Inelastic neutron scattering experiments are performed on a nanoporous metal complex Cu-Trans-1,4-Cyclohexanedicarboxylic Acid (Cu-CHD) adsorbing O2 molecules to identify the spin model of the O2-based magnet realized in the host complex. It is found
that the magnetic excitations of Cu-CHDs adsorbing low- and high-concentration O2 molecules are explained by different spin models, the former by spin dimers and the latter by spin trimers. By using the obtained parameters and also by assuming that the levels of the higher energy states are reduced because of the non-negligible spin dependence of the molecular potential of oxygen, the magnetization curves are explained in quantitative level.
