Asymmetric Reversal Modes in Ferromagnetic/Antiferromagnetic Multilayers

Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems. From a numerical investigation of the domain state model evidence is gained that this effect depends on the angle between the easy axis of the antiferromagnet and the applied magnetic field. Depending on this angle the ferromagnet reverses either symmetrically, e.g., by a coherent rotation on both sides of the loop, or the reversal is asymmetric with a nonuniform reversal mode for the ascending branch, which may even yield a zero perpendicular magnetization.

Figure 1

Projection of the magnetization along (a) and perpendicular to (b) the applied field $\underline{B}$. (c) The AFM interface magnetization (along the $z$ direction).

Figure 2

Magnetization $m_y$ versus $m_z$ for different angles $\theta$; 1 and 3 indicate points with maximum external field $|\underline{B}|$ and 2 and 4 points where $m_{\parallel }$ vanishes. The horizontal dashed line corresponds to the easy axis of FM and AFM and the other dashed line to the field axis.

Figure 3

Sketch of the effective field contributions from external field, $H_Z$, exchange field of AFM $H_X$, and uniaxial anisotropy $H_A$ acting on the FM close to the coercive fields $B^{+}$ and $B^{-}$ before the reversal sets in.

Figure 4

Coercive field values $B^{+}$ and $B^{-}$ versus angle between field and easy axis of FM and AFM.