Exchange-Bias Phenomenon: The Role of the Ferromagnetic Spin Structure

The exchange bias of antiferromagnetic-ferromagnetic (AFM-FM) bilayers is found to be strongly dependent on the ferromagnetic spin configuration. The widely accepted inverse proportionality of the exchange bias field with the ferromagnetic thickness is broken in FM layers thinner than the FM correlation length. Moreover, an anomalous thermal dependence of both exchange bias field and coercivity is also found. A model based on springlike domain walls parallel to the AFM-FM interface quantitatively accounts for the experimental results and, in particular, for the deviation from the inverse proportionality law. These results reveal the active role the ferromagnetic spin structure plays in AFM-FM hybrids which leads to a new paradigm of the exchange bias phenomenon.

Figure 1

Hysteresis loops for ${\mathrm{FeF}}_2(70\mathrm{nm})/\mathrm{NiFe}$ (30 and 100 nm) bilayers at (a) $T=10$ and (b) $T=70\mathrm{K}$. Symbol: Experimental data. Blue solid line in (a): Simulation.

Figure 2

FM thickness dependence of the exchange bias field for experimental data at 10 and 70 K (symbols), Eq. (1) (dashed lines), and calculated from the model in Eqs. (2)–(5) (solid line).

Figure 3

Temperature dependence of the exchange bias field for ${\mathrm{FeF}}_2(70\mathrm{nm})/\mathrm{NiFe}$ (140 nm). Solid line is a guide for the eyes.

Figure 4

Schematic orientation of net magnetic moments in AFM (red) and FM (blue) layers. The model assumes domain walls parallel to the AFM-FM interface [28].