  The recording of planetary magnetic fields is often attributed to
uniformly-magnetised nanoscopic iron oxides, called single-domain (SD). Yet,
the main magnetic constituents of rocks are more complex, non-uniformly
magnetised grains in single or multi-vortex states. We know little about their
behaviour due to limitations in defining their precise shape and internal
magnetic structure. We propose a novel approach combining non-destructive
synchrotron-based ptychographic nanotomography with micromagnetic modelling to
explore the magnetic stability of remanence-bearing minerals. Applied to a
microscopic rock sample, we identified hundreds of nanoscopic grains of
magnetite/maghemite with diverse morphologies. For some grains, shape
irregularities near the transition from SD to the single-vortex state allow for
multiple domain states, some unstable and unable to record the field for
significant periods. Additionally, some other grains exhibit
temperature-dependent occupancy probabilities, potentially hampering
experiments to recover the intensity of past magnetic fields.
