Control of Magnetism by Oxygen Adsorption in Magnetic Sponge Materials

A class of porous metal-organic framework systems, called "magnetic sponge" is attracting interest as gas-sensing material with high designability. One of the systems, [{Ru₂(3,5-F₂PhCO₂)₄}₂{TCNQ(MeO)₂}]3(DCM)1.5(DCE), is a magnetically active compound with a high capability of gas adsorption. While it shows uniform magnetic ordering below around 80 K at normal pressures, its magnetization profile strongly depends on the species of adsorbed gas. In particular, oxygen adsorption sensitively changes the magnetization process according to the gas pressure and alters the low-temperature state to antiferromagnetic. In this contribution, we focus on the antiferromagnetic interaction between the adsorbed oxygen (O) molecules and TCNQ complex and construct an effective spin model with temperature-dependent magnetic interactions by integrating out the O degrees of freedom. The resultant model well explains the magnetization process and the adsorption isotherm at the same time. In particular, we clarify why magnetism can be controlled by oxygen, despite the considerable difference in energy scale between the magnetic interaction and adsorption energy.