Sharp Change in the Exchange Bias and the Magnetic Anisotropy Symmetry at a Subthreshold Interlayer Copper Content in NiFe/Cu/IrMn Heterostructures

The NiFe/Cu/IrMn heterostructures with a variable number of interlayer Cu atoms exhibit a sharp change in the exchange-bias field, the coercive force, and the unidirectional anisotropy field at an effective copper layer thickness t(Cu) approximate to 0.5 nm, which corresponds to incomplete coverage of the interface, i.e., to an island deposited structure. The symmetry of the angular dependence of the ferromagnetic resonance changes sharply at an incompletely coated interface (t(Cu) = 0.5 nm), which corresponds to a transition from isolated islands to a magnetic fractal structure. This phenomenon, which can be called a "magnetic" percolation threshold is not related to the electrical resistance of the heterostructure, which decreases sharply at a significantly larger effective threshold copper layer thickness (t(Cu) = 1.3 nm) when the interface is completely covered.