Theory of trapping of muon and muonium and associated hyperfine interactions in the organic ferromagnet p-NPNN (β-phase)

The ab initio unrestricted Hartree-Fock procedure has been applied to determine the trapping sites for the positive muon and muonium in β-phase ferromagnetic para-nitrophenyl nitronyl nitroxide and to calculate the associated electronic wave functions from which the corresponding contact and dipolar terms in the spin Hamiltonians have been obtained. For muonium, trapping sites were found near the oxygens of the two NO groups, resulting in a singlet electronic state for the overall molecular system, and also near the two oxygens of the NO₂ group, resulting in a triplet state for the overall system. For the muon a total of four trapping sites was found, corresponding to the oxygen and nitrogen atoms of the two NO groups. Using the easy axis along the b-axis of the orthorhombic sublattice, as found from muon spin rotation (μSR) measurements, and the calculated magnetic hyperfine interaction parameters, the observed 2.1 MHz zero-field μSR signal is assigned to the singlet state corresponding to muonium trapping near the oxygen of one of the NO groups. The large hyperfine constant of about 400 MHz inferred from longitudinal field repolarization measurements is assigned to a positive muon trapped near the nitrogen atom of one of the NO groups.