High-temperature 2D ferromagnetism in conjugated microporous porphyrin-type polymers

Abstract

The need for magnetic 2D materials that are stable to the enviroment and have high Curie temperatures is very important for various electronic and spintronic applications. We have found that two-dimensional porphyrin-type aza-conjugated microporous polymer crystals are such a material (Fe-aza-CMPs). Fe-aza-CMPs are stable to CO, CO₂, and O₂ atmospheres and show unusual adsorption, electronic, and magnetic properties. Indeed, they are semiconductors with small energy band gaps ranging from 0.27 eV to 0.626 eV. CO, CO₂, and O₂ molecules can be attached in three different ways where single, double, or triple molecules are bound to iron atoms in Fe-aza-CMPs. For different attachment configurations we find that for CO and CO₂ a uniform distribution of the molecules is most energetically favorable while for O₂ molecules aggregation is most energetically preferable. The magnetic moments decrease from 4 to 2 to 0 for singly, doubly, triply occupied configurations for all gasses respectively. The most interesting magnetic properties are found for O₂ molecules attached to the Fe-aza-CMP. For a single attachment configuration we find that an antiferromagnetic state is favorable. When two O₂ molecules are attached, the calculations show the highest exchange integral with a value of J = 1071 μeV. This value has been verified by two independent methods where in the first method J is calculated by the energy difference between ferromagnetic and anitferromagnetic configurations. The second method is based on the frozen magnon approach where the magnon dispersion curve has been fitted by the Ising model. For the second method J has been estimated at J = 1100 μeV in excellent agreement with the first method.