Origin and properties of an unexpected exchange bias of Ta/Ni80Fe20/Ir20Mn80/Ta heterostructure in ultrathin limit: Impact of the oblique deposition and Ta/Ni80Fe20 alloying

•Shadow effect and interfacial alloy impacting ultrathin film magnetic properties.•Unexpected exchange bias of Ta/NiFe/IrMn/Ta heterostructures.•Investigation of ultrathin dimension system allows sublayer resolution. Ultrathin Si(100)/Ta/Ni80Fe20/Ir20Mn80/Ta heterostructure deposited by oblique incidence in a confocal magnetron sputtering exhibits an unexpected exchange bias effect, despite the rotation of the sample holder and absence of external magnetic field during the deposition. Its magnetic loops have anomalous shifts along the field axis, whose features are non-uniform and strongly dependent on the orientation of the applied magnetic field. Structural, magnetic and thermal properties of the heterostructure were investigated using x-ray scattering and magnetic measurements to highlight the role of interfaces between ultrathin films and the related magnetic effects on the setting of the exchange bias. We show that the shadowing effect due to the oblique incidence of the sputtering flux combined to the atomic diffusion at the Ta/Ni80Fe20 interface favors the emergence of a uniaxial anisotropy in the ferromagnetic layer formed by a Ni80Fe20 sublayer at the top and a NiFeTa alloy at the bottom. The later increases the uniaxial anisotropy constant of the ferromagnet, which was strong enough to impose a well-defined direction for the spontaneous magnetization and, consequently, set the unidirectional anisotropy during the Ir20Mn80 layer formation. The thermal processes can reversibly be applied up to 460 K and, for higher temperatures, there were the vanishing of the exchange bias at 600 K and a thickening of the NiFeTa alloy which led to a reduction in the saturation magnetization of the heterostructure. Therefore, our findings suggest that the role of the interfacial NiFeTa alloy is only revealed in the ultrathin dimension and is imperceptible in thicker samples.