Critical Co Thickness for Magnetic-Field-Induced Reorientation of the NiO Néel Vector

Antiferromagnets offer intrinsic stability against external magnetic fields, but this robustness also makes controlled manipulation of their spin structure challenging. Here we demonstrate that a thin ferromagnetic Co overlayer enables magnetic-field-induced reorientation of the Néel vector in NiO, mediated by strong interfacial coupling rather than conventional exchange bias. Using x-ray magnetic circular and linear dichroism, we show that the NiO spin structure partially aligns with the Co magnetization under applied fields, resulting in a direct correlation between ferromagnetic and antiferromagnetic domain patterns. Systematic variation of the Co thickness reveals a crossover between two distinct coupling regimes. For thin Co layers, NiO follows the Co magnetization during field cycling, whereas thicker Co layers exhibit conventional exchange bias, with NiO acting as a pinning layer. This crossover occurs at a Co thickness of approximately 3 nm, which marks the transition between inverse coupling and exchange-bias-dominated behavior.