Magnetic multicore nanoparticles were produced by crystallisation of a glass with the composition Na2O center dot Al2O3 center dot B2O3 center dot SiO2 center dot Fe2O3. During cooling of the melt, in a first step, phase separation occurred and droplets enriched in boron and iron oxide were formed. These droplets crystallised spontaneously during cooling. The phase-separated droplets had sizes in the range from 200 to 800 nm. Inside the droplets, magnetite crystals with a mean size of 33 nm occurred. Magnetometer measurements showed the occurrence of a small hysteresis which indicates predominantly superparamagnetic behaviour of the magnetite crystals. The magnetic domains of the phase-separated droplets were studied by magnetic force microscopy. In this article, a glass which is exposed to a magnetic field shows droplet-shape phase separations where the single-domain magnetite crystals do not have a preferred orientation of the magnetic dipoles. By contrast, the whole droplet shows one magnetic dipole parallel to the external field if the glass is exposed to a magnetic field during measurement.