Temperature-dependent domain investigations on exchange-biased IrMn-NiFe and IrMn-CoFe systems

The magnetic microstructure and magnetization processes of sputtered IrMn-NiFe exchange-biased films were studied by Kerr microscopy and magneto-optical magnetometry in a temperature range from room temperature to 200°C. A ferromagnetic film in the exchange-biased state (as created by cooling a regular domain pattern or a saturated state from above the blocking temperature) is reversed in a patch-like way in applied magnetic fields, indicating inhomogeneities in the coupling. The domain processes are reversible far below the blocking temperature T_B and get increasingly irreversible within a range of some 10 K around T_B. Magnetic anisotropies in the ferromagnetic layer, though clearly visible above the blocking temperature, have no significant influence on exchange bias. A granular frozen-in domain pattern is observed in as-deposited bilayers, indicating the presence of local exchange bias even without annealing.