Temperature dependence of magnetic coupling in ultrathinNiO/Fe3O4(001)films

We have studied the evolution of nanometer-sized magnetic domains in exchange-coupled $\text{NiO}/{\text{Fe}}_3{\text{O}}_4\left(001\right)$ bilayers as a function of temperature. To image magnetic domains in thin antiferromagnetic (AF) NiO and in ferrimagnetic (FM) ${\text{Fe}}_3{\text{O}}_4\left(001\right)$ layers we have used element specific x-ray photoemission electron microscopy. The epitaxial growth of $\text{NiO}/{\text{Fe}}_3{\text{O}}_4\left(001\right)$ leads to a parallel orientation between the two oxide lattices. The magnetic order-disorder transition temperature of a 2-nm-$\text{NiO}/{\text{Fe}}_3{\text{O}}_4\left(001\right)$ film is found to be $T_{\text{N}}=375\text{K}$, much lower than for bulk $\text{NiO}$ $\left(T_{\text{N}}=520\text{K}\right)$. Setting the temperature below or above the Néel temperature $\left(T_{\text{N}}\right)$ of ultrathin NiO films, the influence of the interlayer exchange coupling on the microscopic domain patterns could be studied. Below $T_{\text{N}}$, the two layers reveal parallel coupling and an induced twofold degenerated spin configuration. Small nanometer domains are observed inside the larger micrometer magnetic domain of ${\text{Fe}}_3{\text{O}}_4\left(001\right)$. Alternating direction of the spins, pointing along $⟨100⟩$ and $⟨120⟩$ are present in both the AF and FM layers. Above $T_{\text{N}}$, the AF contrast in the magnetic image of NiO vanishes and simultaneously large micrometer magnetic domains appear in ${\text{Fe}}_3{\text{O}}_4\left(001\right)$, showing magnetic contrast images of uncoupled spins oriented parallel to $⟨100⟩$. The consequences of the microscopic magnetic domain patterns in FM/AF coupled systems are discussed in terms of macroscopic magnetic properties.