Theoretical studies of implanted muons in organic magnets

Wholly organic ferromagnets are a new class of materials of great potential interest, but the early examples exhibit only very weak magnetic order at very low temperatures (M. Tamura et al., Chem. Phys. Lett., 186 (1991) 401). In these circumstances, muon spin rotation (μSR) is among the most sensitive sources of information about the magnetic structure and interactions on a molecular scale (F.L. Pratt et al., Synthetic Metals, 61 (1993) 171). However, interpretation of the μSR data is complicated because of the large number of binding sites made possible by the relatively complicated molecular structure. We have performed semi-empirical and ab initio density functional calculations of the electronic and molecular structure of muonium incorporated into the organic ferromagnets para-nitrophenyl α-nitronyl nitroxide (p-NPNN), 3-quinolyl nitronyl nitroxide (3-QNNN) and para-pyridyl nitronyl nitroxide (p-PYNN). We find evidence for a wide variety of possible muon binding sites including sites in the conjugated ring systems of the materials as well as in the nitronyl nitroxide group. Our calculations also suggest the formation of local triplet electronic states near the muon within the spin-1/2 magnetic system.