  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.
