Most of the large terrestrial bodies in the solar system display evidence of past and/or current magnetic activity, which is thought to be driven by thermo-chemical convection in an electrically conducting fluid layer. The discovery of a large number of extrasolar planets motivates the search of magnetic fields beyond the solar system. While current observations are limited to their radius and minimum mass, studying the evolution of exoplanetary magnetic fields and their interaction with the atmosphere can open new avenues for constraining interior properties from future atmospheric observations.
Here, we investigate the evolution of massive planets ($0.8-2$~$M_{\rm Earth}$) with different bulk and mantle iron contents. Starting from their temperature profiles at the end of accretion, we determine the structure of the core and model its subsequent thermal and magnetic evolution over $5$~Gyr. We find that the planetary iron content strongly affects core structure and evolution, as well as the lifetime of a magnetic field. Iron-rich planets feature large solid inner cores which can grow up to the liquid outer core radius, shutting down any pre-existing magnetic activity. As a consequence, the longest magnetic field lifetimes ($\sim 4.15$~Gyr) are obtained for planets with intermediate iron inventories ($50-60$~wt.\%). The presence of a small fraction of light impurities keeps the core liquid for longer and extends the magnetic field lifetime to more than $5$~Gyr. Even though the generated magnetic fields are too weak to be detected by ground facilities, indirect observations can help shedding light on exoplanetary magnetic activity.