Magnesium has a first ionization energy of approximately 738 kJ/mol, while aluminum has a lower first ionization energy of approximately 578 kJ/mol due to the loss of a 3p electron rather than a paired 3s electron.
The claim is a well-established fact of inorganic chemistry and standard periodic trends (aluminum's outer electron resides in a 3p orbital, which is slightly higher in energy and shielded better than magnesium's 3s valence electrons). None of the retrieved papers specifically address or measure first ionization energies of Mg and Al, as it is foundational textbook knowledge.