Iron nanoparticles (NPs) prepared by inert gas condensation were studied using high resolution transmission electron microscopy and Wulff construction shape analysis. The NP size and shape show strong dependence on the magnetic field above the target surface. The effect of the magnetic field could be tuned by adjusting the thickness of the protective backing plate positioned in-between the target and the magnetron head. With increasing backing plate thickness, the particle size decreases and the NP morphologies evolve from faceted to close-to-spherical polyhedral shapes. Moreover, with changes in size and shape, the particle structure also varies so that the NPs exhibit: (i) a core-shell structure for the faceted NPs with size similar to 15-24 nm; (ii) a core-shell structure for the close-to-spherical NPs with size similar to 8-15 nm; and (iii) a fully oxidized uniform structure for NPs with sizes less than similar to 8 nm having a void in the center due to the Kirkendall effect. The decrease of NP size with the increasing backing plate thickness can be attributed to a reduced magnetic field strength above the iron target surface combined with a reduced magnetic field confinement. These results pave the way to drastically control the NP size and shape in a simple manner without any other adjustment of the aggregation volume within the deposition system. Published by AIP Publishing.