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

We have studied the evolution of nanometer-sized magnetic domains in exchange-coupled NiO/Fe3O4(001) bilayers as a function of temperature. To image magnetic domains in thin antiferromagnetic (AF) NiO and in ferrimagnetic (FM) Fe3O4(001) layers we have used element specific x-ray photoemission electron microscopy. The epitaxial growth of NiO/Fe3O4(001) leads to a parallel orientation between the two oxide lattices. The magnetic order-disorder transition temperature of a 2-nm-NiO/Fe3O4(001) film is found to be T-N = 375 K, much lower than for bulk NiO (T-N=520 K). Setting the temperature below or above the Neel temperature (T-N) of ultrathin NiO films, the influence of the interlayer exchange coupling on the microscopic domain patterns could be studied. Below T-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 Fe3O4(001). Alternating direction of the spins, pointing along < 100 > and < 120 > are present in both the AF and FM layers. Above T-N, the AF contrast in the magnetic image of NiO vanishes and simultaneously large micrometer magnetic domains appear in Fe3O4(001), 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.