Dual behavior of antiferromagnetic uncompensated spins in NiFe/IrMn exchange biased bilayers

We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfacial spin disorder involved in the data analysis is further supported by the observation of the dual behavior of the interfacial uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we have observed that the antiferromagnetic uncompensated spins are dominantly frozen with nearly no rotating spins due to the chemical intermixing, which correlates to the inferred mechanism for the exchange bias.

Figure 1

(Color online) Panel (a) The dependence of the ${\text{H}}_{\text{EB}}$ and the ${\text{H}}_c$ as a function azimuthal angle $\varphi$ for a representative sample with $t_{\text{AF}}=35\text{Å}$. Solid line and dotted line are numerical simulations (see text). Panel (b) The thickness dependence of the ${\text{H}}_{\text{EB}}$ and ${\text{H}}_c$ (phase diagram). In the inset a schematic view of the samples is shown. The arrow marks the critical antiferromagnetic thickness. Panel (c) The anisotropy of the AF layer extracted through numerical analysis of the phase diagram. The line is a fit to the databased using Eq. (1) revealing a finite size effect of the AF anisotropy.

Figure 2

(Color online) Vertically resolved element specific hysteresis loops measured at the $L_3$ resonant energy of Fe (filled circles) and Mn (open circles) for the sample measured by soft x-rays (see Fig. 3).

Figure 3

(Color online) Panel (a) The element specific reflectivities measured in a saturation field with positive and negative helicities at the $L_3$ resonant energy of Mn (639 eV). Panel (b) The asymmetry extracted from the reflectivities shown in panel a). Panel (c) The element specific reflectivities measured with positive helicity at the $L_3$ resonant energy of Mn and for two opposite magnetic fields. Panel (d) The asymmetry extracted from the reflectivities shown in panel (c). Fits to the experimental data are shown in all panels. Panel (e) Depth dependence of the frozen and rotating AF (Mn) uncompensated spins.