental ones [22][23][24][25][26], where an exchange bias was not found in the Fe/FeF 2 (100) system even though the FeF 2 (100) orientation possesses uncompensated spins at the interface. Therefore, domain wall formation in AFM systems is a good candidate to explain the zero exchange bias in experiments.…”

“…This is evidence that blocking temperature is partly related to interface roughness. Interestingly, a peak of coercivity occurs at the so-called blocking temperature when the exchange bias field becomes zero, qualitatively consistent with experimental results [22][23][24][25][26]. Fig.…”

Interface-induced phenomena in magnetism

“…A great advantage of time-resolved techniques over frequency domain techniques is the ability to measure spin precession even for large damping. Spatial resolution requires combining the optical excitation with optical imaging or by using a small optical spot, enabling investigation of how damping of laser-induced precession is affected by interlayer exchange interactions and modified spin-orbit interactions at the interfaces [Engebretson et al ., 2005; Djordjevic et al ., 2006; Weber et al ., 2006; Zhang et al ., 2010; Fan et al ., 2014a]. Analysis of spin precession induced by femtosecond laser excitation was similarly used to reveal a modified spin-orbit interaction and interfacial effects in ferromagnetic exchange-coupled multilayers ( e.g ., [Barman et al ., 2007; Michalski et al ., 2007, Rzhevsky et al ., 2007; Ren et al ., 2008]).…”

Exchange bias relaxation in reverse magnetic fields

“…Experiments including those in Refs. [10][11][12][13][14] have been conducted to investigate the relaxation of the FM magnetization of a bilayer in a reverse field and the training effect, which describes the decrease of exchange bias with cycling of the field. These studies suggest that changes are occurring in the state and structure of the polycrystalline AF layers in response to the dynamical environment.…”