  In this letter, we address the magnetization switching by oxygen adsorption,
observed in porous metal-organic framework systems. To this aim, we construct a
simple localized spin model combined with the Langmuir-type formula for the
oxygen adsorption and study its finite-temperature properties with Monte Carlo
simulation. We successfully explain the main features of this phenomenon, such
as discontinuous changes of magnetic states, the sensitivity of magnetic
transition temperatures to oxygen pressure, and absence of singularity in
adsorbed oxygens. On the basis of this model, we also reproduce the observed
magnetic transition temperatures from a typical value of oxygen adsorption
energy.
