Magnetization dynamics of exchange biased films
The interfacial exchange coupling between the spins of a ferromagnetic (F) layer and an antiferromagnetic (AF) layer has been extensively investigated in the past years. Experimentally, the exchange bias phenomenon manifests itself in a field shift of the magnetic hysteresis loop, referred to as the exchange bias field Heb, and an increase of coercivity Hc measured in the F film. Numerous theories have been developed that predict values for Heb in reasonable agreement with experimental results. The models consider compensated or uncompensated interfaces, polycrystalline thin-film systems, spin-flop coupling, and interface roughness. The internal AF grain and domain structure add many additional aspects to the exchange bias phenomena. Several mechanisms have been proposed to explain the enhanced Hc in F/AF bilayers. Models include FM domain wall pinning at AF domains and other F/AF interactions. Particularly, time-dependent effects are reported in F/AF systems related to thermally activated switching of AF grains and connected to a rotatable anisotropy field contribution Hrot. This anisotropy can be understood as an anisotropy that has an energetic minimum which aligns parallel to a (possibly changing) F layer magnetization direction. It is related to irreversible changes in the AF layer. Related is the training effect, manifesting itself in changes in the hysteretic characteristics depending on magnetic history.
We are using combined PIMM and hysteresis measurements to separate the rotatable and the unidirectional anisotropy contributions with varying AF layer thickness. In our research we focus on time-dependent relaxation effects in the AF, important for the understanding of the F/AF magnetic structure. The stability of exchange bias and rotatable anisotropy is probed by different techniques. The results are relevant for the understanding of exchange bias in polycrystalline thin films.
Example PIMM data for an bias field Hbias = −20 Oe applied along the induced anisotropy axis (left column) and change of fres with Hbias (right column) (Physical Review B 70, 094420 (2004))