Magnetic hysteresis of individual Janus particles with hemispherical exchange biased caps

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.