Origin of the Asymmetric Magnetization Reversal Behavior in Exchange-Biased Systems: Competing Anisotropies

The magnetization reversal in exchange-biased ferromagnetic-antiferromagnetic (FM-AFM) bilayers is investigated. Different reversal pathways on each branch of the hysteresis loop, i.e., asymmetry, are obtained both experimentally and theoretically when the magnetic field is applied at certain angles from the anisotropy direction. The range of angles and the magnitude of this asymmetry are determined by the ratio between the FM anisotropy and the interfacial FM-AFM exchange anisotropy. The occurrence of asymmetry is linked with the appearance of irreversibility, i.e., finite coercivity, as well as with the maximum of exchange bias, increasing for larger anisotropy ratios. Our results indicate that asymmetric hysteresis loops are intrinsic to exchange-biased systems and the competition between anisotropies determines the asymmetric behavior of the magnetization reversal.

Figure 1

Kerr magnetization curves $M(H)$ of a $18\mathrm{nm}\mathrm{Co}/5\mathrm{nm}\mathrm{IrMn}$ film with the field applied at 13.5° with respect to the easy axis. This angle was chosen to illustrate the clear asymmetry in the hysteresis loops. The symbols represent experimental $M_{\parallel }$ (a) and $M_{\perp }$ (b) loops, and the continuous lines correspond to simulated curves (see text).

Figure 2

Kerr magnetization curves $M(H)$ of $6\mathrm{nm}\mathrm{Co}/5\mathrm{nm}\mathrm{IrMn}$ (left panels) and $18\mathrm{nm}\mathrm{Co}/5\mathrm{nm}\mathrm{IrMn}$ (right panels) films from series I with the field applied at different angles with respect to the easy axis direction. The symbols are experimental loops $M_{\parallel }$ (solid) and $M_{\perp }$ (open).

Figure 3

Angular dependence of the normalized exchange bias (top panels), coercivity (middle), and asymmetry (bottom) of exchange-coupled $\mathrm{Co}/\mathrm{IrMn}$ films from series I. The lines are simulations of the experimental data (symbols) using a modified Stoner-Wohlfarth model (see text). The range of angles where only reversible processes take place during the reversal are marked by shadowed areas.

Figure 4

(a) Astroid representation of the switching behavior. The continuous, dashed, and dotted straight lines represent fields applied at angles with the easy axis of 10° (asymmetric-irreversible), ${\alpha }_C$ (critical angle), and 60° (symmetric-reversible), respectively. The corresponding switching fields of the irreversible transitions in the decreasing (increasing) field branch are depicted by filled white (black) circles in the asteroid. (b) Dependence of the experimental critical angle, ${\alpha }_C$, on the uniaxial/unidirectional anisotropy ratio $K_U/K_E$ for series I (black circles) and series II (gray circles). The continuous line is the expected evolution from the asteroid representation.