  We use sensitive dynamic cantilever magnetometry to measure the magnetic
hysteresis of individual magnetic Janus particles. These particles consist of
hemispherical caps of magnetic material deposited on micrometer-scale silica
spheres. The measurements, combined with corresponding micromagnetic
simulations, reveal the magnetic configurations present in these individual
curved magnets. In remanence, ferromagnetic Janus particles are found to host a
global vortex state with vanishing magnetic moment. In contrast, a remanent
onion state with significant moment is recovered by imposing an exchange bias
to the system via an additional antiferromagnetic layer in the cap. A robust
remanent magnetic moment is crucial for most applications of magnetic Janus
particles, in which an external magnetic field actuates their motion.
